COVER PHOTOGRAPH of San Francisco Bay Region taken April 14, 1972, 

at altitude of 65,000 feet from U-2 aircraft. Courtesy National Aeronautics 

and Space Administration (Ames Research Center, Moffett Field, Calif.) Front 

shows city of San Francisco and Golden Gate at bottom, San Francisco Bay and 

city of Oakland in middle, Sacramento-San Joaquin Delta and crest of Sierra 

Nevada at top. Back shows Bolinas Lagoon and trace of San Andreas fault at 

bottom, San Pablo Bay in middle, Sacramento valley and crest of Sierra Nevada 

at top. 
Flood-prone areas and land-use planningSelected examples from the San Francisco 

Bay Region, California 

By A. 0. WAANANEN, J. T. LIMERINOS, and W. J. KOCKELMAN, ... U.S. GEOLOGICAL SURVEY, and 

W. E. SPANGLE and M. L. BLAIR, WILLIAM SPANGLE & ASSOCIATES 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 942 

jointly supported by the U.S. Geological Survey 

and the Department of Housing and Urban 

Development, Office of Policy Development and Research, 

as part of a program to develop and apply earth-science 

information in support of land-use planning and decisionmaking 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1977 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 77-77832 

First printing 1977 

Second printing 1977 

For ~ale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-02970-7 
FOREWORD 

This report is a product of the San Francisco Bay Region 

Environment and Resources Planning Study, an experimental 

program designed to facilitate the use of earth-science information in regional planning and decisionmaking. The study is 

supported jointly by the U.S. Geological Survey, Department of 

the Interior, and the Office of Policy Development and Research, 

Department of Housing and Urban Development. The Association of Bay Area Governments participates in the study and 

provides a liaison and communication link with other regional 

planning agencies and with county and local governments. 

Although the study focuses on the 9-county 7,4000-squaremile San Francisco Bay region, it bears on an issue that is of 

national concern. This issue-how best to accommodate orderly 

development and growth while conserving our natural resource 

base, insuring public health and safety, and minimizing degradation of our natural and manmade environment-is difficult and 

complex. The complexity, however, can be greatly reduced if we 

understand the natural characteristics of the land, the processes 

that shape it, its resource potential, and its natural hazards. These 

subjects are chiefly within the domain of the earth sciences: 

geology, geophysics, hydrology, and the soil sciences. Appropriate 

earth-science information, if available, can be applied rationally 

in guiding growth and development, but the existence of the information does not assure its effective use in the day-to-day decisions 

that shape development. Planners, elected officials, and the public 

rarely have the training or experience needed to recognize the 

significance of the basic earth-science information, and many of 

the conventional methods of communicating earth-science information are ill-suited to their needs. 

It was hoped that the study would aid the planning and 

decisionmaking community by (1) identifying important problems that are rooted in the earth sciences and related to growth 

and development in the bay region; (2) understanding the relationships between the problems; (3) providing the earth-science 

information that is needed to solve these problems; ( 4) interpreting 

and publishing findings in forms understandable to and usable by 

nonscientists; (5) establishing new avenues of communication 

between scientists and users; and (6) exploring alternate ways of 

applying earth-science information in planning and decisionmaking. 

Since the study was started in 1970, it has produced more than 

100 reports and maps. These cover a wide range of topics: flood 

and earthquake hazard reduction, unstable slopes, engineering 

characteristics of hillside and lowland areas, mineral and waterresources management, solid and liquid waste disposal, erosion 

and sedimentation problems, bay water circulation patterns, and 

others. The methods used in the study and .the results it has 

produced have elicited broad interest in a wide range of applications from planners, government officials, industry, universities, and the general public. 

The present report, "Flood-Prone Areas and Land-Use 

Planning-Selected Examples from the San Francisco Bay 

Region, California," examines the problem of flooding in the San 

Francisco Bay region, describes the preparation and use of 

various types of flood maps and flood information reports, lists 

sources of information on flooding and flood plains, discusses 

flood-loss prevention and reduction measures, and discusses the 

role of comprehensive planning in flood-plain management. 

Adequate information on flood-prone areas and flood hazards 

is essential to effective planning and management. Many sources 

of such information are available, and representative samples of 

importance in planning are discussed in this report. How this 

information is used by agencies involved in managing flood 

plains is shown by a case study involving Napa County. The 

section also shows the interplay among these agencies. 

The methods of gathering and using flood-plain information 

for planning in the San Francisco Bay region are applicable in 

other parts of the country. The examples given in this report are 

selected to show both the wide range of tools available to the 

planner, and how those tools may be applied to deal most 

effectively with flood hazards. 

Project Director 

San Francisco Bay Region Study 

III 

CONTENTS 

Page 

Foreword ------------------------------------------------------------------------------------------------· III 

Conversion factors ------------------------------------------------------------------------------ VI 

Definition of terms ------------------------------------------------------------------------------·VII 

Abstract ---------------------------------------------------------------------------------------------------- 1 

Introduction --------------------------------------------------------------------------------------------· 1 

The San Francisco Bay region --------------------------------------------------· 2 

Acknowledgments ------------------------·-------------------------------------------------· 6 

Floods and the flood plain, by A. 0. Waananen ------------------------· 7 

Major floods in the region ----------------------------------------------------------- 7 

Characteristics of floodflows ----------------------------------------------------- 8 

Flood frequency -----------------------------------------------------------------·---·- 8 

Stage-discharge relation ·-·-·-·-------------------------------------------····· 9 

Characteristics of the flood plain --------------------------------------------- 9 

Manmade influences on potential flooding ___________________________ 11 

Influence of urbanization on potential flooding ......... 12 

Watershed management ------------------------------------------------------- 13 

Structural measures --------------------------------------------------------------- 13 

Obstructions ----------------------------------------------------·------------------------14 

Delineation of flood plains, by J. T. Limerinos --------------------------- 14 

Flood-plain inundation maps -------------------------····------------------------ 14 

Maps of flood-prone areas --------------------------------------------------- 15 

Flood-hazard maps ----------------------------------------------------------------- 17 

Flood-plain information reports ------------------------------------------------- 19 

Flood insurance studies --------------·------------------------------------·----------' 21 

Other flood-plain mapping --------------------····--------------------------------- 21 

Interpretation of flood-plain maps and information for 

site evaluation ---------------------------------------------------------------------------- 22 

Sources of flood-plain maps and information ----------------------· 23 

Flood-loss prevention and reduction measures, by 

W. J. Kockelman -------------------------------------------------------·-·········-------------- 23 

Protection of existing development ---------------------------------------- 24 

Removal or conversion of existing development ---------------- 26 

Discouragement of development ----------------------------------------------- 27 

Regulation of flood-plain uses -------------------------------------------------- 28 

Planning for flood-loss reduction, by W. E. Spangle and 

M. L. Blair --------------·---------------------·-···----------------------------------·--······ 30 

The planning process ---------------------------------------------------------·-------- 31 

Flood-prone areas-Planning for appropriate land uses 32 

Role of land capability studies ------------------------------------------ 34 

Page 

Planning for flood-loss reduction-Continued 

Implementation of land-use plans for flood-loss reduction 35 

Land-use and development regulation ............................... 36 

Program development and execution .................................. 37 

Project review ------------------------------------------------------------------------------ 38 

Governmental organization for flood-loss reduction in the San 

Francisco Bay region, by M. L. Blair and W. E. Spangle 38 

Federal programs ------------------------------------------------------------------------------ 39 

National Flood Insurance Program -------------·-···------------------ 39 

Federal Disaster Assistance Program -----------·-------------------- 41 

U.S. Army Corps of Engineers--------------·-----------------------······· 42 

Comprehensive Planning Assistance Program -------------- 42 

Community Development Block Grant Program .......... 43 

U.S. Soil Conservation Service and resource 

conservation districts --------------------------------------------------------- 43 

Regional and local programs ------------------------------------------------------- 44 

Flood control districts ------------------------------------------------------------- 44 

Regional agencies --------------------------------------------------------------------- 45 

Planning for flood-loss reduction in the Napa Valley, by 

M. L. Blair ---------------------------------------------------------------------------------------- 46 

Description ------------------------------------------------------------------------------------······ 46 

Flooding problems in the Napa Valley ------------------------------------- 48 

Planning for Napa Valley-Regional perspective --------------- 50 

Plan formulation -----------------------------------------------------------------------· 50 

Plan implementation ---------------------------------------------------------------- 53 

Planning for flood-prone areas-Napa County ---------------------·54 

Plan formulation -----------------------------------------------------------------------· 54 

Plan implementation -----------------------------------------------·--------------- 55 

The Napa County Flood Control and Water 

Conservation District --------------------------------------------------------- 58 

Planning for flood-prone areas-City of Napa------------------····· 58 

Plan formulation -------------------------------·--------------------------------------- 59 

Plan implementation -----------------------·····----------------------------------- 59 

Significance of the Napa Valley experience --------------------------- 63 

Summary ----------------------------------------------------------------------------------------------------- 65 

References cited -------------------------------------·--------------------------------------------------- 68 

Selected readings -------------------------------------------------------------------------·------------- 70 

Sources of flood-plain maps and information -------------------·------------- 70 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing principal streams and drainage basins in the San Francisco Bay region --------------------------------------·········------------- 3 

2. Map showing mean annual precipitation in the San Francisco Bay region ------------------------------------------------------------------------------------ 5 

3. Diagram showing the hydrologic cycle -------------------------------------------------------------------------------------------------------------------------------------------------------·- 7 

4. Graph showing flood-frequency curve for a hypothetical basin in the San Francisco Bay region ----------------------------------------- 9 

5. Diagram showing typical sections and profiles in an unobstructed reach of stream valley ----------------------------------------------------- 10 

6. Diagram showing riverine flood-hazard areas of a regulatory flood plain --------------------------------------------------------------------------------------- 11 

7. Graph showing ratios of peak flows for the 50- and 100-year floods in urbanized and unurbanized basins in the 

San Francisco Bay region ---------·--------------------------------------------------·------------------------------------------------------------------------------------------------------------------ 12 

8. Map showing part of the flood-prone area map for the Napa area ----------------------------------------------------------------------------------------------------- 16 

9. Diagram showing areas encompassed by the regional flood-prone area map sheets for the San Francisco Bay 

region ······------------·------------------------------------------------------------·------------------------------------------------------------------------------------------------------------------------------------- 17 

v 
VI CONTENTS 

Page 

FIGURE 10. Regional flood-prone area map for the Napa area ----------------------------------------------------------------------------------------------------------------------------------- 18 

11. Flood-hazard map for the Napa area ----------------------------------------------------------------------------------------------------------------------------------------------------------- 20 

12. Graph showing profiles of floods on Napa River downstream from mouth of Dry Creek ------------------------------------------------------- 23 

13. Graph showing frequency of floods above 39-foot elevation at gaging station on Napa River near Napa, California 23 

14. Flow diagram showing the planning process ------------------------------------------------------------------------------------------------------------------------------------------- 31 

15. Map showing resource conservation districts and U.S. Soil Conservation Service field offices, San Francisco Bay 

region ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 43 

16. Map showing Napa County and Napa Valley planning area------------------------------------------------------------------------------------------------------------- 47 

17. Aerial photograph showing Napa River, city of Napa ------------------------------------------------------------------------------------------------------------------------- 48 

18. Graph showing annual floods above 47-foot elevation, 1900-1967, Napa River near Napa, California ----------------------------- 49 

19-22. Maps showing19. Flood plains in Napa County ----------------------------------------------------------------------------------------------------------------------------------------------------------------- 50 

20. Prime agricultural lands in Napa County ----------------------------------------------------------------------------------------------------------------------------------------- 51 

21. Urban areas and areas with slopes less than 15 percent in Napa County ---------------------------------------------------------------------------51 

22. Association of Bay Area Governments regional plan for Napa County -------------------------------------------------------------------------------52 

23. Diagram showing Napa Valley Area Plan------------------------------------------------------------------------------------------------------------------------------------------------- 55 

24. Diagram showing proposed land-use plan for city of Napa ----------------------------------------------------------------------------------------------------------------- 60 

25. Map showing U.S. Army Corps of Engineers flood-control project and related projects in the city of Napa ------------------- 61 

26. Map showing Parkway Plaza Redevelopment Project in the city of Napa ----------------------------------------------------------------------------------- 64 

27. Photographs showing part of Parkway Plaza during and after redevelopment--------------------------------------------------------------------------- 66 

28. Index map showing the topographic map sheets and the flood-prone area maps available in the San Francisco Bay 

region ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 71 

TABLES 

Page 

TABLE 1. Flood-loss prevention methods and devices ------------------------------------------------------------------------------------------------------------------------------------------------ 24 

2. Expected costs associated with stream flooding --------------------------------------------------------------------------------------------------------------------------------------· 35 

3. Primary functions of flood control districts in the San Francisco Bay region ---------------------------------------------------------------------------- 44 

4. Major floods at the U.S. Geological Survey gaging station Napa River near Napa, California, 1900-1967 ----------------------· 49 

5. Estimated damage from three floods on the Napa River ---------------------------------------------------------------------------------------------------------------------- 50 

6. Categories ofland serving open-space purposes -------------------------------------------------------------------------------------------------------------------------------------- 52 

7. Recommended acreage for each open-space category in Napa County-------------------------------------------------------------------------------------------- 52 

8. Population and land-use projections for the Napa basin, 1960-2060 ------------------------------------------------------------------------------------------------ 62 

9. Sources of maps and flood information ------------------------------------------------------------------------------------------------------------------------------------------------------ 70 

10. Flood-plain maps and information available for areas in the San Francisco Bay region -------------------------------------------------------- 72 

CONVERSION FACTORS 

[Factors for converting English units to metric units are shown to four significant figures. However, in the text the metric equivalents are shown only to the number of significant 

figures consistent with the values for the English units] 

English 

acres 

acre-ft (acre-feet) 

ft (feet) 

ft3/s (cubic feet per second) 

in. (inches) 

mi (miles) 

mi2 (square miles) 

Multiply by4.047 X 10"3 

1.233 X 10"3 

1.233 X 103 

3.048 X 10"1 

2.832 X lQ-2 

2.540 X 101 

1.609 

2.590 

Metric 

km2 (square kilometres) 

hm3 (cubic hectometres) 

m3 (cubic metres) 

m (metres) 

m 3 Is ( cu hie metres per second) 

mm (millimetres) 

km (kilometres) 

km2 (square kilometres) 
CONTENTS VII 

DEFINITION OF TERMS 

Physiographic, h~drologic, hydraulic, and selected administrative terms used in this report are defined as follows: 

Acre-foot is the quantity of water required to cover 1 acre to a 

depth of 1 foot, and is equivalent to 43,560 cubic feet, 325,871 

gallons, or 1,233 cubic metres. 

Area plan establishes policy, expanding upon and consistent 

with a comprehensive plan, for a part of a planning jurisdiction, 

such as a watershed or a central business district. 

Channel is an open conduit either naturally or artificially 

created which periodically or continuously contains moving 

water, or which forms a connecting link between two bodies of 

water. River, creek, run, branch, and tributary are some of the 

terms used to describe natural channels. 

Comprehensive plan is a document setting forth official 

governmental policy for the long-term future development of an 

area considering all major determinants of growth and changeeconomic, political, social, and physical. 

Cubic foot per second is the rate of discharge representing a 

volume ofl cubic foot passing a given point during 1 second, and is 

equivalent to 7.48 gallons per second, 448.8 gallons per minute, or 

0.02832 cubic metres per second. 

Discharge is the volume of water (or more broadly, total 

. fluids) that passes a given point within a given period of time. 

Drainage area of a stream at a specified location is that 

area, measured in a horizontal plane, which is enclosed by a 

drainage divide. 

Flood, or flooding, is the general and temporary condition 

of partial or complete inundation of normal dry land areas from 

(a) the overflow of streams, rivers, and other inland water, or 

(b) abnormally high tidal water or rising coastal waters resulting 

from severe storms, hurricanes, or tsunamis. Also, any relatively 

high streamflow overtopping the natural or artificial banks in any 

reach of a stream; or a relatively high flow as measured by either 

gage height or discharge quantity. 

Flood discharge is a quantitative measure of the rate of flow 

at a point during a flood event; it is usually expressed in cubic feet 

per second or cubic metres per second. 

Flood frequency is the average interval of time between 

floods equal to or greater than a specified discharge or stage; it is 

generally expressed in years (see recurrence interval). 

Flood-frequency curve is a graph which shows the probability of any given discharge being exceeded in any year, or 

which shows the recurrence interval, or return period, within 

which, on the average, a given discharge will be exceeded once by 

the annual peak discharge. 

Flood-hazard map is a map, based on a thorough technical 

study of waterways in a given locality, of sufficient scale and 

clarity to permit ready identification of individual sites as being 

either within or outside an area having special flood hazards. 

Flood insurance is the insurance coverage for both floods 

and mudslides obtained under the National Flood Insurance 

Program. 

Flood peak is the highest value of the stage or discharge 

attained by a flood; thus, peak stage or peak discharge. 

Flood plain is the land area adjoining a river, stream, 

watercourse, ocean, bay, or lake that has been or may be covered 

by floodwater. (This definition of flood plain may differ from that 

used in geologic and geomorphic writing.) 

Flood-plain management is the operation of an overall 

program of corrective and preventive measures for reducing flood 

damage, including but not limited to emergency proparedness 

plans, flood-control works, and land-use and control measures. 

Flood profile is a graph of elevation of the water surface of a 

river in flood in relation to the distance along the stream. A flood 

profile may be drawn to show elevation at a given time, crests 

during a particular flood, or stages for floods of any designated 

frequency. 

Flood-prone area map, as used in this report, is a map 

based on limited studies that indicates areas likely to be flooded by 

virtue of their location adjoining a river, stream, bay, or other 

watercourse or water body. 

Floodproofing is any combination of structural and nonstructural measures, changes, or adjustments to properties and 

structures which reduce or eliminate flood loss oflands, water and 

sanitary facilities, structures, and contents of buildings. 

Flood stage is the stage at which overflow of the natural 

banks of a stream begins to cause damage in the reach in which 

the elevation is measured. 

Flood wave is a distinct rise in stage culminating in a crest 

and followed by recession to lower stages. A flood wave may move 

downstream at a velocity greater than the average velocity of flow 

in a stream. 

Flood way is the channel of a river or other watercourse and 

the adjacent land areas required to carry and discharge a flood of a 

given magnitude. 

Functional plan describes facilities and operations for a 

specific function of government such as transportation, flood 

control, or flood-plain management; it is more specific and usually 

is shorter in range than a comprehensive plan. 

Gage height is the water-surface elevation referred to some 

arbitrary gage datum. Gage height is often used interchangeably 

with the more general term "stage" although gage height is more 

appropriate when used with a reading on a gage. 

Gaging station is a particular site on a stream, canal, lake, 

or reservoir where systematic observations of gage height or 

measurements of discharge are obtained. 

General plan is another term for a comprehensive plan. 

Infiltration is the flow of a fluid into a substance through 

pores or small openings, as the infiltration of water from precipitation or overland flow into the underlying soil. 

Land-use plan is a key component of a comprehensive plan 

including objectives, policies, and proposals for the type, pattern, 

and intensity ofland use in a planning area; it is also called a landuse element. 

Mudslide is the general and temporary movement down a 

slope of a mass of rock or soil, artificial fill, or a combination of 

these materials, caused or precipitated by the accumulation of 

water on or under the ground. 

National Flood Insurance Program is the Federally 

subsidized program established by the National Flood Insurance 

Act of 1968 (U.S. Congress, 1968) that provides previously unavailable flood-insurance protection to property owners in floodprone areas. Insurance protection is provided only in return for the 

participating communities' implementation of a flood-plain management program. 

One-hundred-year flood (100-year flood) is the level of 

flooding that will be equaled or exceeded once in 100 years and has 

a 1-percent chance of occurring each year, on the average. 

Peak discharge is the highest rate of streamflow at a point 

during a rise in the stream. Peak discharge is usually expressed in 

cubic feet per second or cubic metres per second. A peak discharge 
VIII CONTENTS 

that is the greatest in a 12-month period-usually October 

through September-is an annual peak discharge; a peak die: 

charge that causes flooding is a flood discharge. 

Peak stage is the maximum water-surface elevation during a 

rise in a stream and generally is coincident with the peak 

discharge. 

Recurrence interval, or return period, of a flood of a given 

magnitude is the average interval of time within which the given 

flood will be exceeded once by the annual maximum discharge. 

Regulatory flood discharge is the flood discharge selected 

to provide the basis for delineating that part of the flood plain that 

is to be managed or regulated to reduce flood damage (often the 

100-year flood). 

Regulatory flood fringe is the shoreward part of the 

regulatory flood plain that would carry shallow slow-moving 

water when inundated by the regulatory flood discharge. 

Regulatory flood level or profile is the stage along the 

stream that corresponds to the regulatory flood discharge. 

Regulatory flood plain is that part of the flood plain that 

would be inup.dated by the regulatory, or 100-year, flood discharge 

and consists of a regulatory floodway and a regulatory flood 

fringe. 

Regulatory flood profile; see regulatory flood level. 

Regulatory floodway includes the incised stream channel 

and the unobstructed part of the flood plain that would carry deep 

fast-moving water when inundated by the regulatory flood discharge. 

Runoff is that part of the precipitation that appears in 

surface streams. It is the same as "streamflow" unaffected by 

artificial diversions, storage, or other works of man in or on the 

stream channels. 

Stage is the height of a water surface above an established 

datum plane; also see gage height. 

Stage-discharge relation is the relation between gage 

height and the quantity of water flowing in a channel. 

Storm runoff is that portion of the total runoff that reaches 

the point of measurement within a relatively short period of time 

after the occurrence of precipitation. Also called direct runoff. 

Storm water is the excess water running off from the surface 

of a drainage area during and immediately after a period of rain or 

snowmelt. It is that portion of the rainfall and resulting surface 

flow that is in excess of that which can be absorbed through the 

infiltration capacity of the surface of the basin. See storm runoff. 

Streamflow is the discharge that occurs in a natural 

channel. Although the term "discharge" can be applied to the flow 

of a canal, the word "streamflow" uniquely describes the discharge in a surface stream course. The term "streamflow" is more 

general than "runoff' as streamflow may be applied to discharge 

whether or not it is affected by diversion or regulation. 

Stream valley includes both the incised stream channel and 

its flood plain. 

Watershed management is the analysis, protection, development, operation, or maintenance of the land, vegetation, and 

water resources of a drainage basin for the conservation of all its 

resources for the benefit of man ami other living creatures. 

Water-surface elevation is the height in relation to mean 

sea level expected to be reached by floods of various magnitudes 

and frequencies at pertinent points in the flood plains of coastal or 

riverine areas. 
FLOOD-PRONE AREAS AND LAND-USE PLANNINGSELECTED EXAMPLES FROM THE SAN FRANCISCO BAY REGION, 

CALIFORNIA 

By A. 0. Waananen, J. T. Limerinos, and W. J. Kockelman 

U. S. Geological Survey 

and 

W. E. Spangle and M. L. Blair 

William Spangle & Associates 

ABSTRACT 

Flood-plain occupance and use is often based on the economic 

advantages of level ground, fertile soils, ease of access, and 

available water supplies, without full consideration of flood risk. 

Structural measures have provided substantial protection in 

many areas, but full protection of flood plains from all floods is 

economically infeasible, and flood losses have been increasing. 

Thus, planning for flood-plain use to minimize flood damage is 

urgently needed. Flood-prone area maps and other flood information facilitate effective land-use planning by providing a 

regional overview of the potential flood hazard or detailed information for local planning. Various measures are available for 

reducing flood losses through protection, removal, or conversion 

of existing development, discouragement of development in highrisk areas, and regulation of uses on flood plains. 

Many Federal, State, and local governmental agencies are 

involved in flood-plain management and regulation, but often 

without coordinated authority and scope. The common goal of 

flood-plain regulation and use is protecting life, minimizing public 

expenditures, and reducing flood loss. A comprehensive program 

combining structural and nonstructural measures can yield substantial benefits and may present a practical approach for 

managing a flood plain. 

A review of flood-plain planning, management, and regulation in the San Francisco Bay region, as shown by a study ofN apa 

County, demonstrates complex multijurisdictional involvements. 

Yet, need exists for more adequate comprehensive planning and 

coordination to provide for sound development and use of flood 

plains. 

INTRODUCTION 

Floods are natural and recurrent events. They 

become a problem when man competes with rivers 

for the use of the flood plains-the high-water 

channels of rivers. 

Flood plains are occupied by man because they 

are usually accessible and profitable to develop. But 

continued use may be possible only at a price-to 

sustain flood loss or provide flood-control facilities. 

Full flood control often is not possible or feasible; yet, 

abandonment of the flood plain may not be reasonable because the flood plain is a valuable resource 

and, in many areas, will continue to be occupied. The 

nature and extent of flood-plain use should be compatible with the risk involved and the degree of 

protection that would be practicable. 

The alleviation or reduction of flood loss has 

been achieved traditionally through construction of 

protective works such as dams, dikes, levees, channel improvements, and seawalls. These structural 

measures have partially reduced hazards. But increasing development of urban flood plains, as reported by White (1960, 1975), has resulted in the 

paradox of continuing expenditures for flood control 

and rising flood losses. 

Recognition of the mounting flood losses despite 

flood-control efforts in the United States was contained in the report "A Unified National Program 

for Managing Flood Losses" prepared by a special 

Task Force on Federal Flood Control Policy (U.S. 

Congress, 1966a). The President, in his letter transmitting the report to the Speaker of the House of 

Representatives, observed that: 

Nature will always extract some price for use of her flood plains. 

However, this Nation's annual flood damage bill*** is excessive 

•••. Beyond the dollar loss the accompanying toll in personal 

hardship cannot be calculated. In addition, opportunities are 

being lost to use flood plain lands effectively for recreation and 

wildlife purposes. 

The Federal interest in this matter is beyond doubt. The Federal 

effort to cope with the problem will be unsparing. But I cannot 

overemphasize that very great responsibility for success of the 

program rests upon State and local governments, and upon 

individual property owners in hazard areas. The key to resolving 

the problem lies, above all else, in the intelligent planning for and 

State and local regulation of use of lands exposed to flood hazard. 
2 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

The Task Force report cited the increasing 

damage potential under existing policies through 

inadvertent encouragement of uneconomic use of 

flood plains. It presented recommendations for modifying and broadening policies and programs for flood 

control and flood-plain management so that any 

future flood-plain development would yield benefits 

in excess of costs. Executive Order 11296 (U.S. 

President, 1966), issued in response to these recommendations, directed the heads of the Federal executive agencies to provide leadership in preventing 

uneconomic uses and development of the Nation's 

flood plains. This order further instructed Federal 

agencies to evaluate flood hazards associated with 

Federal development proposals and instructed Federal grant, loan, and mortgage insurance agencies to 

evaluate flood hazards insofar as practical to preclude the uneconomic, hazardous, and unnecessary 

use of flood plains. 

Federal and State policies on flood management 

are now shifting from protective measures to a 

balance between structural and regulatory controls. 

Flood-plain management, including land-use regulation, can effectively reduce flood losses. Land-use 

regulations can be used alone orin combination with 

protective works. Information on the extent of the 

flood hazard, land use, and development is essential 

for effective application of regulatory controls. 

The move toward flood-plain management and 

the need for flood-plain information has been intensified by Federal legislation such as the National 

Flood Insurance Act of 1968 (U.S., Congress, 1968) 

and the Flood Disaster Protection Act of 1973 (U.S. 

Congress, 1973). 

The delineation on suitable maps of areas susceptible to flooding can provide a means for identifying and evaluating flood risk. Such information 

can be helpful to elected officials, administrators, 

planners, designers, engineers, and developers concerned with the effective and wise management and 

use of flood plains. 

Comprehensive discussions of the management 

and regulation of flood plains, including legal aspects, are presented in the reports "Regulations for 

Flood Plains," by Kusler and Lee (1972), and "Regulation of Flood Hazard Areas to Reduce Flood 

Losses," by the U.S. Water Resources Council (1971, 

1972). White (1975) presents a comprehensive discussion of the state of the art in flood-hazard reduction, with emphasis on research needs. Further, 

consideration ofnonstructural alternatives to reduce 

flood losses is required by Section 73 of the Water 

Resources Development Act of 1974 (U.S. Congress, 

1974c). 

The purpose of this report is to describe the 

development and use of flood-prone area maps for 

the San Francisco Bay region, the. application of 

these maps and other flood-plain information in 

regional and local land-use planning and in the use, 

regulation, and management of flood plains, and the 

measures which can prevent or reduce flood loss 

through control of flood-plain use and protective 

measures. The maps and related materials described 

are intended to serve as tools useful to regional and 

local planning and decisionmaking agencies. Part of 

the information and techniques presented can also 

be used by those involved in the reduction of flood 

losses in communities throughout the nation. The 

section "Planning for Flood-Loss Reduction in the 

Napa Valley" describes how earth-science information has been used in one area of the Bay region and 

shows the practical benefits and difficulties of applying such information. 

THE SAN FRANCISCO BAY REGION 

The San Francisco Bay region is composed of 

nine counties contiguous to San Francisco Bay 

(fig. 1). It includes all stream basins draining into 

San Francisco, San Pablo, and Suisun Bays within 

these counties, and coastal basins draining into the 

Pacific Ocean along about 150 mi (241 km) of coast 

from about Pigeon Point on the south to the north 

boundary of Sonoma County. The region encompasses an area of 7,416 mi2 (19,207 km2 ). 

The population of the region was more than 4.8 

million in 1975. Major urban centers include the 

Oakland, San Francisco, and San Jose metropolitan 

areas. Highly developed manufacturing, industrial, 

and service activities dominate the economy. Agriculture and related activities, shipping, and distribution are also major segments of the economy of the 

area. 

Transportation facilities are extensive. A highly 

developed Federal, State, and county highway and 

road system provides access to all parts of the region 

and adjacent areas. Three major railroads serve the 

region. Airlines provide passenger and cargo service 

throughout the world. Seven major seaports permit 

ocean-going vessels to serve important industrial 

and agricultural centers and transport commercial 

cargoes. 

Drainage from the Sacramento and San Joaquin 

Rivers flows through the region. The eastern part of 

the region includes part of the Sacramento-San 

Joaquin delta area east of Suisun Bay and part of the 

lower Sacramento River basin south of Putah Creek. 

The Gualala, Russian, and Napa Rivers and Ala-
0 

I 

0 

EXPLANATION 

Boundary of stream basin 

10 

10 

I 

I 

20 

20 

I I 

30 

I 

40 

30 

I 

I 

50 

I 

INTRODUCTION 

40 Ml LES 

I 

60 Kl LOMETRES 

FIGURE !.-Principal streams and drainage basins in the San Francisco Bay region. 

3 
4 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

meda and Coyote Creeks are the principal streams in 

the region; their combined drainage area is about 42 

percent of the land area of the region. Other streams 

include the Petaluma River and Sonoma and Suisun 

Creeks in the north, Sar.. Ramon-Walnut, San 

Lorenzo, and San Pablo Creeks in the east, and Los 

Gatos Creek, through Guadalupe River, in the south; 

all are tributary to Suisun Bay, San Pablo Bay, and 

San Francisco Bay. Coastal streams that flow directly into the Pacific Ocean include the Gualala and 

Russian Rivers and Lagunitas Creek north of San 

Francisco and Pilarcitos, San Gregorio, and Pescadero Creeks south of San Francisco. 

Topography.-The San Francisco Bay region 

lies completely within the Coast Ranges geomorphic 

province of California, and in part has moderate to 

high relief with peaks ranging up to more than 4,300 

ft (1 ,300 m) in elevation. The Coast Ranges extend 

from Santa Barbara County on the south to Humboldt County on the north and form a nearly continuous barrier between the Pacific Ocean and the 

Great Central Valley. The only significant break in 

this barrier is in the San Francisco Bay region where 

the Carquinez Strait between San Pablo and Suisun 

Bays and the Golden Gate at San Francisco provide 

gaps through which the Sacramento and San Joaquin River flows are discharges (fig. 1). Mountains 

are present on the peninsulas north and south of the 

Golden Gate at San Francisco and along the east 

side of San Francisco Bay. The region also contains 

several alluviated valleys of moderately large size. 

Owing to the hilly and mountainous character of 

many parts of the San Francisco Bay region settlement and development has tended to concentrate in 

the foothills, river valleys, and coastal areas. Urban 

and suburban development has been extensive in the 

areas bordering San Francisco Bay, the Santa Clara 

Valley south of the bay, the San Ramon-Walnut 

Creek Valley east of the bay, and stream valleys in 

the north bay area, as well as in the adjacent 

foothills. 

Climate.-The climate of the San Francisco Bay 

region is marked by wide contrasts within short 

distances. The Pacific Ocean and the topography are 

the two major features that influence the climate. 

Precipitation in the region is highly seasonal; almost 

90 percent of the annual precipitation occurs during 

the 6-month period November through April, and the 

summer months are dry. Concentration of rainfall in 

wet months can result in serious floods from intermittent streams which seem to be of small consequence during the dry season. Most of the precipitation occurs in a series of general storms that reach 

all parts of the region, but the storm centers usually 

pass to the north of the regjon, and the result is 

a general tendency for precipitation to decrease from 

north to south. 

Elevation has a strong local influence on the 

depth of precipitation and, because elevations range 

from sea level to more than 4,300 ft (1,300 m), 

there is a wide range in mean annual precipitation 

-from 10 in. (250 mm) in low-lying valley areas 

in the east to 80 in. (2,000 mm) in some mountain 

areas in the north. The distribution of mean annual 

precipitation in the San Francisco Bay region is 

shown in figure 2. Temperatures are generally mild, 

notably through the winter season, reflecting the 

moderating influence of the Pacific Ocean. Thus 

precipitation in the region occurs principally as rain. 

However, at elevations above 2,000 ft (610 m), the 

winter precipitation sometimes occurs as snow; but, 

the snowfalls are generally light, and the snow does 

not remain on the ground for more than a few days. 

Snow thus does not have a significant role in the 

hydrology of the region. Intense local convective 

storms are almost unknown in the region. Sustained 

heavy rains during general storms are the principal 

cause of riverine flooding in the San Francisco Bay 

region. 

Flooding problems.-The San Francisco Bay 

region periodically experiences damaging floods. 

These floods usually are of riverine origin and affect 

flood-plain lands adjacent to the streams. Occasionally extreme high tides in the bay and coastal areas, 

or combinations. of high tides, winds, and floods in 

the streams, cause coastal flooding and inundation 

of lands adjacent to San Francisco Bay. The resulting annual flood loss in the region is estimated to 

average more than $22 million (1965 prices), based 

on studies by the California Region Framework 

Study Committee (1971). These losses occur even 

though millions of dollars have been expended on 

flood-loss reduction measures. On a National basis, 

despite a Federal investment of more than $9 billion 

in flood-protection and prevention measures since 

1936, the annual flood loss was estimated to average 

more than $1.7 billion, as of 1966 (U.S. Water Resources Council, 1968, p. 5-2-6), and as much as $2 

billion in 1972 (U.S. Office of Emergency Preparedness, 1972, v. 1, p. 15). 

Flood-control and flood-management measures 

such as reservoirs, levees, and channel improvements provide flood protection for many areas. But 

flood protection can seldom be complete. There is 

always a probability that a greater flood will occur 

than has been experienced in recorded history. Par-
390 

0 

I 

0 

0 

"' 0 

" ~ 

0'1 "' ,("k " o<fl 

~'1 ' \ 

) 

... -----_.,; 

EXPLANATION 

---50---

SAN 

7 

~ 

INTRODUCTION 

\ 

! 

Boundary, San Francisco Bay region 

j 

Line of mean annual precipitation. 

Interval 10 inches. Supplemental lines 

I I 

10 

10 

I 

20 

20 

I 

I 

30 

I 

40 

30 

I 

I 

50 

I 

40 MILES 

I 

60 Kl LOMETRES 

Hydrology from S. E. Rantz (1971) 

FIGURE 2.-Mean annual precipitation in the San Francisco Bay region. 

5 
6 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

tial protection from flooding may encourage greater 

use and developmentofthefloodplain. Then, when a 

flood exceeds that for which protection is provided, 

losses may be greater than if no protection had been 

provided. Further, average annual flood loss from 

lesser floods has not been reduced as the continuing 

demands for land development have brought about 

further encroachment on and more intensive use of 

the flood plains. Annual flood loss has increased in 

the San Francisco Bay region and throughout the 

Nation (U.S. Congress, 1966a) as a result of increases 

in property values, in the magnitude of floods, and in 

building and other uses on flood-plain lands. 

Extensive flooding occurred throughout the San 

Francisco Bay region in1955 and 1958 and in parts of 

the region in 1940,1952,1963, and 1964. Losses from 

the four later floods were about $23 million in 1955, 

$14 million in 1958, $4 million in 1963, and $17 

million in 1964, estimated at the time of occurrence, 

or reported by the California Region Framework 

Study Committee (1971, p. SF-2). These floods include some of the greatest in the region since the turn 

of the century. On an areawide basis, they have a 

probability of being exceeded once in 50 years, on the 

average (2 percent chance of being exceeded in any 

one year). An appreciation of the flood hazard in the 

region may be gained from the realization that 

planning for flood-plain management is usually 

based on the flood level that would be exceeded once 

in 100 years, on the average. The 100-yearfloodflows 

would be about 20 percent greater than the maximum flows that have occurred in this century. 

Potential losses from the 100-year flood in all streams 

in the region would exceed $200 million (1965 prices), 

based on estimates by the California Region Framework Study Committee (1971). 

Relationships and benefits of study.-The delineation of flood-prone areas represents an interpretation of water-resources data useful in land-use 

planning and decisionmaking. This study is part of 

the San Francisco Bay Region Environment and 

Resources Planning Study, and is directed toward 

minimizing flood losses. The identification of areas 

that may be subject to flooding from either riverine 

or tidal sources, as shown by flood-prone area maps 

and flood-inundation studies, is important to the 

wise use and development of such areas. Awareness 

of potential flooding can lead to a more complete 

appraisal of the extent of possible flooding at specific 

sites and to improved decisions on site selection, 

design, and protection. Further, in flood-prone areas, 

regional and local administrators and planners 

would be alerted to the desirability of considering 

and evaluating alternative uses of flood plains. 

The San Francisco Bay region study of floodprone areas and land-use planning should provide 

an example to other metropolitan areas throughout 

the Nation of potential uses of flood information and 

inundation maps in regional and local land-use 

planning and decisionmaking. The section "Planning for Flood-Loss Reduction in the Napa Valley" 

shows the practical application of this information. 

Similar applications can be made in other areas 

throughout the Nation. Techniques for developing 

the maps may vary, however, depending on the I hydrologic and hydraulic data available. 

I 

ACKNOWLEDGMENTS 

This report was prepared by the U.S. Geological 

Survey in cooperation with the U.S. Department of 

Housing and Urban Development. Counsel and advice in the initial phases of the study were provided 

by a multidisciplinary review committee that included the following members: Dan Coleman, Dan 

Coleman Associates; Arnold B. Jonas, Alameda 

County Planning Department; Bruce Knopf, Alameda County Flood Control and Water Conservation 

District; Ray Laird, Association of Bay Area Governments; Jack A. Lindley, Alameda County Flood 

Control and Water Conservation District; Charles E. 

Murphy, Marin County Department of Public Works; 

and Romaine F. Repair, U.S. Army Corps of Engineers, San Francisco District. 

Acknowledgment is made of the extensive data, 

maps, and reports on floods and flood damage, floodcontrol projects, flood-plain information, land-use 

and watershed planning, and flood-plain management, prepared by Federal, State, county, and local 

agencies and by numerous public and private agencies and organizations, that were used in the preparation of maps of flood-prone areas and the studies 

leading to this report. The cooperation of regional 

and local agencies in the San Francisco Bay region 

such as the Association of Bay Area Governments, 

and agencies of the city of Napa and Napa County, 

are gratefully acknowledged. S. E. Rantz, U.S. Geological Survey, provided helpful guidance in the initiation of the report and in earlier studies related to the 

preparation of maps of flood-prone areas. 

In particular the authors want to thank the 

following government officials of Napa County for 

background information and materials that were 

very helpful for our discussion of the Napa Valley: 

Joseph V. Reynolds, District Engineer, Napa Courity 

Flood Control and Water Conservation District; 

Anthony R. McClimans, Senior Planner, Napa 

County Conservation, Development and Planning 

Department; and Michael Joell, Principal Planner, 
FLOODS AND THE FLOOD PLAIN 7 

City of Napa Planning Department. 

Thoughtful review comments on a preliminary 

report draft led to significant improvements in the 

final report. Appreciation is extended to the following individuals who participated in the review process: James H. Hickey, Director, Napa County Conservation, Development and Planning Department; 

Christopher Hartzell, Land Resources Division, and 

Yvonne San Jule, Senior Regional Planner, Association of Bay Area Governments; William H. Fraley, 

Director, Alameda County Planning Department; 

and Charles E. Murphy, Flood Control Engineer, 

Marin County Department of Public Works. The 

description of the National Flood Insurance Program was provided by Nicholas Lally, Director, 

Flood Plain Management Division, Federal Insurance Administration, U.S. Department of Housing 

and Urban Development. 

FLOODS AND THE FLOOD PLAIN 

By A. 0. w AANANEN 

The occurrence of floods is a part of a natural 

pattern of water circulation from the seas to the 

atmosphere, to the ground, and back to the seas 

again called the hydrologic cycle (fig. 3). In this cycle 

water from the surface of the oceans and other water 

bodies evaporates into the atmosphere. This vapor is 

condensed by various processes and falls to the earth 

as precipitation. Part of the precipitation that falls 

on the land surface is retained temporarily in the 

soil, in surface depressions, and on vegetation and 

other objects until it is returned to the atmosphere by 

evaporation and transpiration. The remainder 

moves through surface and underground channels to 

rivers, lakes, and eventually the sea, and is likewise 

subject to evaporation and transpiration. Precipitation in its various forms is the source of freshwater 

on the earth's surface. Through the hydrologic cycle 

freshwater thus becomes a renewable resource. 

Floods are natural and normal events. Streams 

have, from time immemorial, periodically overflowed 

their banks and inundated their natural flood plains 

and flood ways. In many areas the rich fertile soils 

deposited by such overflows, and the replenishment 

of soil moisture by the floodflows, have sustained an 

abundant agriculture. In semiarid areas such overflows may constitute the only source of enrichment 

and irrigation. Prior to intensive development periodic flooding was accepted as a natural, if at times 

frightening, event. Mter the floods the streams 

usually would revert to their normal channels, leaving the flood plains suitable again for agriculture 

and other compatible uses. 

FIGURE 3.-The hydrologic cycle. 

Typically, a stream will overflow its normal 

channel about once in 2 or 3 years and invade low 

places on its flood plain. The overflow occurs when 

the volume of water entering a stream channel 

exceeds the hydraulic capacity of the channel. 

Greater floods occur at less frequent intervals, with 

concomitant increase in the area affected; but, only 

about once in a century or longer, a great flood will 

submerge all the alluvial deposits in the stream 

valley. Floods vary in size, area inundated, duration, 

and frequency depending on natural and certain 

manmade conditions. The natural conditions include the total quantity, intensity, and geographical 

distribution of rainfall and snowmelt, storm patterns, antecedent moisture conditions, temperature, 

and season of the year, as well as the physical 

features of the watershed, such as topography, soils, 

geology, and drainage pattern. The manmade conditions include the various rural and urban land 

uses, storage, diversion, and regulation of streamflows, as well as changes in drainage and other 

factors that affect storm-water runoff. 

MAJOR FLOODS IN THE REGION 

The San Francisco Bay region is rarely subjected 

to areawide floods. The mountains and bays subdivide the region into relatively small and independent valleys thus eliminating the cumulative effect 

that might be found in a single-stream system. 

Floods on the various streams in the region are 

caused by intense rains and usually are of short 

duration. Runoff is derived almost entirely from 

rain, as the snow that does fall is not extensive. 

Outstanding floods have occurred. Records extending back to 1787 mention storms during the 

winter 1798-99 that were reported to have lasted for 

28 days, and rains in January and February 1819 

caused floods that changed the courses of many 

streams. 
8 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Floods in 1861-62 affected extensive areas in 

California, and in many areas they were of such 

magnitude that they represent the greatest floods 

known. In the San Francisco Bay region considerable damage was caused in the towns of Napa, 

Alvarado, and San Leandro and in San Mateo and 

Santa Clara Counties (fig. 1). Floods on the Napa 

River and Petaluma Creek in January 1881 exceeded 

any other known flood. During the 1889-90 season 

record precipitation occurred at many points in the 

region. The flood of January 1890 near San Jose was 

the greatest since 1862 and may not have been 

equaled since. The 1890 flood in Napa River at Napa 

was reported to be only a foot lower than the record 

height set in 1881. Flows in the Russian River in 1890 

exceeded the levels set in 1861 and approached those 

of the great flood of 1879 in this basin; the basin 

experienced another great flood in 1895. In March 

1907 floods in Los Gatos Creek and other streams in 

the Santa Clara Valley were especially severe, and 

lands near San Jose were flooded. Alameda Creek 

reached levels as high as those in 1895. Flooding was 

also severe in the Russian River basin. In addition, 

many other floods of moderate extent have been 

experienced and reported in some part of the San 

Francisco Bay region since 1849, as reported in U.S. 

Geological Survey Water-Supply Paper 843, "Floods 

of December 1937 in Northern California," 

(McGlashan and Briggs, 1939, p. 434-437), and in 

reports on floods subsequent to 1937. 

The floods of December 1955 generally were the 

greatest floodflows of the present century over the 

San Francisco Bay region except for those in San 

Lorenzo Creek at Hayward in 1962 and Coyote Creek 

at Madrone in 1911. Other notable recent floods 

occurred in 1952, 1958, 1963, 1964, and 1969 and 

affected different parts of the region. The December 

1964 and January-February 1963 floods were severe 

principally in the northern part of the region, while 

the January 1969 floods were limited to the southern 

part. 

The flood of December 23, 1955, was the highest 

in Alameda Creek in the record of peak discharge 

that started in 1916, and probably the highest since 

the flood of November 1892. The four highest floods 

in Alameda Creek near Niles occurred in 1950, 1952, 

1955, and 1958 despite a storage capacity of 96,800 

acre-ft (119 hm3 ) in Calaveras Reservoir which was 

completed in 1925. 

Precipitation records and historical accounts 

appear to indicate that the San Francisco Bay region 

has not experienced a major or areawide flood since 

1889-90. Only the flood of December 1955 has 

approached the 1889-90 flood in magnitude and 

areal extent. 

Many severe floods of limited extent have 

occurred in different parts of the region. Conservation and flood-control reservoirs in many of the 

basins in the region have reduced the peak 

discharges that otherwise would have occurred. 

Floodwaters from many storms have been retained 

in reservoirs operated by the Santa Clara Valley 

Water District (formerly Santa Clara County Flood 

Control and Water District) since about 1935 for 

subsequent release at controlled rates to percolation 

basins for replenishment of ground-water supplies. 

But most of the streams in the region have insufficient storage or no storage for the control or 

attenuation of major floods. 

CHARACTERISTICS OF FLOODFLOWS 

Two characteristics offloodflows-the frequency 

of occurrence and the relation between the quantity 

of flow and the water-surface elevation-are of particular significance to the use of flood-plain lands. 

These characteristics are described in the following 

sections. 

FLOOD FREQUENCY 

In any evaluation of flood hazard or flood potential the probability of recurrence of floods of a given 

magnitude must be estimated. The probability of 

occurrence of floods of various magnitudes at a site 

may be determined by statistical analysis of annual 

peak discharges for all years of record at the site, 

whether or not all of these events caused inundation. 

Peak discharge is the highest rate of flow in a stream 

and generally occurs coincident with the greatest 

water-surface elevation during a rise in the stream. 

The annual peak discharge, by conventional usage 

in most areas, is the greatest flow in a 12-month 

period-usually October through September (the socalled water year). 

The magnitude of the annual peak discharge at a 

site varies from year to year, and it is customary to 

compare those magnitudes in terms of their probability of occurrence. The comparison is made by 

means of a flood-frequency curve for the site which 

shows the probability of any given discharge being 

exceeded in any year. A common practice is to refer to 

a peak discharge of a given magnitude in terms of its 

recurrence interval-the inverse of the probability of 

occurrence. The recurrence interval, or return period, 

is the average interval of time within which a given 

peak discharge will be exceeded once by the annual 
FLOODS AND THE FLOOD PLAIN 9 

peak discharge. Thus, a peak discharge that has a 2 

percent (1 in 50) chance, or probability, of being 

exceeded in any year is a 50-year flood peak, while 

the peak discharge that has a 1 percent (1 in 100) 

chance is a 100-year flood peak. 

. A flood-frequency curve is illustrated in figure 4, 

wh1ch shows a typical curve for a basin in the San 

Francisco Bay region that has a drainage area of 5 

mi2 (13 km2 ) and mean annual precipitation of 40 in. 

(1,000 mm), Figure 4 shows the 50-year flood discharge at the hypothetical site to be 1,740 ft3/s (49.3 

m3/s). Thus, a flood discharge of 1,740 ft3/s (49.3 

m3/s) has a 2 percent chance of being exceeded in 

any year; or it may recur at intervals of 50 years, on 

the average. Similarly, the magnitude of the 100-year 

flood discharge is 2,090 ft3/s (59.2 m3/s). These 

values show the ratio of the 100-year to the 50-year 

flood discharge to be 1.2, rather than 2, the relation of 

the time intervals. The ratios between flood discharges of various recurrence intervals depend primarily on the regimen of storm precipitation and 

thus vary from region to region. The average ratio 

between the 100- and 50-year flood discharges in the 

San Francisco Bay region is 1.20, but it ranges from 

1.25 for basins with a mean annual precipitation of 

10 in. (250 mm) to 1.10 for basins with a mean annual 

precipitation of 80 in. (2,000 mm). 

Recurrence intervals are average periods based 

on historical data; because the occurrence of floods is 

erratic the 50-year floodflow may not necessarily 

occur in any given 50-year period, or floods of this 

magnitude may occur several times during that 

period. A similar relation is true for a peak discharge 

of any given recurrence interval. 

2090 

2000 

Cl 

z 

0 

uiU 1740 

(!)w a:(/) 

~ :!i 1500 ()Q. 

~~­

ow w ~11. 

<(() 

w- 1000 Q.m 

::I 

(.) 

~ 

500 Drainage area=5 mi2 

Mean annual precipitation=40 in. 

60 

59 

Cl 

50 z 

49 8 

•W 

W(f) 

~a: 40<( w J:Q. 

()(f) 

(f)W 

-a: 

01- 30~ w <(::i! 

~~ m 

20 B 

z 

10 

2~~3--~5~~~1~0~--~20--~30--~50-LLLU100 

RECURRENCE INTERVAL, IN YEARS 

FIGURE 4.-Flood-frequency curve for a hypothetical basin in the 

San Francisco Bay region. 

STAGE-DISCHARGE RELATION 

For every peak discharge there is a corresponding peak stage, or water-surface elevation. The stage 

depends not only on the magnitude of the discharge, 

but also on the physical features of the stream valley . 

Pertinent physical features include ground or 

streambed slope in the direction of flow, crosssectional size and shape of the stream channel and 

the valley, and the roughness and alinement of the 

streambed, banks, and overflow area. The slope of 

the water-surface profile during periods of high 

water usually is approximately parallel to the average slope of the channel, with no abrupt changes in 

elevation in the water-surface profile if the flow is not 

obstructed or constricted. However, some pronounced changes in cross-sectional size and shape 

are common in a reach or length of stream valley. 

Stream valleys, for example, may range from wide, 

flat flood plains to steep-sided canyons. Such 

pronounced differences in shape and ground 

elevation, combined with the relatively uniform 

water-surface levels, may cause wide variations in 

the extent of flooding and the depth of flow in the 

main channel and on the flood plain. This is 

illustrated in figure 5, which shows two valley 

sections and the profiles of the streambed and of the 

water surface for the 10- and 100-year flood 

discharges in a reach. The sections show that the 

channel is more deeply incised at section A than at 

section B. Flow velocities in an unobstructed 

waterway are usually greater in the part of the 

section with the greater depths. 

The relation between stage and discharge is 

normally used as a basis for computing or 

determining streamflows. Conversely, this relati'on 

provides a means for determining the water-surface 

elevation at a site or in a reach corresponding to 

floodflows of designated frequency, such as the 10- or 

the 1 00-year flood. 

CHARACTERISTICS OF THE FLOOD PLAIN 

The flood plain is the normally dry land area 

adjoining rivers, streams, lakes, bays, or ocean that 

is likely to be flooded. Most commonly flooding of 

these lands results from the overflow of streams and 

rivers or from abnormally high tidal water or rising 

coastal water resulting from severe storms, hurricanes, or tsunamis (seismic sea waves). Along 

streams the flood plain may include the full width of 

narrow stream valleys or broad areas along the 

streams in wide, flat valleys. The hydrologist and the 

hydraulic engineer often use the term flood to de-
10 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

SECTIONS 

SECTION A 

PROFILES 

FIGURE 5.-Typical sections and profiles in an unobstructed reach 

of stream valley. 

scribe a relatively high streamflow that overtops the 

banks of a stream or channel or a relatively high flow 

as measured by either the elevation of the water 

surface or the volume of flow (see "Definition of 

Terms"). 

The principal characteristics of a stream valley 

are shown in figure 5. The channel and flood-plain 

sections are integral parts of the natural conveyance 

system of a stream. The erosive power of a stream 

carves a channel large enough to carry the most 

frequent flow. Occasionally heavy rains or melting 

snow contribute a flow greater than the capacity of 

.the channel. Then the flood plain carries the flow in 

excess of channel capacity. Flows that occur at 

intervals of 3-5 years may overtop the natural banks 

and occupy low-lying sections adjacent to the stream 

that constitute part of a natural floodway. Greater 

floods, which occur at less frequent intervals, extend 

over the remainder of the flood plain. During these 

greater floods the flood way may transport the principal part of the floodflows. In fringe areas on the flood 

plain the flooding usually consists of shallow inundation with low flow velocities. In the San Francisco 

Bay region some overflow generally occurs at intervals averaging from 3-8 years in streams not markedly affected by urban development. 

Any encroachment on the flood plain interferes 

with the natural flow pattern during high stages. 

The areal extent of inundation of the flood plain is 

therefore related to the magnitude of the flood discharge and the physical characteristics-both natural and manmade or man-influenced-of the 

stream valley, which includes both the incised 

stream channel and its flood plain. Uses of the flood 

plain, such as for agriculture and recreation, that 

involve little physical change have minor effects on 

the stage and the extent of inundation. But the 

modification of natural drainage systems and the 

concentration of various structures that are part of 

the pattern of urban, suburban, industrial, and 

commercial development present obstructions to or 

alterations of flow that may affect the stage, depth, 

and extent of flooding. 

Occupance of the flood plain can thus result in 

two kinds of flood losses. An individual who uses the 

flood plain for any development invites flood losses 

to himself; the potential for loss depends on the 

vulnerability of his specific use. But, more importantly, his entry on the flood plain may create 

problems and costs to others by causing higher flood 

stages through impedance or obstruction of the 

normal pattern of floodflow and reduction of floodplain storage capacity. The increased flood depths 

would affect not only his flood-plain neighbors but 

could also affect residents outside the area who 

would otherwise not be flooded. Hillside developments may also affect flood-loss potential as a result 

of accelerated runoff discharged to flood-plain areas 

downstream. 

The periodic inundation of flood-plain lands 

may result in losses to occupants including loss of 

life and property, hazards to health and safety, disruption of commerce and governmental services, and 

expenditures for flood protection and relief. These 

losses may be caused by both the cumulative effect of 

obstructions in flood plains that cause increased 

flood heights and velocities and uses of areas that 

are vulnerable to floods or present health and safety 

hazards because they are inadequately elevated or 

protected from flood damage. 

The problems resulting from flood-plain occupance and use have led to the development of plans 

and means for reducing or alleviating flood damage. 

Measures generally applied fall into the two broad 

categories-structural and nonstructural. Structural 
FLOODS AND THE FLOOD PLAIN 11 

measures include reservoirs, levees, floodways, and 

channel improvements; nonstructural measures 

(flood-plain management practices) generally include flood-plain regulation or control of land use. 

The purpose of flood-plain regulation is to promote 

beneficial use of flood plains with a minimum of 

flood damage and expense for flood protection. Nonstructural measures may be used effectively in combination with structural measures to achieve maximum benefits. The development and application of 

flood-plain regulations are described in the section 

"Planning for Flood-Loss Reduction." 

A basic feature of flood-plain regulation is the 

establishment of a regulatory flood discharge, commonly the 100-year flood. The topography of the 

flood plain, the magnitude of the floodflow, and the 

corresponding water-surface elevation and profile 

would determine the areal extent of the inundation. 

The part of the flood plain affected by this discharge 

can be described as the regulatory flood plain. It 

would include the channel, flood way, and fringe area 

required to transport the regulatory flood discharge. 

Figure 6 shows the flood-hazard areas of a regulatory 

flood plain along a river. A regulatory floodway 

would include the channel of a stream and the 

unobstructed adjacent land areas necessary to convey floodflows for a selected flood discharge without 

substantially increasing flood heights above unrestricted levels. The width of a regulatory flood way 

could be reduced for the same size flood, if some backwater effects are permissible, through channelization or installation of dikes, levees, or other constrictive embankments or fill. 

Coastal areas may be subject to high hazard 

from high energy winds and wave action, in addition 

to inundation, with resultant possible destruction of 

structures and facilities or severe erosion. 

Comprehensive discussion of the regulation of 

flood plains and flood-hazard areas is presented in 

reports by Kusler and Lee (1972) and the U.S. Water 

Resources Council (1971, 1972). 

MANMADE INFLUENCES ON POTENTIAL FLOODING 

The use of the flood plain by man has been 

accompanied by a variety of changes in the characteristics of the flood plain and floodflows. Under natural conditions the flood stage, or level of flooding, 

depends on both the peak discharge and the physical 

features of the stream valley. Under the changed 

conditions resulting from manmade influences the 

nature and extent of developments on the flood plain 

and upstream affect both the peak discharge and the 

three pertinent physical features-slope, crosssectional area, and the roughness and alinement of 

, rr~ 

}"-, 

::56~~~~·~·f·~-~ .. ~··~·~!"'~"fo~~~~· ·~oo~~:a-=-~-~'\ 

EXPLANATION 

REGULATORY FLOODWAY- Kept open to carry 

floodwater-no building or fill. 

\ 

\ 

\ 

\ 

\ 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

REGULATORY FLOODWAY FRINGE- Usepermitted 

if protected by fill, flood proofed, or otherwise 

protected. 

REGULATORY FLOOD LIMIT- Based on technical 

study-outer limit of the floodway fringe. 

STANDARD PROJECT FLOOD (SPF) LIMIT - Area 

subject to possible flooding by very large floods. 

FIGURE 6.-Riverine flood-hazard areas of a regulatory flood 

plain. (Adapted from U.S. Water Resources Council, 1971.) 

the water course-that govern the stage-discharge 

relation and the consequent peak (flood) stage. These 

manmade influences thus affect the potential for 

flooding and damage. Some changes, such as those 

resulting from urban development, may be inadvertent and have adverse effects. Others, such as reservoirs and channel improvements, are intentional 

and are directed toward alleviating the flood hazard. 

Some projects reduce flood hazards in addition to 

serving the primary or multiple-purpose uses for 

which they were constructed. 
12 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

INFLUENCE OF URBANIZATION ON POTENTIAL FLOODING 

A characteristic of urban growth is the continuing increase in the extent and intensity of flood-plain 

use. The level and fertile lands and the ease of access 

and transportation initially made the flood plain 

attractive. With continued growth an increasingly 

greater part of the flood plain and adjacent foothill 

areas was used for homes, industry, and associated 

commercial and municipal purposes. Such resulting 

urban development usually leads to pre-emption of 

the flood plains of streams and encroachment on the 

natural floodways, often without regard to the periodic flood hazards and concomitant dangers to 

property, health, and life. 

The principal hydrologic effect of urbanization 

is an increase in the peak flows and the corresponding peak stages for floods of all recurrence intervals. As increasingly large percentages of the drainage basins of streams are made impervious by roofs 

and paving, and as drainage channels are lined, 

paved, or replaced by pipe, both the infiltration of 

rain and the lag time, or time response of runoff to 

rainfall, are decreased. The result is usually a greater 

and earlier concentration of storm runoff in a channel and greater peak flows than would occur under 

natural conditions. The magnitude of the increase in 

flow depends also on the location of the impervious 

areas in the basin and the manner in which the 

runoff from the impervious surfaces reaches the collector channels. Storm runoff from developed areas 

in the lower part of a basin may cause sharp, shortduration peak flows that precede the runoff from the 

upper basin, while accelerated and intensified runoff 

from such areas in the upper basin may coincide with 

and accentuate downstream peak flows. The peak 

flows thus may be augmented in most basins but 

reduced in some. 

The increases in flow resulting from urban development, in relation to natural flows in streams, 

usually are greatest for the smaller peak flows that 

have short recurrence intervals, and for which the 

peak flows may be increased manifold. The greater 

floods, those with long recurrence intervals, generally reflect basin-wide runoff after substantial 

saturation., a condition which would also occur under 

natural conditions. The saturation effect is similar to 

the ·urbanization effect, a reduction in infiltration 

and an increase in runoff. Thus the magnitude of the 

urban influence is less for the greater floods. The 

average ratios of peak flows for urbanized and unurbanized basins for the 50- and 1 00-year floods in 

the San Francisco Bay region are shown in figure 7. 

Floodflows from basins that are 80 percent urban-

~3 

(/) 

<( 

m 

0 

.w 

~~ 

oz 

...J<( 

u.m 

~a: 

<(:::) 

wZ 

a..::::l 2 

u_CJ) 

0~ 

oa: 

-w 

~> 

a:o w 

N 

z 

<( 

m 

a: 

50-year recurrence interval 

:::) 1-=~--~------~------~------~----~ 0 80 100 

PERCENTAGE OF BASIN URBANIZED 

FIGURE 7.-Ratios of peak flows for the 50- and 100-year floods in 

urbanized and unurbanized basins in the San Francisco Bay 

region. 

ized, for example, are about twice those from the 

unurbanized basins, and the ratios are only slightly 

smaller for the longer recurrence interval. Urbanization is but one complex factor affecting the ratios. 

Other studies have shown that ratios are also 

smaller for drainage basins with the greater mean 

annual precipitation; the effect of mean annual precipitation is significant, however, only for peak flows 

with recurrence intervals less than 50 years. 

Thus, for any site downstream from an area that 

has become urbanized, the increased peak discharge 

at the upstream urbanized area will be reflected in 

increased peak discharge downstream. If the site is 

downstream from an area that has been leveed (see 

succeeding section "Structural Measures") the increase in flow velocity at the leveed area will increase 

the downstream velocity of the flood peak. The 

change in timing of the flood peak may cause an 

increase in the peak discharge at the downstream 

site. Upstream channel improvements that increase 

the velocity of the streamflow generally have a 

tendency to increase peak flows downstream. 

An associated effect of increased floodflows 

resulting from urban development is an increase in 

flood stage, as determined from the stage-discharge 

relation. Further, developments on the flood plain 

and encroachment on the floodway may obstruct 

floodflows and reduce floodwater storage sufficiently to cause backwater, flow retardation, and increased flood depths in the channel and the flood 

plain. 
FLOODS AND THE FLOOD PLAIN 13 

Urban development also has other effects in 

addition to the impact on peak flows. Total yield and 

annual runoff usually increase as the shortened time 

of surface runoff reduces infiltration opportunity. 

Low flows increase as a result of the discharge of 

wastewater from lawn irrigation and industrial and 

municipal operations, and effluent from sewage 

treatment plants and septic tanks. In contrast, 

recharge to the underlying ground-water basins, 

decreases owing to reduced infiltration. 

WATERSHED MANAGEMENT 

Watershed management generally refers to 

practices and measures designed to improve land 

use, alleviate flooding, and reduce erosion and sedimentation. These benefits may be achieved through 

combinations of land treatment and structural measures. The land-treatment measures may consist of 

agricultural practices to enhance the vegetative 

cover designed to reduce overland flow and runoff 

and to control erosion, stabilization of stream channels, drainage mains and laterals, and irrigationwater management. The methods include revegetation of bare soil, elimination of overgrazing, contour 

plowing, strip cropping, terracing, and mulch tillage. 

The objectives are accomplished by increasing the 

volume of surface storage for water, the rate of 

infiltration of water into the soils, and the capacity of 

the soil to store water. While these practices are 

effective in reducing the magnitude of the more 

frequent peak flows, the effect on the peak flows of· 

major floods is minor. The practices do, however, 

reduce erosion and the subsequent volume of sediment in streams, with consequent reduction in flood 

damage. 

Some agricultural practices, such as harvesting 

timber or substituting shallow-rooted grasses that 

have an economic grazing potential for noneconomic 

deep-rooted grasses, tend to increase peak discharge. 

The rate of runoff from logged areas, for example, 

may be increased with resultant higher peak flows. 

Because deep-rooted vegetation draws water from 

greater soil depths than shallow-rooted vegetation, 

the soil penetrated is left with a greater capacity to 

accept infiltration from rain during subsequent 

storms. The substitution of shallow-rooted vegetation may result in some increase in runoff. The effect 

would be significant primarily for the small, more 

frequent peak flows. 

STRUCTURAL MEASURES 

Constructing and operating reservoirs for water 

supply, flood control, power, irrigation, and conservation, developing and operating watershedmanagement programs, and constructing channel 

improvements for flood control and navigation all 

have an influence on flood discharge and stage. The 

general impact of these operations and measures is a 

reduction in the magnitude of floodflows. 

Reservoirs.-Operating storage reservoirs that 

have gates to control the release of water will reduce 

downstream peak discharge when storage space is 

available at the time of peak inflow to the reservoirs. 

These reservoirs may provide full control, or serve 

multipurpose needs. Even reservoirs that are full and 

spilling at the time of peak inflow may reduce the 

peak discharge downstream by attenuating the 

floodflow. 

Detention (ungated) reservoirs will normally 

have a similar effect on downstream peak discharge. 

However, discharge openings in detention reservoirs 

should be designed to ensure that the reduced peak 

outflows from such reservoirs will not be synchronized with other peak flows downstream in such a way 

that the combined flows produce greater peak discharge than would result from the natural, unsynchronized discharges had the reservoirs not been 

built. 

Channel improvements.-The stage corresponding to a given discharge may be reduced by structural measures such as channel improvements that 

increase channel capacity by increasing the crosssectional area of a stream channel or the stream 

velocity. These improvements may include straightening, realining, and eliminating bends in the channel, installing stabilization structures, changing 

flood way and bank vegetation, and paving. The area 

of a section may be increased by deepening or widening the main channel. Removing snags, trees, and 

brush increases the effective cross-sectional area, 

but a greater effect is an increase in velocity through 

reduction of flow-retarding ihfluences. 

The velocity of flow may be increased in many 

instances without reducing the cross-sectional area. 

Straightening and realining the channel and eliminating bed and bank irregularities will remove 

impediments to flow. Eliminating bends will reduce 

the effective length and increase the slope of a 

channel because the drop in streambed elevation in 

the reach occurs in a shorter distance. An increase in 

slope causes a corresponding increase in the velocity 

of flow. Deepening a channel will increase the 

hydraulic capacity more than widening and will also 

increase the velocity. Thus, for the same increase in 

section area, channel deepening is more effective in 

reducing stage. Reducing stage by improving the 

channel will be reflected in lower stages that extend 
14 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

a short distance upstream from the improved reach 

of channel, but will have little effect downstream. 

In some instances improving the channel may 

have adverse effects. Straightening an alluvial 

channel by removing bends, and possibly increasing 

flow velocities, for example, may result in erosion 

and sedimentation problems. Paved channels could 

provide the ultimate in eliminating flow-retarding 

influences, but the measure may not be fully acceptable because of esthetic, ecological, and other environmental considerations, such as interference with 

natural alluvial deposition and ground-water recharge. Although channel modifications may benefit lands adjacent to the treated reach, they will tend 

to pass the flood problems on downstream. 

Levees._:_Levees are embankments along 

streams or on flood plains that confine river and 

overbank flows to a definite width for the protection 

of lands that are landward from the levee. The major 

or principal levees are usually designed to provide 

protection from the 100-year and greater floods. 

Levees reduce the cross-sectional area of flow and 

increase the velocity of flow within the constricted 

(leveed) reach of channel and flood plain. The stage 

of the flood discharge may be raised in and upstream 

from the leveed reach, but it will be little affected 

downstream. 

OBSTRUCTIONS 

The stages reached in streams and on the flood 

plain during major floods may be affected by several 

factors. Bridges that have limited waterway openings and raised approach embankments present 

impediments to flow that raise stream stages at the 

bridge. The effects sometimes extend for some distance upstream. Obstructions in the flood way and on 

the flood plain such as commercial and indus trial 

buildings and residences may also cause substantial 

reductions in cross-sectional area and raise the flood 

stage. Flow velocities may be reduced immediately 

downstream from such obstructions but may be accelerated in the intervening flow paths. An obstruction in the floodway such as an isolated building 

may cause a local rise in water level immediately 

upstream. Concentrated development in urban areas 

may cause sufficient resistance to flow to cause a 

substantial increase in flood stage and associated 

greater damage from inundation. 

DELINEATION OF FLOOD PLAINS 

By J. T. LIMERINOS 

Areas subject to flooding are commonly identified in reports on floods and in studies of measures 

for the alleviation or control of flooding. Reports on 

major floods prepared by various governmental 

agencies may include maps showing the extent of 

inundation from the floods. Studies of flood-control 

or flood-management projects similarly include appraisals of the probable extent of the areas subject to 

inundation from floods of designated recurrence 

intervals before and after installation of the proposed measures. 

Flood-plain information in more detail is also 

available for many areas in several series of maps 

and information reports prepared by Federal, State, 

and local agencies. These maps generally provide 

sufficient information for identifying areas of potential flood hazards; for some areas the data in the 

flood-plain information reports may permit detailed 

evaluation of the hazards. This information is useful 

to planners, developers, public agencies, and private 

citizens concerned with future land development and 

use. 

Flood-plain inundation maps provide basic information useful in developing land-use plans and 

policies for flood plains and adjacent areas and in 

formulating regulations for managing flood plains. 

Some of the principal types of maps and reports 

that identify and delineate areas subject to inundation from floods and the sources of this information 

are described in the following sections. 

FLOOD-PLAIN INUNDATION MAPS 

Two general types of flood-plain inundation 

maps-flood-prone area and flood-hazard mapshave been issued by the U.S. Geological Survey. 

Maps of flood-prone areas show the areas likely to be 

flooded by virtue of their proximity to a river, stream, 

bay, ocean, or other watercourse or water body, as determined from readily available information. Floodhazard maps show the extent of inundation as 

determined from a thorough technical study of flooding in a given locality. The detail and accuracy may 

be sufficient for readily identifying the relation of 

special flood hazards to individual building sites. 

Flood-plain inundation maps have also been 

prepared by other governmental and private agen-
DELINEATION OF FLOOD PLAINS 15 

cies. Inundation rna ps are an integral part of com prehensive flood-plain information reports prepared by 

the U.S. Army Corps of Engineers and of watershed 

work plans and water- and land-management reports prepared by soil conservation agencies. Flood 

plains of many streams have been mapped by local 

agencies and special districts. 

MAPS OF FLOOD-PRON E AREAS 

A report by the Task Force on Federal Flood Control Policy (U.S. Congress, 1966a), included a recommendation that flood-prone area maps be prepared to 

assist in minimizing flood losses by quickly identifying areas of potential flood hazards. As part of a 

national program the U.S. Geological Survey in 1969 

began compiling a series of maps for California to 

identify flood-prone areas and to alert owners, planners, public agencies, and developers to the areal 

extent of flood hazards. The maps prepared in 1969 

showed areas "occasionally flooded," the delineated 

areas generally being those inundated by the greatest recent floods. Maps prepared since 1969 show the 

approximate limits of flooding for the 100-year flood. 

Through 1974 nearly 500 map sheets had been prepared for California; 119 of these are for the San 

Francisco Bay region (see table 10 and fig. 28). 

The flood-prone area maps were prepared on 

topographic quadrangle maps which include contour lines-imaginary lines connecting points on the 

ground surface that have the same elevation-showing the configuration and elevation of the land surface. Each quadrangle covers an area encompassed 

by 7 1/2 minutes of lati_tude and longitude, or approximately 57 mi 2 (148 km2 ). The scale of these 

maps is 1:24,000, or lin. equals 2,000 ft (10 mm equals 

240m). The areas subject to flooding were delineated 

from readily available information for quick appraisal rather than by detailed field surveys. The 

information included data on flood inundation, flood 

frequency, peak stage and discharge, and the results 

of some hydrologic and hydraulic studies. Estimated 

100-year flood levels were used to identify flood limits 

at specific points along the margin ofthe flood plain. 

Lines denoting the limit of inundation were then 

drawn in general conformance with the stream slope 

and the topographic contours of the land surface. 

More detailed information than is shown on the 

flood-prone area maps usually is required for purposes such as structural design, economic studies, or 

formulation of land-use p_lans and regulations. A 

part of the flood-prone area map prepared for the 

Napa Quadrangle is shown in figure 8. 

The maps of flood-prone areas in the San Francisco Bay region are listed in table 10. 

A regional map of flood-prone areas in the San 

Francisco Bay region in California, prepared by 

Limerinos, Lee, and Lugo (1973) as part of the San 

Francisco Bay Region Study, provides an overview 

of areas subject to the 100-year flood. The map was 

prepared at a scale of 1:125,000, o.r 1 in. equals 

approximately 2 mi (10 mm equals 1,250 m), by transfering and compositing information from the 

1:24,000-scale flood-prone area maps available for 

the entire area. The three-sheet map covers the 7,400 

mi 2 (19,000 km2) in the nine-county region, as shown 

in figure 9. A section of the map, for the Napa area in 

Napa County, is shown in figure 10. The topographic 

contour interval for this map is large-40 ft (12 m) 

in the flatlands and 200ft (61 m) elsewhere-but the 

small map scale precludes greater refinement. The 

small scale of the map, however, permits coverage of 

extensive areas on a few map sheets of manageable 

size, thus furnishing a convenient and illuminating 

appraisal of the areas in the region that are vulnerable to flooding. 

A regional flood-prone area map provides the 

regional and county-level planners and decisionmakers with information useful in formulating 

broad policies to guide the future development of 

flood plains. The inundation map may be used, for 

example, in conjunction with a regional map of present land use and other information in developing a 

regional land-use plan. The map may also serve to 

identify problem areas where detailed study of the 

flood hazard would be desirable; thus, the map may 

also be an aid in local planning. 

The small scale of a regional map and the 

generalized delineation of the areas subject to inundation may not permit sufficient identification of 

flood problems at specific sites to be helpful for direct 

use in local land-use planning and decisionmaking. 

Thus, when construction or occupancy is planned on 

or adjacent to the flood-prone areas shown on the 

map, a more rigorous description of the flood hazard 

should be established through comprehensive flood 

studies. The regional and the standard quadrangle 

maps of flood-prone areas may be helpful, however, 
16 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

122°17'30" 

FIGURE 8.-Part of the flood-prone area map for the Napa area. (Area of fig. 24 not shown on original map.) 
DELINEATION OF FLOOD PLAINS 

to the local planner in instances when time and FLOOD-HAZARD MAPS 

17 

funding restrictions preclude the more rigorous 

flood-hazard mapping studies. 

The flood-prone areas delineated on the regional 

and quadrangle maps for the San Francisco Bay 

region included areas contiguous to the ocean and 

the bay that have a potential flood hazard associated 

with high tides. The tidal elevations, in relation to 

sea level, were based on data compiled by the U.S. 

Coast and Geodetic Survey, as interpreted and reported by the U.S. Army Engineer District, San 

Francisco, Corps of Engineers. (Tide data for San 

Francisco Bay are usually reported locally in relation to datum at mean lower low water, MLLW.) Inundation resulting from tsunamis, or earthquakegenerated ocean waves, was not considered in the 

study of flood-prone areas. Although no devastating 

tsunami has been recorded in the San Francisco Bay 

region, and the likelihood of one is small, the possibility still exists. Ritter and Dupre (1972) have 

evaluated the impact of a possible damaging tsunami, as shown on two maps entitled "Maps Showing Areas of Potential Inundation by Tsunamis in 

the San Francisco Bay Region, California." A tsunami having a waveheightof20ft(6m)witharunup 

of 10ft (3 m) could be disastrous to people participating in shoreline-recreational activities. Shore, 

dock, and harbor facilities, marinas, and boats also 

might suffer extensive damage from inundation and 

from swift currents in the bay generated by tsunamis. A 20-ft (6-m) tsunami also could inundate 

many inhabited areas-suburban, commercial, and 

industrial-around the perimeter of the bay, with 

attendant potential hazard and damage. 

COUNTIES 

A. MARIN 

B. SONOMA 

C. NAPA 

D. SOLANO 

E. CONTRA COSTA 

F. ALAMEDA 

G. SANTA CLARA 

H. SAN MATEO 

I. SAN FRANC ISCO 

FIGURE 9.-Areas encompassed by the regional flood-prone area 

map sheets for the San Francisco Bay region. 

A flood-hazard map, as prepared by the Geological Survey, generally is a detailed map based on 

a thorough technical study that shows the extent of 

areas subject to potential hazard from severe floods. 

Map scales may range from 1:1200 (1 in. equals 100ft 

or 10 mm equals 12 rn) to 1:24,000 (1 in. equals 2,000 ft 

or 10 mm equals 240m), and contourintervals from 1 

to 20ft (0.3 to 6 m), depending on the type and density 

of the existing or planned flood-piain development. 

The large scale maps generally are helpful for developing plans and programs for flood-plain management. 

The cost of a flood-hazard map and the associated study increases greatly with increase in map 

scale and corresponding refinement of the study. 

Thus, cost may preclude preparing maps at large 

scale for all the flood plains in a large area. The 

larger-scale maps may be essential, however, for 

areas where the existing or potential development on 

the flood plain is significant. Where little structural 

development exists or is planned the smaller map 

scales, such as 1:24,000 and contour intervals of 5 ft 

(1.5 m), may be adequate. Then, individual sites or 

local areas can be studied and mapped in greater 

detail for definition of the potential hazard with 

respect to depth and duration of inundation and flow 

velocity. 

In 1954 the Geological Survey began publishing 

a series of hydrologic investigations atlases for the 

presentation of hydrologic information. This series 

has been used extensively for documenting floods 

and flooding in urban areas, river basins, and 

coastal areas (Bue, 1967). The maps presented are 

useful in evaluating flood potential and hazard. 

The atlases generally use topographic quadrangle maps or aerial photomosaics as bases. The 

extent of inundation from floods of record is usually 

delineated on the basis of detailed surveys and 

investigations. Photomosaics permit ready identification of specific sites, structures, properties, and 

extent of inundation but lack technical data on landsurface elevations and depth relations. Topographic 

maps provide the specific data on land elevations 

and location basic to engineering studies and determination of the depths of flooding. The extent of 

inundation may be determined from aerial photographs taken during major flood or from field and 

office studies of peak stages and water-surface profiles. The atlases usually include, in addition to the 

map, a brief test that presents information on peak 

stages and discharges, floods frequency, watersurface profiles, depth of flooding, and the extent of 
18 

0 

I 

0 

FLOOD-PRONE AREAS AND LAND-USE PLANNING 

2 

I 

I 

2 4 

I 

6 

4 

I 

I 

8 

6 MILES 

I 

Kl LOM ETRES 

FIGURE 10.-Regional flood-prone area map for the Napa area. (From Limerinos, Lee, and Lugo, 1973.) 
DELINEATION OF FLOOD PLAINS 19 

regulation. Some also include data on flood history 

and flow velocity. Illustrations may include floodflow hydrographs or stage-duration curves, discharge-frequency curves, graphs of annual peak 

discharges, and suitable photographs. 

Maps showing areas subject to flooding at selected frequencies are most meaningful in evaluating flood-protection proposals, establishing floodinsurance rate zones, and developing flood-plain 

management programs. The extent of areas subject 

to inundation by the 100-year flood is sometimes 

delineated in the atlases by extending the flooddepth orelations shown by the stage- and dischargefrequency curves and the water-surface profiles. 

Depth of flooding may be estimated from the watersurface profiles and ground elevation or from the 

spot water-surface elevations and depth to the general ground surface shown on some atlases. 

Seven hydrologic atlases related to floods have 

been published for areas in California, including two 

in the San Francisco Bay region (table 10). Hydrologic Investigations Atlas HA-54, "Floods at Fremont, California," by Young (1962), delineates the 

flooding in 1955 and 1958 in Alameda Creek in the 

vicinity of Fremont in Alameda County. Atlas HA348, "Floods on Napa River at Napa, California," by 

Limerinos (1970), delineates the flooding in 1940 and 

1955 in Napa River in Napa County. • 

A section of the map in the Napa River atlas 

(HA-348) is reproduced in figure 11. The area encompassed by figure 11 (same as fig. 8), is indicated, 

for comparison, on the section of the regional floodprone area map shown in figure 10. The inundation 

by the floods of 1940 (recurrence interval, 40 years) 

and 1955 (recurrence interval, 8 years) is shown in 

figure 11. The map was prepared on the basis of 

detailed field surveys, inspection, and hydraulic 

studies. Circled figures at points along the main 

stream denote river miles measured from Dutton 

Landing (Brazos). The map (1:24,000; contour interval, 5 ft or 1.5 m) is generally adequate to identify 

the hazards in the lightly developed areas upstream 

from river mile 9.5, iflittle structural development is 

planned for those areas. More detail at a larger scale 

may be desirable for planning and design in the 

highly developed areas downstream from river mile 

9.5. 

Flood-hazard maps require updating when 

changes have occurred in the channel, on the flood 

plain, and in upstream areas. These changes may 

include structural modifications in the channel and 

in upstream areas such as deepening, widening, or 

realining. Development on the flood plain, obstructions, or other land-use changes in the basin, may 

affect the streamflow, water-surface elevations, and 

flow velocities. The usual effect is a change in the 

flood profile. In some instances, recomputation of the 

100-year flood profile may be appropriate. 

FLOOD-PLAIN INFORMATION REPORTS 

A series of flood-plain information reports prepared by the U.S. Army Corps of Engineers presents 

a comprehensive appraisal of the flood hazard in the 

areas studied. The studies were authorized initially 

in the Flood Control Act of 1960 (U.S. Congress, 

1960) to provide technical information to local planning agencies. The authorization was amended and 

.extended by the Flood Control Act of 1966 (U.S. 

Congress, 1966b) in response to recommendations by 

the Task Force on Federal Flood Control Policy (U.S. 

Congress, 1966a). Under the 1966 Act the Corps of 

Engineers is authorized to compile and disseminate 

information on floods and flood damages, including 

identification of areas subject to inundation by 

floods of various magnitudes and frequencies, and 

general criteria for guiding Federal and non-Federal 

interests and agencies in the use of flood-plain areas. 

The principal purpose of the studies, guides, and 

flood-plain management services provided by the 

Corps of Engineers is to furnish local agencies appropriate information and guidance for development of 

plans and regulations for improved management 

and use of flood-plain lands. The surveys and studies 

are made and guides are prepared in response to 

approved requests. 

A flood-plain information report typically includes maps or aerial photomosaics, flood profiles, 

charts, tables, photographs, narrative material on. 

the extent, depth, and duration of past floods, and 

similar data on floods that may reasonably be 

expected in the future. The maps generally are of 

sufficient detail and scale for ready identification of 

areas subject to hazard from the 100-year flood (intermediate regional flood) and a possible greater flood 

(standard project flood). Depths of flooding may be 

determined from flood profiles and ground elevations. The reports may cover small or large basins or 

parts of basins. 

Ten flood-plain information studies have been 

completed for selected streams in the San Francisco 

Bay region, as shown in table 10; studies on other 

streams are in progress. 

Some flood reports published by the Corps of 

Engineers for specific floods, such as the floods of 

December 1955 (U.S. Army Corps of Engineers, 1956) 

and December 1964 (U.S. Army Corps of Engineers, 
20 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

••• 

Hosp.\al ;.;J' 

EXPLANATION 

-1940-

--1955---

Boundary of 1940 flood 

Boundary of 1955 flood 

River mile measured along 

stream channel upstream 

from Dutton Landing 

FIGURE ll.-F1ood-hazard map for the Napa area. (From limerinos, 1970. Area of fig. 24 not shown on original map.) 
DEUNEATION OF FLOOD PLAINS 21 

1965a), include generalized maps of the extent of 

flooding or maps showing elevations and location of 

high-water marks. Similarly, survey reports of the 

Corps of Engineers for flood control and allied 

purposes frequently contain maps giving generalized information on areas subject to inundation 

(table 10). Data from these reports and maps were 

utilized in the preparation of the maps of flood-prone 

areas issued by the Geological Survey; the individual 

reports are not listed herein. 

The U.S. Soil Conservation Service, California 

Department of Conservation, and local resource 

conservation districts (often soil conservation districts) provide flood-plain information in their studies and reports on watershed work plans for watershed protection and flood prevention and on watershed investigations. These studies were authorized 

in Public Law 83-566, the Watershed Protection and 

Flood Prevention Act of 1954 (U.S. Congress, 1954). 

Projects authorized under Public Law 83-566 are 

listed in table 10. The objective of the studies is to 

formulate plans for improvements to alleviate flooding and to reduce erosion and sedimentation. The 

reports may include maps that indicate the probable 

limits of flooding, as well as information on flood 

depth, flow velocities, and agricultural damage from 

floods. Copies of the reports and other unpublished 

information may be consulted at district offices 

or the Soil Conservation Service and cooperating 

agencies. 

FLOOD-INSURANCE STUDIES 

The National Flood Insurance Act of 1968 (U.S. 

Congress, 1968) was enacted to provide previously 

unavailable flood-insurance protection to property 

owners in flood-prone areas. Qualification for inclusion in the flood-insurance program included agreement by a community to adopt and enforce land-use 

control measures consistent with designated criteria. The program is administered by the Federal 

Insurance Administration, U.S. Department of 

Housing and Urban Development. 

Initially flood-insurance coverage under an 

emergency program was extended to communities 

that had established eligibility. Subsequent detailed 

flood-insurance ratemaking studies provide a basis 

for actuarial insurance rates and coverage under a 

regular program. The detailed studies, performed for 

the Federal Insurance Administration by Federal 

agencies and other organizations, include development of inundation maps, water-surface profiles for 

designated floods, floodway delineation, and depthdamage curves. The Federal Insurance Administration then furnishes flood-insurance rate maps to 

insuring agencies and the local communities. These 

usually delineate areas of special flood hazard (inundation by the 100-year flood) and elevations of the 

100-year flood levels within these areas. 

The flood-insurance rate maps and topographic 

maps define the extent of the flood-hazard area and 

the depths of flooding on the flood plain. The reports 

on the detailed studies are filed in each community 

studied but generally are not distributed. 

The Flood Disaster Protection Act of 1973 (U.S. 

Congress, 1973) requires the purchase of flood insurance after March 2, 1974, or 1 year after the community's identification as being flood prone, as a 

condition for receiving any form of Federal or Federally related financial assistance for acquisition or 

construction purposes in an identified flood-plain 

area having special flood hazards that is located 

within any community currently participating in the 

National Flood Insurance Program. Flood-hazard 

insurance would be required when a community 

enters the program, and special flood-hazard areas 

have been identified on flood-hazard boundary 

maps. These flood-hazard boundary maps are the· 

first maps prepared in the identification process, and 

they show the location of special hazard areas. They 

are separate from the Geological Survey's maps of 

flood-prone areas and have been prepared by the 

Federal Insurance Administration specifically for 

insurance purposes. These maps provide only an 

approximate delineation of the extent of the 100-year 

flood, and they do not provide information on watersurface elevations usable in estimating depths of 

flooding. Copies are filed in each eligible community 

and with insuring agencies. 

Communities in the San Francisco Bay region 

participating in the National Flood Insurance Program are shown in table 10. The coordination of 

flood-insurance studies in California has been provided by the California Department of Water Resources through the Flood Control Development 

Branch of the Division of Resources Development. 

OTHER FLOOD-PLAIN MAPPING 

The flood plains of many streams in the San 

Francisco Bay region have been mapped by county 

flood control and water conservation districts, municipal agencies, and other departments and agencies. 

Some maps show the extent of inundation by major 

floods, or probable inundation by floods of designated recurrence intervals. Other maps have been 

prepared in connection with special studies for flood 
22 FLOOD-PRONE AREAS ~ ND LAND-USE PLANNING 

control and drainage, zoning, planning, and development. 

The map scales and detail are generally adequate to provide information comparable to that 

from the flood-hazard and tlood-plain inundation 

maps. The maps may be particularly helpful in 

studying small local areas in and near cities and the 

adjacent suburban areas. 

Flood-plain maps prepared by county and local 

agencies generally are not published but may be 

available for consultation in the offices of the issuing 

agency. 

INTERPRETATION OF FLOOD-PLAIN MAPS AND 

INFORMATION FOR SITE EVALUATION 

Planning for the use and management of floodplain lands includes consideration of the magnitude 

and extent of the flood risk. The several types of 

flood-plain maps and information reports available 

identify flood-prone areas and provide estimates of 

the depth of flooding in some areas. 

The accuracy of the delineations of flood-prone 

areas depends on the quality and scale of the flood 

maps. The regional maps (scale 1:125,000) of floodprone areas generally are useful for broad area 

appraisal of possible flood hazard. The standard 

quadrangle maps (scale 1:24,000) used for both the 

flood-prone area and flood-hazard maps may provide 

sufficient detail for specific site location and hazard 

evaluation, especially in nonurban areas. Adequate 

appraisal of the vulnerability of a particular building site may require maps of larger scale and greater 

detail. The use of flood-hazard maps such as those in 

the hydrologic atlases in flood-risk evaluation and in 

planning for urban development and other land use 

is illustrated in a report, "Flood-Hazard Mapping in 

Metropolitan Chicago," by Sheaffer, Ellis, and Spieker (1970). 

The flood-hazard maps for the San Francisco 

Bay region presented in hydrologic atlases and 

flood-plain information reports permit some evaluation of flood risk. However, these maps are available for only a few areas. The following appraisal of 

flood risk at a site in Napa, Calif., illustrates use of a 

flood-hazard map. 

The hydrologic atlas for the lower Napa River 

area, by Limerinos (1970), provides information 

appropriate for flood-hazard evaluation. Figure 11 

shows part of the flood-prone area within the city of 

Napa. The flood hazard might be evaluated, for 

example, for a site such as the fairgrounds, 0.3 mi (0.5 

km) east of the NapaRiverataboutrivermile 7.3, the 

distance upstream from Dutton Landing. The 

ground elevation at the site is about 15 ft (4.6 m) 

above mean sea level. 

One of the graphs in the atlas is a profile of the 

water-surface elevations along the Napa River downstream from Dry Creek for the floods of February 

1940 and December 1955. This profile (fig. 12) indicate~ an elevation of 16.5 ft (5.03 m) for the flood of 

February 27, 1940, at river mile 7.3, and presumably 

also at the fairgrounds. Flood depths at the fairgrounds thus would be about 1.5 ft (0.5.m). A second 

graph in the atlas is a flood-frequency curve for Napa 

River (fig. 13) at a gaging station upstream at river 

mile 13.9 (not shown in fig. 11). The curve indicates a 

recurrence interval of about 40 years for the 1940 

flood. Flood-plain lands within the limits of flooding 

by the 1940flood thus are subject to inundation to the 

levels shown by the flood profile (fig. 12) about once 

in 40 years, on the average. 

The 100-year flood generally has been accepted 

as the base flood for flood-hazard evaluation, flood 

insurance, and flood-plain planning. The potential 

hazard from a flood of this magnitude therefore 

needs to be evaluated. 

The extent and depth of inundation from the 100-

year flood in Napa River may be estimated from the 

increase in water-surface elevation above that for the 

1940 flood. The stage difference at the gaging station 

at river mile 13.9 is about 1.5 ft or 0.5 m (1940 flood, 

57.5 ft or 17.5 m; 100-year flood, 59.0 ft or 18.0 m) (fig. 

13). Owing to greater flood-plain width downstream 

the stage difference in the vicinity of Third Street 

and the fairgrounds may be only about two-thirds of 

that at the gage, as shown by the 1940 and 1955 flood 

profiles in figure 12. The 100-year flood elevations in 

the city of Napa accordingly would be approximately 

1 ft (0.3 m) higher than the 1940 levels. The corresponding depth of flooding at the fairgrounds would 

be 2.5 ft (0.8 m). 

In a similar manner the flood maps in the Corps 

of Engineers flood-plain information reports provide 

information for flood-risk evaluation in the areas 

covered by the reports. The maps provide data on 

ground elevations and show the limits of flooding 

from the Intermediate Regional Flood (IRF), equivalent to the 100-year flood, and from a greater 

Standard Project Flood (SPF). Water-surface profiles 

for these floods along the major streams are included. 

The potential for flooding at a site from the 100-year 

flood (IRF) can be determined readily from the map, 

and the depth of flooding can be estimated from the 

ground elevations and flood profiles. 
FLOOD-LOSS PREVENTION AND REDUCTION MEASURES 23 

DISTANCE UPSTREAM FROM DUTTON LANDING, IN KILOMETRES 

22 20 18 8 0 

_j 50 

w 

> 

w 

..J 

<( 40 w 

en 

z 

~ 30 

~ 

w 

> 

~ 20 

<( 

z 

0 

1- 10 

<( 

> 

w 

..J 

w 0 

0 

c: ~ 

~ Q) 

Q) 

~ 

<tl u 

0 <tl 

"0 

0 

(/) 

Geological Survey gaging station 

Napa River near Napa, California. 

Datum 24.74 feet above mean 

sea level. Mile 13.9 

~ Q) Q) 

Q) ::J 

u c: 

Q) 

> c: <( Q) 

~ c: 

0 ~ u 

.~ 

.... 

Q) 

~ .... ci5 Q) 

.~ ~ (/) 

Flood of February 27, 1940 

Flood of December 22, 1955 

Floodmark elevation 

0 February 27, 1940 

£::. December 22, 1955 

Dutton Landing 

20 

en 

w 

a: 

1-

w 

~ 

15 ~ 

..Jw 

> 

w 

..J 

<( 

10 ~ 

z 

<( 

w 

~ 

w 

> 

5 0 

m 

<( 

z 

0 

1-

<( 

0 ~ ..J 

w 

15 11 10 9 8 7 6 5 4 3 2 0 

DISTANCE UPSTREAM FROM DUTTON LANDING, IN MILES 

FIGURE 12.-Profi.les of floods on Napa River downstream from mouth of Dry Creek. (From Limerinos, 1970.) 

"' 

~ w 

a: 

25 60 1-

w 18 :;: 

~ 

uf ~ u. 

20 uf 6 0 0 

~ 55 51 z 

1i; 0 

~ 16 ...J 5 a: 

0 ~ 

15 ~ z a: 

z 50 :::l w 4 J: Q, 

:5 z z ~ :;: :5 - a: 

w :;: u.i ..... 

10 > 45 14 w 3 (.? UJ 

g > a: ::;; 

g <( u <( J: a; 

z <( u :::l 

0 z 2 ~ u 

j::: 40 0 

<( 

38 12 j::: 

~ <( 

1 10 100 

RECURRENCE INTERVAL, IN YEARS ~ 

FIGURE 13.-Frequency of floods above 39-foot elevation at 

g~gin~ station on Napa River near Napa, California. (From 

Limennos, 1970.) 

SOURCES OF FLOOD-PLAIN MAPS AND 

INFORMATION 

The Geological Survey and other Federal 

county, and local agencies have prepared flood-plai~ inundation maps and other flood studies for many 

areas in the San Francisco Bay region. Information 

on flooding and flood hazards has been presented in 

pertinent maps and reports. The principal sources of 

the maps and flood information are indicated in 

table 9; the maps and reports available are listed in 

table 10. 

FLOOD-LOSS PREVENTION AND 

REDUCTION MEASURES 

By W. J. KocKELMAN 

The need for preventing and reducing flood loss 

has grown with increasing use of, and encroachment 

upon, flood plains. Many methods and devices, both 

structural and nonstructural, are available for the 

prevention and reduction of flood loss. These 

methods and devices may be used in a variety of 

combinations to provide relief from existing and 

potential flood problems. Effective programs for 

flood-loss prevention 1 should be based on an adopted 

comprehensive plan, although individual methods 

and devices are often applied without a plan. The 

comprehensive planning process is discussed in the 

following section, "Planning for Flood-Loss Reduction." Any flood-loss prevention program is most 

effective if based on an adopted comprehensive plan 

or flood-plain management program that is being 

firmly implemented. 

1 Flood-loss prevention and flood-loss reduction are mutually 

inclusive terms as used in this report. Prevention is usually the 

desired goal; however, it is recognized that under certain circumstances reduction may be the more feasible goal. 
24 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

A regional land-use plan can serve as a general 

framework for subregional, watershed, and community land-use plans. An adopted regional land-use 

plan that recommends protecting flood plains from 

incompatible development and establishes specific 

land-use regulations, if implemented, could prevent 

flood loss, protect certain important elements of the 

natural resource base, including water quality, and 

partially satisfy a need for park and open-space 

lands. 

PROTECTION OF EXISTING DEVELOPMENT 

Coordinating land-use and water-use plans 

would assure, for instance, that plans for using flood 

plains for park and recreational purposes would 

include concurrent establishment of water-quality 

standards that would accommodate recreational 

uses. Therefore, land- and water-use plans should be 

prepar.ed together and carefully adjusted to the 

capability of the resource base to sustain the proposed land and water uses. Because water-use plans 

are best prepared for a watershed, the mutual adjustment of land-use and water-use plans may be best 

accomplished as part of a comprehensive watershed 

planning program. 

Once land-use ,and related water-use plans have 

been prepared, adopted, and implemented, many 

methods and devices are available for preventing or 

reducing flood loss. These include the protection, 

removal, discouragement, and regulation of certain 

types of development on flood plains. These methods 

and devices are listed in table 1 and are briefly 

described in the following sections. 

Development has occurred and will continue to 

occur on flood plains within the San Francisco Bay 

region. Such development, though often convenient, 

advantageous, and profitable to some individuals 

and economic sectors of a community, sometimes 

imposes a heavy burden on society. For economic 

and humanitarian reasons, local, State, and Federal 

governmental agencies may find it necessary to 

protect existing developments on flood plains. This 

protection may be provided by flood-control works, 

flood warning and evacuation, and floodproofing. 

Flood-control works.-Loss from floods often 

leads to persistent demands for public-works programs to provide protection for existing developments through construction of structures and improvements such as dams, ditches, canals, sluices, 

holding basins, and detention reservoirs; channel 

deepening, straightening, widening, and paving; 

bypass or diversion channels, dikes, revetments, 

flood walls, levees, and underground drainage facilities; or combinations of several of these. 

Construction of flood-control works can be selfdefeating. As the urban development of flood plains 

continues, the number of persons and the value of 

property in areas subject to flooding tend to increase 

at rates greater than that at which protection can be 

provided. Most flood-control works are extensive and 

costly. For example, to be effective, levees must be of 

substantial construction, continuous throughout 

areas with low banks, set back to allow for highwater flows, and may need to be supplemented by 

drainage of interior areas. 

TABLE 1.-Flood-loss prevention methods and devices 

Purpose 

Protection of existing 

development. 

Removal or con version of 

existing development. 

Discouragement of 

development. 

Regulation of flood-plain 

uses. 

Method or device 

Flood-control works: 

Reservoirs 

Channel improvements 

Diversions 

Floodwalls and levees 

Flood warning and evacuation 

Flood proofing 

Public acquisition 

Urban redevelopment 

Public-nuisance abatement 

Nonconforming uses 

Conversion of use 

Public-facility reconstruction 

Public information 

Warning signs 

Recordation of hazard 

Tax-assessment practices 

Financing policies 

Public-facility extensions 

Flood-insurance costs 

Zoning ordinance districts 

Special flood-plain regulations 

Subdivision ordinances 

Sanitary ordinances 

Building ordinances 

The costs of constructing single-purpose floodcontrol structures and improvements may be greater 

than the benefits from reduced flood-loss risk. However, multipurpose structures and improvements, 

such as low-flow augmentation or recreation, that 

provide benefits in addition to flood control may 

prove economically feasible when evaluated within 

the context of a comprehensive watershed plan. 

Environmental, recreational, and esthetic costs and 

benefits should be considered even though their 

evaluation might be difficult. 

Actions and improvements by upstream communities often impose expenditures on downstream 

communities for public works. Channel improvements, for example, may increase flood peaks downstream and require public expenditures for downstream flood-control works. Some facilities for flood 
FLOOD-LOSS PREVENTION AND REDUCTION MEASURES 25 

control, such as dams and levees, may encourage 

development of flood plains in anticipation of additional works being constructed. The public may 

believe that the flood problem has been eliminated, 

rather than simply abated, by newly constructed 

works. Also, such works may not prevent losses from 

great and infrequent floods that exceed the design 

flood, often with catastrophic effects. The intelligent 

management and regulation of the flood plain is still 

required in conjunction with public works. 

Several Federal programs are available to assist 

communities in the construction, repair, or rehabilitation of structures and other improvements to protect existing development. These Federal programs 

include the Snagging and Clearing for Flood Control, Flood Control Works, Emergency Bank Protection, and Small Flood Control Projects programs 

authorized under the Flood Control Acts of 1937, 

1941, 1946, and 1948. These programs are discussed 

in the "Catalog of Federal Domestic Assistance" 

(U.S. Office of Management and Budget, 1974). 

Flood warning and evacuation.-Reliable and 

timely flood warnings would permit temporary evacuation of people and some personal properties from 

flood plains to reduce loss of life and property 

damage. In areas where the time interval between 

the onset of rainfall and flooding is short, such 

information relayed immediately to residents on the 

flood plain can result in saving of life and movable 

property. 

The Joint Federal-State River Forecast Center at 

Sacramento, Calif., a cooperative program of the 

National Oceanic and Atmospheric Administration 

and the California Department of Water Resources, 

monitors weather conditions and river stages in the 

Pacific Coast area. For the San Francisco Bay 

region, the Center issues forecasts of river stages for 

Walnut Creek and the Napa and Russian Rivers 

during the flood season. The Center also provides 

information to the National Weather Service Forecast Office at Redwood City for other streams in the 

San Francisco Bay region. 

The National Weather Service River District 

Office at San Francisco International Airport relays 

forecasts and issues flood warnings, when necessary, to the appropriate county flood-control districts, 

State and Federal agencies, and the American Red 

Cross. When flash floods are occurring or imminent, 

the Forecast Office at Redwood City issues flashflood watches and flash-flood warnings to Weather 

Service offices, various public agencies, and the 

news media. Flash floods are not a common phenomenon in the San Francisco Bay region. 

I Floodproofing.-Floodproofing prevents or reduces flood loss to existing structures that cannot be 

economically removed from, or that need to be 

maintained on, the flood plain, such as certain public 

utilities, navigation facilities, depots, and warehouses. Floodproofing should be considered, however, only where floods are likely to be of short 

duration and have low stages and velocities. 

Floodproofing measures range from structural 

modifications and installation of special equipment 

or materials to operational and management safeguards. Many of these measures are discussed and 

illustrated in "Introduction to Floodproofing" 

(Sheaffer and others, 1967) and "Flood-Proofing 

Regulations" (U.S. Army Corps of Engineers, 1972c). 

Structural modifications and special installation 

methods are the most effective as they are not 

dependent on warning, judgment, decision, and action immediately prior to a flood. 

The feasibility of modifying structures and 

installing protective equipment or applying special 

materials would depend on the materials, age, design 

features, and the existing and proposed use of the 

structures to be floodproofed. Structural modifications include reinforcing basement walls and floor 

underpinnings to withstand the hydrostatic pressures of floodwaters; permanently sealing exterior 

openings to basements; using masonry construction; 

erecting low floodwalls; installing water-tight bulkheads, shutters, and doors; and elevating the lowest 

floor and access roads to at least 2ft (0.6 m) above the 

100-year flood. 

Installing special equipment or applying special 

materials may include using special cements for 

flooring; providing adequate electric fuse protection; 

anchoring buoyant tanks; sealing of the outside 

walls of basements; installing automatic sump 

pumps, sewer-check valves, seal-tight windows and 

doors, and door and window flood shields; and using 

wire-reinforced glass. Care must be exercised, however, to avoid high hydrostatic pressures which 

might cause structural damage. 

Operational and management safeguards include taking special actions during, or in anticipation of, floods such as waterproofing valuable items 

and merchandise or removing them from areas 

subject to flood loss; protecting or removing electrical equipment; discontinuing personal use of areas 

subject to flooding; postponing orders of freight ship-

-ments; operating emergency pump equipment and 

sewer valves; and placing movable bulkheads 

against doors and windows. Though floodproofing 

.on-site soil absorption systems for sewage disposal is 
26 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

sometimes suggested, substituting alternative disposal systems, such as public sanitary sewer systems 

or holding tanks, would be more effective. 

REMOVAL OR CONVERSION OF EXISTING 

DEVELOPMENT 

Recurrent and frequent damage or hazard to life 

from flooding may be avoided by permanently evacuating the flood plain. This may be accomplished by 

removing structures or converting them to some use 

less vulnerable to damage by floods. The feasibility 

of such action depends on the value of the structures, 

their adaptability to floodproofing, their effect upon 

the movement, depth, and storage of floodwaters, 

and sufficient citizen interest and concern to support 

the necessary action by local public officials. Special 

attention should be given to removing structures 

within floodways because of their effect on flood 

stage and flood velocity. Methods for removal or 

conversion include public acquisition, urban redevelopment, public-nuisance abatement, nonconforming-use provisions in zoning ordinances, 

con version of use, and reconstruction of existing 

public facilities. 

Public acquisition.-One approach to removing 

or converting flood-plain development is for a governmental agency to acquire the developed land. The 

flood plains can be acquired through negotiation, 

condemnation, tax-delinquency default, dedication, 

devise, or donation. The governmental agency can 

then control development in the public interest. It 

may elect to sell or lease part or all of the acquired 

lands, restricting any development or uses that 

would be vulnerable to flood damage. If agricultural 

land has been acquired, it might then be leased for 

appropriate agricultural uses, thus permitting part 

of the acquisition costs to be recovered. 

Under the Federal Land and Water Conservation 

Fund Act of 1965 and the Federal Housing and 

Community Development Act of 1974, grants are 

available to assist communities in acquiring flood 

plains for park and recreation purposes. These grant 

programs are discussed in the "Catalog of Federal 

Domestic Assistance" (U.S. Office of Management 

and Budget, 1974). 

Acquiring less-than-fee interest in flood plains 

costs the public less than purchasing the land because only certain property rights need to be purchased. Such interest may be in the form of scenic 

easements for vista protection, conveyance of development rights to assure continuance of private parks 

and open spaces, and grants of public access and 

development rights for construction and use of park 

facilities. By purchasing easements or development 

rights, flood-plain development can be limited to 

certain uses, while the owner receives fair compensation for the release of these rights. Easement lines 

need not be based on documented flood data because 

the boundaries of the lands to be acquired can be 

determined by agreement. The use of easements 

should include periodic inspection and enforcement 

of the land use by the agency holding the easement. 

Easements should be obtained in perpetuity, or for as 

long as the flood hazard exists. 

Urban redevelopment.-Flood plains can be 

redeveloped publicly or privately by purchasing land 

that has been determined to be blighted or to have 

deteriorated and then clearing and redeveloping the 

land. 

The California Community Redevelopment Law 

(California Health and Safety Code, 1976c) authorizes the creation of public redevelopment agencies. 

The law provides for the preparation and adoption of 

redevelopment plans; acquisition, clearance, disposal, reconstruction, and rehabilitation of blighted 

areas; and relocation of persons displaced by a 

redevelopment project. Redevelopment agencies 

generally are empowered to issue bonds, receive a 

portion of taxes levied on property in the project, and 

use Federal grants or loans available under the 

Urban Growth and New Community Development 

Act of1970 and the Federal Housing and Community 

Development Act of 1974. These programs are discussed in the "Catalog of Federal Domestic 

Assistance" (U.S. Office of Management and 

Budget, 1974). 

Public-nuisance abatement.-Buildings and 

structures subject to periodic flood loss often go 

unrepaired after a flood, thus initiating a cycle of 

deterioration. These buildings and structures can be 

removed or razed by local units of government 

applying their public-nuisance abatement powers. 

For example, Section 203 of the Uniform Building 

Code, prepared by the International Conference of 

Building Officials (1973), which many cities and 

counties have adopted, provides: 

All buildings and structures which are structurally unsafe or not 

provided with adequate egress, or which constitute a fire hazard, 

or are otherwise dangerous to human life, or which in relation to 

existing use constitute a hazard to safety or health, or public 

welfare, by reason of inadequate maintenance, dilapidation, 

obsolescence, fire hazard, disaster damage, or abandonment*** 

are***unsafe buildings. All such unsafe buildings are hereby 

declared to be public nuisances and shall be abated by repair, 

rehabilitation, demolition, or removal***. 

Nonconforming uses.-The adoption and enforcement of zoning ordinances, discussed in a 
FLOOD-LOSS PREVENTION AND REDUCTION MEASURES 27 

following section, may make some existing uses of 

the flood plains nonconforming. These are uses in 

existence at the time of the adoption of a zoning 

ordinance that do not conform to the use restrictions 

of the ordinance. For example, if residential uses of 

flood plains are prohibited by a zoning ordinance, 

residences existing on the flood plain at the time of 

adoption of the ordinance would become nonconforming. 

Zoning ordinances may provide that nonconforming uses may be continued but not extended or 

enlarged and, if discontinued for some designated 

period, 'any future use must conform with the ordinance. The total structural repairs or alterations 

over the lifetime of a nonconforming use may also be 

limited to a percentage of the assessed or market 

value. State enabling legislation may also permit 

eliminating nonconforming uses by providing for 

the amortization of such uses over a reasonable 

period of time. 

Conversion of use.-Land uses, improvements, 

and structures that are vulnerable to flood damage 

can be voluntarily converted to uses less vulnerable 

to loss by floods. Farm feed-storage buildings, for 

example, could be converted to sheds for farm equipment that would not be damaged by floodwaters or 

disrupt farm operations by being temporarily unavailable; tilled lands might be used for pasture or 

sown to crops that are less susceptible to flood loss; 

and urban redevelopment areas could be used for 

parking lots or recreation areas. 

Public-facility reconstruction.-Reconstructing 

public facilities located on the flood plain, such 

as roads, bridges, utilities, and community facilities, 

that are subject to renewal by reason of functional or 

structural obsolescence may afford opportunity to 

reduce the risk of flood loss. This might be done by 

elevating roads and utilities above flood peaks, floodproofing, or relocating them in areas not subject to 

flooding. 

DISCOURAGEMENT OF DEVELOPMENT 

Several methods or devices are available to 

discourage development on flood plains. They include public-information programs, warning signs, 

recordation of hazard, tax-assessment practices, 

financing policies, and public-facility extension 

policies. Flood-insurance requirements and costs 

may be an additional deterrent. 

Public information.-Public-information programs can help to bring important flood information 

to the attention of the public. Prudent citizens, when 

advised of flood hazards, would be reluctant to risk 

property losses and expose their families to the 

danger and trauma that may accompany floods. As 

any program of land-use control will depend on the 

support of an informed public, using educational 

measures is important. 

Preparing, announcing, and disseminating 

flood-hazard information are effective means for 

informing the public. Flood-prone area maps, floodhazard maps, and flood-plain information reports, 

such as those described in the preceding section, 

"Delineation of Flood Plains," are some of the ways 

to provide the needed information. 

Warning signs.-Warning signs draw public 

. attention to flood hazards, especially the attention of 

potential land purchasers and developers who inspect sites prior to purchase. Such signs may be most 

effective if they are readily visible to buyers, developers, and the public, are based upon adequate 

flood data, and are posted where the 100-year flood 

boundaries intersect public rights-of-way. Warning 

signs might take other forms also, such as rubberstamp impressions on subdivision plats and on 

building and zoning permits stating that the sites 

are in areas subject to flooding at a given frequency 

or recurrence interval. 

Recordation of hazard.-Recordation provides a 

possible means for alerting land purchasers, local 

assessors, and lenders to potential flood hazards. 

This might be done by filing flood-hazards maps with 

the appropriate county recorder, together with listings of the subdivisions or the sections, as identified 

under the public lands survey system, and requesting 

entry into tract indexes. Abstracts of titles for affected properties and subsequent conveyances then 

would contain an entry referencing the hazards. 

Adopting local subdivision ordinances that require 

potential flood boundaries to be shown would automatically result in the filing of pertinent floodhazard data with the county recorder along with the 

subdivision plat. 

Tax-assessment practices.-Real-property assessment is an important factor influencing land -use 

patterns. In urban and urbanizing areas, the appraisals, assessments, and tax rates on real property 

reflect the high demand for land. Owners of flood 

plains may seek to relieve tax burdens by selling the 

less agriculturally productive flood plains for development. The public-information, warning, and recordation programs can alert potential purchasers and 

local property-tax assessors to those lands subject to 

flood hazards. A lower assessment and reduced 

demand can result in less economic pressure on the 

owner to convert flood plains to urban use. 
28 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

When constructing public works for flood-loss 

prevention becomes necessary, the costs of these 

flood-control works may be assessed in whole or in 

part against those lands that will benefit from the 

works. 

Financing policies.-Almost all building or construction today involves loans or mortgages by 

private lenders, many of which are insured by the 

Federal Housing Administration, Farmers Home 

Administration, or the Veterans Administration. 

Compliance by private lenders and Federal agencies 

with policies denying loans or loan insurance for 

construction on flood plains would discourage floodplain development. 

Public-facility extensions.-The availability of 

utilities and other community facilities in flood 

phtins may attract subdividers, developers, and 

home purchasers. Metropolitan and municipal utilities could design and install water-supply and 

sewerage systems that would not have the capacity 

to serve urban developments proposed on flood 

plains. Local governing bodies and agencies could 

adopt and announce policies that they will not 

authorize, finance, or construct community facilities, 

such as roads and schools, to serve areas subject to 

flooding. These actions should be taken well in 

advance of developmental pressures; but they can 

also be used with removal and regulatory methods 

and devices. 

Flood-insurance costs.-Flood insurance traditionally has not been available from private sources 

because floods lack the essentially random nature 

necessary to a sound insurance program. The American Insurance Association (1956) has reported that: 

flood insurance covering fixed-location properties in areas subject 

to recurrent floods cannot feasibly by written because of the 

virtual certainty of loss, its catastrophic nature, and the reluctance or inability of the public to pay the premium charge required 

to make the insurance self-sustaining. 

Flood insurance apparently can be sold at 

feasible rates only with sizable government subsidies. Unless premiums are related to the risk 

involved, the cost of flood insurance may be considered a taxation measure. 

The National Flood Insurance Program attempts 

to discourage development in flood-hazard areas. 

This program requires the purchase of flood insurance as a condition for receiving any form of Federal 

financial assistance for construction or acquisition 

in identified flood-hazard areas. This program is 

discussed in the section "Governmental Organization for Flood-Loss Reduction in the San Francisco 

Bay Region." 

I 

REGULATION OF FLOOD-PLAIN USES 

It is costly to undertake public-works programs 

for the protection of development, difficult to remove 

or convert existing development, and probably unrealistic to assume that all future development on the 

flood plain will be discouraged by indirect action. 

Prohibiting and regulating uses vulnerable to flood 

loss under local police powers, however, provides an 

efficient and economical method for preventing or 

reducing flood loss. For example, the California 

Legislature has declared that the primary responsibility for planning, adopting, and enforcing landuse regulations to manage flood plains rests with 

local government (California Water Code, 1971). 

Generally, flood ways should be restricted to 

open-space uses, such as parks, grazing, some types 

I 

of agriculture, drive-in theaters, parking lots, and 

outside storage areas, and to selected structures that 

can withstand flood velocities without obstructing 

the movement of floodwaters. The flood plains 

should also be limited to open-space uses, but certain 

uses that are not flood-vulnerable might be permitted 

if so regulated that reductions in floodwater storage 

capacity and increases in flood stage are minor. 

Isolated filling operations on flood plains might be 

allowed, but numerous individual fills may have substantial adverse effects on storage capacity and flood 

stage. Therefore, the U.S. Army Corps of Engineers 

has stated in one of its flood-plain information 

reports that, "The best rule is to avoid any filling of 

the flood plain***" (U.S. Army Corps of Engineers, 

1966, p. 26). 

Devices for prohibiting or regulating flood-plain 

development include establishing regulatory zones 

compatible with the flood hazards involved and 

incorporating flood-plain regulations in zoning, subdivision, sanitary, and building ordinances. An extensive discussion of state enabling legislation, 

selected legal issues, program administration, and 

local ordinances relating to flood -plain regulations is 

contained in the U.S. Water Resources Council report 

"Regulation of Flood Hazard Areas to Reduce Flood 

Losses" (1971, 1972). Examples of zoning districts 

and special flood-plain, subdivision, sanitary, and 

building regulations, which can be incorporated into 

existing local ordinances, are set forth in Appendices 

H through L of the "Floodland and Shoreland 

Development Guide" (Southeastern Wisconsin Regional Planning Commission, 1968). Planningrelated aspects of these regulatory devices are 

discussed in the section "Planning for Flood-Loss 

Reduction." 
FLOOD-LOSS PREVENTION AND REDUCTION MEASURES 29 

Zoning ordinance districts.-Zoning ordinances, 

adopted and administered by local governmental 

agencies, can regulate and restrict the use of land, 

water, air, and structures in the public interest. The 

ordinances usually consist of a text setting forth 

regulations that apply to each zoning district and a 

map delineating the boundaries of each district to 

which the regulations apply. 

Zoning is a widely accepted and effective 

technique for controlling flood-plain development. 

This device can provide direct benefits by restricting 

future development of vacant lands in flood plains to 

avert potential damage and by limiting expansion of 

existing development in flood-hazard areas. 

The California Legislature has provided for the 

adoption and administration of zoning ordinances 

by counties and cities and specifically declared its 

intention that counties and cities may exercise the 

maximum degree of control over local zoning matters 

(California Government Code, 1976a). 

The most common general zoning districts 

which contain uses compatible with flood hazards 

are agricultural, open-space, conservancy, and park 

districts. These districts permit such uses as general 

farming, woodlands, wildlife refuges, and public and 

private recreation. The general zoning regulations 

can be supplemented by incorporating flood-plain 

regulations into the zoning ordinance text to prohibit 

those agricultural, open-space, conservancy, and 

park uses which would be vulnerable to flood damage. For example, the use of flood plains for agricultural purposes can be regulated to prohibit farm 

dwellings, the permanent sheltering or restrictive 

confinement of animals, and tillage of the flood ways 

without soil conservation practices. 

Flood plains which can be zoned for commercial 

and industrial use because of proximity to existing 

development can be regulated to permit only parking 

and outside storage and to prohibit the storage of 

buoyant, toxic, flammable, and explosive materials. 

Selected parts of flood plains adjacent to proposed 

residential development may be placed in a planned 

residential district. Regulations can then be imposed 

so that such flood plains are reserved or dedicated for 

public or private neighborhood parks to serve the 

adjoining residential development. 

Special flood-plain regulations.-Regulations in 

zoning ordinances concerning the use of flood plains 

supplement the basic use and site regulations and 

can be designed to: 

1. Prohibit flood-vulnerable uses and structures 

within the flood plain and its floodway, 

including on-site sewage disposal. facilities, 

residential uses, sheltering and confining 

of animals, and the storing of buoyant, 

toxic, flammable, and explosive materials. 

2. Regulate other uses and structures within the 

flood plain to require flood proofing without 

impeding drainage, reducing storage capacity, increasing flood peaks, or raising flood 

stages. 

3. Prohibit uses and structures within the floodway, including filling, dumping, bridge embankments, permanent structures, and private roads that would obstruct the floodway, raise flood stages, increase flood 

velocities, or retard the movement of 

floodwaters. 

4. Prohibit dumping, filling, and erecting any 

structures within the channel that might 

obstruct floodwaters, and prohibit the 

building of bulkheads, wharves, and piers 

except by special permit. 

Subdivision ordinances.-Regulating the design 

and improvement of subdivisions is a less frequently 

used method for controlling flood-plain development. 

The California Legislature has provided that every 

city and county "***shall by ordinance regulate and 

control subdivisions" (California Government Code, 

1976b). These ordinances may prohibit subdividing 

lands not suited to the intended uses, subdividing 

flood plains, and altering flood plains and floodways. They may require flood-protection measures 

for building sites and improvement of streets and 

building sites prior to dedication and sale. 

The approval of subdivisions, acceptance of 

public rights-of-way, and extension of utilities, 

coupled with the developer's investments in road and 

site improvements and the erection of dwellings, 

creates a dilemma for lo<!al officials. This dilemma 

concerns the legality, reasonableness, and economy 

of subsequently applying zoning and other regulations that would prohibit further development on 

lands already subdivided and partially improved. 

This problem could be avoided by prohibiting the 

creation of new building sites in areas subject to 

flooding. 

Such prohibition can be accomplished by a 

subdivision ordinance that is designed to: 

1. Prohibit the creation of building sites on flood 

plains subject to the 100-year flood. 

2. Require the delineation and designation of 

flood-prone areas on subdivision plats and 

certified survey maps. 
30 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

3. Require dedication or reservation of floodprone lands for public or private parks or 

other community purposes. 

4. Require that public and private roads, bridges, 

and other facilitieo be designed and constructed to withstand flood velocities; prevent isolation, utility outages, and disruption of transportation; and not obstruct the 

movement of floodwaters, increase flood 

velocities, or raise flood stages. 

5. Require dedication of, or easements along, 

those drainageways necessary for adequate 

watershed drainage. 

Sanitary ordinances.-Sanitary ordinances are 

public laws adopted by local units of government to 

protect the health of the citizens within their jurisdiction. For example, the California Legislature has 

provided that cities "***shall take such measures as 

may be necessary to preserve and protect the public 

health, including the regulation of sanitary matters" 

(California Health and Safety Code, 1970). 

These ordinances can be used to eliminate the 

health problems resulting from the disruption of 

private sewage-disposal systems or contamination 

of private water-supply systems caused by flood 

inundation. On-site soil absorption sewage-disposal 

systems, for example, including septic tanks, absorption fields, and seepage beds and pits, do not function 

during floods and may become inoperative or clogged 

after floodwaters have receded. Such problems can 

be avoided by a sanitary ordinance that is designed 

to: 

1. Require a permit prior to installing any 

system or constructing or modifying any 

building, and require the application for 

such permit to show the flood-plain boundaries. 

2. Prohibit on-site soil absorption sewagedisposal systems and private water-supply 

systems on lands subject to flooding. 

3. Require the replacement of on-site soil 

absorption sewage-disposal systems on 

flood plains with alternate systems, such 

as public sanitary sewerage or floodproofed holding tanks. 

Building ordinances.-Building ordinances are 

public laws adopted by local units of government to 

ensure the safety of structures within their jurisdiction. Section 17958 of the California Health and 

Safety Code (1975b) provides that "***every city or 

county shall adopt ordinances or regulations imposing the same requirements as are contained in the 

regulations adopted pursuant to Section 17922***." 

Section 17922 of the California Health and Safety 

Code (1976b) provides for imposing "***substantially 

the same requirements as are contained in the most 

recent edition of the ***Uniform Building Code of the 

International Conference of Building Officials." 

These ordinances can be used to ensure that the 

structures and their contents are protected from 

flood loss, do not aggravate flood problems, and 

provide sound and safe occupancy during floods. The 

opportunity to protect structures from flood loss can 

be increased by a building ordinance that requires: 

1. Foundations, base supports, footings, and 

other anchorages that can withstand flood 

velocities and hydrostatic pressures. 

2. Use of materials that will not be damaged 

if submerged. 

3. Elevation of floors and electrical equipment 

at least 2 ft (0.6 m) above the level of the 

100-year flood. In addition, the ground level 

around buildings should be at least 1 ft 

(0.3 m) above the 100-year flood for at least 

15 ft (4.6 m) out from the exterior walls. 

4. Appropriate floodproofing measures, including structural modifications and installation of special equipment. 

5. Bridge and culvert openings adequate to pass 

high-flood discharges and designed for 

maximum passage of debris. 

A good example of minimum building standards 

and requirements that could be used to supplement 

existing building codes is contained in "FloodProofing Regulations" (U.S. Army Corps of Engineers, 1972c). 

PLANNING FOR FLOOD-LOSS 

REDUCTION 

By W. E. SPANGLE and M. L. BLAIR 

In general, planning is the process of devising 

and carrying out a course of action to reach an 

identified objective. As an organized governmental 

activity, planning is directed toward improving the 

quality of decisionmaking and may result in several 

kinds of plans serving different purposes. The principal kinds of plans discussed in this report include: 

Comprehensive plan.-A plan for the long-term 

future development of an area considering all major 

determinants of growth and change-economic, political, social, and physical. When adopted by the 

governing body having planning authority, the plan 

becomes the official policy of the agency. A comprehensive plan is often called a general plan or master 

plan. In California, each city and county is required 
PLANNING FOR FLOOD-LOSS REDUCTION 31 

by State law to prepare and adopt a "comprehensive, 

long-term general plan for the physical development 

of the county or city" (California Government Code, 

1966). Unless otherwise specified or made clear by 

the context, the terms "plan" and "planning," as 

used in this report, refer to comprehensive (or general) plans and planning. 

Land-use plan.-A key component of a comprehensive plan providing a link between more general 

goals and policies and the pattern of land development. A land-use plan includes objectives, policies, 

and proposals for the type, pattern, and intensity of 

land use. The land-use plan may be called a land-use 

element, as in the Federal Comprehensive Planning 

Assistance Program in Section 701 of the Housing 

and Community Development Act of 1974 (U.S. 

Congress, 1974b) and in the California Planning and 

Zoning Law (California Government Code, 1976d). 

Functional plan.-A plan for facilities and 

operations for a specific function of government such 

as transportation, water development, flood control, 

or flood plain management; it is more specific and 

usually shorter range than the comprehensive plan. 

Area plan.-A plan, expanding upon and consistent with the comprehensive plan, for a part of 

a planning jurisdiction, such as a watershed, central 

business district, or other particular area. 

THE PLANNING PROCESS 

Planning is described herein as a six-step 

process: (1) Identifying problems and defining goals 

and objectives; (2) collecting and interpreting data; 

(3) formulating the plan; (4) evaluating impacts; (5) 

reviewing and adopting the plan; and (6)implementing the plan. These steps, as depicted in figure 14, are 

closely interrelated. Plan formulation often indicates 

the need for additional iRformation. Additional information may alter the concept of the objectives and 

problems, and plan implementation may reveal the 

need for additional information or modification of 

the plan. 

Identify problems 

and define goals 

and objectives 

Collect and 

interpret 

data 

Formulate 

plans 

Under our democratic form of government, 

public participation is essential throughout the planning process. In fact, most planning efforts funded 

by Federal programs are required to provide for significant public participation. "Public" may refer to 

elective or appointed political bodies, special-interest 

groups, or interested individuals. Success in implementing a plan depends on widespread public support which is difficult to achieve if major segments of 

the public are excluded from participating in the 

planning process. 

Decision occur throughout the process, ranging 

from the decision to engage in a planning effort, to 

the final approval of a plan and adoption of implementing regulations, programs, and procedures. 

Elected public officials have final responsibility for 

most key policy decisions, although persons in nonelective positions actually make many important 

day-to-day decisions. It is particularly important 

that public input precede major policy decisions 

made by elected officials. 

The steps in the planning process compose a 

rational, systematic approach to informed decisionmaking generally applicable to land-use, functional, 

and area planning as well as comprehensive planning. Ideally, the product of the process is an orderly, 

logical, and internally consistent plan, or a set of 

plans and programs, to guide public and private 

decisions. 

Land-use planning is directly relevant to reducing flood losses. The extent of loss from flooding is a 

direct consequence of the use of land. Through a 

land-use planning and decisionmaking process, 

agreement concerning an acceptable level of risk 

from flooding can be reached, appropriate uses of 

flood-prone areas can be determined, and adverse 

effects of flooding can be mitigated. A land-use plan 

prepared by planners in collaboration with earth 

scientists, hydrologists, and other specialists thus 

provides an excellent framework for developing specific programs to reduce flood losses. 

Review and 

adopt plans 

Implement 

plans 

Feedback for review and revision 

FIGURE 14.-The planning process. 
32 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Logical decisions concerning flood-loss reduction are likely to result when flooding problems are 

considered in each step of the land-use planning 

process. The outline below lists, under each step of 

the planning process, tasks or actions ordinarily 

needed for effective land-use planning and decisionmaking in flood-prone areas. In many cases, specific 

procedures to be followed are defined by State law. 

Identify problems and define goals and objectives.-Obtain readily available data for preliminary 

identification of flooding problems. 

Review the data in relation to existing development, current land-use plans and policies, projected 

growth trends, and anticipated changes. 

Based on community consideration of the 

acceptable level of risk, develop a tentative set of 

objectives and priorities related to reduction of future 

flood losses. A usual objective is to prevent losses 

from floods of varying severity up to that of the 100-

year flood. 

Collect and interpret data.-Evaluate adequacy 

of available hydrologic and engineering data for 

land-use planning and develop a program for compiling needed new data. 

Arrange with earth scientists and engineers for 

needed hydrologic studies including maps of floodprone areas, flood profiles, and discharge-frequency 

relationships. Map information should relate in 

scale and detail to the physical characteristics of the 

flood plain and stream basin and the degree of development of the area, present and potential. 

Estimate the probable future demand for land 

considering projections of population growth and 

distribution, economic activity, social and cultural 

needs, and transportation requirements. 

Use flooding information in combination with 

other data to prepare land-capability maps showing 

the relative natural capability of each land unit to 

accommodate each contemplated use. 

Formulate plans.-Consider feasible alternative 

arrangements of land uses, based on land-capability 

maps, appropriate projections, and economic, social, 

and political analyses. 

Prepare and test alternative land-use plans 

incorporating as much detail as necessary to provide 

a policy guide for future actions and decisions. 

Prepare functional water-resources plans for 

stream basins with flooding problems that provide 

enough depth and detail to guide the selection of 

appropriate flood-loss reduction measures. 

Evaluate impacts.-Evaluate alternative landuse plans for environmental, economic, and social 

impacts. 

Evaluate the impact of alternative land-use 

patterns on the extent and severity of flooding. 

Evaluate the impact of alternative land-use 

patterns in terms of potential flood loss. 

Review and adopt a plan.-Present alternative 

plans for review and selection by the appropriate 

legislative body. 

Schedule and hold public hearings. 

Adopt plan with such modifications as may be 

needed to respond to information provided and 

opinions expressed at public hearings. 

Implement the plan.-Prepare and seek adoption 

of regulations and any land-acquisition and capitalimprovement programs needed to carry out the plan. 

Establish guidelines and a procedure for 

evaluating the effect of development proposals on 

flood potential. 

Develop procedures and staff or consultant 

capability for reviewing hydrologic, soils, and 

geologic reports, environmental impact assessments, 

and project proposals. 

Arrange for modification of previous steps as 

new or more detailed data become available. 

The generalized model of the planning process 

and list of actions undertaken in each step are 

necessarily idealized and simplified. Actual practices vary widely depending on the responsibility, 

authority, and financial position of the planning 

agency; the diversity of the planning area; scope of 

the planning effort; and availability of data. For 

example, planning by regional councils of government (COGs) is likely to emphasize the development 

of objectives, policies, and criteria for use in 

reviewing projects and plans because the COG's 

primary source of power derives from Federally 

mandated review processes. Local planning, on the 

other hand, is more likely to emphasize the development of objectives, policies, and criteria to serve as a 

basis for land-use and development regulationprimarily a local responsibility. 

In addition, planning practices are not static. 

Planning is in a state of flux, with planners, 

legislators, and citizens searching for new ways to 

make the process more effective. The scope of 

planning is expanding and its role changing, fresh 

approaches are being tried, and new relationshipslocal, metropolitan-regional, State, and Federal-are 

emerging. 

FLOOD-PRONE AREAS-PLANNING FOR APPROPRIATE 

LAND USES 

In planning for flood-loss reduction, the local 

and regional land-use planner is particularly 
PLANNING FOR FLOOD-LOSS REDUCTION 33 

concerned with defining appropriate land uses for 

flood-prone areas. The determination of appropriate 

land uses is a technical-social-political process 

involving personal and corporate aspirations and 

legal rights, as well as public goals and objectives. A 

well-conceived and implemented land-use plan can 

lead to significant reduction in flood losses with 

relatively small public cost, particularly in areas 

where little or no development of the flood plain has 

occurred. 

More is at stake in deciding land uses in floodprone areas than the potential reduction of flood 

losses. The flood plain is an important environmental and ecological resource meriting attention in 

its own right. Flood-prone areas often provide wildlife habitats, water-recharge areas, fertile soils, 

scenic areas, and lands suitable for recreation and 

park uses. Thus, the land-use planner must evaluate 

proposed land uses in terms of both the potential 

flood risk and the beneficial use of the natural 

attributes of the flood plain. 

Planning for appropriate flood-plain uses must 

also be done in the context of land-use planning for 

the entire drainage basin. The extent and frequency 

of flooding and the resource potential of the flood 

plain may be affected by land-use decisions in upstream areas. In situations where the drainage basin 

extends beyond a local planning agency's jurisdiction, effective planning for flood-prone areas 

requires interjurisdictional coordination or multijurisdictional (regional) planning. 

If development is proceeding in a stream basin 

or if flood-control works are being considered, plans 

more specific than local land-use plans are often 

needed to focus directly on the flooding problem. 

Such plans, usually called functional plans, consider 

in depth the land-use/hydrologic relationships in the 

stream basin. Functional plans can provide a bridge 

between land-use plans and engineering plans for 

flood-control works or specific flood-plain management regulations. Such functional planning may be 

carried out by a public works department or a flood 

control district, or by the general planning agency. 

The comprehensive plan and planning information 

should provide a framework for the functional 

planning effort whether or not the planning agency 

is directly involved. 

More than one governmental agency is frequently involved in planning for a given stream 

basin. The plans of the various agencies differ in 

scope and specificity, but should be mutually consistent. A regional land-use plan ordinarily provides 

broad policy guidance for evaluating future urban 

development, open space, large scale commercial 

and industrial development, and major public 

facilities ·including transportation facilities. 

Ideally the regional plan provides a framework 

for more detailed county and city land-use planning 

in which both urban and nonurban uses are usually 

considered in more detail and attention is given to 

local conditions and problems. A regional plan 

might recommend open-space uses for flood-prone 

areas; a county or city plan would typically specify 

the particular open-space use, such as grazing or 

water-related recreation. 

In undeveloped flood-prone areas, nonintensive 

land uses should be given high priority. Because 

flood plains are usually flat and are often overlain 

with fertile alluvial soils, agricultural uses are frequently appropriate. When natural and economic 

factors favor agricultural uses, more specific determinations need to be made concerning the kind of 

agricultural use. Grazing may have a different 

impact on flood-loss potential than crop cultivation. 

The impacts of crop cultivation can also vary 

depending on the kind of crop, the growing season, 

the structures needed (irrigation structures, storage 

structures, fencing), the use of pesticides, erosion 

potential, and land-treatment practices. Where crop 

production is permitted on the flood plain, maintenance of riparian cover adjacent to the streambanks 

can prevent undue erosion or bank collapse and loss 

of wildlife habitats. Farm buildings or other 

structures should be located outside the floodway to 

avoid obstructing the flow of floodwater. 

Agricultural use of flood-prone areas reduces 

flood-loss potential below that of urban uses, but 

does not eliminate it. In fact, flood-control works are 

often proposed and constructed to protect agricultural lands from flooding. Such projects should be 

evaluated for both short- and long-term effects. 

Periodic flooding and deposition of sediment is a 

natural process which forms the rich alluvial soils of 

flood plains. While the loss of a crop may cause 

immediate economic hardship to the region, the 

farmer, or the locality, the replenishment of the soil 

by flooding may be a vital long-term benefit. 

Agriculture is only one of many open-space uses 

which may be appropriate for flood-prone areas. 

Recreation, especially water-related activities such 

as fishing, boating, and swimming, often has little 

flood-loss potential and may provide valuable public 

benefits.Open-space and other low-intensity uses of 

flood-prone areas may be appropriate to preserve 

scenic, archeological, and scientific resources or to 

conserve mineral and timber resources for future use. 
34 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Uses such as parking lots, drive-in theaters, or uses 

involving structures with relatively little flood-loss 

potential may be appropriate for flood-prone areas. 

However, the impact of such uses on the natural 

functions of the flood plain must be carefully 

evaluated. In particular, the extent of impervious 

surfaces (roofs and paving) must be strictly limited 

in ground-water recharge areas if such areas are to 

continue to function. Determining the appropriate 

uses for specific flood-prone areas thus involves 

achieving beneficial use from its natural features as 

well as minimizing flood-loss potential. 

Planning for developed flood-prone areas is 

more difficult. Detailed plans for conversion of 

developed flood-prone areas to less intensive uses 

can be prepared to guide day-to-day decisions concerning changes in land use within the flood plain. 

Such plans can also provide the basis for decisions 

concerning reconstruction and extension of public 

facilities and utilities and public acquisition of land 

for park or other open-space purposes. Well 

conceived and publicized plans for conversion of an 

area can also affect private investment decisions. 

Detailed plans prepared to guide gradual conversion 

of use of the flood plain can also serve effectively in 

guiding recovery activity following a damaging 

flood. The availability of such a plan can speed 

recovery, provide a rational basis for administering 

State or Federal disaster assistance, and encourage 

rebuilding in a way that minimizes future flood 

losses. 

Flood-loss potential may also be reduced 

through planned redevelopment of the flood-prone 

area. Redevelopment permits public acquisition and 

clearance of parcels in a blighted area. Parcels 

acquired are then made available for re-use according to a specific redevelopment plan. Redevelopment 

can be an effective method of reducing flood-loss 

potential under certain circumstances. If the 

redevelopment area includes both flood -prone and 

flood-free lands, the plan can specify intensive 

development of the flood-free area and nonintensive 

uses compatible with the flood risk for the floodprone area. Where urban uses are planned for a floodprone area, the redevelopment plan may require 

floodproofing or related safeguards for new structures and those structures which are to remain. 

In many developed flood-prone areas, particularly those with economically viable uses and sound 

structures, clearance may not be socially or economically feasible. In such areas, safeguards such as 

floodproofing and measures to limit occupancy can 

reduce potential flood losses to some extent. For 

major reduction in flood-loss potential, flood-control 

works will probably be required. If flood-control 

works are constructed, it is important that floodplain uses be regulated to prevent an increase in 

damage potential from floods greater than the 

design flood. Even with flood-control structures 

designed for the 100-year flood, it is usually wise 

planning to restrict occupancy of flood plains to uses 

which can accept occasional flooding. 

ROLE OF LAND CAPABILITY STUDIES 

The evaluation of land capability can assist 

planners and decisionmakers in assessing alternative land uses in flood-prone areas. In any area the 

natural features and processes present a range of 

advantages and disadvantages for different land 

uses. Land-capability studies systematically record 

judgments about the relationship between the 

physical characteristics of land and particular land 

uses. Such studies provide at least a generalized view 

of the relative physical merits of lands in a study 

area for particular uses. 

Information delineating flood-prone areas is 

important in assessing land capability for most land 

uses. In fact, a capability study may be designed to 

eliminate, on a first cut, lands subject to flooding 

from consideration for residential or intensive urban 

uses. Considering only the objective of reducing 

flood losses, flood plains would almost always have a 

high capability rating for low intensity, open-space 

uses. If flood-prone area information is combined 

with data describing other natural features often 

occurring in flood plains (for example, good agricultural soils, fish and wildlife habitats, groundwater recharge areas, and scenic sites), the high 

capability of the flood-plain lands for low intensity 

uses may be reinforced. 

Flood-prone areas are typically part of larger 

flatland areas which, if flooding is not considered, 

often have high physical capability for urban uses as 

well as agriculture and other open-space uses. 

Resolving the conflicts among the competing land 

uses is rarely done on the basis of natural factors 

alone. Economic, social, and political data are 

needed in this process. Although an area may have 

poor physical capability to support intensive uses, 

other factors, such as location and accessibility, land 

cost, absence of alternative lands, or overriding 

public need, may indicate that it be intensively 

developed. In cases where intensive development of a 

flood plain is warranted, flood-control works to 

reduce the flood-loss potential may be appropriate. 
PLANNING FOR FLOOD-LOSS REDUCTION 35 

The use of flooding information in a land 

capability study is illustrated in the Ohio Department of Natural Resources (1974) report "Big Darby 

Creek Corridor Study.'' The purpose of the study was 

to prepare a land-use plan for a stream corridor in 

central Ohio "to permit the best use of flood plains 

while avoiding flood hazardous construction on 

them***(and)***to properly evaluate and use the 

recreation and natural preservation potential of 

sceni~ river valleys" {preface). For the capability 

analysis, the corridor was divided into 2.35-acre 

(0.0095-km2) grid cells. Each grid cell was given a 

numerical rating for physical variables such as soils, 

slope, vegetation, flood plains, ground-water availability, sand and gravel deposits, and surface water. 

The variables selected and the ratings assigned 

differed with each land use considered. The 

individual ratings were added for each cell giving an 

overall rating of relative capability for the land use. 

The total ratings were then divided into three capability classes, A, B, and C, and one "incapable" class. 

A cell was classed as "incapable" if it had "severe 

limitations" in certain physical variables. For 

example, in evaluating capability for residential 

development, an "incapable" classification was 

automatically given to cells within the flood plain. 

The capability classifications provided key 

input to a suitability analysis which considered 

transportation, existing land use, utility availability, and other cultural aspects of the stream 

corridor. In the system used, cells classed as 

"incapable" for a particular use were also automatically considered "unsuitable." Cells in the A, B, 

and C capability classes were rated for suitability 

and classed as A, B, or unsuitable. The suitability 

classification was applied directly in plan formulation. The resulting plan, incorporating objectives 

and policies derived from public participation in the 

planning process, recommended primarily park and 

recreational uses in the flood plain and ·local 

adoption of flood-plain zoning and subdivision 

regulations. 

A somewhat different approach to land capability analysis has been taken by the Association of 

Bay Area Governments (ABAG) in a recent study 

(1976). ABAG, supported by the San Francisco Bay 

Region Study (SFBRS), developed a method for 

expressing land capability in terms of the dollar 

costs associated with hazard mitigation measures, 

potential property damage from natural hazards, 

and loss of natural resources. The method was tested 

in a demonstration area in a part of the Santa Clara 

Valley. 

The ABAG study focuses on geologic and hydrologic hazards and resources, making excellent use of 

many SFBRS products. Natural factors considered 

in evaluating land capability include earthquakes, 

flooding, bearing materials, slope stability, erosion/ 

sedimentation, and natural resources. Land uses 

considered include agricultural or rural, semirural 

residential, single-family residential, multifamily 

residential, regional commercial, downtown commercial, industrial manufacturing, and freeways. 

The total expected cost associated with each 

natural constraint and resource for each land use 

was calculated. For stream flooding, expected costs 

are assigned to areas within the 100-yearflood plain 

as mapped by Limerinos, Lee, and Lugo (1973) at a 

scale of 1:125,000. Insurance rates developed for the 

National Flood Insurance Program are used to estimate all future expected costs per acre associated 

with flooding. Table 2 lists these costs for each land 

use. 

TABLE 2.-Expected costs associated with stream flooding 

[From Association of Bay Area Governments, 1976, p. 113] 

Land use 

Rural or agricultural ................................................... . 

Semirural residential ................................................... . 

Single-family residential ........................................... . 

Multifamily residential ............................................... . 

Regional shopping centers ......................................... . 

Down town commercial ............................................... . 

Industrial ......................................................................... . 

Freeways ......................................................................... . 

_Expected cost 

per acre 

$200 

700 

9,000 

40,000 

40,000 

50,000 

40,000 

0 

Cost information for all natural resources and 

problems for each 24.9-acre (0.10-km2) grid cell was 

aggregated for each land use. The resulting number 

indicates, for each cell, "the dollar cost per acre 

expected to be incurred by developing that cell with 

that land use" (Association of Bay Area Governments, 1976, p. 168). The range of total costs was 

divided into six capability levels and a land capa-

. bility map for each use was printed by computer. The 

study shows that the expected costs associated with 

flooding are quite high for most land uses, but less 

significant than the costs associated with other 

problems, particularly dike failure, landslides, and 

soil creep. 

IMPLEMENTATION OF LAND· USE PLANS 

FOR FLOOD-LOSS REDUCTION 

Plan implementation with a focus on flood-loss 

reduction is primarily a function of local government. It can be accomplished in a variety of ways 

depending on the degree of existing development, the 
36 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

natural characteristics of the flood plain, the kind 

and level of detail of flood information, and the 

policies and proposals of the land-use plan. 

Three categories of plan implementation actions 

may be effective in reducing flood losses: 

1. Establishing and administering regulations 

on land-use and development through zoning, subdivision, building and housing 

codes, and grading and sanitary ordinances (most effective in undeveloped 

areas); 

2. The execution of programs directly by the 

government which has jurisdiction in the 

planning area, including actions such as 

land acquisition for public sites and construction of public facilities; and 

3. The review of projects, both public and private, as required by the National Environmental Policy Act of 1969 (U.S. Congress, 

1970), various State environmental laws, 

and the U.S. Office of Management and 

Budget Circular A-95 (1969). 

Effective plan implementation depends on the 

active support of residents and organizations within 

a planning jurisdiction. The implementing process is 

intensely political, involving public decisions which 

have direct impact on the legal rights, economic and 

social status, and living and working environment of 

individuals in a community. Measures to insure 

public participation in decisions concerning implementing programs and regulations are critical to the 

realization of plan objectives. 

LAND-USE AND DEVELOPMENT REGULATION 

Land-use regulation is largely a prerogative 

granted by the States to local units of government. 

Although case law and planning legislation differ 

from state to state, general legal principles pertain to 

all efforts to control land-use and development. 

These principles may be summarized as follows: 

1. Because the authority to regulate the use of 

private land derives from a local jurisdiction's "police power," regulations, to be 

legal, must promote the "health, safety, 

morals, and general welfare" of the community. 

2. Regulation of land use must be "reasonable." 

While the concept of "reasonableness" has 

been variously interpreted, it still remains 

the most crucial test of the legality of a regulation. Prevailing considerations in court 

determinations of reasonableness include 

the existence of a rational basis for the 

regulation and equity in its application. 

3. Regulations are viewed more favorably by the 

courts if they are the product of a consistent, 

well-thought-out planning program which 

reflects explicit, community-determined 

goals. 

Zoning.-A zoning ordinance divides a jurisdiction into districts or zones and establishes for each 

district requirements governing the use of land and 

the bulk, height, coverage, and use of structures. Certain zoning classifications and procedures may reduce allowable intensity of development in flood 

plains and (or) require concentrating structures on a 

portion of a site out of the flood plain thus reducing 

flood-loss potential. Some of the more commonly 

applied zoning techniques include planned unit development (PUD), planned community zoning, 

cluster zoning, agricultural zoning, and flood-plain 

zoning. 

Planned unit development or planned community zoning allows development of a large parcel as a 

single unit according to a plan approved by the 

legislative body. This technique provides the developer the flexibility to plan for the economic use of a 

large area within constraints posed by natural features and processes. It also gives the public agency 

the opportunity to review all aspects of the proposal 

and place legally binding conditions on the development of the property. Flood-prone areas are typically 

left as open space. 

Cluster zoning specifies the density of permitted 

development (the number of dwelling units per unit 

of land) and requires that the houses be placed close 

enough together to leave a considerable portion of 

the site in open space. Under this device, flood-prone 

areas may be left in open space with little or no loss of 

development potential of a large parcel. 

Agricultural zoning restricts use to specified 

agricultural uses. Agricultural zoning applied to 

flood-prone areas can be effective in reducing losses 

particularly if farm structures are prohibited in the 

flood plain and appropriate measures are taken to 

prevent encroachment on the stream channel and to 

reduce erosion. 

Flood-plain zoning restricts the use of officially 

designated flood-prone areas. The zoning is often 

applied to the 100-yearflood plain although different 

restrictions may apply to different parts of the flood 

plain depending on flood frequency or loss potential. 

The flood way may be under more severe use limitations than the less frequently flooded fringes of the 

flood plain (figs. 5 and 6). 

Subdivision regulations.-The process of dividing undeveloped land into buildable parcels is governed by subdivision regulations. These typically 
PLANNING FOR FLOOD-LOSS REDUCTION 37 

prescribe lot sizes and relationships, requirements 

for the dedication of lands for public use, limitations 

on use of hazardous areas, standards for street 

improvements, standards for storm drainage, and 

requirements for other public utilities and facilities. 

Subdivision regulations also prescribe procedures 

for review and approval of proposed subdivisions. 

In the San Francisco Bay region, the local flood 

control and water conservation district typically reviews subdivision proposals involving flood-prone 

areas and recommends steps to reduce potential 

flood losses. These may include dedication of easements along water courses for public access, parks, 

or some other legitimate public purpose; prohibition 

of stream channel encroachments; relocation of utilities, roads, or other public facilities; protection of 

riparian vegetative cover; and elevation of structures 

above the flood level. Recommendations of the flood 

control districts are advisory, but the local legislative body has the power, under State law, to attach 

any of these conditions to the approval of a subdivision or to deny approval of a subdivision proposal for failure to comply with such conditions. 

Where there is no flood control district, the responsibility for review and recommendations regarding 

flood problems and solutions usually rests with the 

city or county engineer. 

Building and sanitary codes.-Building and 

sanitary codes are regulations adopted by local units 

of government to ensure the safety of structures and 

residents within their jurisdictions. Such regulations 

can be effectively used to reduce health hazards and 

damages from flooding. Typical local ordinance 

provisions were described in a preceding section 

"Flood-Loss Prevention and Reduction Measures." 

Administration of land-use regulations.-Development proposals are evaluated for compliance with 

appropriate policies, plans, and regulations according to procedures which are ordinarily specified in 

the regulatory ordinances. When ordinances contain detailed restrictions on land use and development, administration is straightforward. For example, administering flood-plain zoning that prohibits construction within a flood plain specifically 

delineated in the ordinance involves reference to a 

map to determine whether or not a proposed structure is within the designated flood plain. 

Regulations which indicate desired results in 

more general terms permit greater discretion and 

require greater expertise at the time of review of 

development proposals. Planned unit development 

regulations which call for avoiding natural hazards, 

for example, can be appropriately administered only 

if the agency staff has detailed information concerning the physical characteristics of the site or requires· that such information be provided by the 

developer. In either case, the agency staff should 

have sufficient information to set proper data requirements for the developer, and sufficient expertise to review the developer's reports. 

The administration of land-use and development regulations is a staff function of the reviewing 

agency, and the staff ordinarily makes ministerial 

decisions. But, in cases where discretion is allowed, 

the decision to approve or deny a development 

proposal is usually made by the local planning 

commission or legislative body. In these cases, the 

responsibility of the staff is to provide the decisionmaking body with all relevant and legally required 

information. 

PROGRAM DEVELOPMENT AND EXECUTION 

Program development and execution involves 

the systematic use of governmental powers and 

resources to achieve public objectives. Programs 

describe the direct actions a public agency intends to 

undertake including land acquisition, construction 

of public facilities, and provision of public services. A 

typical example is a local capital improvement program which schedules, over a multiyear period, the 

capital expenditures proposed for each project or 

activity. Constructing a flood-control project, for 

example, requires scheduling land acquisition and 

construction components in a logical sequence related to availability or commitment of funds. 

A funding schedule or program is also needed to 

carry out public acquisition of flood-plain lands. 

Within limits established by Federal and State law, 

public agencies may acquire land for public purposes. Many agencies may acquire land through 

eminent domain (condemnation at fair market 

value). Public land may be maintained in open-space 

uses such as parks, natural preserves, or parking 

lots, or, in some cases, sold or leased for appropriate 

private development or use. 

Land acquisition programs may be critical to 

flood-loss reduction in both developed and undeveloped flood-prone areas. In undeveloped floodprone areas, public acquisition of land may be 

effectively used if the land is suitable for a public use 

such as recreation or wildlife conservation, or if 

regulation to reduce flood-loss potential would discriminate against individual land owners. In developed flood-prone areas, public acquisition and 

clearance of land may be appropriate where incentives are lacking, or cannot be reasonably provided, 

for private redevelopment to uses less vulnerable to 

flooding. 
38 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Construction of public facilities strongly affects 

realization of a land-use plan. The timing and location of utility extension, street and highway construction, and park and school development, for 

example, all influence the timing and location of 

urban development. Development of flood-prone 

areas can be discouraged by restricting utility extensions, transportation access, and public services 

consistent with an adopted land-use plan. If construction of flood-control projects is needed to reduce 

flood losses, programming the actions of several 

governmental agencies-Federal, State, and localmay be involved. 

PROJECT REVIEW 

Planning agencies are regularly called upon to 

review proposals for projects within or having potential impact on their planning areas. Even in the 

absence of specific procedures or requirements, careful review for consistency with the general plan is a 

matter of good planning practices. In recent years, 

through Federal requirements for the assessment of 

environmental impact and the U.S. Office of Management and Budget A-95 review (1969), such review 

has become increasingly formalized. These review 

processes are the major plan-implementing techniques of many regional agencies. 

A-95 review.-A-95 review is a procedure designed to coordinate Federally funded projects with 

State and local planning objectives. Under this procedure, notification of application for Federal funds 

for a wide variety of projects must be submitted to 

designated State and regional clearinghouse agencies for review for consistency with State, area-wide, 

and local plans and programs. The clearinghouse 

agency forwards the notification to agencies potentially affected by the project for their review and 

comment. The comments are only advisory, but a 

Federal agency must defend in writing any decision 

to fund a project which has received a negative 

review. This assures at least consideration of the 

review comments of affected public agencies. 

Environmental impact assessment.-The National Environmental Policy Act of 1969 [NEPA] 

(U.S. Congress, 1970) requires that an environmental impact statement (EIS) be prepared for proposals 

for legislation and other Federal actions significantly affecting the quality of the human environment. The statement must include the environmental impact of the proposed action, any adverse environmental effects which cannot be avoided should 

the proposal be implemented, alternatives to the 

proposed action, the relationship between local 

short-term uses of man's environment and the maintenance and enhancement of long-term productivity, 

and any irreversible and irretrievable commitments 

of resources which would be involved in the proposed action should it be implemented. 

Environmental impact assessment requirements, similar to those set forth in the Federal legislation, were adopted by California in the Environmental Quality Act of 1970 (California Public Resources Code, 1976). This act requires an environmental impact report (EIR) for all public and private 

projects which may have significant environmental 

impacts, and which involve a discretionary decision 

by a public agency. Other states have adopted 

similar requirements since 1970. 

The requirement for environmental impact assessment, whether Federal or State, is, in effect, 

superimposed on the normal review process of the 

jurisdiction in which a project is proposed. Projects 

subject to environmental impact assessment are still 

reviewed for conformity with the general plan and 

regulations of the appropriate jurisdiction. Local 

regulations may require more environmental information than is required by Federal or State law. 

GOVERNMENTAL ORGANIZATION FOR 

FLOOD-LOSS REDUCTION IN THE 

SAN FRANCISCO BAY REGION 

By M. L. BLAIR and W. E. SPANGLE 

Planning and decisionmaking rMated to floodloss reduction in the San Francisco Bay region is 

carried out and influenced by a diverse group of 

governmental agencies. Typically, efforts to reduce 

flood losses are initiated by local government. If 

flood-loss reduction is to be accomplished primarily 

through regulation, local decisions dominate the 

effort. However,ifmajorflood-control works are to be 

constructed, State and Federal agencies are usually 

more directly involved. Because flood-loss reduction 

often involves a combination of regulatory and 

structural measures, the interplay of government 

agencies can become very complex. 

Even when no flood-control works are anticipated, flood-loss reduction is influenced by Federal, 

State, and regional agencies. Federal and State 

agencies affect local decisionmaking through requirements for funds, criteria for programs, shared 

responsibility for functions such as transportation, 

and regulations such as those relating to local 

planning or environmental quality. 

Local governmental agencies are increasingly 

dependent on Federal and State funds to carry out 
GOVERNMENTAL ORGANIZATION FOR FLOOD-LOSS REDUCTION 39 

their responsibilities. This means that local plans 

and programs are often framed with an eye to 

Federal and State program funding requirements as 

well as to locally expressed objectives and concerns. 

The fact that, until recently, Federal funding has 

been more readily available for flood-control structures than for purchase of land or development 

rights in flood-prone areas has influenced local decisions concerning flood-loss reduction measures. 

Key governmental programs which influence planning for flood-loss reduction and selected governmental agencies most often directly involved in the 

planning and decisionmaking process are described 

in the following sections. 

FEDERAL PROGRAMS 

NATIONAL FLOOD INSURANCE PROGRAM 

The National Flood Insurance Program was 

enacted through passage of the National Flood 

Insurance Act of 1968 (U.S. Congress, 1968) and is 

administered by the Federal Insurance Administration (FIA) in the U.S. Department of Housing and 

Urban Development. The program was established 

to promote the public interest by providing appropriate protection against flood losses through the 

availability of flood insurance coverage and the 

requirement of sound flood-plain management regulatory measures to minimize the exposure of lives 

and property to flood risk. The economic justification for the program, which initially requires extensive public subsidies to bring premiums in line with 

what the public can afford, is the reduction of the 

need for and dependence on increasing flood disaster 

relief appropriations through safer construction 

practices in flood-hazard areas. 

The program was amended by the Flood Disaster Protection Act of 1973 (U.S. Congress, 1973) 

under which available limits of flood insurance 

coverage were substantially increased. Further, such 

coverage became a statutory requirement to secure 

eligibility for Federal financial assistance for acquisition or construction purposes within the identified 

special flood-hazard areas of the Nation. Federal 

agencies responsible for the supervision of lending 

institutions are required to direct such institutions to 

require flood insurance in connection with real estate, mobile home, or personal property loans in 

identified flood-hazard areas. State-owned properties may be covered by approved self-insurance 

plans. 

Section 202 of the Act prohibits Federal financial assistance for acquisition or construction purposes and Federally related financing by private 

lending institutions for properties in identified special flood-hazard areas, unless the community in 

which the area is located is participating in the program within one year of its notice of identification as 

being flood prone. 

The Federal Insurance Administration is 

charged with the responsibility to identify all the 

Nation's flood-plain areas that have special flood 

hazards. Such an area is defined as being that area of 

the flood plain which, on the average, is likely to be 

flooded once every 100 years (that has a one percent 

chance of flood occurrence in any given year). The 

Federal Insurance Administration (written commun., 1977) has identified 96 communities in the San 

Francisco Bay region that are eligible for the sale of 

flood insurance. Flood-hazard areas have been identified in all but two of these communities (see table 

10). The eligible communities are all participating in 

the emergency flood insurance program in which the 

limits of coverage are one-half of that of the regular 

program, and the premiums are subsidized. Upon 

completion of flood-insurance ratemaking studies, 

each community will be transferred to the regular 

program with full insurance coverage available at 

actuarial rates based on the degree of flood risk. 

The specificity of the flood-plain management 

requirement of the National Flood Insurance Program is directly related to the degree of technical 

data which FIA has made available to a participating community. When a community has submitted an application to participate in the NFIP but has 

not yet received a Flood Hazard Boundary Map 

(FHBM) delineating the special flood hazard area, it 

is required to adopt and enforce general flood-plain 

management measures (Federal Insurance Administration, 1976) which are applicable throughout its 

entire jurisdiction. These measures are performance 

standards; the development of specific implementation criteria is left to the discretion of the community, 

which has the best knowledge of its own flood 

problem and administrative resources. At all times, 

however, technical assistance is available from both 

FIA and the coordinating officials designated to 

carry out such responsibilities by each State government. Since some of the following requirements are 

paraphrased by the FIA, and since the map zones in 

which each of them apply are omitted for the sake of 

brevity, the appropriate section of the regulations 

(Federal Insurance Administration, 1976) is noted 

and should be referred to for the specific regulatory 

language and area of applicability. 

When the FHBM is not yet available, the com-
40 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

munity must meet the following requirements of Section 1910.3.a: 

1. Require permits for all proposed construction or other development2 (including placement of prefabricated buildings and 

mobile homes) to determine if it lies within a flood-prone area 

(1910.3.a.l). 

2. Review such development proposals to assure that all necessary Federal or State permits have been obtained (e.g., Section 404 

permits under the Federal Water Pollution Control Act Amendments of 1972) (1910.3.a.2). 

3. Review all permit applications to assure that all new construction and substantial improvements be: (a) Anchored to 

prevent displacement or collapse. (b) Constructed with materials 

and utility equipment which are resistant to flood damage. 

(c) Constructed according to methods and practices that minimize 

flood-damage potential (1910.3.a.3). 

4. Review subdivision proposals and other proposed new development to determine whether such proposals will be reasonably 

safe from flooding. If a subdivision proposal or other proposed 

new development is in a flood-prone area, any such proposal shall 

be reviewed to assure that: (a) Such proposals are consistent with 

the need to minimize flood damage within the flood-prone area. 

(b) New or replacement utilities are located and constructed in a 

manner which will minimize or eliminate flood damage. (c) Adequate drainage is provided to reduce exposure to flood hazards 

(1910.3.a.4). 

5. Require new or replacement water systems to be designed to 

minimize or eliminate infiltration of flood waters into the systems 

and discharges from the systems into flood waters (1910.3.a.5). 

6. Require new or replacement sanitary sewage systems to be 

designed to minimize or eliminate infiltration of flood waters into 

the systems and discharges from the systems into flood waters, 

and require that on-site waste-disposal systems be located to avoid 

impairment of them or contamination from them during flooding 

(1910.3.a.6). 

After receiving its FHBM, the community must 

continue to enforce the same controls within the area 

defined on the map, as well as the following additional requirements of Section 1910.3.b: 

1. Require that all subdivision proposals and other proposed 

new developments greater than 50 lots or 5 acres, whichever is the 

lesser, include base flood-elevation data (1910.3.b.3). 

2. Obtain, review, and reasonably utilize base flood-elevation 

data from alternative sources, prior to its being provided by FIA 

through its Flood Insurance Rate Study, as criteria for requiring 

that all new residential structures and substantial improvements 

to existing structures have their lowest floor (including basement) 

elevated to or above the base flood level, and new nonresidential 

structures and substantial improvements to existing ones have 

the lowest floor (including basement) elevated or floodproofed to 

or above the base flood level (1910.3.b.4). 

3. Obtain and record the lowest floor level when a permit is 

issued for new construction and substantial improvements for use 

in the determination of applicable flood insurance risk premium 

rates (1910.3.b.5). 

2'fhe term "development" is defined to mean "any manmade change to improved or 

unimproved real estate including, but not limited to, buildings or other structures, 

mining, dredging, filling, grading, paving, excavation or drilling operations." 

4. In riverine situations, notify adjacent communities and the 

State Coordinating Office prior to any alteration or relocation of a 

watercourse and submit copies of such notifications to FIA 

(1910.3.b.6). 

5. Require that the flood carrying capacity within the altered or 

relocated portion of any watercourse be maintained (1910.3.b. 7). 

6. Require that mobile homes meet FIA anchoring standards to 

resist flotation, collapse, or lateral movement (1910.3.b.8). 

7. Require that an evacuation plan for all mobile home parks 

and subdivisions be filed with appropriate Disaster Preparedness 

Authorities (1910.3.b.9). 

Up to this point, the community has been participating in the Emergency Flood Insurance Program 

and has received none of the advanced technical 

data (base flood data, floodway, or coastal highhazard area delineation) which becomes available 

upon conversion to the Regular Program with the 

completion of the Flood Insurance Rate Study and its 

accompanying Flood Insurance Rate Map (FIRM). 

Mter a community con verts to the Regular Program, 

it must meet the requirements of either Section 

1910.3.c, d, ore, depending upon the type of information which the study provides. If the study provides 

only base flood data, the community must adhere to 

Section 1910.3.c; if it provides a flood way delineation 

which is typical in riverine situations, as well as the 

base flood data, Section 1910.3.d must be adhered to; 

if it provides a coastal high-hazard area delineation 

for coastal areas subject to wave action, Section 

1910.3.e must be adhered to; and in certain circumstances where communities include both riverine 

(floodway) and coastal high-hazard areas, Section 

1910.3.d and 1910.3.e must be adhered to. 

The requirements of Section 1910.3.c include 

those of Section 1910.3.b, as well as the following 

additional requirements: 

1. Require that all new construction and substantial improvements of residential structures have the lowest floor (including 

basement) elevated to or above the base flood level, unless an 

exception for the allowance of basements and/or storm cellars is 

granted to the community by the FIA (1910.3.c.2). 

2. Require that new construction and substantial improvements of nonresidential structures have the lowest floor (including basement) elevated to or above the base flood level, or floodproofed to or above that level (1910.3.c.3). 

3. Require that, where floodproofing is used for structures in 

lieu of elevation, a registered professional engineer or architect 

certify that the floodproofing methods used are adequate to 

withstand flood depths, pressures, impact and uplift forces, and 

other forces associated with the base flood and record such 

certifications o~submit to FIA for approval of local regulations 

containing detailed floodproofing specifications which meet the 

watertight performance standards (1910.3.c.4). 

4. For new mobile home parks and subdivisions, expansion to 

existing ones, and in those where access, utilities, and pads are 

substantially improved, require that stands and lots are elevated 

so that the lowest floor of the mobile home will be at or above the 

base flood level, that adequate surface drainage and access for 
GOVERNMENTAL ORGANIZATION FOR FLOOD-LOSS REDUCTION 41 

haulers is provided, and that where piles or columns are used for 

elevation, lots are large enough to permit steps, and piling 

foundations are stabilized and reinforced (1910.3.c.5). 

5. Require that mobile homes which are not to be located in 

mobile home parks or subdivisions meet the foregoing requirements (1910.3.c.6). 

6. In areas subject to shallow flooding, require that all new 

construction and substantial improvements have the lowest floor 

(including basement) elevated above the crown of the nearest 

street to the height specified on the FIRM (1910.3.c. 7). 

7. Require that nonresidential structures in areas subject to 

shallow flooding be either floodproofed or meet the foregoing 

requirement (1910.3.c.8). 

8. Until a regulatory flood way is designated, require that, prior 

to such designation, no new construction or other development is 

permitted which, when combined with all other existing and 

anticipated development, will increase the water-surface elevation of the base flood more than one foot at any point within the 

community (1910.3.c.9). 

The requirements of Section 1910.3.d include 

the requirements of Sections 1910.3.c.1-8, as well as 

the following: 

1. Select and adopt a regulatory flood way based on the principle 

that the area chosen for the regulatory flood way must be designed 

to carry the waters of the base flood without increasing the water 

surface elevation of that flood more than one foot at any point 

(1910.3.d.2). 

2. Prohibit encroachment, including fill, new construction and 

substantial improvements, and other development within the 

adopted regulatory flood way that would result in any increase in 

flood levels in the community during the occurrence of the base 

flood discharge (1910.3.d.3). 

3. Prohibit the placement of any mobile homes, except in an 

existing mobile home park or subdivision, within the adopted 

regulatory floodway (1910.3.d.4). 

The requirements of Section 1910.3.e include 

those of Section 1910.3.c.1-9 for the portions of the 

flood-hazard area outside of the coastal high-hazard 

area as well as the following additional requirements 

which apply within the coastal high-hazard area: 

1. Obtain the elevation of the lowest habitable floor of all new or 

substantially improved structures, for the determination of applicable flood insurance risk premium rates, and maintain records of 

all such information (1910.3.e.2) 

2. Provide that new construction is located landward of the 

reach of mean high tide (1910.3.e.3). 

3. Provide that (i) all new construction and substantial improvements are elevated on adequately anchored piles or columns, 

and securely anchored to such piles or columns so that the lowest 

portion of the structural members of the lowest floor (excluding 

piles or columns) is elevated to or above the base flood level, and 

(ii) that a professional engineer or architect certify that the 

structure is securely anchored to adequately anchored piles or 

columns in order to withstand velocity waters and hurricane wave 

wash (1910.3.e.4). 

4. Provide that all new construction and substantial improvements have the space below the lowest floor free of obstructions or 

be constructed with "breakaway walls" intended to collapse under 

stress without jeopardizing the structural support, so that the 

impact on the structure by abnormally high tides or wind-driven 

water is minimized. Such temporarily enclosed space cannot be 

used for human habitation (1910.3.e.5). 

5. Prohibit the use of fill for structural support (1910.3.e.6). 

6. Prohibit the placement of mobile homes, except in existing 

mobile home parks and mobile home subdivisions (1910.3.e.7). 

7. Prohibit man-made alteration of sand dunes and mangrove 

stands which would increase potential flood damage to existing 

structures (1910.3.e.8). 

The implementation of a flood-plain management regulatory effort based on the design and highhazard area criteria discussed above as required 

under the National Flood Insurance Program will 

have significant effects on land-use planning in 

many San Francisco Bay region communities. Large 

areas of the region are subject to either tidal or 

riverine flooding-including many highly urbanized 

areas. Planners in these areas need detailed, up-todate information about the flood-insurance requirements of the Federal Insurance Administration. 

Because the regulations are subject to change, the 

planner is well advised to contact the Federal Insurance Administration for current requirements pertaining to his jurisdiction. 

FEDERAL DISASTER ASSISTANCE PROGRAM 

Federal disaster assistance supplements the resources of State and local governments to alleviate 

the suffering and damage caused by emergencies 

and major disasters, as defined by the Disaster Relief 

Act of 1974 (U.S. Congress, 1974a). This Act, implemented by the Federal Disaster Assistance Administration (1975) contains three significant provisions 

in Federal preparedness and disaster assistance 

relating to flood-prone areas. First, financial and 

technical assistance, funded by the Federal government, is offered to the States to develop plans, programs, and regulations for hazard reduction, disaster preparedness, and disaster telief. Second, buildings and mobile homes located within identified 

flood-prone areas which are to be replaced, repaired, 

or restored with Federal disaster assistance funds 

must conform to the National Flood Insurance Program prior to receipt of that assistance. The major 

effect of the program on flood-loss reduction efforts is 

to reinforce the provisions of the National Flood 

Insurance Program that discourage flood-plain development. Also, Section 314 of Public Law 93-288 

(U.S. Congress, 1974a) requires insurance for other 

hazards on properties assisted under Section 402 or 

419 of the Act, provided that such insurance is 

reasonably available, adequate, and necessary. 

Third, as a condition of any loan or grant, the State 

or local government must agree that the natural 

hazards in the areas in which the funds of the grants 
42 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

or loans are to be used will be evaluated. The State 

and local government must also agree that appropriate action will be taken to mitigate these hazards, 

including safe land-use and construction practices. 

U.S. ARMY CORPS OF ENGINEERS 

As set forth in the Flood Control Act of 1936 (U.S. 

Congress, 1936), the U.S. Army Corps of Engineers 

has a major responsibility for flood control throughout the United States. The Corps studies flood problems, recommends solutions, and constructs and 

maintains flood-control projects. The Corps undertakes major flood-control projects specifically authorized by Congress and small flood-control projects that do not require Congressional authorization. 

The purpose of major flood-control projects is to 

prevent flood losses by construction of channel improvements, levees, or dams. Most major projects 

involving construction of dams are multipurpose, 

also serving needs for "hydroelectric power, irrigation, navigation, municipal and industrial water 

supplies, water-quality control, recreation and enhancement of fish and wildlife resources" (U.S. 

Army Corps of Engineers, 1975d, p. 215). 

Small flood-control projects provide floodcontrol structures at a Federal cost usually less than 

$1 million per project. Both major and small floodcontrol projects are preceded by thorough hydrologic studies, cost/benefit analyses of alternative 

solutions to flood problems, and detailed project 

reports. The costs of Corps projects are shared 

between the Federal and local governments. States 

may assume some or all of the local costs. 

The Water Resources Development Act of 1974 

(U.S. Congress, 1974c), authorizing Corps projects 

for the coming year, requires, for the first time, that 

nonstructural alternatives to reduce flood losses be 

analyzed in flood-control studies. The local share of 

the cost of nonstructural alternatives, such as land 

acquisition and relocation of buildings, is the same 

as for structural projects. Three nonstructural projects are authorized by the Act, representing a significant change in the Corps' traditional approach. 

Under the Flood Plain Management Services 

Program, the Corps prepares flood-plain information reports at the request of local agencies, undertakes hydrologic studies for the Federal Insurance 

Administration, and provides technical assistance 

to government agencies in flood-plain planning and 

management. Corps of Engineers flood-control 

studies, flood-plain information reports, and projects in the San Francisco Bay region are listed in 

table 10. 

COMPREHENSIVE PLANNING ASSISTANCE PROGRAM 

The Comprehensive Planning Assistance Program, authorized by Section 701 of the Housing and 

Community Development Act of 1974 (U.S. Congress, 1974b), currently provides grants to finance 

comprehensive planning programs. The purpose of 

this assistance is to increase the capacity of States, 

regional governmental bodies, units of general local 

government, and other eligible grant recipients to 

plan and manage all 'the resources available to them 

in order to achieve the following goals: 

1. Community betterment, in both rural and 

urban areas, that is responsive to the needs 

of the public; 

2. Adequate housing, public facilities, and public services that are required to support an 

improved quality of life; 

3. Conserving and protecting the environment 

and natural resources for future generations; and 

4. Coordination and management of all functional planning activities. 

Currently, grants usually cover two-thirds of project 

costs. The Secretary of Housing and Urban Develop-

, ment may allocate assistance directly to eligible 

recipients or through States (U.S. Department of 

Housing and Urban Development, 197 4). 

All agencies applying for grants must have an 

adopted land-use element by August 22, 1977 (U.S. 

Department of Housing and Urban Development, 

I 1975, p. 36862). The element is to identify areas where 

growth should and should not take place giving comprehensive consideration to environmental factors. 

In addition, the regulations require that assisted 

planning activities be conducted in accord with the 

National Environmental Policy Act of 1969 (U.S. 

Congress, 1970) through the inclusion of environmental planning in the comprehensive planning 

process. Specifically each agency shall (U.S. Department of Housing and Urban Development, 1975, p. 

36860): 

1. Identify salient elements of the natural and manmade 

environments, their interrelationships, and major problems and/ or opportunities they present for community 

development; 

2. Assess those environmental factors which will: (i) Minimize or prevent undue damage, unwise use, or unwarranted pre-empting of natural resources and opportunities; (ii) Recognize and make prudent allowance for 

major latent environmental dangers or risks (e.g., floods, 

mud slides, earthquakes, air and water pollution); and 

(iii) Foster the human benefits obtainable from use of 

the natural environment by wise use of the opportunities available (e.g., use of natural drainage systems for 

park and recreational areas). 
GOVERNMENTAL ORGANIZATION FOR FLOOD-LOSS REDUCTION 43 

Under these regulations, problems of flooding must 

be considered in the development of land-use plans. 

COMMUNITY DEVELOPMENT BLOCK GRANT PROGRAM 

The Community Development Block Grant Program, administered by the Department of Housing 

and Urban Development, was instituted by the 

Housing and Community Development Act of 1974 

(U.S. Congress, 1974b). Under this program, Federal 

funds are made available to local governments for a 

wide variety of community activities, many of which 

were previously funded under separate programs. 

Among the activities eligible for grants are the 

conservation of open space, natural resources, and 

scenic areas; provision of recreational opportunities; 

and installation of flood and drainage facilities (U.S. 

Department of Housing and Urban Development, 

1976). Funds are also available to meet emergency 

community development needs in Federally declared 

disaster areas. 

U.S. SOIL CONSERVATION SERVICE AND 

RESOURCE CONSERVATION DISTRICTS 

The U.S. Soil Conservation Service (SCS) is 

concerned primarily with watershed management 

and land-treatment practices to reduce soil erosion 

and sedimentation; it is also involved in floodcontrol projects through Public Law 83-566 (U.S. 

Congress, 1954). Under this law, Federal funds are 

provided for flood-control projects in small watersheds. The projects are undertaken cooperatively 

with local sponsors and may have water supply, 

recreation, or erosion-control functions. The projects 

are financed partly by State or local funds. Six small 

watershed projects, as identified in table 10, have 

been authorized by Congress for the San Francisco 

Bay region; one, the Ulatis Creek project, has been 

completed. 

County flood control and water conservation 

districts together with resource conservation districes serve as local sponsoring agencies for all 

authorized small watershed projects in the San 

Francisco Bay region. In California, resource conservation districts (RCD), like soil conservation districts throughout the United States, are served by 

SCS field offices, each headed by a district conservationist. Resource conservation districts may be 

formed under provisions set forth in the California 

Public Resources Code (1970) for "the control of 

runoff, the prevention or control of soil erosion, the 

development and distribution of water, and the 

improvement of land capabilities." Eight SCS field 

offices, located in Concord, Dixon, Half Moon Bay, 

Livermore, Morgan Hill, Napa, Santa Rosa, and 

Petaluma serve the 15 resource conservation districts in the San Francisco Bay region (fig. 15). 

....._r, 

"'-r ""-.. "~ 

SOTOYOME- ( 

SANTA ROSA SONOMA VALLEY 

I 

/ Boundary, 

San Francisco 

Bay region 

( 

N 

I 

\ ~ '\....._ 

\ LOMA PRIETA \ 

, ../ , ~ Ol---.--1..l...0...,----'20 M I L ES 

~............,-!,_.. 0 10 20 KILOMETRES 

' ~ L-----------------------~~------------~ 

EXPLANATION 

Nondistrict areas 

District areas 

----District boundary 

FIGURE 15.-Resource conservation districts and U.S. Soil Conservation Service field offices, San Francisco Bay region. 
44 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

REGIONAL AND LOCAL PROGRAMS 

FLOOD CONTROL DISTRICTS 

Flood-plain regulations in the Bay region are 

often drafted and administered by county flood 

control districts. All the San Francisco Bay region 

counties except San Francisco have such districts, 

but as each district was established by separate 

State legislation, the powers and form of organization are not uniform. For example, the Santa Clara 

Valley Water District is governed by a separately 

elected board, while the other districts are governed 

by the local County Board of Supervisors sitting as 

the district governing board. All the districts are 

empowered by their establishing legislation to levy 

taxes, acquire property and water rights, enter into 

contracts, issue bonds, establish zones for levying 

assessments, and exercise the power of eminent 

domain in carrying out their functions. 

The districts usually are responsible for drainage, flood control, water-supply development, water 

conservation, and several other functions related to 

the use of the county's water resources. They maintain channels, remove debris and other ohstructions 

to waterflow, and provide emergency services in the 

event of a flood. The primary functions actually 

performed by the eight districts in the region are 

shown in table 3. 

The flood control districts typically draft and 

administer regulations responsive to the California 

Cobey-Alquist Flood Plain Management Act (California Water Code, 1976), which requires prohibition 

of development within the right-of-way of a proposed 

flood-control project. In addition, the districts may 

administer county, and sometimes city, flood-plain 

regulations. In most San Francisco Bay region 

counties, permits from the district are required for 

encroachment on or obstruction of stream channels. 

TABLE 3.-Primary functions of flood control districts in the San Francisco Bay region 

Name of 

district 

Date 

established 

Alameda County 1949 

Flood Control 

and Water 

Conservation 

District. 

Contra Costa 1951 

County Flood 

Control and Water 

Conservation 

District. 

Marin County 1953 

Flood Control 

and Water 

Conservation 

District. 

Napa County 1951 

Flood Control 

and Water 

Conservation 

District. 

San Mateo 1959 

County Flood 

Control District. 

Santa Clara 1951 

Valley Water 

District. 

Solano County 1951 

Flood Control 

and Water 

Conservation 

District. 

Sonoma County 

Water Agency. 

1949 

Plan, construct, Conserve, 

maintain import, develop 

flood control water 

facilities supply 

X X 

X 

X 

X X 

X 

X X 

X X 

X X 

Monitor 

water 

quality 

X 

X 

X 

Project 

review 

X 

X 

X 

X 

X 

X 

X 

X 

Land use and development regulations 

administered by district 

Permit for work within easements, rights-of-way owned 

by District. Building permit denial in flood-prone areas 

along certain creeks. Two city ordinances regulating 

"designated floodways" for proposed flood-control 

projects. 

Permit for work within stream channels in unincorporated 

areas. Subdivision regulations setting minimum drainage standards, requiring easements. Permit for grading, 

soils and engineering geology reports. Prohibition on 

obstructing water flow, damaging levees, work in 

county easements. 

Flood-plain zoning applied in Novato area-primary and 

secondary zones. Permit for work in channels in unincorporated and some incorporated areas. Prohibition 

on obstructing water flow. 

Permit for encroachment on channels including 50-foot 

strip on either side of channel operated and maintained 

by District. Limited flood-plain zoning applied to rightof-way of proposed U.S. Army Corps of Engineers 

project on the Napa River. 

Permit for construction activities in San Francisquito 

Creek channel and 15 feet from San Mateo County side. 

Permit for construction within "designated floodway" or 

within 50 feet of top of streambank in both unincorporated and incorporated areas. Policy for dedication to 

District of land in "designated floodway." 

Permit for construction in channels or altering existing 

water drainage· in unincorporated areas. Subdivision 

regulations setting drainage criteria. Limited floodplain zoning. 

Limited flood-plain zoning. Permit for construction in or 

near any drainageway. 
GOVERNMENTAL ORGANIZATION FOR FLOOD-LOSS REDUCTION 45 

REGIONAL AGENCIES 

Regional agencies influence flood-loss reduction 

through an extension of their planning, project 

review, and regulatory powers. These agencies include the Association of Bay Area Governments, the 

San Francisco Bay Conservation and Development 

Commission, and the California Coastal Zone Conservation Commission. 

The Association of Bay Area Governments 

(ABAG) is the only regional agency covering the 

entire nine-county San Francisco Bay region with 

responsibility for comprehensive planning. Established in 1961 to develop plans and policies pertinent 

to region-wide problems, ABAG is a voluntary association of city and county governments. Implementation of ABAG's regional plans and policies depends on decisions by State and Federal agencies, 

other regional agencies, and local units of government. However, ABAG can indirectly influence local 

decisions through its designation by the Federal 

government as the A-95 Review clearinghouse 

agency for the San Francisco Bay region. In this 

capacity, ABAG reviews requests for Federal funds 

available through most Federal programs. A major 

function is the review of local applications for Community Development Block Grants. ABAG reviews 

proposed Federal projects for conformity with adopted regional plans. ABAG also reviews Federally 

required environmental impact statements and most 

State-required environmental impact reports. In the 

usual situation in which many projects are competing for limited funds, a negative finding by 

ABAG, although advisory, is likely to be heeded by 

the funding agency. 

The San Francisco Bay Conservation and Development Commission (BCDC) was created by the 

State legislature to prepare a comprehensive plan for 

San Francisco Bay and its shores. The plan was 

adopted by the State legislature, and BCDC became 

a permanent agency charged with carrying out the 

plan. The adopted plan has legal status and serves as 

a guide in the review of projects. BCDC shares 

jurisdiction over land-use decisions with the cities 

and counties which retain normal land-use and 

building permit controls. However, with certain 

minor exceptions, a permit from BCDC is required 

for all projects within its area of jurisdiction (the 

water of San Francisco Bay and up to 1,000 ft or 

305m inland from the Bay shoreline). Thus, in effect, 

it holds veto power over any project proposal in 

conflict with the San Francisco Bay Plan. 

With respect to flooding the plan states as a 

major policy (Bay Conservation and Development 

Commission, 1969, p. 17): 

To prevent damage from flooding, buildings on fill or near the 

shore-line should have adequate flood protection as determined by competent engineers. As a general rule, buildings 

near the shoreline should be at least nine feet above mean sea 

level (standard U.S.G.S. datum) or should be protected by 

dikes of an equivalent height and by any necessary pumping 

facilities. In the southern half of the South Bay, this height 

should be at least ten feet. Exceptions to the general height 

rule may be made for developments specifically designed to 

tolerate periodic flooding. 

The California Coastal Zone Conservation Commission (CCZCC) and six subordinate regional commissions have powers comparable to BCDC's over 

California's coastal areas. In 1972 California voters 

adopted, by initiative, legislation creating these commissions. The CCZCC, working with the regional 

commissions, was charged with preparing a plan for 

managing the California coastal zone. While the 

plan was being prepared, the commissions controlled development, through a permit process, to ensure 

consistency with the objectives of the establishing 

legislation and the emerging plan policies. Coastal 

areas of the San Francisco Bay region are represented by two regional commissions: Central (San 

Mateo County) and North Central (San Francisco, 

Marin, and Sonoma Counties). The plan was presented to the Governor and State Legislature in 

December 1975 for adoption and implementation. 

Under the terms of the initiative, the CCZCC and the 

six regional commissions were to expire on .January 

1, 1977, unless legislation was enacted to create successors to them. 

In September 1976 the California Coastal Act of 

1976 was enacted (California Public Resources Code, 

1976b), establishing the California Coastal Commission and six regional coastal commissions as successors to the commissions created by the 1972 

initiative. Under the terms of the Act, the six regional 

commissions will expire 30 days after the last required local coastal program has been certified, but 

no later than January 1, 1981. 

The California Coastal Plan is interesting in its 

emphasis on the beneficial effects of flooding. According to the plan, these effects include (California 

Coastal Zone Conservation Commission, 1975, p. 

83): 

the maintenance of salmon and steelhead spawning ground; 

the continued supply of beach sands; 

the removal of vegetation choking the river channel, restoring the 

channels capacity to contain minor flood flows; 
46 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

the long-term deposition along the floodplain of sediments that 

provide highly fertile soils; 

flushing of undesirable salts from the surface layers of soils, and 

the preservation of valuable plant communities on overflow lands, 

such as giant redwood groves. 

Recognizing that development in flood-hazard 

areas not only removes these benefits, but is exposed 

to loss from flooding, the plan contains the following 

policy statement (p. 84): 

To avoid the need for new flood control works and interference 

with natural watershed processes that adversely affect coastal 

resources such as sand supply and anadromous fisheries, development in flood-hazard areas shall be regulated as follows: 

a. Criteria for New Developments in Unprotected Flood-Hazard· 

Areas. Only new developments that can sustain periodic 

flooding and that will not create public burdens by aggravating the flood problem, impeding floodwater storage capacity, 

or increasing pressure for new flood control projects shall be 

allowed in presently unprotected flood-hazard areas (those 

subject to inundation by a 100-year flood), consistent with the 

existing Federal insurance program. Examples of permissible uses include agriculture and recreation, with necessary 

incidental structures. 

b. Restrict Use of Flood-Hazard Areas during Flood-Prone 

Periods. During flood-prone periods, flood-hazard areas shall 

not be used for log decks or storage of materials that can be 

carried downstream by flood waters unless mitigation (such 

as anchoring devices or berms) is adequate. 

c. Review Inland Flood-Hazard Area Projects That Could 

Mfect Coastal Zone. It is recommended that the Legislature 

establish procedures to ensure opportunities for public review of proposed inland flood-hazard area projects that 

could adversely affect lives and property in the coastal zone. 

PLANNING FOR FLOOD-LOSS REDUCTION 

IN THE NAPA VALLEY 

By M. L. BLAIR 

Planning for flood-loss reduction, through structural or nonstructural measures, usually involves 

several agencies with differing powers and responsibilities. The pattern of relationships among the 

agencies is highly variable and often difficult to 

trace. Therefore, examining the actual responses of 

agencies to a particular flooding problem can be 

more instructive than attempting to generalize from 

very diverse situations. This section describes planning and decisionmaking related to flood-loss reduction in the Napa Valley area of the northern San 

Francisco Bay region. This area was selected for 

detailed study because it has agricultural, urban, 

and urbanizing lands subject to severe flooding; 

agencies at all jurisdictional levels have been involved in planning for the flood-prone areas; and 

several different approaches have been tried or 

proposed to reduce flood losses. 

This study describes the plan formulation and 

implementation efforts at the regional, county, and 

city levels with respect to the flood-prone areas of the. 

Napa Valley. The case study format necessarily 

emphasizes what has been done rather than what 

ought to be done. However, in describing the efforts 

of the agencies involved, the effectiveness of various 

actions within a specific context is made clear. 

Because flooding problems are widespread and variable, and there are many possible responses, a 

description of actual responses to a particular problem can illuminate possibilities and problems which 

may pertain to many different situations. 

DESCRIPTION 

The Napa River rises just north of Calistoga in 

northern Napa County and flows south through the 

Napa Valley to San Pablo Bay-a distance of 39 mi 

(63 km). Numerous tributaries flow into the river 

which broadens onto tidal sloughs about 9 mi (14 km) 

north of San Pablo Bay. North of the city of Napa, 

the valley floor is flanked by a series of foothills that 

-reach a maximum elevation of 4,343 ft (1,323 m) at 

Mount St. Helena on the county line 7 mi (11 km) 

north of Calistoga. The location of Napa County in 

the San Francisco Bay region is shown in figure 1. 

The location of the Napa Valley Planning Area is 

shown in figure 16. 

With the exception of the flat tidal marshes just 

north of San Pablo Bay, the river valley is entirely 

within Napa County. The valley, liowever, is a 

natural feature significant to the entire San Francisco Bay region because of the economic importance 

of its renowned vineyards, the significance of its 

large non urban areas to a regional system of open 

space, and its attractiveness as a locale to accommodate a substantial share of projected regional population growth. Figure 17 is an aerial view of the Napa 

River as it winds through the city of Napa. 

Although only a 1-hour drive from San Francisco, the valley is bypassed by the Bay area freeway 

network and has experienced far less rural-residential development and suburbanization than neighboring counties. Napa, Yountville, St. Helena, and 

Calistoga, the only incorporated cities in Napa 

County, are all located in the Napa Valley and have 

a combined population of approximately 52,400 

(44,200 in Napa). Another 34,600 people live in the 

unincorporated areas of the county, for a total county 

population of 87,000 (Napa County Conservation, 

Development and Planning Department, 1974a). 

Napa County plans and regulations are presently 

designed to accommodate a population of 115,000 by 
38°45' 

38°30' 

38°15' 

0 

I 

0 

PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 

Napa Valley 

Planning Area 

I 

5 

5 

I 

I 

10 

122°30' 122°15' 

\ 

\ 

' \ I 

_/ ' \_, 

\ 

10 MI LES 

I 

San Pablo Bay 

FIGURE 16.-Napa County and Napa Valley planning area. 

47 

\ 

""' ' ) 
48 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

FIGURE 17.-Napa River, city of Napa. (Photograph by U.S. Army Corps of Engineers.) 

the year 2000. Regional and local plans indicate that 

population growth is to be accommodated in or near 

the existing cities along the Napa River, especially 

the city of Napa. 

The soils, climate, and topography of Napa 

Valley are exceptionally well-suited to the cultivation of grapes. Some of California's finest wines are 

produced in the valley, and the value of land for 

viticulture has in recent years exceeded its value for 

subdivision development. Although this situation 

may be temporary, it is presently (1976) in sharp 

contrast to the economic situation which contributed 

to the suburbanization of prime agricultural land in 

other areas of the Bay region such as the Santa Clara 

Valley. 

FLOODING PROBLEMS IN THE NAPA VALLEY 

Flooding along the Napa River is a persistent 

problem. The city of Napa has experienced damaging floods 18 times since 1900, or an average of more 

than once every 4 years (fig. 18). According to 

Limerinos (1970), 

Flooding of low-lying areas in the city occurs when streamflow 

exceeds 12,000 cfs (cubic feet per second) or at stages above an 

elevation of about 4 7 feet above mean sea level (gage height, 23 

feet) at the gaging station (at Oak Knoll Avenue) on Napa River 

near Napa, Calif. 

The date, elevation, and discharge of the major 

floods, in order of magnitude, are shown in table 4. 

The extent of flooding in the valley, caused by 

storms of the same magnitude, may vary for several 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 49 

35 

25 

l-6o~----~------~,----~,,-----,.------.-----,,,---~ en 

w w a: w 

u. -18 ~ 

z ~ 

~ eno 

Oz 

zo 

<(() 

enw 

:>en 20 Oa: 

1-

w 

w 

g • I cl 

~55~ I I -17 ~ 

en 

w 

a: 

1- u. w 

z 

-30 ~ 

Iw 

1-a.. 

Zl-

-w 

·w Wu. 

~() 

<(co 

I:> 

()()15 

~u. 

00 

..: 

I 

C) 

w 

I 

w 

C) 

<( 

C) 

~ I -wi 

i I I 

9 ~ 

..: 

I 

(!) 

w 

I 

25 6 50 f- ;:: 15 6 

1- I 

8 ~ <( 

(!) 

<( ,, :[~1 ~ 

~47LEL-JLdam_m~~~L_L_'~--~~----a~~~ a*~---L-'~m.LB,~~~u[rn~®~w~~~ ~ 7 

1900 1910 1920 1930 1940 1950 1960 1967 w 

WATER YEAR 

FIGURE 18.-Annual floods above 47-foot elevation, 1900-1967, Napa River near Napa, California. (From Limerinos, 1970.) 

reasons. First, tidal action can affect flood elevations 

in the lower end of the valley. A high tide can 

increase the height of a flood up to Trancas Road3 in 

the north side of the city (Limerinos, 1970). Second, 

although of limited capacity and not operated for 

flood control, several small water-supply reservoirs 

on the tributaries can retain a portion of the runoff if 

a storm occurs at a time when the reservoirs are not 

full. Limerinos reported that: 

At the onset of the storm that caused the December 1955 flood, 

the water level in Lake Hennessey was 14 feet below the spillway elevation, and a considerable part of the flood runoff was 

stored in the reservoir. On January 31, the water level was 

only 1 foot below the spillway elevation, and most of the 

ensuing flood runoff spilled. This caused extensive damage 

along Conn Creek and increased the flow of the Napa River 

through Napa. 

Third, the construction of levees and other channel 

modifications or the presence of debris or channel 

obstructions can affect the extent of damage from a 

given storm. Fourth, runoff characteristics of the 

220-mi2 (570-km2) drainage basin upstream from the 

Oak Knoll Avenue gaging station above Napa also 

affect the degree and frequency of flooding from a 

given storm. If the storm occurs when the soil is 

saturated from the rain of preceding storms, the 

amount of runoff increases. The effect is similar in 

areas where the drainage basin has been denuded of 

3 Although shown on Geological Survey maps as Trancas Road, 

and often referred to as Trancas Avenue, the official city name is 

Trancas Street. 

natural vegetation or cleared for cultivation, subdivision development, highway construction, or 

other purposes. 

TABLE 4.-Major floods at the U.S. Geological Survey gaging 

station Napa River near Napa, California, 1900-19671 

[From Limerinos, 1970] 

Date of flood 

March 18, 1907 

February 27, 1940 

February 6, 1942 

December 31, 1913 

February 24, 1902 

January 31, 1963 

December 22, 1955 

January 3, 1916 

January 21, 1967 

February 12, 1925 

Stage 

(feet) 

233.8 

32.8 

232.8 

2 31.8 

2 28.5 

27.5 

227.5 

226.8 

26.5 

22~.4 

Elevation above 

mean sea level 

(feet) 

258.5 

57.5 

257.5 

256.5 

253.2 

52.2 

252.2 

251.5 

51.2 

251.1 

1No significant floods have occurred since 1961. 2Estimated. 

Discharge 

(cubic feet 

per second) 

223,800 

22,500 

222,500 

221,500 

218,000 

16,900 

216,800 

2 16,200 

15,800 

2 15,700 

The U.S. Army Corps of Engineers (1965b) 

estimated the damage from flooding on the Napa 

River for three recent ·floods (table 5). 

Response to the problem of flooding in the Napa 

Valley is shared, often without formal coordination, 

by a variety of governmental agencies. ABAG, the 

Napa County Conservation, Development and Planning Department, and the incorporated cities are 

involved in general planning for all or part of the 

Napa Valley. Detailed water-resources planning 

with emphasis on providing flood protection has 

been done by the Corps of Engineers, the Soil 
50 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Conservation Service, and the Napa County Flood 

Control and Water Conservation District. The plans, 

programs, and functions of these agencies with 

respect to the flood-prone areas in Napa County and 

the nature of hydrologic information used at each 

jurisdictional level for various purposes are described in the following sections. 

TABLE 5.-Estimated loss from three floods on the Napa River 

[From U.S. Army Corps of Engineers, 1965b] 

Type of flood loss Loss, in dollars, at 1963 price levels 

1955 1958 

Commercial .................. $198,000 $17,000 

Residential .................... 61,000 23,000 

Agricultural .................. 187,000 288,000 

Roads and bridges .... 170,000 28,000 

Public utilities ------······ 24,000 

Federal property -------- 1,000 

Total .................... $641,000 $356,000 

Area flooded 

(in acres) ........ 11,900 9,300 

PLANNING FOR NAPA VALLEYREGIONAL PERSPECTIVE 

1963 

$237,000 

136,000 

198,000 

80,000 

16,000 

$667,000 

7,200 

Regional planning for the Napa Valley provides 

policies and procedures related to urban and openspace uses of the valley lands. ABAG's "Regional 

Plan 1970:1990" discusses the relationship between 

the physical character of the land and the pattern of 

urban development in the following terms (1970, 

p. 4): 

It is on the Bay plain area that circles the Bay, and the larger 

valleys of Santa Clara, Napa, Sonoma, Petaluma, Livermore, 

and Ygnacio that the greatest amount of urban development 

has occurred. These valley lands, separated only by intervening ridges, are being steadily converted from agricultural 

to urban use to serve the needs of a growing population. Due to 

the difficulty of building on steeper slopes, those valley lands 

that remain unurbanized are prime targets for future urban 

development. The region will have to choose either to retain 

the prime agricultural lands and unique natural settings that 

these lands provide, or to allow them to be transformed by 

urbanization. 

The flatlands of the Napa Valley have excellent 

agricultural soils (Class II and III according to the 

Soil Conservation Service soil capability rating 

system) and are prone to flooding; but, because of low 

slope and stable soils, they are considered reasonably well suited for urban development. Figures 19, 

20, and 21 show the relationship of these characteristics for the Napa Valley. The geographic coincidence of these features is typical of the Bay region. It 

is within this general framework of conflicting 

potentials and problems that regional plans attempt 

to provide policy direction. 

... 

\~-'---'-- \ \ 

<r-J \ 

/ \ 

/ \ ---- L ) 

~ ~ 

(A 'I 

\ 

' \ 

\ 

\ ___ _j 

\ 

c 2 4 

I 1 I 1

1 

0 2 4 6 

EXPLANATION 

Flood plains 

FIGURE 19.-Flood plains in Napa County. (From Napa County 

Conservation, Development and Planning Department, 1973.) 

PLAN FORMULATION 

ABAG's regional plan (1970) provides a policy 

framework for considering the future growth of the 

Bay region. A major plan policy is that new urban 

growth should be accommodated in two ways: by the 

infilling and controlled extension of existing cities 

and by the development of entirely new, selfcontained communities. The land-use implications 

of this and other ABAG policies adopted by 1970 are 

presented graphically on the land-use diagram, 

figure 22, which indicates infilling and extension of 

existing urban areas in Napa County. 

Nonurban lands are classed in the ABAG plan 

as either permanent open space or lands for controlled development. Lands in the con trolled develop-
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 51 

'I 

\ 

' \ 

4 6 

I I II I I 

2 4 6 8 

EXPLANATION 

Prime agricultural lands 

FIGURE 20.-Prime agricultural Yands in Napa County. (From 

Napa County Conservation, Development and Planning Department, 1973.) 

ment category are to be held in reserve to provide 

land for urban development beyond 1990, if needed. 

According to the plan, major flood plains and 

drainage channels should be accorded priority for 

public acquisition to avoid the costs of further floodcontrol protection and to provide a means of linking 

the region's open-space areas. Other open-space 

lands, like those with exceptional qualities for the 

specialized agriculture of the Napa Valley, are to be 

preserved by regulation. The plan diagram thus 

shows the area north of the city of Napa as 

permanent open space, primarily on the basis of 

agricultural value of the land. The open-space strip 

along Napa River in the city of Napa provides flood 

protection and links open -space areas a hove and 

.. 

\"'--'--- \ \ 

<r--1 \ 

/ \ 

I \ 

----I \,. ) 

~ ~ 

(A 'I 

\ 

\ 

~ \ 

\ 

\ ___ _j 

\ 

c 2 4 

I I I I I 0 2 4 6 

EXPLANATION 

Hills and mountains 

Flat land 

Water 

FIGURE 21.-Urban areas and areas with slopes less than 15 

percent in Napa County. (From Napa County Conservation, 

Development and Planning Department, 1975.) 

below the city. 

Thus, the regional plan policies recognize and 

address, in general terms, the flooding problem of the 

Napa Valley and the conflict arising from the high 

potential of the land for both urban and agricultural 

uses. The ABAG plan clarifies the potentials and 

conflicts on a regional level and provides a 

conceptual framework to guide local land-use planning and decisionmaking. Detailed data were not 

needed for formulating the plan. The Geological 

Survey regional map of flood-prone areas (Limerinos, 

Lee, and Lugo, 1973), for example, can be used effectively for planning at this degree of generality. 
52 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

I I 0 2 4 

. : : : : : ::t: : 

4 6 

I I I I I 6 8 10 

. ::: :)7.:: 

. : : : :,¢: : . 

: .... :::1:: 

EXPLANATION 

'I 

\ 

\ 

\ 

\----' 

. : . \ ... ~ 

.. ~ t_ 

,... 

: :~ 

:::; 

··~ 

... :f 

Developed space 

......... Open space -Controlled 

development 

Open space - Permanent 

FIGURE 22.-Association of Bay Area Governments regional plan 

for Napa County. (From Association of Bay Area Governments, 

1970.) 

The open-space policies of the ABAG regional 

plan are amplified in the "Regional Open Space 

Plan-Phase II" (Association of Bay Area Governments, 1972). Six functional categories of open space 

were defined as shown in table 6. Lands designated 

permanent open space and controlled development 

in the Regional Plan were then assigned to the 

appropriate categories. 

Map overlays were prepared showing lands in 

each open-space category to allow visual identification of lands serving multiple open-space purposes. 

Priority is given to these lands in the open-space 

preservation program. Flood plains were included in 

the public safety category (table 6). Significant parts 

of the Napa Valley fall into all six categories, and 

these areas are consequently given high priority for 

open-space preservation. Table 7 shows the acreages 

in Napa County recommended for open space, 

categorized according to function. 

TABLE 6.-Categories of land serving open-space purposes 

[From Association of Bay Area Governments, 1972, p. 4] 

Category 

Open space for managed 

resource production. 

Open space for preservation 

of natural and human 

resources. 

Open space for health, 

welfare, and well-being. 

Open space for public safety. 

Open space for outdoor 

recreation. 

Open space for shaping 

urban growth. 

Uses 

Prime agricultural lands and 

lands for specialty crops. 

Lands for grazing. 

Lands for mineral production. 

Lands for water supply. 

Water areas for fish and 

marine life production. 

Lands, tidelands, marsh, and 

water areas for fish and 

wildlife refuge. 

Notable geological features. 

Historical and cultural sites 

and places. 

Areas to provide visual amenity. 

Land to protect the quality of 

water resources, including 

ground water . 

Land for disposal of sewage, 

garbage, etc. 

Open areas to improve airshed 

quality. 

Flood control reservoirs, flood 

plains, drainage channels, 

and areas below dams. 

Unstable soil and fault areas. 

Airport flight path zones. 

Critical fire zones. 

Lands for developed recreation 

uses. 

Lands for water-oriented 

recreation. 

Lands for natural environmental experience. 

Lands for scenic and 

recreational travel. 

Lands to preserve community 

identity. 

Lands to prevent inefficient 

urbanization. 

TABLE 7.-Recommended acreage for each open-space category in 

Napa County. 

[From Association of Bay ~ea Governments, 1972, p. 6] 

Open·space category 

Total acres serving at least one purpose 

Managed resource production ......................................... . 

Preservation of natural and human resources ........ .. 

Health, welfare, and well-being .................................... .. 

Public safety ......................................................................... . 

Outdoor recreation ............................................................. . 

Shaping urban growth .................................................... .. 

Acres 

399,850 

294,931 

177,855 

270,519 

31,585 

188,417 

161,813 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 53 

The plan illustrates how information concerning areas subject to flooding can be combined with 

other data to arrive at a system of priorities for openspace preservation based on numerous factors. 

Classifying the 4-million acres (16,200 km2) in 

the Bay region allocated to open space in the 

Regional Plan was accomplished with existing and 

readily available information. The classification can 

be refined as more detailed information becomes 

available. Infprmation of a much more detailed 

nature will be needed to implement the open-space 

preservation program. Identification of specific land 

areas (with well-defined boundaries) to be preserved 

for open space is required. The plan, however, 

presents a broadly conceived target for preserving 

regionally significant open-space areas. Information developed and presented at a scale of 1:125,000, 

such as the regional flood-prone area maps 

(Limerinos, Lee, and Lugo, 1973), is usually adequate 

for this purpose. 

PLAN IMPLEMENTATION 

As a voluntary association, ABAG has very 

limited powers to implement its plans. Successful 

implementation really depends on the actions of 

Federal, State, and local governmental agencies. 

ABAG's implementing program, therefore, emphasizes measures to encourage other governmental 

units to act. ABAG proposes the following five 

actions to implement the "Regional Open Space 

Plan-Phase II" (Association of Bay Area Governments, 1972, p. 7). 

!.-presenting the two five-year "exemplar" programs as 

developed; 

2.-reviewing grant applications from jurisdictions within the 

region; 

3.-encouraging the preparation of individual open space 

elements consistent with the Regional Open Space Plan-. 

Phase II; 

4.-developing a regional Open Space Preservation Information 

System (OSPIS); 

5.-participating in a program of coordination and legislative 

advocacy for open space purposes. 

The two exemplar programs-a minimal preservation program and an augmented preservation 

program-are proposed as alternative 5-year programs. Under the minimal program, 18,954 acres 

(76.7 km2) in Napa Valley (presumably prime 

agricultural land) would be preserved by local 

zoning, and development rights to 4,154 acres (16.8 

km2) of the Napa River flood plain would be acquired 

for $7,504,000 (1972 estilnates). Under the augmented program, the State would acquire development rights to an additional10,954 acres (44.3 km2) 

of Napa River marshlands for an estimated 

$1,664,000 (Association of Bay Area Governments, 

1972, p. 9). 

The ABAG minimal program indicates that 

financial responsibility for acquiring development 

rights to the Napa River flood plain belongs to the 

region. However, ABAG has neither the authority 

nor the funds to acquire land or development rights 

to land. It may influence decisions through review of 

grant applications of local governments and special 

districts for State and Federal funds available for the 

purchase of open space. 

Through the A-95 review process, ABAG may 

also review development projects involving Federal 

funds for conformity with its open-space plan. The 

agency reviews Federally required environmental 

impact statements. There is no requirement for 

ABAG review of environmental impact reports 

mandated for public and private projects by State 

law. However, most EIRs are voluntarily submitted 

to ABAG for review. 

To assist in the review function, ABAG's recent 

report "Areas of Critical Environmental Concern" 

(Association of Bay Area Governments, 1975) sets 

forth policies concerning and criteria for identifying 

areas of regionally significant environmental concern which "because of intrinsic qualities demand 

special consideration in the comprehensive planning process" (p. 1). The report extends the earlier 

work on the "Regional Open Space Plan-Phase II" 

(Association of Bay Area Governments, 1972) by 

exploring "in much greater detail those land areas 

that merit protection because of intrinsic environmental characteristics" (unpaged). 

With respect to stream flooding, the report sets 

forth the following regional policy (p. 43): 

Protect floodplains of stre~ms that flow through more than 

one jurisdiction from development -that will alter floodplain 

extent or that will be significantly damaged by flooding. 

Lands within the 100-year floodplain are subject to 

the policy. The following statement is made concerning compatible uses: 

Within floodplains, uses should be limited to those types of 

activities which will be minimally damaged by flooding, such 

as certain types of agricultural uses, low intensity recreational uses and parking. 

The map "Flood-Prone Areas in the San Francisco 

Bay Region, California" (Limerinos, Lee, and Lugo, 

1973) is cited as a reference for identifying 100-year 

flood plains. It is emphasized, however, that the 

maps are not a part of the plan; they are listed to help 

identify locations where the policies might apply. 
54 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

PLANNING FOR FLOOD-PRONE AREASNAPA COUNTY 

The Napa River, its environs, problems, and 

potentials, dominates planning in Napa County. 

Almost all of the county's population lives in the 

river valley, and prospects for substantial development in other areas of the county are currently 

remote because of steep slopes, inaccessibility, and 

distance from major metropolitan centers. A possible 

exception to this general observation is the potential 

for additional recreational development in the Lake 

Berryessa area in northeastern Napa County. 

Within the last few years, advocates of limited 

growth, preservation of environmental quality, and 

conservation have achieved a majority on the 

County Board of Supervisors. Recent elections in the 

city of Napa, including a 1973 plebiscite favoring a 

limited growth concept, indicate a similar trend. The 

new priorities of the residents of the county were 

revealed by the response to a survey on growth 

policies. The survey questionnaire was included with 

a summary of the county general plan sent to all 

registered voters in February 1974. The document 

outlined the impacts associated with three possible 

population levels (growth options) for the year 2000 

(Napa County Conservation, Development and 

Planning Department, 197 4a): 

(Current population 87,000) 

Low growth ---------------- 115,000 

Medium growth ________ 150,000 

High growth -------------- 200,000 

Over 76 percent of the respondents favored the lowgrowth alternative and associated policies to limit 

growth. As a result, new general plans are being 

developed for both the county and the city, reflecting 

the public consensus on growth limitation. 

PLAN FORMULATION 

The Napa County Board of Supervisors recently 

adopted the following new set of land-use planning 

goals (Napa County Conservation, Development 

and Planning Department, 1975): 

Goal 1 To plan for agriculture and related activities as the 

primary land uses in Napa County and concentrate 

urban uses in the County's existing cities and 

urban areas. 

Goal 2 To develop and implement a set of planning policies 

which combine to define a population size, rate of 

population growth and the geographic distribution 

of that population in such a manner that the desired 

quality of life is achieved. 

Goal 3 To determine what the land is best suited for; to match 

man's activities to the land's natural suitability; to 

take advantage of natural capabilities and minimize conflict with the natural environment. 

Goal4 To work with cities, other governmental units, citizens, 

and the private sector to plan for services, facilities 

and accommodations, including housing, transportation, economic development, parks and recreation, open space, and other total County needs. 

These goals, developed through extensive public 

participation, are a logical extension of work done on 

the Napa County Open Space and Conservation 

Plan and a subarea plan for the Napa Valley. The 

goals guided the preparation of a new land-use 

element and provide a framework for future 

revisions or additions to the County General Plan. 

The Napa County land-use element, adopted in 

September 1975 by the Board of Supervisors, 

designates lands for urban growth sufficient to 

accommodate a population of 115,000 by the year 

2000. The element explicitly recognizes constraints 

to urban development including potential flooding. 

Flood plains are designated as "limited development 

areas" to be retained in large parcel sizes primarily 

for agricultural use (Napa County Conservation, 

Development and Planning Department, 1975, p. 16). 

The Napa Valley Area Plan, a subarea plan 

adopted by the Board of Supervisors in 1975, set forth 

more specific goals and policies covering the use oP 

flood plains. The goals are (Napa County Conservation, Development and Planning Department, 

197 4b, p. 22-23): 

Restrict and regulate urban development in areas of flood 

risk. 

Protect the vegetation and animal habitats of the waterways 

and flood plains as well as their hydraulic carrying capacity 

from encroachment of urban development. 

Protect existing areas of urban development from flooding. 

The related policies are: 

Seasonal flooding from streams, deposits of rock and sediment and bank undercutting make some areas difficult to 

develop. Occasional high water levels in the lakes and reservoirs flood adjacent areas for short durations. Encourage 

development and implementation of flood plain management programs that protect homes and property, as well as 

stream side vegetation, and control obstruction of natural 

waterways. 

Maintain watercourses and related vegetation as components 

of an open space system. Develop pedestrian and riding 

trails if compatible with riparian (stream side) vegetation 

and wildlife habitat. Develop public access at frequent 

intervals. 

Maintain watercourses and vegetation within rural areas as 

components of an open space system and develop public 

access or roadside rests at cross roads where compatible 

with surrounding land uses. 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 55 

The land-use implications of these and other plan 

policies are shown in the plan diagram (fig. 23). 

The county-wide Conservation and Open Space 

Element, approved by the Board of Supervisors in 

.June 1973, contains the same goals and policies for 

land use in flood plains as the adopted Napa Valley 

Area Plan, with the following additions (Napa 

County Conservation, Development and Planning 

Department, 1973, p. 22): 

Investigate the feasibility of obtaining data to establish a 

floodway and restrictive zone outside the floodway for all unincorporated areas subject to flooding, based on a 100 year storm. 

Identification of flood prone areas will be accomplished by the 

Napa County Flood Control and Water Conservation District. 

Adopt flood plain zoning in all applicable areas, and investigate the compatibility of zoning areas adjacent to flood plains for 

recreational uses. Flood plains along streams which feed Lake 

Berryessa, the Napa River, and the Suisun Marsh are zoned for 

agricultural uses in the majority. 

Encourage provision for flood insurance. The Napa County 

Flood Control and Water Conservation District and the Napa 

County Board of Supervisors have obtained Federal Government 

approval of Napa County for flood insurance and have agreed, in 

return, to enact local land use and control measures for areas 

having special flooding problems. The controls are to be consistent with Federal criteria. 

Establishing flood-plain boundaries can be quite 

general at this phase of planning. However, applying the policies, through either regulation or project 

review, ordinarily requires data which are both 

specific and accurate. The Geological Survey floodprone area quadrangle maps of the Napa Valley 

(Cuttings Wharf, Napa, Rutherford, St. Helena, and 

Yountville, scale 1:24,000) showing the areal extent 

of the 100-year flood (fig. 28 and table 10) can be used 

in framing general plan policies (U.S. Geological 

Survey, 1971). 

The success of these county planning efforts 

depends in large measure on the continued economic 

viability of grape cultivation in the Napa Valley. In 

recent years, public decisions to limit population 

growth, preserve agriculture, and limit flood-plain 

development have been consistent with private landuse decisions based on land-use economics. 

PLAN IMPLEMENTATION 

The county of Napa has the same powers to 

implement plans-zoning ordinance, subdivision 

regulations, building codes, project review, for example-as a city. However, the county's authority 

extends only to the unincorporated areas. Most of 

Napa County's unincorporated area is undeveloped 

except for agriculture, small towns, or other lowintensity uses. Thus the county is developing and 

applying policies in essentially nonurban areas. 

EXPLANATION 

Urban 

Low density urban 

{~/}\~M?[f{W~ Agricultural preserve 

............... 

............................ 

Planning area for 

Napa Valley area plan 

2 4 MILES 

I I II 

4 6 Kl LOMETRES 

Agriculture, watershed, and recreation ......... 

FIGURE 23.-Napa Valley Area Plan. (From Napa County Con 

servation, Development and Planning Department, 1974b.) 
56 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Napa County's implementation program has six 

main aspects: flood-plain zoning, establishment of 

agricultural preserves, regulation of watercourse 

obstruction and riparian cover, watershed protection, flood insurance, and project review procedures. 

Flood-plain zoning.-In 1967, the Board of 

Supervisors adopted a very limited flood-plain zoning ordinance to meet the requirements of the 

original Cobey-Alquist Flood Plain Management 

Act (later amended, California Water Code, 1976). 

The ordinance prevents encroachment in the designated floodway for which a flood-control project is 

proposed or anticipated. The ordinance provides for 

three zoning districts to be applied to flood-prone 

areas to regulate uses according to the severity of 

flood hazard (Napa County Board of Supervisors, 

1967): FP-1 applies to the stream channel and 

portions of the flood plain required to carry the 

floodflow. Permitted structures and uses include 

parks, farming, grazing, utility pipelines, and boating facilities, and campgrounds from May to N ovember. FP-2 applies to areas flooded by overflow and 

backwater relatively free of current. Uses normally 

permitted in the zoning districts with which the FP-2 

district is combined are allowed as long as the 

ground-floor level of structures is above the floodprofile level. FP-3 applies to properties in a flood 

zone which are protected by flood-control works. All 

uses permitted in the zoning districts with which the 

FP-3 district is combined are allowed with no 

additional restrictions. 

To date (1976) only the FP-1 zoning has been 

applied. The area now covered by this zoning 

includes the unincorporated areas along Napa River 

from Trancas Road south to Edgerly Island. In 

general, the district conforms to the proposed rightof-way for a Corps of Engineers flood-control project. 

The ordinance as it is now applied regulates only a 

small portion of the county's flood-prone lands. The 

currently zoned districts are designated on a map on 

file with the Napa County Flood Control and Water 

Conservation District. 

The county has adequate information provided 

by a flood-insurance study and the Geological 

Survey study by Limerinos (1970) to apply floodplain zoning to the entire 100-year flood plain of the 

Napa River and its major tributaries. The floodinsurance study delineates the 100-year flood plain 

at a scale of 1:12,000 and includes flood profiles 

which permit estimation of depth of flooding at 

different points in the flood plain. This is important 

when considering various flood-loss prevention measures such as flood proofing requirements. The study 

by Limerinos (1970, scale 1:24,000) shows the areas 

flooded in 1940 and 1955, provides information 

concerning the flood profiles of the 1907, 1940, and 

1955 floods, and gives a flood-frequency curve. This 

information is especially useful in applying differential restrictions to different areas of the flood plain 

based on the probability of a flood of a given frequency occurring in a given year. 

Agricultural preserves.-The county has three 

types of agricultural preserves. The Agricultural 

Preserve District (AP) is intended to apply to fertile 

valley and foothill areas in which agriculture is and 

should continue to be the predominant use. It 

requires a minimum lot size of 20 acres (0.08 km2), 

permits extensive agriculture and processing in 

addition to grazing and cultivation, and has been 

applied primarily to the Napa Valley (Napa County 

Board of Supervisors, 1968). 

The Agricultural Preserve Interim District (API) 

is intended to preserve agricultural uses until an area 

is ready for urbanization. It requires a minimum lot 

size of 20 acres (0.08 km2), and it is applied primarily 

in hillside areas (Napa County Board of Supervisors, 

1969). 

The Agricultural Preserve Extensive District 

(AP-E) is intended to apply primarily to grazing land 

with mostly non-prime soils. It requires a minimum 

lot size of 100 acres (0.40 km2) and mimimum zoning 

district size of 320 acres (1.3 km2). All lands in AP-E 

districts are under Williamson Act contracts (Napa 

County Board of Supervisors, 1969). 4 

North of the city of Napa, most of the valley has 

been zoned Agricultural Preserve (AP) since 1968. 

·This zoning establishes agriculture as the primary 

use of the valley area including the flood plain. The 

AP zone was challenged in the courts by the Napa 

Valley United Farmers, but it was upheld by the 

Superior Court of the county of Napa, in a decision on 

February 17, 1971, as a proper exercise of police 

powers which benefited not only the public at large, 

but also those whose land was regulated (Overview 

Corporation, 1973, p. 20). 

Because much of the 100-year flood plain along 

the Napa River is in agricultural use, betterinformation concerning the impact of flooding on different 

crops and types of f~rming and grazing would be 

useful to the county in determining appropriate 

4The Williamson Act, also called.the California Land Conservation Act of 1965 (California Government Code, 1976c), permits 

landowners to enter into contracts with cities and counties in 

which the landowners agree to maintain land in agricultural 

use in exchange for tax assessment based on the economic return 

from agricultural use of the land. 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 57 

flood-plain uses. More needs to be known about the 

effects of different land-management practices on 

the amount of loss sustained from a given flood. 

However, it is generally accepted that many agricultural uses are appropriate for flood-prone areas. 

Unless the cost of losses from flooding in agricultural areas is excessive, or erosion and sedimentation problems are particularly severe, a single zone 

that covers the 100-year flood plain and permits only 

agricultural and other open -space uses may be 

sufficient for nonurban areas. It is desirable to 

prohibit construction of farm buildings, grading, or 

other farm-related activities within the floodway. 

Watercourse obstruction/riparian cover ordinance.-In February 1974 the Napa County Board 

of Supervisors adopted Ordinance 44 7 (Napa County 

Board of Supervisors, 1974a). The purposes of the 

ordinance include reducing flood losses caused by 

channel obstructions or water-borne debris, preserving wildlife habitats, and preventing streambank erosion. A permit is required from the Planning 

Commission before anyone may deposit or remove 

any material within a watercourse; excavate within 

a watercourse; construct, alter, or remove any 

structure within, upon, or across a watercourse; 

plant or remove any vegetation within a watercourse; or alter any embankment within a watercourse. ExceptfortheNapaRiver, thepermitrequirement applies to the stream channel and a 50-ft (15-m) 

wide strip measured landward from the top of each 

bank of the stream. For the Napa River the requirement applies to a 100-ft (30-m) strip along either side 

of the river from the county's southern boundary to 

St. Helena. The ordinance does not apply in the 

incorporated areas. A map showing streams covered 

by the ordinance is on file in the Flood Control and 

Water Conservation District office. 

This ordinance was intended to alleviate some of 

the potential problems associated with agricultural 

use of the flood plain. It provides the means whereby 

the county may prohibit cultivation of farmland 

immediately adjacent to the banks of the river. 

Cultivation to the river's edge removes riparian 

cover which may cause increases in bank erosion 

and collapse, significant increases in river sedimentation, and loss of wildlife habitats. In addition, the 

ordinance regulates the deposition of debris in the 

river channel. Debris carried by flood waters is a 

major source of damage to agricultural lands. 

Watershed protection.-Tll.e extent of flooding 

may be affected by man's use of watershed lands. 

Thus, control of use of the watershed lands can significantly affect potential damage from flooding. Most 

unincorporated areas in Napa County which are not 

in the Agricultural Preserve District are in the Agriculture, Watershed, and Recreation (A WR) District 

pending completion of the county general plan 

revision. Minimum lot size in this district is 40 acres 

(0.16 km2). The adopted Napa Valley Area Plan 

(Napa County Conservation, Development and 

Planning Department, 197 4b) recommends retention 

of the minimum parcel size of 40 acres (0.16 km2) in 

the A WR zone, with the possibility of reducing the 

minimum lot size to as small as 5 acres (0.02 km2) 

under certain conditions. Residential development 

could be permitted on parcels less than 40 acres (0.16 

km2) after review based on consideration of water 

and sanitation conditions, slope, access, fire hazards, environmental impacts, and growth policies 

and related factors (Napa County Board of Supervisors, 1974b). 

In the Napa Valley watershed, five public 

reservoirs, with a combined firm annual yield of 

23,000 acre-ft (28 hm3) of water, provide most of the 

county's water supply. The assurance of both quality 

and quantity of this local water supply is critical for 

both domestic and agricultural users. Stringent 

watershed protection measures are easily justified in 

this case to protect public health, safety, and welfare. 

Proper watershed management is also important in 

controlling erosion and sedimentation. 

Flood insurance.-On December 10, 1970, the 

Napa County Board of Supervisors adopted Resolution 70-155 (Napa County Board of Supervisors, 

1970) requesting that flood insurance be made available to property owners in the county. The resolution 

stated that the county would meet the following landuse and control standards established by the Federal 

Insurance Administration (Federal Insurance Administration, 1971): 

(1) Require building permits for all proposed construction or 

other improvements in the community; 

(2) Review all building permit applications for new construction or substantial improvements to determine whether proposed 

building sites will be reasonably safe from flooding.lf a proposed 

building site is in a location that has a flood hazard, any proposed 

new construction or substantial improvement (including prefabricated and mobile homes) must (i) be designed (or modified) and 

anchored to prevent flotation, collapse, or lateral movement of the 

stucture, (ii) use construction materials and utility equipment that 

are resistant to flood damage, and (iii) use construction methods 

and practices that will minimize flood damage; 

(3) Review subdivision proposals and other proposed new 

developments to assure that (i) all such proposals are consistent 

with the need to minimize flood damage, (ii) all public utilities and 

facilities, such as sewer, gas, electrical, and water systems are 

located, elevated, and constructed to minimize or eliminate flood 

damage, and (iii) adequate drainage is provided so as to reduce 

exposure to flood hazards; and 
58 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

(4) Require new or replacement water supply systems and/or 

sanitary sewage systems to be designed to minimize or eliminate 

infiltration of flood waters into the system and discharges from 

the systems into flood waters, and require onsite waste disposal 

systems to be located so as to avoid impairment of them or contamination from them during flooding. 

The standards for land-use and control measures 

become more stringent as more detailed information 

is provided to the community by the Federal Insurance Administration. 

As presently enacted and administered, the 

National Flood Insurance Program provides Napa 

County with an incentive to prevent inappropriate 

use of flood-prone areas while providing insurance 

protection to owners of flood -prone property which is 

already developed. 

Project review.-Information defining the extent 

of areas subject to flooding at a given frequency is 

very important in the process of reviewing development proposals for conformity with adopted plans 

and regulations and for assessing environmental 

impact. Any proposal for development within the 

100-year flood plain, as delineated on the 1:12,000 

scale maps used in administering the flood insurance program, is reviewed for the county by theN apa 

County Flood Control and Water Conservation 

District. Detailed delineation of areas likely to be 

flooded, and information on the nature and frequency of flooding, water elevation, and speed of 

movement, are all important in the review of specific 

projects. Requirements which can legally be imposed 

for floodproofing or raising the floor level, for 

example, require site-specific information concerning the nature and depth of potential floodwaters. 

The Geological Survey's maps of flood-prone areas 

do not serve this purpose. However, if the District did 

not have the more detailed information provided by 

the flood insurance study, the Geological Survey 

maps could serve as the basis for determining which 

developments have potential flooding problems and 

should be reviewed by the District. Where subdivision is proposed, detailed information can be 

required from the subdivider under the California 

State Subdivision Map Act of 1974 (California 

Government Code, 1976e) to assure that a project is 

properly designed with respect to the flood risk. 

The Napa County Flood Control and Water 

Conservation District makes its recommendations to 

the County Planning Commission and the Board of 

Supervisors based on the maps and flood profiles 

available from the flood insurance study. The 

District recommendations are also sent directly to 

the applicant for a subdivision or building permit. 

The District usually indicates the elevation of the 

100-year flood and recommends that the ground floor 

of a structure be 1 ft (0.3 m) above that elevation. The 

recommendations are advisory; the authority to 

impose requirements rests with the Board of 

Supervisors. 

THE NAPA COUNTY FLOOD CONTROL AND WATER 

CONSERVATION DISTRICT 

The Napa County Flood Control and Water 

Conservation District is a countywide district with 

the County Board of Supervisors acting as the 

governing board. In addition to responsibilities for 

flood control, the District acts as the contracting 

agency for Napa County for water from the State 

Water Project, plans for and develops adequate 

water supply from all sources within the county, is 

involved with assuring adequate water quality, 

supervises water and sanitation systems of other 

special districts which are governed by the Board of 

Supervisors, and provides technical assistance to the 

Board with respect to flood insurance. 

The District participates in the review of development proposals and the development of land-use 

plans and regulations in flood-prone areas by providing technical assistance and advice. Its role in 

land-use decisions, however, is strictly advisory. The 

District has direct permit power over activities in 

and along the stream channels it operates and 

maintains. The District, in cooperation with the. 

County Planning Commission, exercises considerable control over construction or other development 

in and along all major streams in the county in 

connection with the Water Course Obstruction and 

Riparian Cover Ordinance (Napa County Board of 

Supervisors, 1974a). 

PLANNING FOR FLOOD-PRONE AREASCITY OF NAPA 

The city of Napa is located in the Napa Valley 

upstream from the marshlands and diked lands 

which extend to San Pablo Bay (fig. 18). Nearly half 

the county's population lives in the city of Napa, the 

governmental and urban center of the county. Much 

of the flood damage in Napa County occurs in the 

city of Napa. A major portion of the downtown area 

is within the 100-year flood plain of the Napa River 

and is also subject to flooding from Napa Creek 

which flows through downtown Napa into Napa 

River (fig. 8). 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 59 

PLAN FORM ULATION 

The city, like the county, has experienced a 

political changeover as a result of changes in public 

attitudes towards growth and environmental quality. The 1968 General Plan for the city (Napa City 

Planning Department, 1968) is now considered 

inadequate to address todav's concerns, and the city 

council is presently (1976) reviewing a revised 

general plan (fig. 24, Napa City Planning Department, 1975). The revised plan stresses the establishment of a "residential urban limit line" to confine 

future development to locations close to the presently 

developed parts of the city. This is a reduction from 

55 mi2 (142 km2) to 18 mi 2 (47 km2) in the area 

considered appropriate for residential development. 

Planning for the flood-prone areas within the 

city of Napa is, and has been, based on the 

assumption that flood-control works will be constructed which will eliminate, to a large degree, the 

flood hazard within the city. The new plan calls for 

continuation of a redevelopment project featuring a 

major government, business, and retail center within the 100-year flood plain. The plan diagram shows 

a linear park along the river, but the 100-year flood 

plain extends well beyond the limits of this park. 

Major areas of proposed high density residential use 

(10 to 18 units per acre) and major commercial and 

industrial areas are shown within the 100-year flood 

plain along the river. The intensity of land use 

increases from the foothills to the river with the most 

intense development along the river in the flood 

plain or along Highway 29-the major north-south 

route through the city. 

PLAN IMPLI!MENTATION 

The city of Napa has adopted flood-plain zoning 

virtually identical to that of the county and has 

applied it to the areas needed to construct the 

proposed Corps of Engineers Napa River flood 

control project. The city's efforts to plan, and implement plans, for the flood-prone areas are embedded 

in multijurisdictional programs for redevelopment 

and flood control. An effort has been made to coordinate the provision of flood control with redevelopment, transportation, and open-space planning in 

the city. The main features of this effort are 

discussed below. 

A solution to Napa's flooding problems is the 

flood control project proposed by the U.S. Army 

Corps of Engineers (1973f, p. 60, 61): 

Authorized by the 1965 Flood Control Act, the project provides 

for channel enlargement and realinement, construction of levees, 

floodwalls, and public boat-lau!}ching facilities in an 11-mile 

reach downstream from Trancas Road in the city of Napa to 

Edgerly Island. 

The Federal first cost of the project is estimated at $25,300,000 

(.July 1972). The estimated cost to local interests is $9,680,000. 

The planning process involved in developing the 

flood-control project is somewhat unusual. The 

Corps of Engineers originally proposed to construct 

a straight-walled channel through the center of the 

city of Napa to carry flood waters. Local authorities 

viewed the Corps project as an opportunity to 

revitalize the water-front area of the city and create 

an esthetically pleasing and useful linear parkway 

through the city. They requested that the Corps of 

Engineers alter its plans to provide a somewhat 

wider but less austere channel. The reach of the 

project through central Napa was redesigned with a 

series of steps rather than a straight wall. A landscaped linear park with walkways, bike trails, picnic 

areas, and benches along either side of the channel 

became an integral part of the project design. 

The flood-control project became part of a 

broader effort to redevelop downtown Napa, upgrade 

transportation, and provide recreational facilities. 

This integrated approach involved coordination of 

planning and implementation of activities of the 

Napa County Flood Control and Water Conservation District, city of Napa, Corps of Engineers, 

Department of Housing and Urban Development, 

Soil Conservation Service, California Department of 

Water Resources, and California State Division of 

Highways (now Department of Transportation). 

Figure 25 shows the location of the project and 

related elements. A major part of the cooperative 

project was to be Federally sponsored urban renewal 

for the deteriorating downtown area of Napa, much 

of which is flood prone. Open space in addition to the 

linear park would be provided at selected locations 

along both sides of the channel in both the rural and 

urban areas of the project. 

As originally conceived, the responsibility and 

financing for the project involved five agencies. 

The Corps of Engineers was to design and 

construct the flood channel improvements including 

boat launching facilities at selected locations and 

the landscaping required for the linear park. Recreational facilities in the linear park were to be financed 

jointly with Federal and local funds. 

The California Department of Water Resources 

was authorized by State legislation to finance the 
60 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

0 y, 2 Ml LES 

0 5 2 3 Kl LOMETRES 

FIGURE 24.-Proposed land-use plan for city of Napa. (Generalized from Napa City Planning Department, 1975.) 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 61 

EXPLANATION 

RESIDENTIAL 

MAXIMUM UNITS 

PER ACRE (GROSS) 

Low density 1-2 

Medium density 2-9 

High density 10-18 

NONRESIDENTIAL 

OFFICE AND COMMERCIAL 

Professional office II 

11 

Neighborhood retail, and tourist-oriented 

commercial ' 

INDUSTRIAL 

OPEN SPACE 

------Residential urban limit line 

local share of Federal flood-control projects. The 

local share for purchasing the right-of-way required 

for the multipurpose project is larger than for the 

single-purpose flood-control project. Thus, it became 

important that the State agree to assume the 

additional costs. 

The Department of Housing and Urban Development, under Title I of the Housing Act of 1949, as 

amended (U.S. Congress, 1949), would provide funds 

for acquiring and clearing the land in central Napa 

for a redevelopment project to include a county 

government complex, a commercial area, and open 

space along the river. Planning of the various 

aspects of the project was to be partly financed with a 

HUD planning grant. 

The State Division of Highways (now Department of Transportation) was to construct a northsouth freeway through Napa which would cross the 

river twice. Both the county and city of Napa at that 

time favored the freeway as an important stimulant 

to economic and populatio\1 growth of the county. 

The Soil Conservation Service, under U.S. Public Law 83-566 (U.S. Congress, 1954), was to construct a reservoir on Redwood Creek west of the city 

of Napa to control flood waters on Napa Creek. 

The project, as originally conceived, required an 

unusual degree of cooperation among government 

agencies to coordinate and resolve a number of 

interrelated problems. The Napa River Technical 

Committee, composed of city and county elected 

representatives and staff members, functioned as 

the main coordinating group for the various aspects 

of the plan. The plan was viewed as 8: model for intergovernmental relations and comprehensive planning. However, 10 years after the original concept 

was put together, problems of coordination. and 

changing Federal and State regulations and. priorities and new local preferences are strongly affecting 

the prospects for successfully completing the project. 

38° 

15' 

URBAN RENEWAL 

AREA 

PROPOSED 

2 

PROPOSED 

NAPA RIVER 

2 MILES 

KILOMETRES 

122° 

15' 

FIGURE 25.-U.S. Army Corps of Engineers flood~ontrol project 

and related projects in the city of Napa. (From U.S. Army Corps 

of Engineers, 1975c.) 
62 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Flood control.-Construction of the Corps of 

Engineers flood-control project, authorized by Congress in 1965, has not yet begun. A major problem 

has been raising the local share of project funds used 

to acquire land for the right-of-way and to relocate 

roads, bridges, and utilities. The California Resources Agency originally agreed to provide only 

partial reimbursement of the local share of the 

project cost. The city of Napa was unable to finance 

the remainder. In addition, Federal requirements 

regarding environmental impact statements have 

delayed the start of the construction. 

The problems over State funding seem to be 

resolved. The State has now agreed to provide 75 

percent of the cost of land acquisition and 90 percent 

of the cost of necessary road, bridge, and utility 

relocations. No State funding is presently provided 

for recreation or fish and wildlife enhancement. 

The Corps released a draft environmental impact statement in April 1975 (U.S. Army Corps of 

Engineers, 1975c). The major concern regarding the 

project was voiced by the California Department of 

Fish and Game. The Department objected to the plan 

to deposit spoils from the river dredging on former 

marshland south of the city which was diked many 

years ago for agricultural use. If the dikes were 

removed and the land subjected to tidal action, 

marshes could be restored on the land. The Corps is 

working out a mitigation plan with the Department 

of Fish and Game involving the provision of public 

marshland elsewhere to compensate for the loss of 

potential marshland resulting from the project. 

The Corps of Engineers recently released a 

General Design Memorandum which updates the 

1965 flood-control study-particularly with respect 

to the cost/benefit analysis. Changes in the concept 

and design of the project, as well as sharp changes in 

growth assumptions over the last decade, made such 

an update necessary. The estimate offuture average 

annual damages from flooding in the 1965 report 

(U.S. Army Corps of Engineers, 1965b) was based on 

population projections which foresaw almost an 

eight-fold increase in population from 1960 to 2060 

(table 8). According to the report, the popuiation 

increase would be accompanied by a large shift from 

agricultural to residential and commercial land uses 

(table 8). Changes oflesser magnitude would occurin 

other land uses which account for approximately 40 

percent of the basin area. 

These projections are inconsistent with the desires ofthe valley residents as expressed in plans and 

regulations adopted or under consideration. The 

future average annual damages from flooding would 

presumably be lower on the basis of more realistic 

land use and population projections. However, the 

progress of redevelopment in downtown Napa has 

undoubtedly increased potential future flood losses. 

The General Design Memorandum indicates a current benefit/cost ratio of 1.1 to 1.0 as opposed to a 

ratio of 1.6 to 1.0 in the 1965 study. As the ratio is 

still favorable, although considerably reduced, 

construction of the project may begin in 1978, if 

environmental and funding problems are finally 

resolved. 

The Soil Conservation Service, in cooperation 

with the Napa County Flood Control and Water 

Conservation District and the Napa County Resource Conservation District, intends to construct a 

flood-control project in the city of Napa to reduce 

damages from Napa Creek flooding. Under Public 

Law 83-566 (U.S. Congress, 1954), the Soil Conservation Service is authorized to construct and 

maintain multipurpose flood-control projects in 

small watersheds. In 1962 the Congress authorized a 

project to construct a dam and reservoir on Redwood 

Creek west of the city of Napa. Redwood Creek joins 

Browns Valley Creek just west of the city to form 

Napa Creek which then follows a course through the 

middle of the city, and joins Napa River near the 

downtown area of Napa (fig. 25). To prevent flooding 

in downtown Napa requires coordination between 

the creek and river projects. The Soil Conservation 

Service has officially dropped consideration of the 

dam project because of rising costs and negative 

environmental impact associated with constructing 

the dam and relocating a county highway through 

very steep terrain. 

As an alternative, the SCS is studying the 

feasibility of constructing an underground channel 

to divert excess flow from the stream channel 

through the downtown area into the river. The Corps 

of Engineers may become directly involved with 

structural improvements on Napa Creek if the river 

project is constructed and there is no alternate 

solution to control Napa Creek flooding. 

TABLE B.-Population and land-use projections for the Napa 

basin, 1960-2060 

[Data from U.S. Anny Corps of Engineers, l965b. FiguresincludecityofVallejoand the 

part of Solano County between the Napa County line and San Pablo Bay] 

Land use 

Year Population Agricultural Residential and commercial 

Area Percentage of Area Percentage of 

(acres) basin area (acres) basin area 

1960 ........ 132,000 141,780 52.0 21,250 7.8 

1980 ........ 260,000 127,000 46.5 38,000 13.9 

2000 ........ 390,000 113,000 41.4 53,000 19.4 

2020 ........ 630,000 94,500 34.1 76,000 27.8 

2040 : ....... 840,000 74,000 27.1 92,000 33.7 

2060 .... 1,020,000 59,000 21.6 107,000 39.4 
PLANNING FOR FLOOD-LOSS REDUCTION IN THE NAPA VALLEY 63 

Transportation.-Changing priorities have influenced the evolution of the freeway project. At the 

request of local authorities, the State Department of 

Transportation has removed the proposed northsouth freeway through Napa Valley from its plans. 

With local goals for economic and population growth 

revised downward, the freeway was rejected by local 

authorities because it was considered to be a possible 

stimulant to growth. 

Redevelopment.-In 1969 the Napa Community 

Redevelopment Agency submitted an Urban Redevelopment Plan for the Parkway Plaza Redevelopment Project (Napa Community Redevelopment 

Agency, 1969). The boundaries and land-use plan of 

the project are shown in figure 26. More than onehalf the area is subject to the 100-year flood. In 1970 

funding was received from HUD to acquire and clear 

parcels in the first nine-block section of the project 

(fig. 26). This phase of the project has been completed. New buildings housing government offices 

and commercial enterprises have been constructed. 

New street paving, lighting, brick sidewalks, benches, planters, and trees have been added to the area. 

Figure 27 shows part of the area developed in the first 

phase during and after construction. 

In 1972, the Urban Redevelopment Plan was 

amended (Napa Community Redevelopment 

Agency, 1973) to add 11 blocks to the project, and 

funds from HUD are now available to acquire three 

of these blocks. The 11 blocks (fig. 26) are also within 

the 100-year flood plain. ThecityofNapareceived an 

open-space grant from HUD covering 50 percent of 

the cost of acquiring the first parcel for the linear 

park. 

The redevelopment area remains subject to frequent flooding, and the potential cost of damages 

from a flood on either Napa Creek or Napa River has 

increased because of new construction in the re-

-development area. In addition, under the National 

Flood Insurance Program, the city faces a dilemma. 

Under present regulations the city will be required to 

enter the program or face the loss of all Federal 

funds. However, even if the city enters the program, 

HUD would be violating its own guidelines if it 

continued to fund the redevelopment project in the 

flood-prone area. The city seems to have little choice 

but to push for expeditious construction of floodcontrol structures on both Napa River and Napa 

Creek. If the downtown area is protected by floodcontrol works from the 100-year flood, HUD could, in 

accordance with its guidelines, continue to approve 

the use of Federal funds for the redevelopment 

project. 

SIG1'1FICA1'CE OF THE ~APA VALLE\" EXPERIE~CE 

Flooding problems in the Napa Valley have been 

addressed in the plans and actions of numerous 

agencies with varying degrees of success. Many 

factors influence the success or failure of any particular action, or group of actions, in reducing flood 

losses. Experience in Napa Valley seems to indicate 

the following factors are particularly important. 

Compatibility of flood-loss reduction with general community objectives.-A significant factor in 

reducing flood losses seems to be the compatibility of 

flood-loss reduction actions and techniques with 

general community objectives. The fact that the 

flood-prone areas of Napa Valley have other characteristics important for open space led ABAG to 

place high priority on preserving the flood-prone 

areas in open-space uses. Similarly, Napa County's 

goal to limit population growth and maintain agricultural uses has led to land-use plans and regulations consistent with reducing flood losses. In the 

city of Napa, however, the desire to redevelop floodprone portions of the downtown area for intensive 

and economically valuable uses is inconsistent with 

reducing flood-loss potential. Even with a completed 

flood-control project, the redevelopment area would 

be subject to increased losses from floods exceeding 

the 100-year flood. 

Compatibility of flood-loss reduction efforts with 

land-use economics.-Actions to reduce losses will be 

more widely accepted if economically viable land 

uses are permitted. A major reason for the success of 

Agricultural Preserve Zoning in Napa Valley has 

been the high market value of land for agricultural 

uses in relation to its value for urban uses. 

Compatibility of flood-loss reduction efforts with 

objectives of other agencies.-The chances of reducing flood losses are enhanced if actions by a city 

of county are consistent with programs of other 

agencies with jurisdiction or responsibility in the 

drainage basin. For example, the plans and actions 

of Napa County, with respect to the unincorporated 

flood-prone areas, are reinforced by ABAG's policies 

and the requirements of the National Flood Insurance Program and the Disaster Relief Act. In 

turn, the potential for effective flood-loss reduction 

through proposed Corps of Engineers and Soil Conservation Service projects is enhanced by the planning and regulatory efforts of Napa County. 

Intensity of existing development.-It is usually 

easier and less costly to maintain low flood-loss 

potential in undeveloped areas than to reduce loss 

potential in already urbanized areas. Napa County, 
64 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

0 500 1000 1500 2000 FEET 

0 100 200 300 400 500 600 METRES 

FIGURE 26.-Parkway Plaza Redevelopment Project in the city of Napa. (From Napa Community Redevelopment Agency, 1969.) 
·sUMMARY 65 

EXPLANATION 

Residential, med ium de nsity 

Residen tial, high density 

Residential, high rise 

Administra tive and professio nal offi ces 

Central business 

Commercial 

Park 

I Ongmal redeve lopment area 

Second eleven block redevelopment area 

Block purchased fo r lmear park 

with largely undeveloped flood-prone areas, can 

prevent future increases in flood losses by regulating 

land uses in the flood plain; but, the city of Napa 

would have to convert existing developed areas to 

less intensive uses or provide flood-control works to 

significantly reduce flood-loss potential. These measures are costly and involve coordinating the efforts 

of various Federal, State, and local agencies. 

Location of drainage basin with respect to juris-

. dictional boundaries.-Effective local efforts to reduce flood losses are easier to carry out if the major 

portions of a stream and its drainage area are located 

within a single jurisdiction. The Napa River and its 

drainage basin, with the exception of a small area of 

tidal flats along San Pablo Bay, are within Napa 

County. Thus the county can integrate drainage 

basin and flood-plain plans and regulations in a 

comprehensive program to reduce flood losses. If the 

drainage basin covered a multicounty area, the 

plans and programs of multiple local jurisdictions 

would have to be coordinated to achieve a comprehensive approach to flood-loss reduction. 

Coordination of planning and implementation. 

-When flood-control works are planned to reduce 

flood-loss potential, related land-use decisions and 

public and private facility construction and(or) relocation should be appropriately phased to avert 

undue increases in flood-loss potential. Planning 

related to the Corps of Engineers project in the City 

of Napa dealt with flood control, urban redevelopment, highway construction, and relocation in a 

coordinated manner. Not completing the floodcontrol project prior to redevelopment has resulted in 

an increase (perhaps short term) in flood-loss potential in the city of Napa. 

Availability of hydrologic information.-The 

specificity of plans and regulations dealing with 

flood -prone areas depends on the type, accuracy, and 

detail of information available to delineate the flood 

hazard. With more detailed data, particularly precise 

mapping of the floodway, the Napa County Flood 

Control and Water Conservation District could prepare and seek adoption of flood-plain zoning regulations covering the entire flood plain with restrictions 

reflecting varying degrees of flood risk within the 

flood plain. Such flood-plain zoning regulations 

could be overlain on, or combined with, the use and 

site restrictions of the basic or underlying zoning 

district. 

The Napa Valley case study presents flood-loss 

reduction efforts in the context ofland-use planning 

and decisionmaking proceeding from generalized 

regional policies and plans through county planning 

and regulation to detailed local planning and application of both structural and regulatory measures to 

reduce flood losses. In such a process the information 

requirements are graduated with the most specific 

information on flooding needed by cities with developed flood plains. 

Although flood-prone areas and responses to 

flooding problems are highly variable, the Napa 

Valley study provides a reasonably typical view of 

the interplay among agencies, plans, and actions to 

reduce flood losses within the context of land-use 

planning. Many of the factors affecting success or 

failure of flood-loss reduction efforts in the Napa 

Valley can be important in other flood-prone areas. 

SUMMARY 

Flood plains historically have provided attractive and accessible sites for urban development. Use 

by man has been extensive, and growing urban 

pressures are causing still more intensive development. But use of the flood plain has exacted a price in 

losses from flooding or in costs of suitable protection. 

Flood-loss reduction traditionally has been approached through installing structures such as 

dams, dikes, levees, channel improvements, and 

sea walls. These have provided only partial reduction 

of risk from flooding; flood losses have continued to 

rise as a result of continuing encroachment on flood 

plains. 

In recen~ years Federal and State policies on 

flood-loss reduction have shifted from reliance on 

structural works to a balance between structural and 

nonstructural measures. For example, regulation of 

flood-plain development is a requirement for communities to qualify for subsidized flood insurance 
66 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

FIGURE 27.-Part of Parkway Plaza during and after redevelopment. 
SUMMARY 67 

and, under certain circumstances, flood-disaster relief. Information on the extent of the flood hazard, 

land use, and development is essential to develop a 

sound and effective flood-loss reduction program. 

Flood-plain information reports, flood maps, 

and other information prepared by Federal, State, 

and local agencies provide pertinent information on 

flooding needed to evaluate flood risk. Flood-prone 

areas in the San Francisco Bay region are delineated 

on a series of topographic quadrangle maps (scale 1: 

24,000) prepared by the Geological Survey. Regional 

maps of flood-prone areas (scale 1:125,000) provide 

an overview of the extent of potential flooding. 

Many methods and devices are available toreduce flood losses. These include the protection, removal, discouragement, and regulation of certain 

types of development on flood plains. Programs for 

flood-loss reduction are most effective when based on 

adopted comprehensive plans that are firmly implemented. Effective flood-loss reduction programs 

achieve a balance between the use of flood plains and 

the hydraulic requirements of floods. 

Protection of existing development often is 

necessary for economic or humanitarian reasons 

and can be provided by flood-control works, flood 

warning, evacuation, and floodproofing. Damage 

and hazards from flooding may be avoided through 

removal of existing structures or conversion of structures to less vulnerable uses. Structures within floodways may increase flood stage and velocity. Methods 

for removal or conversion include public acquisition, 

urban redevelopment, public-nuisance abatement, 

non-conforming-use provisions in zoning ordinances, conversion of use, and reconstruction of 

existing public facilities. 

Discouraging development on flood plains may 

be achieved through public-information programs, 

warning signs, recordation of hazard, taxassessment practices, financing policies, and publicutility extension policies. Flood insurance 

requirements and costs may constitute an additional 

deterrent. 

The regulation of uses on flood plains provides a 

direct means for reducing flood losses through prohibition or regulation of uses vulnerable to flood 

damage. In most states the primary responsibility 

for planning, adopting, and enforcing land-use regulations rests with local government. Flood-plain 

development can be controlled through establishment of zoning districts and incorporation of floodplain regulations in zoning, subdivision, sanitary, 

and building ordinances. 

As the extent of damage from floods is a direct 

consequence of the use of the land, efforts to reduce 

losses from flooding logically occur within the context of land-use planning. Land-use planning considers the future development of an area in terms of 

the economic, political, social, and physical determinants of growth and change. The planning process involves identifying problems, defining goals 

and objectives, collecting and interpreting data, 

formulating plans, evaluating impacts, and adopting and implementing plans. Public participation is 

necessary throughout the planning process. 

Many governmental agencies and programs influence planning for flood-loss reduction. Federal 

programs for flood insurance, disaster assistance, 

comprehensive planning grants, and community 

development grants all have impact on planning for 

flood-prone areas-primarily a local responsibility. 

A major portion of the cost of flood-control works is 

also borne by the Federal Government. State legislation defines the powers of local governments to plan, 

regulate, acquire land or development rights, or 

otherwise act to reduce flood losses. In addition, 

regional agencies may have specific authority derived from Federal or State legislation to make or 

influence decisions related to flood-loss reduction. 

Improved coordination of governmental actions 

to reduce flood losses is needed. Flood-loss reduction 

efforts should be integrated into land-use planning 

·for entire drainage basins. There is need also for 

coordination in providing data, notably among Federal agencies such as the Geological Survey, Corps of 

Engineers, and Soil Conservation Service, and the 

county flood control and water conservation districts, to assure comprehensive coverage and availability of pertinent data. 

Study of the response to flooding problems in the 

Napa Valley outlines the complex intergovernmental responsibilities involved in developing plans 

and programs to reduce flood losses. In addition to 

the need for coordinating the actions of numerous 

agencies, the Napa Valley experience indicates that 

factors important in flood-damage reduction include 

compatibility of flood-damage reduction with general community objectives, land-use economics, and 

the objectives of other agencies; the intensity of 

existing development; the availability of hydrologic 

information; and the coordination of planning and 

implementation. 

The Napa Valley study illustrates a graduated 

approach to flood-damage reduction proceeding 

from generalized regional policies and plans through 
68 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

county planning and regulation to detailed local 

planning and application of both structural and 

regulatory measures to reduce flood losses. In such a 

process the information requirements are also graduated with the most specific delineation of flood-prone 

areas needed for urban areas. 

Although flood-prone areas and responses to 

flooding problems are highly variable, the Napa 

Valley study provides a typical view of interplay 

among agencies, plans, and actions in attempting to 

reduce flood losses within the context of land-use 

planning. Factors affecting the flood-loss reduction 

efforts in the Napa Valley can be important in other 

flood-prone areas. 

REFERENCES CITED 

American Insurance Association, 1956, Studies of floods and 

flood damages 1952-1955: New York, N.Y. 

Association of Bay Area Governments, 1970, Regional Plan 

1970:1990, San Francisco Bay region: Berkeley, Calif., 32 p. 

--1972, Regional open space plan-Phase II: Berkeley, Calif., 

15 p. 

--1975, Areas of critical environmental concern: Berkeley, 

Calif., 83 p. 

--1976, Land capability analysis-A method of applying 

earth science information to planning and decisionmaking: 

Berkeley, Calif., 225 p., app. A-D. 

Bay Conservation and Development Commission, 1969, San 

Francisco Bay plan: Sacramento, Calif., 43 p. 

Bue, C. D., 1967, Flood information for flood-plain planning: . 

U.S. Geol. Survey Circ. 539, 10 p. 

California Coastal Zone Conservation Commission, 1975, California coastal plan: Sacramento, Calif., 443 p. 

California Region Framework Study Committee, 1971, Comprehensive framework study, California region-Appendix IX, 

Flood control: Pacific Southwest Inter-Agency Comm., U.S. 

Water Resources Council, 197 p. 

California Government Code, 1966: West's Annot. Gov. Code, 

sec. 65300. 

--1976a, West's Annot. Gov. Code, sees. 65800-65912. 

--1976b, West's Annot. Gov. Code, sec. 66411. 

--1976c, Land conservation act of 1965: West's Annot. Gov. 

Code, sees. 51200-51295. 

--1976d, Planning and zoning law: West's Annot. Gov. 

Code, sees. 65000-66180. 

--1976e, Subdivision map act of 1974: West's Annot. Gov. 

Code, sees. 66410-66499. 

California Health and Safety Code, 1970: West's Annot. Health 

and Safety Code, sec. 500. 

--1976a, West's Annot. Health and Safety Code, sec. 17922. 

--1976b, West's Annot. Health and Safety Code, sec. 17958. 

--1976c, Community redevelopment law: West's Annot. 

Health and Safety Code, sees. 33000-34160. 

California Public Resources Code, 1970, Soil conservation and soil 

cons4;lrvation districts: West's Annot. Pub. Resources Code, 

sec. 9074. 

--1976a, Environmental quality act of 1970: West's Annot. 

Pub. Resources Code, sees. 21000-21176. 

--1976b, California coastal act of 1976: West's Annot. Pub. 

Resources Code, sees. 30000-30900. 

California Water Code, 1971: West's Annot. Water Code, sec. 

8401(c). 

--1976, Cobey-Alquist flood plain management act: West's 

Annot. Water Code, sees. 8400-8415. 

Crippen, J. R., 1975, Index of flood maps for California prepared 

by the U.S. Geological Survey through 1974: U.S. Geol. 

Survey open-file rept., 29 p. 

Federal Disaster Assistance Administration, 1975, Federal disaster assistance-Final regulations: Federal Register, v. 40, 

no. 103,p. 23251-23269. 

Federal Insurance AdminiE?tration, 1971, National flood insurance program: Code of Federal Regulations, 1975 ed., title 24, 

sec. 1910.3(a). 

--1976, National flood insurance program-Rules and regulations: Federal Register, v. 41, no. 207, p. 46962-46992. 

International Conference of Building Officials, 1973, Uniform 

building code: Whittier, Calif., 704 p. 

Kusler, J. A., and Lee, T. M., 1972, Regulations for flood plains: 

Am. Soc. Planning Officials, Planning Advisory Servicerept. 

277, 68 p. 

Limerinos, J. T., 1970, Floods on Napa River at Napa, California: 

U.S. Geol. Survey Hydrol. Inv. Atlas HA-348. 

Limerinos, J. T., Lee, K. W., and Lugo, P. E., 1973, Flood-prone 

areas in the San Francisco Bay region, California: U.S. Geol. 

Survey Water-Resources Inv. 37-73, 3 maps, scale 1:125,000. 

McGlashan, H. D., and Briggs, R. C., 1939, Floods of December 

1937 in northern California: U.S. Geol. Survey Water-Supply 

Paper 843, 497 p. 

Napa City Planning Department, 1968, City ofN apa general plan: 

Prepared and published by Gordon Hall and Associates, San 

Francisco, Calif., 82 p. 

--1975, The revised general plan for the city of Napa: Napa, 

Calif., 53 p. 

Napa Community Redevelopment Agency, 1969, Urban redevelopment plan, Parkway Plaza redevelopment project: 

Napa, Calif., 43 p. 

--1973, Amendments to urban redevelopment plan, Parkway 

Plaza redevelopment project: Napa, Calif. 

Napa County Board of Supervisors, 1967, Flood plain zoning 

classifications: Ordinance 262. 

--1968, Agricultural preserve district: Ordinance 274. 

--1969, Agricultural preserve interim district: Ordinance 292. 

--1969, Agricultural preserve extensive district: Ordinance 

309. 

--1970, Resolution 70-155. 

--1974a, Watercourse obstruction and riparian cover: Ordinance 447. 

--1974b, Agriculture, watershed and recreation zoning: Ordinance 468. 

Napa County Conservation, Development and Planning Department, 1973, Napa County conservation and open space element: Napa, Calif., 37 p., 8 maps. 

--1974a, Napa County summary general plan: Napa, Calif., 

4 p. 

--1974b, Napa Valley area plan: Napa, Calif., 35 p. 

--1975, Land use element, Napa County general plan: Napa, 

Calif., 63 p. 

Ohio Department of Natural Resources, 1974, Big Darby Creek 

corridor study: Columbus, Ohio, 99 p. 

Overview Corporation, 1973, How to implement open-space plans 

for the San Francisco Bay area: Berkeley, Calif., 172 p. 

Ritter, ,J. R., and Dupre, W. R., 1972, Maps showing areas of 

potential inundation from tsunamis in the San Francisco 

Bay region, California: U.S. Geol. Survey Misc. Field Studies 
REFERENCES CITED 69 

Map MF-480, 2 sheets. 

Sheaffer, J. R., and others, 1967, Introduction to floodproofing: 

Chicago Univ. Center for Urban Studies, Chicago Univ. 

Press, Ill., 61 p. 

Sheaffer, J. R., Ellis, D. W., and Spieker, A. M., 1970, Floodhazard mapping in Metropolitan Chicago: U.S. Geol. Survey 

Circ. 601-C, 14 p. 

Southeastern Wisconsin Regional Planning Commission, 1968, 

Floodland and shoreland development guide: Planning 

Guide no. 5, 199 p. 

U.S. Army Corps of Engineers, 1956, Report on floods of December 

1955 and January 1956 in northern California coastal 

streams: San Francisco [Calif.] Engineer Dist. rept., 137 p. 

--1965a, Report on floods of December 1964 in northern 

California coastal streams: San Francisco [Calif.] Engineer 

Dist. rept., 46 p. and appendixes. 

--1965b, Napa River basin, California: Washington, D.C., 

Office of the Chief of Engineers rept., 111 p. 

--1966, Flood plain information report on the Des Plaines 

River, Illinois and Wisconsin: Chicago [Ill.] Engineer Dist. 

rept. 

--1970, Flood plain information, Coyote Creek, San Francisco Bay to Anderson Reservoir, Santa Clara County, California: San Francisco [Calif.] Engineer Dist. rept., 26 p. 

--1972a, Flood plain information, Guadalupe River, Santa 

Clara County, California: San Francisco [Calif.] Engineer 

Dist. rept., 22 p. 

--1972b, Flood plain information, Green Valley, Dan Wilson, 

and Suisun Creeks, Cordelia [Solano County], California: 

Sacramento [Calif.] Engineer Dist. rept., 37 p. 

--1972c, Flood-proofing regulations: Washington, D.C., Office of the Chief of Engineers rept., 79 p. 

--1973a, Flood plain information, Alamitos Creek, including 

Guadalupe Creek, Arroyo Calero, and Santa Teresa Creek, 

Santa Clara County, California: San Francisco [Calif.] 

Engineer Dist. rept., 22 p. 

--1973b, Flood plain information, Alamo and Ulatis Creeks, 

Vacaville [Solano County], California: Sacramento [Calif.] 

Engineer Dist. rept., 44 p. 

--1973c, Flood plain information, Fisher Creek, Santa Clara 

County, California: San Francisco [Calif.] Engineer Dist. 

rept., 18 p. 

--1973d, Flood plain information, San Felipe Lake and Pacheco Creek, San Benito County, California: San Francisco 

[Calif.] Engineer Dist. rept., 17 p. 

--1973e, Flood plain information, Uvas-Carnadero Creek, 

Pajaro River to Uvas Reservoir, Santa Clara County, California: San Francisco [Calif.] Engineer Dist. rept., 19 p. 

--1973f, Water Resources development in California: South 

Pacific Division-San Francisco, Sacramento, and Los 

Angeles Engineer Dists., 244 :p. 

--1975a, Flood plain information, Llagas Creek Unit 1, including Edmundson, Church, San Martin, New, Center, Corralitos, Tennant, Maple, and Foothill Creeks, Santa Clara 

County, California: San Francisco [Calif.] Engineer Dist. 

rept., 31 p. 

--1975b, Flood plain information, Rush Creek-Petaluma 

River to U.S. Highway 101, Marin County, California: San 

Francisco [Calif.] Engineer Dist. rept., 19 p. 

--1975c, Napa River, draft environmental statement, flood 

control project, Napa, California: San Francisco [Calif.] 

Engineer Dist. rept., 213 p., 17 pls. 

--1975d, Water resources development in California: South 

Pacific Division-San Francisco, Sacramento, and Los Angeles Engineer Dists., 231 p. 

U.S. Congress, 1936, Flood control act of1936: Public Law 74-738, 

49 Stat. 1570, 33 U.S.C. 701. 

--1949, Housing act of 1949: Public Law 81-171, title !-Slum 

clearance and urban renewal, 63 Stat. 413, 42 U.S.C. 1441, 

1450-1469. 

--1954, Watershed protection and flood prevention act of 1954: 

Public Law 83-566, 68 Stat. 666, 16 U.S.C. 1001-1009. 

--1960, Flood control act of 1960: Public Law 86-645, title II, 

sec. 206, 74 Stat. 500, 33 U.S.C. 709a. 

--1965, Flood control act of 1965: Public Law 89-298, title II, 

79 Stat. 1073, 33 U.S.C. 701. 

--1966a, A unified national program for managing flood 

losses: House Document 465, 89th Congress, 2d sess., 4 7 p. 

--1966b, Flood control act of 1966: Public Law 86-789, title II, 

sec. 206, 80 Stat. 1418, 33 U.S.C. 709a. 

--1968, National flood insurance act of 1968: Public Law 

90-448, title XIII of Housing and urban development act of 

1968, 82 Stat. 572, 42 U.S.C. 4001-4127. 

--1970, National environmental policy act of 1969: Public 

Law 91-190, 83 Stat. 852, 42 U.S.C. 4321-4347. 

--1973, Flood disaster protection act of 1973: Public Law 

93-234, 87 Stat. 975, 42 U.S.C. 4001-4128. 

--1974a, Disaster relief act of 1974: Public Law 93-288, 

88 Stat. 143, 42 U.S.C. 5121-5202. 

--1974b, Housing and community development act of 1974: 

Public Law 93-383, 88 Stat. 633, 42 U.S.C. 5301-5317. 

--1974c, Water resources development act of1974: Public Law 

93-251, title I, sec. 73, 88 Stat. 16, 33 U.S.C. 701b-11. 

U.S. Department of Housing and Urban Development, 1974, 

Summary of the HUD budget, FY 1975: Washington, D.C., 

Office of the Secretary. 

--1975, Comprehensive planning assistance program: Federal Register, v. 40, no. 164, p. 36855-36865. 

--1976, Community development block grants-Eligible activities: Federal Register, v. 41, no. 12, p. 2766-2768. 

U.S. Geological Survey, 1971, Flood-prone areas in theN apa River 

drainage basin, Napa County, California: Cuttings Wharf, 

Napa, Rutherford, St. Helena, and Yountville quadrangles, 

scale 1:24,000. 

U.S. Office of Emergency Preparedness, 1972, Disaster preparedness-Report to Congress: Washington, D.C., U.S. Govt. 

Printing Office, 3 v. 

U.S. Office of Management and Budget, 1969, Circular no. A-95: 

Washington, D.C., U.S. Govt Printing Office. 

--1974, Catalog of Federal domestic assistance: Washington, 

D.C., U. S. Govt. Printing Office, 744 p. 

U.S. President, 1966, Evaluation of flood hazard in locating Federally owned or financed buildings, roads, and other facilities, 

and in disposing of Federal lands and properties: Executive 

Order 11296, Code of Federal Regulations, 1970 ed., title 3, 

p. 181, 33 u.s.c. 701. 

U.S. Water Resources Council, 1968, The Nation's water resources: Washington, D. C., U.S. Govt. Printing Office, pts. 

1-7. 

-- 1971, 1972, Regulation of flood hazard areas to reduce 

flood losses: Washington, D. C., U.S. Govt. Printing Office, 

v. I, pts. 1-4, 578 p.; and v. II, pts. 5-6, 389 p. 

White, G. F., 1960, Strategic aspects of urban flood plain occupance: Am. Soc. Civil Engineers Proc., v. 86, HY2, Paper 

2376, p. 89-102. 

--1975, Flood hazard in the United States-A research assessment: Colorado Univ. Inst. Behavioral Science, Monograph 

NSF-RA-E-75-006, 141 p. 
70 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

Young, L. E., 1962, Floods at Fremont, California: U.S. Geol. 

Survey Hydrol. Inv. Atlas HA-54. 

SELECTED READINGS 

Federal Disaster Assistance Administration, 1975, Federal disaster assistance-Final regulations: Federal Register, v. 40, 

no. 103, p. 23251-23269. 

Federal Insurance Administration, 1975, National flood insurance program-Proposed criteria: Federal Register, v. 40, no. 

59, p. 13420-13433. 

Liebman, Ernst, 1973, Legal problems in regulating flood hazard 

areas: Am. Soc. Civil Engineers Proc., v. 99, HYll, Paper 

10135, p. 2113-2123. 

Matthai, H. F., Back, William, Orth, R. P., and Brennan, Robert, 

1957, Water resources of the San Francisco Bay area, California: U.S. Geol. Survey open-file rept., 55 p. 

Perrey, J. 1., 1956, Use of zoning principles in flood-plain regulation: Am. Soc. Civil Engineers Proc., v. 82, HY2, Paper 

957, p. 1-10. 

Phippen, G. R., 1974, Can a right go wrong?: Water Spectrum, 

U.S. Army Corps of Engineers, Washington, D.C., v. 6, 

no. 1, p. 31-37. 

Rantz, S. E., 1971, Mean annual precipitation and precipitation-depth-duration frequency data for the San Francisco 

Bay region, California: U.S. Geol. Survey open-file rept., 23 p. 

--1971, Suggested criteria for hydrologic design of stormdrainage facilities in the San Francisco Bay region, California: U.S. Geol. Survey open-file rept., 69 p. 

--1974, Mean annual runoff in the San Francisco Bay region, 

California, 1931-70: U.S. Geol. Survey Misc. Field Studies 

Map MF-613. 

Schneider, W. J., and Goddard, J. E., 1974, Extent and development of urban flood plains: U.S. Geol. Survey Circ. 601-,J, 

14 p. 

Sheaffer, J. R., and others, 1967, Introduction to floodproofing: 

Chicago Univ. Center for Urban Studies, Chicago Univ. 

Press, Ill., 61 p. 

Spangle, William and Associates, and others, 1973, Application of 

earth-science information in urban land-use planning-Stateof-the-art review and analysis: Available from the Natl. 

Tech. Inf. Service, U.S. Dept. Commerce, Springfield, Va., 

NTIS PB-238-081/ AS, 330 p. 

--1976, Earth-science information in land-use planningguidelines for earth scientists and planners: U.S. Geol. Survey 

Circ. 721, 28 p. 

U.S. Water Resources Council, 1968, The Nation's water resources: 

Washington, D.C., U.S. Govt. Printing Office, pts. 1-7. 

--1971, 1972, Regulation of flood hazard areas to reduce flood 

losses: Washington, D.C., U.S. Govt. Printing Office, v. I, 

pts. 1-4, 578 p.; and II, pts. 5-6, 389 p. 

--1972, Flood hazard evaluation guidelines for Federal execu· 

tive agencies: Washington, D.C., U.S. Govt. Printing Office, 

22 p. 

Weber, E. W., and Sutton, W. G., 1965, Environmental effects 

of flood-plain regulations: Am. Soc. Civil Engineers Proc., 

v. 91, HY4, Paper 4402, p. 59-70; discussion v. 91, HY6, 

p. 249-251. 

White, G. F., and Haas, .J. E., 1975, Assessment of research on 

natural hazards: Mass. Inst. Tech. Press, Cambridge, Mass., 

and London, England, 487 p. 

SOURCES OF FLOOD-PLAIN MAPS 

AND INFORMATION 

Flood-plain inundation maps have been prepared and other 

flood studies made by the Geological Survey and other Federal, 

county, local, and private agencies for many areas in the San 

Francisco Bay region. Information on flooding and flood hazards 

has been presented in pertinent maps and reports. The principal 

sources of the maps and flood information are summarized in 

table 9; the maps and reports available are listed in table 10. 

Figure 28 shows the quadrangle maps available in the Bay region. 

TABLE 9.-Sources of maps and flood information 

Types of maps and reports 

Flood-prone area Floodmaps Flood- plain Flood- Other 

hazard information insurance flood-plain 

Regional Quadrangle maps reports studies maps 

u.s. 

Geological 

Survey x (1) x 

U.S. Army 

Corps of 

Engineers ------------------------------------------------ (2) 

U.S. Soil 

Conservation 

Service ------------------------------------------------------ (

3

) 

Federal 

Insurance 

Administration, 

U.S. Department of 

Housing 

and Urban 

Development --------------------------------------·------------------------ x 

County and 

municipal 

agencies --------------·----------··········-----------------------------·--··--·-------------------- (

4

) 

'Quadrangle maps available are shown in figure 28. 2ln flood-plain information, flood, and flood-control project reports. 3Generally in watershed work plan or watershed investigation reports. 4Generally unpublished. 
SOURCES OF FLOOD-PLAIN MAPS AND INFORMATION 71 

EXPLANATION 

Map sheets showing inundation from 

• 100-year flood 

• Occasional flooding 

0 10 20 30 40 50 MILES 37° 

I I I I I I I I I I I I 

I 

0 10 20 30 40 50 60 70 KILOMETRES 

i~~ 

<:> ~~ ~ 

~ ' 

FIGURE 28.-Topographic map sheets and the flood-prone area maps available in the San Francisco Bay region. 
72 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

TABLE 10.-Flood-plain maps and information available for areas in the San Francisco Bay region 

A. Flood-prone area maps 

Regional maps: 

U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA, 94025 

1. "Flood prone areas in the San Francisco Bay region, California," by J. T. Limerinos, K. W. Lee, and 

P. E. Lugo (1973), in three sheets, scale 1:125,000. Map shows areas in the region subject to inundation by the 100-year flood. Coastal areas subject to a potential hazard from high tides are delineated 

with respect to the 100-year tide. (SFBRS Interpretive Report 4). 

2. "Map showing areas of potential inundation by tsunamis in the San Francisco Bay region, California," by .J. R. Ritter and W. R. Dupre (1972), in two sheets, scale 1:125,000. Map shows estimated 

extent of potential inundation from a tsunami wave height of 20 feet at the Golden Gate (recurrence 

interval, about 200 years). Inundation hazard from tsunamis was not included in the delineation of 

flood-prone areas shown on the regional map. (SFBRS Basic Data Contribution 52). 

Quadrangle maps (fig. 28): 

U.S. Geological Survey, California District, Water Resources Division, 855 Oak Grove Avenue, Menlo Park, 

CA, 94025 

1. Maps, on topographic map sheets, scale 1:24,000, showing areas prone to occasional flooding, 

prepared in 1969: 

Calistoga 

Dublin 

La Costa Valley 

Livermore 

Niles 

Palo Alto 

San .Jose West 

2. Maps, on topographic map sheets, scale 1:24,000, showing areas prone to flooding from the 100-year 

flood, prepared 1971-75 (location shown in fig. 28): 

Aetna Springs Franklin Point 

Allendale Geyserville 

Altamont Gilroy 

Antioch North Gilroy Hot Springs 

Antioch South Glen Ellen 

Asti Gualala 

Benicia Guerneville 

Bethany Half Moon Bay 

Big Basin Hayward 

Birds Landing Healdsburg 

Bolinas Honker Bay 

Bouldin Island Hunters Point 

Brentwood Inverness 

Briones Valley Isleton 

Byron Hot Springs Jersey Island 

Calaveras Reservoir Jimtown 

Camp Meeker Kenwood 

Capell Valley La Honda 

Castle Rock Ridge Las Trampas Ridge 

Cazadero Laurel 

Chiles Valley Liberty Island 

Chittenden Los Gatos 

Clayton Mare Island 

Cloverdale Merritt 

Cordelia Milpitas 

Cotati Mindego Hill 

Courtland Montara Mountain 

Cupertino Morgan Hill 

Cuttings Wharf Mountain View 

Davis Mt. George 

Denverton Mt. Madonna 

Detert Reservoir Napa 

Diablo Newark 

Dixon Nova to 

Dozier Oakland East 

Duncans Mills Oakland West 

Elmira Pacheco Peak 

Fairfield North Petaluma 

Fairfield South Petaluma Point 

Petaluma River 

Pigeon Point 

Point Bonita 

Port Chicago 

Redwood Point 

Richmond 

Rio Vista 

Rutherford 

San Felipe 

San Francisco 

North 

San Francisco 

South 

San Gregorio 

San .Jose East 

San Leandro 

San Mateo 

San Quentin 

San Rafael 

Santa Rosa 

Santa Teresa 

Hills 

Saxon 

Sears Point 

Sebastopol 

Sonoma 

St. Helena 

Tassajara 

Three Sisters 

Two Rock 

Valley Ford 

Walnut Creek 

Walter Springs 

Watsonville East 

Winters 

Woodside 

Woodward Island 

Yountville 

These maps and flood-prone area maps available for other areas in California are listed in an "Index of 

Flood Maps for California Prepared by the U.S. Geological Survey through 1974," compiled by J. R. Crippen 

(1975) and supplements. The maps are listed alphabetically and by counti~s, with inl'ormation on location, 

map scale, and year mapped. Copies of the index may be obtained from the California District, Water 

Resources Division, U.S. Geological Survey, 855 Oak Grove Avenue, Menlo Park, Calif. 94025. 

B. Flood-hazard maps 

U.S. Geological Survey, Denver Federal Center, Lakewood, CO, 80225 

1. Hydrologic Investigations Atlas HA-54, "Floods at Fremont, California," by L. E. Young (1962). 

2. Hydrologic Investigations Atlas HA-348, "Floods on Napa River at Napa, California," by .J. T. Limerinos 

(1970). 
SOURCES OF FLOOD-PLAIN MAPS AND INFORMATION 

TABLE 10.-Flood-plain maps and information available for areas in the San Francisco Bay region-Continued 

C. Flood-plain information reports 

U.S. Army Engineer District, San Francisco, 211 Main Street, San Francisco, CA, 94105. Copies of reports 

generally available from the flood control and water conservation districts and collaborating county agencies. 

Flood-plain information for: 

Rush Creek-Petaluma River to U.S. Highway 101, Marin County (1975). 

Alamo and Ulatis Creeks, Vacaville, Solano County (1973). 

Green Valley, Dan Wilson and Suisun Creeks, Cordelia, Solano County (1972). 

Alamitos Creek, including Guadalupe Creek, Arroyo Calero, and Santa Teresa Creek, Santa Clara 

County (1973). 

Coyote Creek, San Francisco Bay to Anderson Reservoir, Santa Clara County (1970). 

Fisher Creek, Santa Clara County (1973). 

Guadalupe River, Santa Clara County (1972). 

Llagas Creek Unit 1, including Edmundson, Church, San Martin, New, Center, Corralitos, Tennant, 

Maple, and Foothill Creeks, Santa Clara County (1975). 

San Felipe Lake and Pacheco Creek, San Benito County and part of Santa Clara County (1973). 

Uvas-Carnadero Creek, Pajaro River to Uvas Reservoir, Santa Clara County (1973). 

D. Flood-insurance studies 

Communities participating in the National Flood Insurance Program as of October 31, 1975, that have special 

flood hazard areas identified, and wherein the sale of flood insurance is authorized, include the following: 

County and 

community 

Alameda County: 

Unincorporated area 

Alameda 

Berkeley 

Emeryville 

Fremont 

Hayward 

Livermore 1 

Newark 

Oakland 

Piedmont 

Pleasanton 

San Leandro 

Union City 

Contra Costa County: 

Unincorporated area 

Antioch 

Brentwood 

Clayton 

Concord 

El Cerrito 

Hercules 

Lafayette 

Martinez 

Pinole 

Pittsburg 

Pleasant Hill 

Richmond 

San Pablo 

Walnut Creek 

Marin County: 

Unincorporated area 

Belvedere 

Corte Madera 

Fairfax 

Larkspur 1 

Mill Valley 

County and 

community 

Marin County-Continued: 

Novato 

Ross 

San Anselmo 

San Rafael 

Sausalito 

Tiburon 

Napa County: 

Unincorporated area 

Calistoga 

Napa 

St. Helena 

Yountville 

San Francisco County: 

San Francisco City 

and County2 

San Mateo County: 

Unincorporated area 

Atherton 

Belmont 

Brisbane 

Burlingame 

Colma 

Daly City 

Foster City 

Half Moon Bay 

Hillsborough 

Menlo Park 

Millbrae 

Pacifica 

Portola Valley 

Redwood City 

San Bruno 

San Carlos 

San Mateo 

South San Francisco 

Woodside 

1

Participating in program, but special flood hazard areas not yet delineated. 

~Determined to have no special flood·hazard areas. 

County and 

community 

Santa Clara County: 

Campbell 

Cupertino 

Gilroy 

Los Altos 

Los Altos Hills 

Los Gatos 

Milpitas 

Monte Sereno 

Morgan Hill 

Mountain View 

Palo Alto 

San Jose 

Santa Clara 

Saratoga 

Sunnyvale 

Solano County: 

Benicia 

Dixon 

Fairfield 

Rio Vista 

Suisun City 

Vacaville 

Vallejo 

Sonoma County: 

Unincorporated area 

Cloverdale 

Cotati 

Healdsburg 

Petaluma 

Rohnert Park 

Santa Rosa 

Sebastopol 

Sonoma 

73 
74 FLOOD-PRONE AREAS AND LAND-USE PLANNING 

TABLE 10.-Flood-plain maps and information available for areas in the San Francisco Bay region-Continued 

E. Other flood-control and watershed projects 

U.S. Army Engineer District, San Francisco, Corps of Engineers, 211 Main Street, San Francisco, CA, 94105 

Flood control projects: 

Name 

San Lorenzo 

Creek. 

Alhambra Creek 

County 

Nature of 

work 

Alameda ---------------------------- Levees and channel 

improvements. 

Contra Costa ------------------ Channel 

improvements, 

diversion. 

Walnut Creek ________________ -------------------- do -------------------- Channel enlargement and 

stabilization. 

Corte Madera 

Creek. 

Napa River 

basin. 

Alameda 

Fairfield 

vicinity streams. 

Sonoma Creek 

basin. 

Marin --------------------------------

Napa----------------------------------

Santa Clara --------------------

Solano --------------------------------

Sonoma ----------------------------

Channel 

improvements. 

Channel enlargement, realinement 

levees. 

Channel 

improvements. 

Channel 

improvements, 

diversion. 

Channel 

improvements. 

Warm Springs Dam, 

Lake Sonoma. 

_____________________ do ____________________ _ Dam and lake, 

channel 

improvements. 

Small flood-control projects: 

Name County 

Nature of 

work 

San Leandro Alameda -------------------------- Channel 

Creek. improvements. 

Pinole Creek ------------------ Contra Costa ------------------ -------------------do ---------------------

Rheem Creek ------------------ --------------------- do -------------------- ------------------·do ____________________ _ 

Rodeo Creek ------------------ ---------------------do ____________________ ------------------· do---------------------

Coyote Creek ------------------

Green Valley 

Creek. 

Flood control studies: 

Name 

Wildcat and 

San Pablo Creeks. 

Nova to Creek and 

tributaries. 

Pacific coastal 

streams. 

South San Francisco 

streams. 

Guadalupe River. 

Russian River. 

Marin -------------------------------- Channel lining. 

Solano -------------------------------- Channel 

enlargement, 

realinement. 

County 

Contra Costa _________________ _ 

Marin --------------------------------

San Mateo ------------------------

San Mateo ------------------------

Santa Clara -------------------

Sonoma ----------------------------

Date authorized 

1960 

1941 

1965 

1958 

1941 

1958 

Status 

Completed 1962. 

Not started. 

Under construction 

(1977). 

Under construction, 

in court. 

Planning completed. 

Under construction. 

Planning completed 

1973. 

Planning and EIS 

completed 1973. 

Planning, land 

acquisition, road 

relocation in progress. 

Status 

Near completion. 

Completed 1966. 

Completed 1960. 

Completed 1966. 

Completed 1965. 

Completed 1962. 
SOURCES OF FLOOD-PLAIN MAPS AND INFORMATION 

TABLE 10.-Flood-plain maps and information available for areas in the San Francisco Bay region-Continued 

E. Other flood-control and watershed projects-Continued. 

U.S. Soil Conservation Service. Information pertaining to these and other studies in the region may be obtained 

from offices ofthe Soil Conservation Service (Concord, Dixon, Half Moon Bay, Livermore, Morgan Hill, Napa, 

Santa Rosa, Petaluma) or from offices of cooperating soil conservation districts and county flood control and 

water conservation districts. 

Small watershed projects under Public Law 83-566 (U.S. Congress, 1954): 

Project Sponsoring agencies Purpose 

Ulatis Creek ........................................................................................................................................ .. 

Year authorized Status 

Complete. 

75 

Central Sonoma ............... Sonoma County Water Agency. Flood control. 

1961 

1958 Nearing completion. 

Lower Uagas Creek 

and Upper Llagas 

Creek. 

Santa Rosa Resource Conservation 

District. 

Santa Clara Valley Water District. 

South Santa Clara County Water 

District. 

Lorna Prieta Resource Conservation 

District. 

Lower Pine Creek ............. Contra Costa County Flood Control 

Marsh-Kellogg 

Creek. 

and Water Conservation District. 

Contra Costa County Resource 

Conservation District. 

East Bay Regional Park District. 

Contra Costa County Flood Control 

and Water Conservation District. 

Contra Costa County Resource 

Conservation District. 

Napa River .................... Napa County Flood Control and 

Water Conservation District. 

Napa County Resource Conservation 

District. 

*U.S. GOVERNMENT PRINTING OFFICE: 1977 - 789-025/66 

.............. do ................ .. 

Flood control, 

water storage, 

recreation. 

Flood control. 

.............. do ................ . 

1969 

1970 

1959 

1962 

Not yet under 

construction. 

................. do ................ .. 

Nearing completion. 

Stalled .