GEOLOGICAL SURVEY 

CIRCULAR 601-J 

Extent and Development 

Of Urban Flood Plains 

Despite the flooding hazard, flood plains have always been prime areas for industrial, commercial, and residential 

development. 
EXTENT AND DEVElOPMENT 

OF URBAN FlOOD PlAINS 
Extent and Development 

Of Urban Flood Plains 

By William J. Schneider and James E. Goddard 

WATER IN THE URBAN ENVIRONMENT 

G E 0 L 0 G I CAL SURVEY CIRCULAR 601-J 

Washington 197 4 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to the U.S. Geological Survey, National Center, Reston, Va. 22092 
FOREWORD 

Urbanization-the concentration of people in urban areas and the 

consequent expansion of these areas-is a characteristic of our time. It has 

brought with it a host of new or aggravated problems that often make new 

demands on our natural resources and our physical environment. Problems 

involving water as a vital resource and a powerful environmental agent are 

among the most critical. These problems include the maintenance of both 

the quantity and quality of our water supply for consumption, for 

recreation, and general welfare and the alleviation of hazards caused by 

floods. drainage, erosion. and sedimentation. 

A prerequisite to anticipating, recognizing. and coping intelligently with 

these problems is an adequate base of information. This series of reports is 

intended to show the relevance of water facts to water problems of urban 

areas and to examine the adequacy of the existing base of water information. 

J. S. Cragwall, Jr. 

Chief Hydrologist 

v 

CONTENTS 

Page 

Forevvord ---------------------------------------------------------------------------- \' 

1\bstract ---------------------------------------------------------------------------- J1 

Introduction__________________________________________________________________________ 1 

Objectives and scope------------------------------------------------------------------ 2 

1\pproach ---------------------------------------------------------------------------- 2 

Defining the urban area ---------------------------------------------------------- 2 

Defining the flood plain ---------------------------------------------------------- 3 ~easuringthe areas______________________________________________________________ 4 

Defining the flood-plain development ---------------------------------------------- 5 

Thedata ---------------------------------------------------------------------------- 5 

1\nalyses of the data-------------------------------------------------~---------------- 5 

Discussion -------------------------------------------------------------------------- 14 Reference ____________________________________________________________________________ 14 

ILLUSTRATIONS 

FRONTISPIECE. Photograph shovving development in the flood plain. 

Page 

FIGURE 1. ~ap shovving location of cities used in this study ----------------------------------------------------- J3 

2. ~ap of flood-prone area, Harrisburg, Pa --------------------------------------------------------------- 6 

3. Photographs shovving types of uses of flood plain, Harrisburg, Pa --------------------------------------- 7 

4. ~ap of flood-prone area, Spokane, Wash_______________________________________________________________ 8 

5. Photographs shovving types of uses of flood plain, Spokane, Wash --------------------------------------- 9 

6. ~ap of flood-prone area, Phoenix, 1\riz --------------------------------------------------------------- 10 

7. Photographs shovving types of uses of flood plain, Phoenix, 1\riz ----------------------------------------- 11 

8. Graph shovving correlation betvveen percent of urbanized area in flood plain and index of depth of flooding___ 14 

TABLES 

Page 

TABLE 1. 1\rea and population of selected urbanized areas ---------------------------------------------------- ·-- J12 

2. 1\reas of selected urban flood plains------------------------------------------------------------------- 12 

3. Selected indices related to extent of flood plains in selected urbanized areas ----------------------------- 13 

-vii 

Water in the Urban Environment 

Extent and Development of Urban Flood Plains 

By William J. Schneider and James E. Goddard1 

ABSTRACT 

A study of26 urbanized areas in the United States indicates 

that the amount of urban area in flood plains ranges from 2.4 

percent for Spokane, Wash., to 81 percent for Monroe, La. The 

median value is 10.5 percent, and the weighted average is 16.2 

percent. 

The amount of development on these flood plains also varies 

widely, from 11.3 percent for Lorain-Elyria, Ohio, to 97 percent 

for Great Falls, Mont. The median value is 57 percent, and the 

weighted average is 52.8 percent. 

Attempts to correlate either the extent of urban area in flood 

plain or amount of development with three readily available 

indices-depth of flooding, precipitation, and physiography-showed no strong relationships. Some correlation was 

found between the extent of urban area in flood plain and index 

of depth of flooding. 

INTRODUCTION 

uA flood is when the water gets up to your pocketbook." 

Flood plains-the natural overflow channels of 

rivers, streams, oceans, and other bodies of 

water-have always attracted man. The rich fertile soils deposited by the floodwaters have sustained an abundant agriculture. The water supplied his needs and carried away his wastes, and 

the rivers and oceans served as his main artery of 

transportation long before the railroad or the 

highway linked the urban centers. 

The general and predictable pattern of urban 

growth has been one of settlement on the flood 

plain, followed by expansion onto higher ground to 

the foothills or mountains or less desirable hinter- · 

lands. The major cities of the United States, indeed 

of the world, have followed this pattern. Each also, 

to one degree or another, has paid the penalty of 

flooding as complacency or ignorance or sheer 

gambling has exposed inhabitants to floodwaters. 

It has been said that uNature has a preemptive 

right to the flood plain," but when Nature does not 

1Consulting engineer, Tucson, Ariz. 

J1 

exercise that right for 20 or 50 years, man too often 

has become complacent, careless, or a holder of 

short odds. 

The fact is that much of our urban areas occupy 

flood plains. But how much? And how much have 

been developed? Answers to these questions are 

being sought to place better perspective on the 

problems involved in the national goal of effective 

flood-plain management. 

Currently, no data are compiled on the extent of 

flood plains in our urban areas. Nor are there data 

on the capital investment in development of these 

areas. Delineation of flood plains, however, are 

available through many agencies, at all levels of 

government, including the U.S. Geological Survey. The Survey is actively engaged in programs 

related to flood-plain management. Yet despite 

the management efforts of these agencies, according to the Federal Water Resources Council, flood 

damages continue to increase each year. The magnitude of factors related to the problem of increasing flood damage-the amount of urban land in 

flood plains, the extent of development of these 

flood plains, and an economic evaluation of this 

occupancy and efforts to protect this occupancyhave never been fully assessed. 

Late in 1973, the American Society of Civil Engineers, through its Urban Water Resources Research Program sponsored by the Office of Water 

Resources Research, U.S. Department of the Interior, completed a study of the flood-plain problem as a part ofits program. Because the study was 

an addition to the program, it of necessity was 

limited, both in scope and in time. Because of relevance to their programs, several Federal agencies 

concerned with flooding-the U.S. Geological Survey, the U.S. Army Corps of Engineers, and the 
Tennessee Valley Authority-provided data to 

that study, particularly on the extent of flooding in 

the urban areas. 

The U.S. Geological Survey, as its contribution 

to the American Society of Civil Engineers study, 

provided flooq-plain information for 16 cities included in the RALI (Resource and Land Information Program) of the Department of the Interior. 

The effort was undertaken to demonstrate how 

available resources data, along with some additional data collected especially to augment the 

existing data, could be analyzed to meet a specific 

need. 

OBJECTIVES AND SCOPE 

The objective of this RALI study is to evaluate 

both the amount of flood plain in urban areas and 

the degree of development of these flood plains. 

The scope of the study is limited by the time-frame 

of the broader study conducted by the American 

Society of Civil Engineers (Goddard, 1974). 

The broader study has five objectives: (1) To 

determine that part of urban lands in flood plains, 

(2) to determine that part of urban flood plains that 

are developed, (3) to determine the economic investment in the developed urban flood plains, ( 4) to 

determine the average annual flood damages in 

urban areas, and (5) to determine the economic 

investment in protecting urban areas from flood 

damage. Clearly, the efforts described here relate 

to objectives (1) and (2) of the broader study. 

This report contains data on areal extent and 

degree of occupancy of flood plains in the urbanized areas of26 cities (see fig. 1). Ofthese, data 

were supplied for 16 cities by the U.S. Geological 

Survey, for 1 city by the Tennessee Valley Authority, and for 9 cities by the American Society of 

Civil Engineers who used basic data provided by 

the U.S. Army Corps of Engineers. These cities 

represent a statistical sampling based upon variations in physiography, topography, climate, population, river size, and political entity. They were 

selected to represent: 

Varying populations (50,000 to 7 million) 

Varying types of architecture 

Varying lifestyles 

Total population of 20.3 million 

10 percent of total U.S. population 

Inland, coastal, and estuarine conditions 

All major physiographic regions 

28 of the 50 states 

15 of the 20 major water-resources regions 

28 of the 4 7 U.S. Geological Survey Districts 

J2 

21 of the 38 U.S. Army Corps of Engineers 

Districts 

TV A in the Tennessee Valley. 

APPROACH 

For each sample city the general approach for 

this study was to determine the areal extent of the 

urban area, to delineate and measure the area of 

the flood plains within each urban area, and to 

determine and measure the present extent of 

usage of the areas of those flood plains. 

DEFINING THE URBAN AREA 

The UA (urbanized area) as defined by the U.S. 

Bureau of Census for the 1970 census was used for 

this study. An urbanized area consists of a central 

city or cities and surrounding closely settled territory. The specific criteria for the delineation are 

as follows: 

1. City areas. 

a. A central city of50,000 inhabitants or more 

in 1960, in a special census conducted by 

the Census Bureau since 1960, or in the 

1970 census; or 

b. Twin cities, that is, cities with contiguous 

boundaries and constituting, for general 

social and economic purposes, a single 

community with a combined population 

of at least 50,000, and with the smaller of 

the twin cities having a population of at 

least 15,000. 

2. Surrounding closely settled territory, including the following (but excluding the rural 

portions of extended cities): 

a. Incorporated places of 2,500 inhabitants or 

more. 

b. Incorporated places with fewer than 2,500 

inhabitants, provided that each has a 

closely settled area of 100 housing units 

or more. 

c. Small parcels of land normally less than 1 

square mile in area having a population 

density of 1,000 inhabitants or more per 

square mile. The areas of large nonresidential tracts devoted to such urban land 

uses as railroad yards, airports, factories, parks, golf courses, and cemeteries 

are excluded in computing the population density. 

d. Other similar areas in unincorporated territory with lower population density 

provided that they serve: 
POPULATION 

Over 1,000,000 

• 225,000-1,000,000 

0 

Less than 225,000 

FIGURE 1.-Map showing location of cities used in this study. Population given is defined as that of the urbanized area. 

to eliminate enclaves, or 

to close indentations in the urbanized 

areas of 1 mile or less across the open 

end, or 

to link outlying enumeration districts 

of qualifying density that are not 

more than 11h miles from the main 

body of the urbanized area. 

The urbanized areas based on the above criteria 

were chosen for several reasons. Primarily, the 

urbanized area is exactly what its name implies: it 

is urbanized. It does not include large farmland, 

open land, or sparsely populated expanses under 

private or public control that are a major part of 

political or other uareas" selected for statistical 

purposes, therefore, its use more accurately 

reflects the percentage of flood plain in urban 

areas. Furthermore, the urbanized area is a unit 

for which the Bureau of Census publishes demographic statistics of widely varying types, thus 

facilitating comparisons of the flood-plain data 

with other statistics. Finally, the use of the urbanized area provides homogeneity in occupancy 

characteristics of the selected urban areas. 

J3 

DEFINING THE FLOOD PLAIN 

The flood plain as defined for this study is that 

area which would be inundated by the natural 

100-year flood (the flood level that, on an average, 

is exceeded once in 100 years). 

It should be recognized, however, that there is 

great variance and freedom in the usage of the 

term uflood plain" that leads to frequent misunderstandings. The geologic definition of a flood 

plain is the relatively fiat area or low lands adjoining the channel of a river, stream, or watercourse 

or along an ocean, lake, or other body of standing 

water, which has been or may be covered by 

floodwater. However, large parts of geologic flood 

plains may seldom be flooded or may be 

insufficiently defined to describe the flood potential. For that reason the current concept of 

flood-plain management generally recognizes the 

engineering-type definition of areas subject to 

specific probabilities of flooding. 

Data on the 100-year flood are available from 

several sources. The Corps of Engineers' 

flood-plain information reports provide information for their SPF (standard project flood) and for 
the IRF (intermediate regional flood) which is approximately equivalent to the 100-year flood. The 

TV A local flood reports provide information for 

flood generally equivalent to the SPF and the IRF. 

The U.S. Geological Survey flood-prone area maps 

show the areas that are subject to flooding by the 

100-year flood, and their hydrologic atlas maps 

show areas inundated by one or more large historic 

floods with information to generally relate the 

data to 50-year or 100-year flood. The FIA (Federal 

Flood Insurance Administration) ~~Flood Information and Floodway" maps show the areas inundated by the 100-year and 500-year floods plus a 

basic floodway. FIA flood-insurance studies also 

include a rate map intended primarily for use by 

insurance agents to calculate rates. The rate map 

shows areas inundated by the 100-year and 

500-year floods extended to identifiable physical 

features. FIA studies made prior to 197 4 often 

contain only the rate map. The ~~Flood Hazard 

Evaluation Guidelines for Federal Executive 

Agencies," issued by the Federal Water Resources 

Council in 1972, uses the 100-year flood as one of 

its major criteria. Hundreds of local communities 

and cities and counties have adopted flood-plain 

regulations and some States have established programs designed around the 100-year flood or its 

equivalent. 

Upstream regulating reservoirs, channel improvements, levees, walls, and similar structures 

have reduced flood hazards at many sites and 

changed the heights of the 100-year flood. Many of 

the areas that have been protected in this manner 

are now developed-some of them prior to the 

flood-control works and others after their construction. 

The purpose of this study wa's to learn the extent 

of the flood plain and its relation and (or) effect on 

urban development; thus, to be consistent with 

generally accepted practices and programs of 

other agencies, the term ~~flood plain" in this study 

refers to the area, except the channel that will be 

inundated by a 100-year flood, without consideration of present or future flood-control storage, 

channel modifications, levees, or other works that 

may reduce flood levels. 

Data for both natural and regulated conditions 

were obtained for three urban areas. These provide an indication of the relations and effect of 

flood plains on urban development at those sites. 

MEASURING THE AREAS 

Flood-prone area maps and hydrologic atlases of 

the U.S. Geological Survey, maps accompanying 

the floodplain information reports of the Corps of 

Engineers, and the flooded-area maps of the Tennessee Valley Authority were the principal 

sources of data. Maps of 1:24,000 or larger scale 

were used if available. 

Measurements of areas were made in accordance with the following criteria: 

A. Watersheds greater than 5 square miles. 

1. The 100-year floods were outlined on 

maps. 

2. The large flood-plain areas outlined on 

maps were planimetered using standard techniques. 

3. Some small, narrow, elongated floodplain areas were divided into reaches. 

The length of each reach was measured, and the average width of each 

respective reach was estimated or 

measured. These areas constitute a 

small part of the total flood plain for 

any urbanized area. 

4. Water-surface areas of streams, lakes, 

and reservoirs, large enough to be 

shown with double lines or in blue on 

standard topographic quadrangles 

were excluded. 

5. Islands in large water areas were included as :flood plain wher~ 1

so designated on the maps. Those of about 20 

acres or larger were measured. A visual estimate of the total area of many 

smaller islands was made. 

B. Watersheds less than 5 square miles. 

1. Streams for which the 100-year flood 

plains were not outlined on maps (less 

than 5 square miles) were divided into 

reaches. The length of each reach was 

measured, and an estimate of the average width of flood plain for each respective reach made. Such cursory estimates were based on available topographic maps, size of watershed, personal knowledge of terrain and hydrologic conditions, and judgment. This 

procedure was applied to all streams or 

watercourses that are more than 3 

miles in length or have a drainage area 

J4 
exceeding 2 square miles. Measurements extended upstream to a point 

where the drainage area was approximately 2 square miles. These totals 

were generally less than 2 percent of 

the total flood-plain area. 

C. Total urbanized areas. 

1. Total area of each urbanized area, except 

Asheville and Texarkana, was taken 

from Bureau of Census records. Areas 

for Texarkana and Asheville were 

measured by planimeter. 

DEFINING THE FLOOD PLAIN DEVELOPMENT 

The flood plains were classified as ttundeveloped" or ~~developed," and the developed part was 

measured. 

~~Developed" flood plains were defined as those 

flood plains where: 

1. More than one-third of the acreage, whether 

continuous or interspersed, is actively used 

for residential, commercial, or industrial 

purposes; or 

2. Plans exist for development to a degree outlined in (1) and there is assurance that the 

development will be accomplished soon; or 

3. State or local planning and (or) development 

agencies have classified the land as being 

developed; or 

4. The Corps of Engineers has classified them as 

being ~~built-up" and measured for its Civil 

Works Information System records. 

Several types of flood plains and their uses are 

illustrated in figures 2-7. 

THE DATA 

The data measured and compiled for this study 

are given in tables 1 and 2. 

Table 1 gives the area and population of the 

selected urbanized areas. Data are from the U.S. 

Bureau of Census for 1970 except for Asheville and 

Texarkana. For Asheville, the Bureau limits were 

used but new measurements were made of the 

area. Readily available flood-plain data from the 

Corps of Engineers for Texarkana included recent 

expansions of the corporate boundaries. 

Table 2 gives the urban flood-plain areas and 

parts thereof which had been developed. The areas 

shown are for the 100-year flood under natural or 

unregulated conditions. 

J5 

In some areas, local flood-protection works and 

upstream regulation provide some degree of flood 

protection, often considerably reducing the areas 

susceptible to flooding. Measurements were made 

for both natural and regulated conditions at three 

urbanized areas. At Omaha-Council Bluffs, for example, structural flood-control measures have reduced the area susceptible to flooding by the 

100-year flood from 50.6 square miles to 23.2 

square miles with only 8.3 square miles developed. 

At Monroe, flood-control measures have reduced 

the flood-plain area from 32.5 to 3.5 square miles 

with only 0.9 square miles developed; and at St. 

Louis, from 136.1 to 45.9 square miles with only 

7.4 square miles developed. 

ANALYSES OF THE DATA 

There is an extremely wide variation in the extent of flood plains in urbanized areas. For the 26 

urbanized areas selected for this study, the percent 

of area in flood plain ranges from 2.4 percent for 

Spokane, Wash. to 81.0 percent for Monroe, La. It 

should be noted, however, that the extent of flood 

plain in Monroe is exceptional for the areas sampled. The next largest value is less than half of that 

for Monroe-40.1 percent for Charleston, S.C. The 

median value is 10.5 percent, and the weighted 

average is 16.2 percent. 

The development of the flood plains in the urbanized areas also varies widely. The least developed of the flood plains of the urbanized areas 

used in this study is in Lorain-Elyria, Ohio-11.3 

percent of the total flood-plain area-and the most 

highly developed is that of Great Falls, 

Mont.-97.0 percent developed. The median value 

is 57.0 percent, and the weighted average is 52.8 

percent. 

Because of the reconnaissance nature of this 

study and the original objective of defining solely 

the extent and occupancy of flood plains in urbanized areas, no attempt was made at collecting 

detailed supplemental data for analyses. However, readily available data on a few indices related to flooding were developed to provide some 

preliminary analyses of the data. These attempts 

are intended more to provoke further study than to 

provide definitive answers. 

Table 3 gives indices of three aspects of flooding 

(depth of flooding, precipitation, and physiography) for which data were readily available. 
FIGURE 2.-Flood-prone area, Harrisburg, Pa. 

J6 
FIGURE 3.-Types of uses of flood plain, Harrisburg, Pa. 

J7 
FIGURE 4.-Flood-prone area, Spokane, Wash. 

J8 
I_ 

FIGURE 5.-Types of uses of flood plain, Spokane, Wash. 

J9 
FIGURE 6.-Flood-prone area, Phoenix, Ariz. 

JlO 
FIGURE 7 .- Types of uses of flood plain, Phoenix, Ariz. 

Jll 
TABLE 1.-Area and population of selected urbanized areas 

Urbanized areas 

Asheville, NC ----------------------------

Boise, ID -------------------------------- Boston, ~A ------------------------------ Charleston, SC --------------------------

Chicago, IL ------------------------------

Dallas, TX ------------------------------ Denver, CO ------------------------------ Fargo-~oorhead, ND-~N _______________ _ 

Great Falls, ~T -------------------------- Harrisburg, PA -------------------------- Lansing, ~I ------------------------------

Lincoln, NB ------------------------------ Lorain-Elyria, OH ----------------------

~onroe, LA ------------------------------ Norfolk-Portsmouth, VA _______________ _ Omaha-Council Bluffs, NB---IA ___________ _ 

Phoenix, AZ ------------------------------ Portland, OR ----------------------------

Reno, NV -------------------------------- Richmond, VA----------------------------

St. Louis, ~O-IL ------------------------ Salt Lake City, UT _______________________ _ 

San Jose, CA ---------------------------- Spokane, WA ---------------------------- Tallahassee, FL -------------------------- Texarkana, TX-AR ----------------------

Total ------------------------------

Population 

(1000's) 

72.5 

85.2 

2,652.6 

228.4 

6,714.6 

1,338.7 

1,047.3 

85.5 

70.9 

240.8 

229.5 

153.4 

192.3 

90.6 

668.3 

491.8 

863.4 

824.9 

99.7 

416.6 

1,882.0 

479.4 

1,025.3 

229.6 

77.9 

58.6 

20,359.4 

TABLE 2.-Areas of selected urban flood plains 

Urbanized area 

Asheville, NC ------------------------

Boise, ID ---------------------------- Boston, ~A-------------------------- Charleston, SC _____________________ _ Chicago, IL _________________________ _ 

Dallas, TX --------------------------

Denver, CO-------------------------- Fargo-~oorhead, ND-~N _________ _ Great Falls, ~T _____________________ _ 

Harrisburg, PA ---------------------- Lansing, ~I ------------------------ Lincoln, NB _________________________ _ 

Lorain-Elyria, OH _________________ _ ~onroe, LA _________________________ _ 

Norfolk-Portsmouth, VA ___________ _ 

Omaha-Council Bluffs, NB-IA _____ _ 

Phoenix, AZ ------------------------

Portland, OR ------------------------

Reno, NV----------------------------

Richmond, VA ---------------------- St. Louis, ~O-IL ___________________ _ 

Salt Lake City, UT _________________ _ 

San Jose, CA ------------------------ Spokane, WA ---------------'--------- Tallahassee, FL ---------------------- Texarkana, TX-AR ------------------ Total _________________________ _ 

Weighted average _____________ _ 

Flood plain 

Area 

(sq.mi.) 

1.6 

2.5 

62.4 

39.8 

131.8 

146.1 

30.6 

9.4 

2.0 

9.7 

4.8 

13.8 

5.3 

32.5 

59.2 

50.6 

71.2 

14.5 

2.0 

12.9 

136.1 

12.9 

80.0 

1.9 

3.1 

4.7 

941.4 

J12 

Percent of urbanized area 

4.4 

8.5 

9.4 

40.1 

10.3 

21.7 

10.5 

40.0 

9.2 

12.4 

6.5 

26.5 

5.0 

81.0 

19.8 

33.5 

18.4 

5.4 

5.3 

8.9 

29.6 

7.0 

28.8 

2.4 

10.4 

13.8 

16.2 

Land area P~ulation ensity 

(sq.mi.) (1,000/sq.mi.) 

38.2 1.90 

29.4 2.90 

664.4 3.90 

99.2 2.30 

1,277.2 5.26 

674.2 1.99 

292.8 3.58 

23.5 3.64 

21.8 3.25 

78.4 3.10 

73.4 3.13 

52.1 2.95 

106.4 1.81 

40.1 2.26 

299.0 2.24 

151.2 3.25 

387.5 2.21 

266.8 3.10 

37.5 2.66 ~144.6 2.98 

460.6 4.19 

184.3 2.60 

277.2 3.70 

77.8 2.95 

29.8 2.61 

30.8 1.90 

5,831.7 

Developed 

Area 

(sq.mi.) 

1.0 

2.1 

11:9 

21.2 

75.1 

28.0 

19.1 

5.1 

1.9 

8.1 

.9 

6.9 

.6 

26.8 

15.5 

23.1 

63.5 

8.5 

.9 

1.7 

91.7 

10.1 

67.9 

.9 

2.6 

2.1 

497.2 

Percent of floodplain total 

65.0 

84.0 

19.1 

53.3 

57.0 

19.2 

62.2 

54.3 

97.0 

83.5 

18.8 

49.6 

11.3 

82.4 

26.2 

45.5 

89.2 

58.7 

45.0 

13.2 

67.4 

78.3 

84.7 

47.4 

83.9 

44.2 

52.8 
TABLE 3.~elected indices related to extent of flood plains in selected urbanized areas 

Urbanized area 

Asheville, NC -------------------------- Boise, ID -------------------------------- Boston, MA ---------------------------- Charleston, SC -------------------------- Chicago, IL _____________________________ _ 

Dallas, TX ------------------------------ Denver, CO ---------------------------- Fargo-Moorhead, Nl}-MN _____________ _ 

Great Falls, MT --------·---------------- Harrisburg, PA --------------------------

Lansing, MI ---------------------------- Lincoln, NB ---------------------------- Lorain-Elyria, OH ----------------------

Monroe, LA ---------------------------- Norfolk-Portsmouth, VA _______________ _ Omaha-Council Bluffs, NB-IA _________ _ 

Phoenix, AZ ----------------------------

Portland, OR----------------------------

Reno, NV ------------------------------ Richmond, VA -------------------------- St. Louis, MO-IL _______________________ _ Salt Lake City, UT _____________________ _ 

San Jose, CA ---------------------- _____ _ 

Spokane, WA ---------------------------- Tallahassee, FL _________________________ _ 

Texarkana, TX-AR ----------------------

They have been listed to facilitate some crude 

analyses. The indices were derived as follows. The 

index of flooding, to represent possible depth of 

inundation as a measure of extent offlooding, was 

computed as the difference (in feet) in elevation of 

low water and the 100-year flood elevation for the 

major stream in an urbanized area. Data used for 

computation were records of tl1e U.S. Geological 

Survey streamflow-measuring station on the 

major stream located in or nearest to the major 

downtown area. For regulated streams, the index 

was computed on the basis of the elevation of the 

unregulated 100-year flood. The index of precipitation is the average annual precipitation in inches at the principal first-order weather station in 

the area. The index of elevation is the elevation in 

feet of the U.S. Weather Service precipitation gage 

used as the index of precipitation. 

The data in tables 1, 2, and 3 were analyzed by a 

step-forward regression analysis using both the 

percent of flood plain and the percent of development as dependent variables in correlation with 

the total land area, the population density, and the 

indices of flooding, precipitation, and elevation. 

Index of flooding 

24 

10 

5 

14 

19 

31 

12 

25 

11 

26 

16 

25 

21 

38 

8 

31 

20 

30 

15 

28 

50 

10 

17 

13 

10 

18 

Index of precipitation 

38 

11 

43 

37 

33 

35 

15 

18 

14 

38 

31 

27 

35 

51 

45 

28 

7.2 

37 

7 

44 

36 

14 

14 

17 

57 

48 

Index of elevation 

2,242 

2,838 

15 

9 

658 

481 

5,332 

896 

3,664 

338 

841 

1,150 

777 

78 

22 

1,323 

1,117 

21 

4,404 

164 

555 

4,220 

95 

2,349 

55 

390 

There is no significant correlation between the 

size of the urbanized area and the percent of the 

land in flood plain. Nor is there any significant 

correlation between the population density of the 

urbanized area and the percent of occupancy of 

those flood plains. Of the variables considered in 

this study, only one relationship seems to be appreciable: the percent of urbanized flood-plain related to the index of flooding. That relationship 

could be considered: 

PFP = 1.85 + 0.80 FI 

where PFP is the percent of urbanized area in flood 

plain, and FI is the index of flooding. However, the 

low coefficient of correlation 0.49 explains only 24 

percent of the variance. It is based on two extreme 

values-those for Monroe and St. Louis-and 

without those values the relationship would not be 

significant. The relationship is shown graphically 

in figure 8. 

This minimal consideration of a few factors or 

indices indicates there probably is no one factor 

that explains the inclusion of flood plains in urban 

areas or the degree of development of those flood 

plains. However, further studies may develop imJ13 
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ll.. 

80 

70 

60 

50 

40 

30 

20 

10 

0 

land-use pattern of the area. Two examples are 

worth citing. 

At St. Louis, considerable flood-plain area lies 

outside the boundary of the urbanized area but is 

abutted on one or both sides by the urbanized area. 

Determining the area of such flood plains is not 

easy because of the difficulty in associating the 

plains areas with the urbanized area, but an estimate of 5 square miles of developed flood plain and 

60 square miles of undeveloped flood plain is 

reasonable. Inclusion of this area brings the total 

flood-plain area to 38 percent instead of30 percent. 

At Portland, major parts of the Columbia River 

flood plain are not included in the urbanized area. 

Areas outside the urbanized area include the Portland Air Force Base, the Portland International 

Airport, and other commercial developments. Although these areas were not determined accu0 10 20 30 40 50 60 70. 

FLOOD INDEX (FI) 

80 rately because of the difficulty in defining limits, 

as much as 30 square miles of flood plain could be 

considered in this category. This would triple the 

proportion of flood plain, increasing the flood plain 

from 5.4 to 16.7 percent. 

FIGURE B.-Correlation between percent of urbanized area in 

flood plain and index of depth of flooding. 

portant relationships for other combinations of 

factors. 

DISCUSSION 

The analyses presented in this report must be 

evaluated in terms of a dichotomic situation. The 

measurements of the flood plains and their occupancy are precise in terms of the definitions accepted for this study. The flood plains are those of 

the 100-year flood; the urbanized areas are those 

defined by the U.S. Bureau of the Census; the 

occupancy of the flood plains is defined by reasonably precise criteria. These measurements were the 

primary purpose of this study: to determine 

through a statistical sampling of the urbanized 

areas the extent and occupancy of their flood 

plains. The range of data indicates no consistency 

in amounts. 

At least part of the inconsistency is related to 

the arbitrary boundaries of the urbanized areas. In 

many cities, flood-plain areas have been excluded 

from the urbanized areas because of the particular 

It should again be pointed out that the flood 

plains designated for this study are those for the 

natural100-year flood. Flood-protection measures 

in many areas have reduced the areas susceptible 

to flooding. 

Only limited attempts were made to relate the 

extent and occupancy of the measured flood plains 

to associated factors. Again, it must be emphasized 

that the original scope of the effort-to delineate 

the flood plains-precluded any major research 

effort in describing causative factors. The reconnaissance relationships explored here are not intended as definitive attempts, but rather as 

stimuli to further detailed efforts to understand 

the natural physiographic setting and the 

economic and social stimuli that control occupancy 

of flood plains. 

REFERENCE 

Goddard, J. E., 1974, An evaluation of urban flood plains: Am. 

Soc. Civil Engineers, Tech. Memo. 19, 40 p. (Also available 

from the Natl. Tech. Inf. Service, U.S. Dept. Commerce, 

Springfield, Va. 22151, NTIS PB-227337.) 

*U.S. GOVERNMENT PRINTING OFFICE: 1974-543-585/138 

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