GEOLOGICAL SURVEY 

Cl RCULAR 601 - B 

Urban Sprawl and 

Flooding in 

Southern California 

URBAN SPRAWL AND FLOODING 

IN SOUTHERN CALIFORNIA 
I I 

Floodwater rushing through doorway of home in Mandeville Canyon , a Los Angeles suburb. Photograph courtesy 

of Los Angeles Times. 
Urban Sprawl and 

Flooding in 

Southern California 

By 5. E. Rantz 

WATER IN THE URBAN ENVIRONMENT 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 601-e 

Washington 19 70 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

First printing 1970 

Second printing 1970 

Third printing 1974 

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 

tloods, 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. 

6 z ~J?fi,l~£__ / 

E. L. Hendricks, 

Chief Hydrologist 

v 

Foreword 

Abstract 

CONTENTS 

................................................................... 

.................................................................... 

Introduction ................................................................ . 

The flood problem in southern California ......................................... . 

Flood-control facilities in southern California ...................................... . 

Description of the southern California storms of January 18-26, 1969 ................... . 

Description of the southern California floods of January 18-26, 1969 ................... . 

Effect of flood-control measures on flood damage ................................... . 

Effect of urban sprawl on flood damage ........................................... . 

Summary and conclusions ..................................................... . 

ILLUSTRATIONS 

Frontispiece. Photograph showing floodwater rushing through doorway of home in Mandeville 

Canyon, a Los Angeles suburb. 

Page 

v 

Bl 

1 

1 

3 

5 

5 

8 

10 

11 

Page 

Figure 1 . Map showing report area . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . .. . . . . . . . . . . .. . . .. . . .. . . . B2 

2-6. Photographs showing: 

2. Result of mudflow in Glendora, January 22............................................................. 6 

3. Result of landslide (mudslide) in Glendora, January 26 ........................................... 7 

4. Street flooding in El Segundo................................................................................... 7 

5. Floodwater flowing down Casiano Road in Bel-Air.................................................. 7 

6. Inflow and outflow hydrographs of Santa Ana River at 

Prado flood-control basin....................................................................................... 9 

TABLES 

Page 

Table 1. Summary of precipitation data at selected stations........................................................ B5 

2. Peak discharges at selected stations for the floods 

of March 1938 and January 1969.................................................................................. 8 

3. Estimated physical damage in the report area in March 

1938 and January 1969................................................................................................. 9 

VII 

Water 1n the Urban Environment 

Urban Sprawl and Flooding in Southern Califc rnia 

By 5. E. Rantz 

ABSTRACT 

The floods of January 1969 in south-coastal 

California provide a timely example of the effect of 

urban sprawal on flood damage. Despite recordbreaking, 

or near recordbreaking, stream discharges, damage was 

minimal in the otder developed areas that are protected 

against inundation and debris damage by carefully 

planned t1ood-control facilities, including debris basins 

and flood-conveyance channels. By contrast, heavy 

damage occurred in areas of more recent urban sprawl 

where the hazards of inundation and debris or landslide 

damage have not been taken into consideration, and 

where the improvement and development of drainage or 

flood-control facilities have not kept pace with 

expanding urbanization. 

INTRODUCTION 

Intense storms during the period January 

18-February 25, 1969, produced 

recordbreaking, or near recordbreaking floods in 

southern California in late January and again in 

late February. Particularly hard hit were the 

urbanized areas in the coastal region between 

the Santa Ana and Santa Ynez Rivers (fig. 1 ). 

The loss of life and physical damage attributable 

to the storms and ensuing floods in the report 

area- 92 lives lost and an estimated physical 

damage of $62 million in January alone-have 

focused attention on the effect of urban sprawl 

on flood damage. Urban sprawl is defined here 

as the rapid expansion of suburban development 

without complete planning for the optimum 

control and development of water and 

associated land resources. The greatest damage 

during the storms was sustained in areas of 

urban sprawl where the hazards of inundation 

and debris or landslide damage have not been 

taken into consideration, and where the 

improvement and development of drainage or 

flood-control facilities have not kept pace with 

expanded urbanization. 

Because the floods in south-coastal California 

provide a timely example of the effect of urban 

sprawl, a brief case history of that region is 

presented here. The discussions are related to 

the floods of January 1969: the flo':lds of 

February 1969 were somewhat similar with 

respect to both the magnitude of discharge and 

the areal pattern of physical damage. 

THE FLOOD PROBLEM IN 

SOUTHERN CALIFORNIA 

To better understand the flood prot 'em in 

south-coastal California. one should be familiar 

with the characteristics of the stream cl'annels 

and the tloodflows they carry. The upper reach 

of a typical stream channel occupies the floor of 

a steep mountain canyon. At the mouth of the 

canyon near the base of the mountain is a 

well-developed alluvial cone, a cone b~ing a 

built-up slope of water-transported rock debris 

that extends from the canyon mouth down to 

the alluvial valley floor. Intense mountain 

storms cause swift sediment-laden streamflow in 

the canyon. When the streamflow react~s the 

flatter alluvial cone its velocity is reduced and 

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the larger water-transported rock debris is 

deposited on the cone. The discharge- of the 

stream is also reduced in the course of its travel 

down the cone as water seeps into the permeable' 

bed material. From the base of the cone the 

stream crosses the valley floor to empty into the 

ocean. When southern California was first 

settled, channels on the cones and in the alluvial 

valleys were often poorly defined and 

semipermanent, and during periods of flood, the 

waters spread over wide flood plains. 

The rapid growth of California changed 

conditions. On thousands of acres of alluvial 

cones, old river bottoms, and flood plains, where 

tloodtlows once ran relatively unconfined, there 

are now homes, orchards, highways, railroads, 

towns, and cities. The development has created a 

complicated, if not unique, flood problem. 

Flood hazards now are not confined to water 

runoff alone but include debris that is brought 

down the steep mountain slopes and is deposited 

on streets and developed property. To some 

extent the problem has been aggravated by 

increased runoff and erosion in n1ountain areas 

where erodible soils have been laid bare as a 

result of forest and brush fires of both natural 

and manmade origin. 

A permanent solution to the problem is a 

difficult one. Geologically speaking, the 

mountains are young and break down rapidly 

under the combined action of the elements. 

Reservoirs and debris basins fill with sediment 

and rapidly lose their useful capacity. Storm 

rain fall is often excessive and even such 

tremendous floods as those of January 1916, 

January 1934, March 1938, and January 1969 

may not be indicative of the maximum flood 

potential. For exmnple, the storm of January 

21-23, 1943, was the most intense of record in 

much of the region from a standpoint of 

flood-producing potential. However, flooding 

was not disastrous in 1943 because the rains fell 

on dry ground that absorbed much of the 

rainfall. There is no doubt that despite the 

application of our most advanced hydrologic 

principles and flood-control practices, an 

element of risk still exists for those whc occupy 

natural flood channels, flood plains, and alluvial 

cones. However, without application of those 

principles and practices, such occupancies are 

virtually an invitation to future disaster. 

Social and economic considerations 

compound the flood problems associated with 

the physical environment. The region has been 

growing at a tremendous rate. For example, in 

1935 the population of greater Los Angeles was 

about 2.5 million~ it is now about 7 million and 

is still increasing. The pressure to provide 

building sites to house the increasing population 

and attendant industrial and commercial activity 

places a great burden on city and county 

planners. Furthermore, no direct central 

planning authority exists for the heavily 

urbanized areas because local jurisdiction over 

planning resides in the many incorporated 

communities. The local planning, although often 

based on regional concepts, is not uniform in 

quality. The number of municipalities has also 

been increasing over the years. Using the Los 

Angeles area again as an example, the number of 

incorporated areas in Los Angeles County 

increased by 75 percent between 1935 and the 

present ( 1969) from 44 to 78. Many of these 

municipalities are contiguous and form a 

continuous urban sprawl. The difficulty of 

planning on a regional scale to provide a logical 

pattern of zoning to protect residents from the 

hazards associated with storm and flood is 

apparent. 

FLOOD-CONTROL FACILITIES 

IN SOUTHERN CALIFORNIA 

Local organizations, cities, countir-s, the 

State, and the Federal Government have been, 

and are, attempting to remedy the hazardous 

flood situation through the following measures: 

I . The construction of reservoirs designed to 

store as much of the flood waters as possible. 

2. The construction of debris basins to catch 

debris and sediment that otherwise would be 

deposited on the alluvial cones and valley floors. 

B3 
3. The diversion, wherever possible, of 

sediment-laden waters onto areas where 

sediment can be deposited and excess water can 

percolate into the ground-water reservoir. 

4. The realinement enlarging, and paving of 

permanent channels to convey excess runoff to 

the ocean. 

5. Such measures are possible in the 

mountain areas to retard erosion and surface 

runoff through the prevention of forest fires, the 

reseeding of areas denuded by fire, the 

regulation of land use, and the stabilization of 

channels. 

Flood control on a major scale in southern 

California had its inception in 1915 with the 

organization of the Los Angeles County Flood 

Control District. That district, organized under 

the laws of the State, had as its objective the 

control of the waters of the Los Angeles and San 

Gabriel Rivers. the Rio Hondo, Ballona Creek, 

and n1ajor tributaries and washes of Los Angeles 

County. This task was much later described by 

an eminent en gi ncer as being ''well-nigh 

impossible of complete solution on the one hand 

and a necessity on the other." Impetus to the 

tlood-control activities in the region followed 

the passage of several Federal tlood-control acts, 

the first of which was approved in 1936. Those 

acts provided the U.S. Army Corps of Engineers 

with the authority and funds for the 

construction of flood-control basins, debris 

basins, and several hundred miles of 

levee-and-channel improvement. 

In Los Angeles County, the Corps of 

Engineers and the Los Angeles County Flood 

Control District have constructed all 20 of the 

tlood-control reservoirs and 61 of the I 06 debris 

basins planned for the county. They have also 

improved more than 3 50 miles of stream 

channel for the conveyance of tloodwaters. The 

improvement is usually made by fully lining a 

reach of channel with reinforced concrete or by 

constructing levees faced with concrete or 

rip-rap. The improvement of an additional 275 

miks of channel is planned in the county. The 

storage capacities of the t1ood-control reservoirs 

are maintained. as much as possible, by sluicing 

debris through them during periods of low tlow. 

84 

but periodically some mw't be cleaned 

mechanically. The debris basins. too, must be 

cleaned mechanically; the material must be 

removed and then must be transrortcd by truck 

to disposal areas. That process becomes 

increasingly costly as nearby disposal areas are 

filled and longer trucking hauls are required to 

reach new disposal areas. 

Another important function of the Los 

Angeles County Flood Control District is the 

construction and maintenance of a supplemental 

system of storm drains in the urbanized areas of 

the county to collect storm waters close to their 

source and carry them to the main system of 

tlood-conveyance channels. About 950 miles of 

storm drain have been built at a cost of about 

$700 million, and it is estimated that $1 billion 

more will be needed to provide the additional 

625 miles of storm drain that aF~ planned. The 

eventual total cost of the en tire supplemental 

storm-drain system will far exceod that of the 

main system of flood-conveyance channels. 

East of Los Angeles County, in the Slnta Ana 

River basin. the Corps of Engineers has 

undertaken the protection of the urban areas of 

Orange, San Bernardino, and Riverside Countie-s. 

They have constructed five of eight authorized 

flood-control reservoirs and many miles of flood 

channels, such as those on Lytle Creek and 

Cajon Creek. The flood-control program of the 

Corps of Engineers for the Santa Ana River 

basin has been augmented by the work of 

flood-control districts in the three counties 

involved. These organizations cooperate in 

integrating the flood-control programs, in 

consolidating local levee and storm-drainage 

districts, and in building storm drains and flood 

channels. 

West of Los Angeles County, the Ventura 

County Flood Control District ras constructed 

Matilija Reservoir, and along the Ventura River 

downstream from that reservoir the Corps of 

Engineers has buil! 13.000 feet of levees. On the 

Santa Y nez River in Santa Barbara County the 

U.S. Bureau of Reclamation has built Cachuma 

Reservoir downstream from the older J.uucal and 

Gibraltar Reservoirs. 
The U.S. Department of Agriculture is 

another agency active in southern California. 

They operate under a directive to implement a 

program of ''runoff and water-flow retardation . and soil-erosion prevention.'' Funds have been 

used by the Department for the following 

purposes: 

1. Fire control, including the construction 

and improvement of fire-truck roads. 

2. Mountain channel improvements, 

including debris barriers. 

3. Farmland improvements. 

4. Stabilization of highway fills in the 

mountain areas. 

5. Debris basins. 

6. Improvement of vegetative cover. 

Many reservoirs in the region have been 

constructed primarily for water supply by 

public, private, and mutual enterprises. Those 

reservoirs have also proven effective for flood 

control within the limits of the storage space 

they have available at the time a flood starts. 

DESCRIPTION OF THE SOUTHERN 

CALIFORNIA STORMS OF 

JANUARY 18-26, 1969 

Southern California. until January 12, 1969, 

was experiencing an unseasonal winter drought, 

and the moderate precipitation that occurred on 

January 13--14 gave little indication that the 

drought was soon to be broken. The series of 

storms that was to plague the region did not 

begin until the evening of January 17. 

Precipitation was relatively light until January 

1 9 when intensities increased sharply. The 

center of the eastward-moving low-pressure area 

that generated the storms stagnated about 700 

miles off the coast on January 21 , with the 

result that a succession of storm waves passed 

over southern California. Except for a lull on 

January 22, heavy precipitation occurred during 

most of the period January 19-26, and this was 

climaxed by the intense downpour of January 

25. During most of the storm period the freezing 

level was at an altitude 7,000 feet; precipitation 

occurred as rain below that altitude and as snow 

at the higher altitudes. Table 1 summarizes 

precipitation data for selected stations 

throughout the area. Some of the storm totals 

exceed the heaviest January precipitation 

previously recorded at the stations. The wide 

range of precipitation values in the table reflects 

the general decrease of precipitation with 

distance from the storm center and the local 

increase of precipitation with altitude. 

DESCRIPTION OF THE SOUTHERN 

CALIFORNIA FLOODS OF 

JANUARY 18-26, 1969 

Heavy rains during the 4-day period January 

18-21 brought widespread, but generally minor, 

damage to southern California. Damage was 

severe, however, in localized areas. Streams rose 

but even those that were uncontrolled by 

reservoirs generally stayed within their banks 

and flooding was localized. Ten campers, 

Table I.-Summary of precipitation data at selected stations 

Precipitation station Precipitation, in inches, for dates shown 

Name Altitude Jan.25 Jan. 25-26 

I j Entire 

(feet) Jan. 18-21 I Jan. 23-261 storm, I Jan. 18-26 

Big Bear Dam _______ 6,800 11.05 18.59 13.60 21.82 35.42 

San Bernardino ___ 1.100 2.53 4.99 3.35 6.08 9.43 

Mount Baldy _________ 4.280 11.04 21.42 20.08 27.54 47.62 

Glendora West_ ___________ 820 6.00 7.35 8.28 9.22 17.50 

Opids Camp ___________ 4,250 15.56 21.1 1 21.01 24.16 45.17 

Burbank ______________ 680 4.43 5.98 7.10 7.16 14.26 

Topanga ___________________ 745 9.54 12.18 12.98 14.20 27.18 

Matilija Dam ____________ 1,050 9.15 14.76 20.84 17. I 7 38.01 

B5 
however, lost their lives when trapped by the 

rising waters of Sespe Creek in Ventura County, 

and four other drownings were reported 

elsewhere in the region. Transportation was 

snarled as floodwaters in Cajon Canyon cut the 

main east-west lines of the Southern Pacific and 

Santa Fe Railroads, and more than 100 Los 

Angeles streets were blocked by felled trees. The 

greatest monetary damage occurred in the 

Glendora-Azusa foothill area of the San Gabriel 

Mountains. A brush fire in August 1968 had 

burned off erosion-retarding vegetation in the 

canyon area above the town of Glendora. The 4 

days of rain on the bare soil were climaxed by a 

local precipitation burst of 2 inches in 3 hours, 

which brought down a torrent of sediment- and 

debris-laden water. Streets were boulder-strewn 

and sediment spread over streets and lawns and 

into homes (fig. 2). In places the streets were 

covered with sediment to depths of as much as 4 

feet. Damage in Glendora was estimated at $2 

million. 

It was with a feeling of relief, therefore, that 

southern Californians viewed the rainless skies of 

January 22. The relief was short-lived, however. 

The rains of the preceding 4 days had saturated 

the ground and had produced a condition 

favorable for heavy runoff from ensuing rains; 

the stage was set for the deluge that was to 

follow. The rains started again on January 23 as 

the second 4-day phase of the storm arrived. Not 

only were antecedent conditions favorable for 

Figure 2.-Result of mudflow in Glendora, January 

22. Photograph courtesy of Los Angeles Times. 

B6 

heavy runoff, but the time distribution of 

precipitation during this phase was conducive to 

high peak discharges, because the heaviest 

precipitation came near the end of the storm 

when streams were already swollen. 

In Los Angeles County, flooding was confined 

primarily to the headwater tributaries of the 

principal streams-the San Gabriel and Los 

Angeles Rivers-and to the smaller canyon 

streams that are directly tributary to the ocean. 

The heavy runoff of the San Gabriel and Los 

Angeles Rivers was stored in flood-control 

reservoirs and released at rates compatible with 

the capacity of downstream flood channels. In 

the mountain and foothill areas, however, the 

rapidly rising tributaries of those two rivers left 

their banks and created havoc. Bridges, roads, 

and streets were washed out and homes were 

destroyed or damaged; thousands of persons 

were evacuated. The sediment and debris carried 

by the streams added to the misery. Debris flows 

occurred again at Glendora to add to the damage 

suffered in the earlier phase of the storm. 

Damage of a similar nature, but not as severe, 

occurred in Highland Park, Sherman Oaks, 

Verdugo Hills, Brentwood, Bel-Air, Hollywood 

Hills, Encino, and Glendale. Landslides, more 

aptly termed mudslides, also created much 

damage in those towns and buried seven persons 

alive in their beds when the slides entered their 

homes. Topanga Creek, a small stream directly 

tributary to the ocean, swelled to the size of a 

river. Almost I ,000 persons in Topanga Canyon 

were isolated when homes and roads were 

destroyed by water and mudslides, and three 

persons were smothered when mud swept 

through their home. On the coastal plain, street 

flooding occurred in many towns such as 

Manhattan Beach, El Segundo, and Long Beach. 

(See figures 3, 4, and 5.) 

East of Los Angeles County, in the Santa Ana 

River basin, major flooding was confined to the 

upper reaches of the river and its mountain 

tributaries. The rampaging waters of such creeks 

as Cucamonga, Deer, Day, and Cajon damaged 

or destroyed by erosion an aggregate of several 

hundred miles of improved flood channels. 

Roads and bridges were washed out and the 

main lines of the Southern Pacific and Santa Fe 
Railroads, which had received emergency repair 

after being cut during the first phase of the 

storm, were severely damaged as a result of 

repeated attack by the waters of Cajon Creek. 

Nor were homes spared by the deluge; in the 

town of Cucamonga alone, flooding caused the 

evacuation of I ,000 persons. Farther 

downstream, near the junction of the San 

Bernardino, Riverside, and Orange County lines, 

the Santa Ana River was effectively controlled 

by storage in the Prado flood-control basin, and 

released flows were within the capacity of the 

river channel downstream through the populous 

areas of Orange County. 

Figure 3.- Result of landslide (mudslide) in Glendora, 

January 26. Water has cut a channel through the 

mudslide. Photograph courtesy of Los Angeles 

Times. 

Figure 4.-Street flooding in El Segundo. Photograph 

courtesy of Police Department, city of El Segundo. 

Figure 5 .- - Floodwater flowing down Casiano Road in 

Bel-Air (suburban Los Angeles) jams parked car 

against fence and lamppost. Photograph courtesy of World-Wide Photos. 

In Ventura and Santa Barbara Counties, to 

the west of Los Angeles County, peak discharges 

were unprecedented and damage was 

correspondingly high. In the Santa 

Paula-Fillmore-Piru area, the Santa Clara River 

and Santa Paula Creek spilled over their banks 

and caused the evacuation of about 3,000 

persons. Evacuations were also necessary along 

the Ventura River and in the Ojai Valley where 

such communities as Live Oak, Oak View, and 

Meiners Oaks were hard hit. Highway damage 

was heavy throughout Ventura County. In Santa 

Barbara County all the small streams south of 

the Santa Ynez River were in extreme flood; the 

severest damage occurred in the towns of 

Montecito and Carpinteria. In both those towns, 

streams changed their courses and cut new 

channels through residential areas. On the Santa 

Ynez River, the spillways of Gibraltar and 

Cachuma Dams carried flows that equaled or 

slightly exceeded those for which they had been 

designed. There was no damage to the structures 

but damage was severe in the Solvang and 

Lompoc areas downstream. 

In general the flood of January 1969 was 

comparable to that of March 1938. The 1938 

flood had been the most damaging flood of 

recent times in southern California and its peak 

discharges are usually used as a standard for 

B7 
Table 2.--Peak discharges at selected stations for the floods of March 1938 and January 1969 

[Discharge figures for 1969 are provisional and therefore subject to revision] 

Stream-gaging station 

Drainage 

area 

(square 

miles) 

Peak discharge, in 

cubic feet per second 

March 19381 January 1969 

i l 

East of Los Angeles County 

East Twin Creek near Arrowhead Springs _____________ _ 8.8 

4.6 

172 

1,485 

3,360 

2,350 

21 ,500 

100,000 

2,300 

1,180 

14,500 

175 ,000 

Waterman Canyon Creek near Arrowhead Springs ________ _ 

Lytle Creek at Colton _____________________________ _ 

Santa Ana River at Prado Dam __________________________ _ 

Los Angeles County 

San Gabriel River near Azusa __________________________ _ 214 

158 

16.0 

832 

65 ,700 

12,000 

8,620 

99,000 

22,500 

48,000 

13,800 

8,540 

101,000 

15,600 

Los Angeles River at Sepulveda Dam ________________ _ 

Arroyo Seco near Pasadena _____________________ _ 

Los Angeles River at Long Beach ___________________ _ 

Ballona Creek at Sawtelle Blvd., near Culver City ____ _ I 11 

West of Los Angeles County 

Sespe Creek near Fillmore ____________________________ _ 251 

1,595 

188 

789 

56,000 

120,000 

39,200 

45 ,000 

60,000 

165,000 

55,000 

100,000 

Santa Clara River at Saticoy_____ ----------- Ventura River near Ventura ________________________ _ 

Santa Ynez River near Lompoc _____________ -'-----------

1 Inflow of 75 ,000 cfs was reduced by storage to outflow of 5,800 cfs. 

comparing flood magnitudes. West of Los 

Angeles County peak discharges were greater in 

1969, and east of Los Angeles County the 1938 

peak discharges were greater. In Los Angeles 

County itself, the relative magnitude of the two 

floods varied somewhat randomly, but more 

commonly the greater peak discharges were 

those that occurred in 1938. Table 2 lists peak 

flows for the two floods at selected stations. 

EFFECT OF FLOOD-CONTROL 

MEASURES ON FLOOD DAMAGE 

The various storage and conveyance facilities 

for flood control--reservoirs, debris basins, 

improved channels, and storm drains--operated 

effectively during the January flood, and the 

B8 

damage they prevented has been estimated at 

$ 1 . 2 b iII ion. The flows received by the 

flood-control reservoirs during the first phase of 

the storm, January 18-21 , were released 

quickly , and consequently the reservoirs were 

essentially either empty or at conservation-pool 

level when the critical second phase of the storm 

began on January 23 . During this second phase, 

none of the large reservoirs that were built 

primarily for flood control by the Corps of 

Engineers were completely filled by the heavy 

runoff. The other reservoirs in the area have an 

important water supply and conservation 

function, and although most of them filled 

during the storm, many were effective in 

reducing the magnitude of peak flows 

downstream from the reservoirs. The discharge 
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JANUARY 1969 

27 

Figure 6.-lnflow and outflow hydrographs of Santa 

Ana River at Prado flood-control basin. 

hydrographs in figure 6 show the effect of the 

Prado flood-control basin on the floodflows of 

the Santa Ana River: the peak inflow to the 

reservoir of 7 5,000 cubic feet per second was 

reduced by storage to such degree that the pe~k 

outflow was only 5,800 cubic feet per second. 

The debris basins in Los Angeles County 

trapped an estimated 2 million cubic yards of 

debris. Of the 61 debris basins only seven were 

completely filled, and only three of those had 

debris pass over their spillways into downstream 

drains. The improved channels, in general, 

contained the floodflows within their banks or 

levees; the only serious flooding associated with 

such channels occurred on the streams tributary 

to the upper Santa Ana River. Most of the 

storm-drain system in Los Angeles County was 

not overtaxed; where flooding associated with 

drains occurred, it was primarily due to the 

drains becoming filled with sediment and debris. 

The system of storage and conveyance works 

for flood control in Los Angeles County 

prevented damage estimated at $900 million: the 

Prado flood-control basin prevented damage in 

Orange County estimated at $260 million: 

improved channels throughout the remainder of 

the report area prevented damage estin1ated at 

$40 million. 

Although $ 1. 2 billion of damage was 

prevented, losses were still heavy. The death toll 

for the storm and flood of January I96S was 92 

persons, and I 0,000 persons were driven from 

their homes. Of the 92 that died, 19 drowned, 

12 were buried alive in mud and debris. 55 were 

killed in storm-associated automobile accidents, 

four were killed in storm-associatec plane 

crashes. and two died of heart attacks brought 

on by physical exertion connected with the 

flood. Physical damage caused by the storm and 

flood is estimated at $62 million. (Physical 

damage refers to rehabilitation or replacement 

costs of structures or facilities that were 

damaged or destroyed. It does not include such 

emergency costs as those associaterl with 

evacuation or police work, nor does it include 

such indirect costs as loss of income due to 

interruption of commercial activities or 

depreciation of property values in areas that 

suffered damage.) Table 3 shows damage figures 

for urban and rural areas, and for comparison. 

Table 3.-Estimated physical damage, in millions of dollars. in the report area in March 1938 

and January 196 9 

Location 

East of Los Angeles County-----------------

Los Angeles County------------------------

West of Los Angeles County _________________ _ 

Total for report area _____________________ _ 

March 1938 

Urban I Rural I 

I I 3 

25 1 

3 ..., 

39 7 

Total 

I4 

27 

5 

46 

January 1969 

Urban I Rural I Total 

18 10 28 

14 15 

12 7 19 

44 18 62 

B9 
similar figures are given for the flood of March 

1938. It is to be expected that for floods of 

equivalent magnitude, monetary dan1age would 

be greater in 1969 than in 1938 because of the· 

greater degree of development in 1969 and the 

lower purchasing power of the dollar. The 

lesser damage in Los Angeles County in 1969 is 

conclusive evidence that flood-control measures 

taken since 1938 were effective in evidence that 

flood-control measures taken since 1938 were 

effective in protecting areas that have been 

occupied or fully developed for at least the last 

30 years. 

EFFECT OF URBAN SPRAWL 

ON FLOOD DAMAGE 

Damage in areas of urban sprawl is of two 

types. The first type is damage by inundation. 

Encroachment on natural flood plains, in the 

absence of facilities for the storage and 

conveyance of floodwaters, exposes homes and 

other structures to the damaging effect of 

rampaging streams carrying large sediment and 

debris loads. Urban development has encroached 

on alluvial cones, which in their virgin state are 

highly permeable. Urbanization almost 

waterproofs the ground surface, with the result 

that storm runoff from urbanized areas on the 

cones is increased in volume and intensity and is 

virtually dumped on lower-lying developed 

areas. 

The second type of damage in areas of urban 

sprawl is that which occurs as a result of 

landslides or mudflows during storm periods. 

The steep flanks of many mountains and 

foothills in south-coastal California are unstable 

when saturated by heavy rainfall and often slide, 

particularly under stresses associated with the 

construction of buildings and roads. It is not 

enough that one consult an experienced 

engineering geologist before building on or near 

such slopes, because a safely designed structure 

may be endangered by subsequent improper 

design and building practices by others in the 

vicinity. Appropriate studies on a broader scale 

should be made before development, and if 

adequate zoning and (or) building codes are not 

established, prospective developers and residents 

810 

should at least be made aware of the potential 

hazard. 

Urban development that takes place too close 

to the head of an alluvial cone, in the absence of 

an upstream debris basin, exposes structures to 

mudflows. A mudflow is a viscous mixture of 

water, sediment. and rock debris that often has 

the consistency of wet concrete. 

Water-transported rock debris has been 

deposited on alluvial cones from time 

immemorial, and the cones, in fact, have been 

naturally built by that process. The distance the 

large material is carried downslor~ on a cone is, 

to a large degree, dependent on the amount of 

flowing water available to supply the energy for 

transport. Under natural conditions the cone is 

permeable and streamflow decreases in its course 

down the cone. When the streatrflow decreases 

sufficiently the coarser material comes to rest. 

Development that extends far up the cone 

provides an impermeable surface that prevents 

the seepage of large amounts of water, and the 

streamflow reaching the develop~d area will be 

undiminished as it travels downftream, or may 

even be augmented by local runoff from the 

impermeable area. When that occurs, the "mud" 

and rock debris that reaches the developed area 

will be carried downstream until the slope of the 

cone becomes too flat to support such transport, 

or until the water has spread far enough laterally 

to effectively reduce its ener~:y for further 

transport. The situation is often aggravated by 

brush fires in the mountains thrt consume the 

erosion-retarding vegetation and thereby 

increase the potential for damag~ by sediment 

and debris. During the 1969 flood, however, it 

was only in the Glendora-Azusa area that recent 

brush fires were responsible for significantly 

increased production of sediment and debris. 

That area received intense precipitation in 

January. In other areas where re~ent burns had 

occurred, as in the Little Tujunga basin, erosion 

rates were not significantly increased because 

precipitation intensities there were moderate in 

January. 

Because debris damage is usually accompanied 

by water damage, it is net possible to 

differentiate the monetary damage attributable 
to each in the flood of January 1969. However, 

with regard to deaths, 19 lives were lost in 

drownings and 12 lives were lost in landslides. 

The bulk of the urban damage in the flood of 

January 1969 occurred in areas of urban sprawl. 

In those areas, development plans and related 

zoning did not prevent the occupancy of land 

exposed to the hazards of inundation and (or) 

debris damage, and the improvement and 

development of drainage of flood-control 

facilities did not keep pace with expanded 

urbanization. Ideally, adequate zoning and 

drainage design should be incorporated in the 

planning stage of any new development. 

The narrow coastal plain in the region is now 

almost fully occupied and housing is moving 

into the upland areas in the alluvial cones and in 

the canyons. To a large degree the move to the 

uplands is for aesthetic reasons as people look 

for homes with a view. However, it is the upland 

areas that are hardest to protect because of the 

myriad small watercourses, each of which 

requires individual control measures for 

protection against inundation and debris 

damage. A high degree of protection now exists 

along the larger streams in the older established 

areas on the coastal plain. The greatest 

concentration of future effort will most likely 

be on the protection of urbanized areas in the 

uplands. 

SUMMARY AND CONCLUSIONf 

Urban sprawl is defined here as the rapid 

expansion of suburban development without 

complete planning for the optimum control and 

development of water and associate,d land 

resources. By that definition any suburban area 

is an area of urban sprawl if its zoning 

ordinances do not take into consideration the 

hazards of inundation and debris or landslide 

damage, or if its drainage and flood-control 

facilities are not fully developed. During the 

calamitous floods of January 1969 in 

south-coastal California, flood-control measures 

were effective in minimizing flood dan1ages in 

areas that they were designed to protect. It was 

in the areas of urban sprawl that the largest part 

of the total physical damage of $62 million 

occurred. 

The construction of additional 

flood-protection facilities will assure some 

reduction in the damage from future floods. 

However, the problems associated with 

floodtlows and storm-induced mudslides can 

never be completely solved; only an 

accomodation to those problems can be 

achieved. The most effective accomodation is 

that based on adherence to a truly 

comprehensive regional plan. 

Bll 

*u.s. GOVERNMENT PRINTING OFFICE: 1974 O-SB5-46S/2E'