2d Session
No. 452







June 28, 1954.-- Referred to the Committee on Public Works and ordered to be printed with two illustrations


Washington 25, D.C., June 11, 1954.

DEAR MR. SPEAKER: I am transmitting herewith a report dated January 21, 1954, from the Chief of Engmeers, Department of the Army, together with accompanying papers and illustrations, on a review of report on San Lorenzo Creek, Calif., requested oy the Committee on Flood Control, House of Representatives, adopted on May 14, 1945.

In accordance with section 1 of Public Law 534, 78th Congress, the views of the State of California and the Department of the Interior are enclosed with the reply of the Chief of Engineers to State of California. The comments of the Department of Agriculture are also enclosed.

Although the Bureau of the Budget advises that there is no objection to the submission of the report to the Congress, it states that no commitment can be made at this time as to when any estimate of appropriation would be submitted for construction of the project, if authorized by the Congress, since this would be governed by the President's budgetary objectives, as determined by the then prevailing fiscal situation. The complete views of the Bureau of the Budget are contained in the attached copy of its letter.

Sincerely yours,

Robert T. Stevens,
Secretary of the Army








The district engineer finds that future floods on the highly developed flood plain of San Lorenzo Creek may be expected to cause damages averaging $180,500 annually. He finds that a relatively low degree of flood control would be provided by possible multiple-purpose or flood-control dams in the basin, that the benefits accruing from such projects would not justify the expenditures required, and that participation by the United States in the construction of such reservoirs is therefore not justified. He finds, however, that improvement of San Lorenzo Creek channel through its flood plain would produce average annual benefits of $193,200 as compared to average annual costs of $165,700. In view of a resultant ratio of benefits to costs of 1.17 and the need for improvements to alleviate the serious flood problem in this area, the district engineer recommends the adoption of a project for San Lorenzo Creek consisting of a riprapped and leveed channel from San Francisco Bay to a point 1,600 feet below Lorenzo Avenue, a concrete-lined channel from that point to Foothill Boulevard, and channel clearing, sacked concrete bank revetment, and channel bottom stabilizing sills between Foothill Boulevard and B Street, all at an estimated Federal first cost of $3,644,000 and an estimated non-Federal first cost of $631,000, or a total first cost of $4,275,000, The estimated cost of maintenance, operation, and replacements, which would be a responsibility of local interests, is estimated at $14,500 annually.

The district engineer further recommends that the construction of the proposed improvements be subject to such reasonable modifications as, in the discretion of the Chief of Engineers, may be advisable at the time of construction, and subject also to the condition that local interests give assurance satisfactory to the Secretary of the Army that they will --

(a) Provide without cost to the United States the lands, easements, and rights-of-way required for construction of the recommended improvements, now estimated to cost $264,000.

(b) Replace the existing vehicular bridges at Lorenzo Avenue, Hesperian Boulevard, and East 14th Street and remove a private vehicular bridge below Lorenzo Avenue, now estimated to cost $269,000, all in accordance with plans approved by the Chief of Engineers.

(c) Make minor alterations and relocations of utilities, now estimated to cost $2,000, in accordance with the plans approved by the Chief of Engineers.

(d) Contribute in cash an amount now estimated at $96,000, or in lieu thereof, perform additional construction for the project equivalent to the amount of the cash contribution, in accordance with plans approved by the Chief of Engineers.

(e) Maintain and operate the project after completion in accordance with regulations to be prescribed by the Secretary of the Army, estimated to cost $14,500 annually, including replacements; hold and save the United States free from damages due to the construction of the project; and prevent encroachment on the project channels which might interfere with their proper functioning for flood control.

San Francisco, Calif., February 18, 1953.

Subject: Review of Report on San Lorenzo Creek, Alameda County, Calif., for Flood Control and Allied Purposes.
Through: Division Engineer, South Pacific Division, Corps of Engineers, United States Army, San Francisco, Calif.
To: Chief of Engineers, Department of the Army, Washington, D. C.

Scope of Survey
Prior Reports
Economic Development
Runoff and Streamflow Data
Floods of Record
Standard Project Flood
Extent and Character of Flooded Area
Flood Damages
Existing Improvements
Improvements Desired
Flood Problems and Solutions Considered
Flood Control Plan
Estimates of First Cost
Estimates of Annual Charges
Estimates of Benefits
Comparison of Benefits and Costs
Proposed Local Cooperation
Allocation of Costs
Coordination with Other Agencies


1. This report was authorized by the following resolution adopted by the Committee on Flood Control of the House of Representatives on May 14, 1945:

That the Board of Engineers for Rivers and Harbors, created under section 3 of the River and Harbor Act approved June 13, 1902, be, and is hereby requested to review the report on the San Lorenzo Creek, California, submitted to Congress on June 8, 1943, with a view to determining the advisability of undertaking improvements for flood control on the San Lorenzo Creek, California, at this time.


2. Scope of investigation. This report, which is of survey scope, was assigned by the Chief of Engineers on May 29, 1945. The study consists of a review of an unpublished survey report on San Lorenzo Creek,. Alameda County, Calif., for flood control and water conservation which was transmitted to Congress by the Chief of Engineers on June 8, 1943, with the recommendation that no improvement be made at that time.

3. Basic data obtained.-- In connection with the preparation of this report, a public meeting was held on April 23, 1946, at Hayward, Calif., at which the desires of the local people and supporting data were presented. Aerial photographs of the flood plain and adjacent areas were taken in March 1949. Instrumental surveys, soils investigations and flood-damage surveys were made as a basis for office studies which included hydraulic and hydrologic analyses, development of the best plan of improvement, and costs and prospective benefits of alternative pans for effecting flood control. Interested Federal and non-Federal agencies were contacted relative to problems of water-resource development on San Lorenzo Creek.

4. Examination by the district engineer.-- Sites of possible improvements discussed herein were examined by the district engineer.


5. The only prior flood-control report on San Lorenzo Creek is the unpublished report under review, dated June 8, 1943.


6. Geography.-- San Lorenzo Creek drains a small watershed in the foothills of the Coast Range Mountains on the east side of San Francisco Bay. The stream flows in a general westerly direction through the town of Hayward and enters the bay at a point about 10 miles southeast of Oakland. The upper part of the drainage basin, comprising 45 square miles, is roughly fan-shaped and extends approximately 12 miles in a north-south direction and 7 miles in an east-west direction. The lower 4 miles of the creek, from the vicinity of Bridge Street in Hayward to the bay, flows on the crest of an alluvial cone and includes an additional indefinitely defined drainage area of about 16 square miles. The principal tributaries, which consist of Cull and Crow Creeks from the north, Eden and Hollis Creeks from the east, and Palomares Creek from the south, all join San Lorenzo Creek upstream from the alluvial cone. These tributaries converge toward the main stream within short distances of each other, giving rise to the fan-shaped appearance of the drainage basin and the rapid concentration of peak flood flow.

7. Topoqraphy.-- The upper part of the watershed is made up of narrow valleys divided by ridges ranging from 200 to 1,850 feet in elevation. Below this mountainous area, the main stream is confined to a small gorge in which it flows through the southern part of the relatively flat Castro Valley and into the city of Hayward. Downstream from Hayward, the stream is located on the crest of an alluvial cone and its channel becomes progressively shallower and narrower as it approaches the bay.

8. Geology.-- In that part of San Lorenzo Creek Basin above the alluvial cone, the geology is characterized by folded and faulted strata of Cretaceous and Tertiary age. The principal materials present are sandstones shales, silts, and clays. The floor of Castro Valley and the alluvial cone in the lower part of the basin are covered with alluvium derived from these rock types. The Hayward Fault (a major active fault) traverses the basin in a northwest direction east of the city of Havward. The presence of this fault has been recognized in the design of the improvements considered herein. A detailed report on geologic conditions in this area is presented as appendix I. [Not printed.]

9. Channel characteristics.-- Stream-bed slopes in this basin range from more than 100 feet per mile at the headwaters of the tributaries to less than 10 feet per mile in the lower reaches of the main stream. The main-stream channel varies considerably in width due to greater bank erosion in the sandy formations than in the clays and other harder alluvial deposits along the stream. In the reaches upstream from Hayward, the channel ranges from 50 to 100 feet or more in top width and from 20 to 30 feet in depth. In the leveed reach of the stream below Hesperian Boulevard, the depths become progressively less until the creek fans out at its mouth. In general, channel capacities of tributaries and of the main stream above Foothill Boulevard are adequate for all flood flows. The capacity of the main channel varies from about 10,000 cubic feet per second at Foothill Boulevard to about 5,400 cubic feet per second between Bridge Street and Meekland Avenue. Downstream from Meekland Avenue, the capacity decreases progressively downstream, being about 3,500 cubic feet per second at Hesperian Boulevard, 1,900 cubic feet per second at Lorenzo Avenue, and 500 cubic feet per second at the Southern Pacific Co. railroad near the mouth of San Lorenzo Creek. Below the railroad, various structures restrict the channel to the extent that the capacity becomes negligible. There are a number of railroad, highway, and farm bridges crossing San Lorenzo Creek. For the most part, these structures have ample waterway openings so that they do not obstruct the flow to any appreciable extent. Several bridges, however, restrict the flow because of the relatively small cross-sectional area of the bridge opening. The most critical of these is the highway bridge on East 14th Street where the area of the waterway opening under the structure is less than one-half of that of the channel in the immediate vicinity. Other bridges which obstruct the flow, but to a lesser extent, are those on Foothill Boulevard, Bridge Street, and the lower Southern Pacific Co. railroad.

10. Soils and vegetative cover.-- The soils in the alluvial cone area and in Castro Valley consist of alluvial sands, silts, and loams derived from the rock types in the upper basin. Such soils are capable of intensive agricultural production, but most of the area has been developed for residential purposes. The vegetative cover in the upper watershed consists primarily of grasses with small clusters of trees on the eastern slopes. In Castro Valley and on the alluvial cone, those areas that are not urbanized are generally used for agriculture.

11. Maps and charts.-- The drainage basin of San Lorenzo Creek is shown on quadrangles published by the Army Map Service, Corps of Engineers, and the United States Geological Survey. The Army Map Service quadrangles are printed to a scale of 1:50,000. The United States Geological Survey quadrangles include the Pleasanton and Hayward sheets printed to a scale of 1:62,500 and the Hayward and San Leandro sheets printed to a scale of 1:24,000. A general map of the basin showing the principal features of the drainage basin accompanies this report as enclosure 1.


12. Population.-- Based on data obtained from the United States Bureau of the Census, the Hayward and Castro Valley Chambers of Commerce, and the local power company, it is estimated that the 1950 population residing in the drainage area and the alluvial cone flood plain was approximately 40,100. For many years prior to World War II, the population of the area increased at a rate of about 5 percent per year and the total population was fairly evenly divided between urban and rural. During and after World War II, this increase was greatly accelerated by the phenomenal influx of people to the San Francisco Bay area so that the estimated 1950 population is about twice that of 1940. The population of the area is now predominantly urban. Population centers in the area contiguous to San Lorenzo Creek are the incorporated city of Hayward (1950 population 14,272) and the unincorporated communities of Castro Valley and San Lorenzo Village. A publication of the California State Chamber of Commerce states that the 1950 population of the area within a 4-mile radius of the center of Hayward was about 75,000 compared to 31,000 in 1940.

13. Occupations and industries.-- It has been estimated that approximately 75 percent of the wage earners residing in the area under consideration commute to the large cities of the San Francisco Bay area for employment. The remainder are engaged in professional and construction services, retail and service establishments, nurseries, small manufacturing plants, and agricultural processing plants.

14. Land use and development.-- Prior to World War II, the land in the area affected by San Lorenzo Creek floods was used primarily for agriculture, with the exception of the city of Hayward. The areas between Hayward and the bay were occupied by truck farms, orchards, and nurseries, and the areas in Castro Valley and the upper part of the drainage basin were used for raising poultry, rabbits, and cattle, and for miscellaneous orchard, field, and truck crops. Recent residential and industrial developments now occupy a large part of these former agricultural areas, especially in the flood plain between Hayward and the bay. It is apparent that in the near future only the upper reaches of the drainage basin will be available for agriculture. In 1950, the total income from agriculture in this basin was $4,607,000, of which $200,000 was derived from tree and vine crops, $155,000 from truck crops, $1,512,000 from nursery products, $1,865,000 from poultry, pigeon, and rabbit production, and $875,000 from dairy products and livestock. Prior to the change in land use noted above, the total value of vine and truck crops was comparable to that of other crops in the area.

15. Resources. -- Historically, the major natural resource of this basin has been the rich alluvial soil cover in the nonmountainous areas. This resource was developed for intensive agriculturral production for a number of years. With the rapid commercial and industrial development of the San Francisco Bay Area in recent years, much of the agricultural land already has been utilized for residential and industrial sites. Considering the existing state of the economic development of the San Francisco Bay Area and the prospect for continuation of that development, it is evident that the economic future of the San Lorenzo Creek area is assured because of its suitability for home sites for persons employed in the nearby industrial centers.

16. Transporation facilities and water uses. -- The Southern Pacific and Western Pacific Railroads and California State Highway Route No. 17 and U.S. 50 traverse the drainage basin of San Lorenzo Creek. In addition, a freeway which will connect Oakland with San Jose and which is now under construction passes through the area. The Hayward Municipal Airport, which is used by charter and privately owned planes, is located just outside of the flood plain of San Lorenzo Creek. The development of navigation or power on this stream is infeasible because of the limited and intermittent flow. Irrigation use of the water is accomplished by infiltration from the creek into the aquifer in the San Lorenzo Creek zone which is the source of replenishment for shallow wells in the alluvial cone.


17. Climate and precipitation. -- The climate of the San Lorenzo Creek Basin, typical of central California, is characterized by warm, dry summers and mild, wet winters. Due to the moderating effect of the Pacific Ocean and San Francisco Bay, extremes in temperature are rare. Estimates from nearby stations indicate that the average annual temperature for this area is 56 degrees, with monthly average temperatures ranging from 63 degrrees in the warmest month (September) to 48 degrees in the coldest (January). The length of growing season is about 340 days. Rainfall over the basin averages 23 inches and ranges from a minimum of 20 inches to a maximum of 26 inches. Snow occurs very infrequently and has no measurable effect on the flood runoff. Detailed climatic and precipitation data are presented in appendix II. (Not printed.)

18. Precipitation stations and records. -- Precipitation records are available from 35 active and inactive precipitation stations located within, and adjacent to, the San Lorenzo Creek drainage area. The locations of these stations and pertinent data relative to the gages and the station records are given in appendix II. (Not printed.) Two of the gages have comparatively long records: Jensen Ranch, from 1905 to date, and Hayward High School, from 1914 to date. The most useful record is that of the recording gage at Chabot Observatory, located about 6 miles northwest of the drainage basin adn operated since 1905.

19. Storm characteristics. -- Flood-producing storms of the central California coast genreally result from a southward displacement of hte Aleutian low and its accompanying frontal systems. The average duration of such storms is 2 days; however, the passage of more than one storm may result in a continuous and prolonged flood period. The relatively small size of the San Lorenzo Creek Basin limits the effective period of precipitation which results in flood flows. In general, high-intensity rainfall for a period ranging from 4 to 8 hours, preceded by sufficient rainfall to lower the initial high loss rates, results in flood discharges.

20. Storms of record. -- Prior to the installation of adequate rain gages in the basin, newspaper accounts and interviews indicate major stormms occurred in the years 1862, 1866, 1871, and 1891. Thirteen flood-producing storms have been recorded during the period 1905 to 1950.

21. Standard project storm. -- The standard project storm is a hypothetical storm which is used to establish and upper limit of conditions against which protection might be sought. Such a storm was quantitatively estimated by (a) delineating a region within which a storm may be transposed to the San Lorenzo Creek Basin without decreasing its probability of occurrence; (b) determining the most critical storm of record within that region; and (c) transposing this storm to the basin. The stormof December 9-12, 1937, was selected as the most critical. A quantitative estimate of the standard project storm was prepared by considering the December 1937 storm as being transposed so as to be centered over the San Lorenzo Creek drainage basin. Details of the method used in this transposition of the storm are presented in appendix II. (Not printed.)


22. Stream-gaging stations. -- A gaging station on San Lorenzo Creek at Bridge Street was established in November 1920 by the Eden Township County Water District, and intermittent records werre collected until June 1923. In December 1939, the Corps of Engineers installed a wire-weight gage at B Street which was continued to September 1940. This station was reestablished and a recording gage installed in October 1946 by the United States Geological Survey in cooperation with the Corps of Engineers. This station has been maintained since that date and records therefrom have been published in water-supply papers of the United States Geological Survey.

23. Adequacy of streamflow for multiple-purpose uses. -- Since 1946, annual and monthly runoff measurements have been recorded. In order to extend this record, the 37-year runoff records of San Leandro Creek, an adjoining watershed on the north, were correlated with those of San Lorenzo Creek for the period when both streams were being measured. The correlation factor thus determined was applied to the record of San Leandro Creek, which extends back continuously to 1915.

24. The annual runoff for San Lorenzo Creek at B Street, as estimated by the foregoing method, ranged from about 800 acre-feet in water year 1924 to 23,800 acre-feet in water year 1938 and averaged 8,725 acre-feet during the period 1915-49. With an adequate supply of water for domestic and industrial uses being supplied by the East Bay Municipal Utility District and San Francisco's Hetch Hetchy system, there is no immediate water shortage in this basin. Therefore, the conservation of the runoff of San Lorenzo Creek is not a critical matter at the present time. However, the yield which could be developed with the basin eventually may be needed in the ultimate development of the San Francisco Bay area.


25. Floods of record. -- Other than floods recorded in February 1940 and December 1950, no flows of significant magnitude have occurred during the short period of gagings. In February 1940, rains totaling 5.15 inches in the basin resulted in a peak flow of 3,470 cubic feet per second at B Street. In December 1950, rains totaling 3.22 inches produced a peak flow of 2,600 cubic feet per second at B Street. For periods when stream gages were not operated, but subsequent to the installation of rain gages in 1905, peak discharges were estimated from rainfall records. Major floods are those estimated to have occurred in 1911, 1916, 1919, 1925, 1931, and 1942. The maximum during this period occurred in February 1925 with an estimated peak of 6,800 cubic feet per second. The largest historical flood in the basin is generally considered to have occurred in 1862. From information obtained from local newspapers, there were other major floods, also, in 1866, 1871, and 1881. It is quite probably that the major storm of 1891 produced a flood of high magnitude during that year. Additional information on floods in this basin is given in appendix II. (Not printed.)

26. Flood characteristics. -- Floods in the San Lorenzo Creek Basin are characterized by their short duration, with flows above 1,500 cubic feet per second seldom being maintained for more than 6 hours. The peak occurs within 3 to 4 hours following the cessation of the high-intensity, flood-producing rainfall and recedes almost as rapidly. As an example, in the flood of December 1950, the flow at B Street increased from 250 cubic feet per second to 2,600 cubic feet per second in about 3 hours. These flashy peaks develop high velocities which cause scouring in all reaches. Practically all of the large floods of record in the past have occurred during December, January, or February.

27. Flood frequency. -- The frequency of floods was estimated from the measured and estimated annual flood peaks since 1905. The frequency curve thus derived was modified to include the large historical floods during the period 1860-1905. A detailed account of the frequency-curve determination is presented in appendix II. (Not printed.) Based on these studies, it is estimated that flood flows of varying magnitude on San Lorenzo Creek at Bridge Street may be expected with the following frequencies:

Peak discharge equaled or exceeded

Cubic feet
per second
Once in 10 years .........................................................................4,800
Once in 25 years .........................................................................6,100
Once in 50 years .........................................................................7,100
Once in 100 years ........................................................................8,000


28. The standard project flood is a hypothetical flood, usually greater than the maximum flood of record, quantitatively estimated to establish the most severe conditions against which protection might be sought. It is defined as the flood that would be produced at a particular point if the standard project storm should occur over the tributary drainage area when hydrologic conditions were reasonably favorable for producing high runoff. If protection against the standard project flood can be economically justified, it generally becomes the upper limit of design for flood-control improvements. The peak discharge of the standard project flood for San Lorenzo Creek Basin is estimated at 9,700 cubic feet per second at Bridge Street. Its fequency of occurrence is estimated at not more than once in several hundred years. Further details on the standard project storm and resultant flood are given in appendix II. (Not printed.)


29. Flood-plain area. -- The channel capacity of San Lorenzo Creek is sufficient to carry most floods in the reaches upstream from about Hazel Street in Hayward. From Hazel Street to a point immediately above East 14th Street, the creek is generally confined within its banks except for high peak flows. Accounts of floods which have occurred since 1905 indicate that the creek has topped its banks immediately upstream from East 14th Street on several occasions, resulting in inundation from East 14th Street to the bay. Downstream from East 14th Street to Meekland Avenue, the channel has a fairly uniform capacity estimated as 5,400 cubic feet per second for existing conditions. Because the channel follows the crrest of the alluvial cone and the channel banks are several feet higher than the adjacent ground, the flow which overtops the channel banks does not return to the main channel but makes its way down to the flood plain. For this reason, the maximum flow in the channel at Meekland Avenue is not much in excess of 5,400 cubic feet per second regardless of the magnitude of the flood. Downstream from Meekland Avenue, the channel becomes progressively more restricted due to decrease in cross-sectional area and to decrease in bottom slope. As a result, overflow in the lower reaches of the alluvia cone takes place with a relatively small discharge. In the vicinity of Lorenzo Avenue, overtopping and breaching of the levees have occurred when the peak exceeded about 1,500 cubic feet per second. The failure of hte levees tends to alleviate the threat of the channel being overtopped immediately upstream. During the minor floods of the past few years, the right and left levees have been alternately breached. As a result, the extent of inundation, particularly for minor floods, is dependent upon the location of the initial failure of the levee. It is estimated that the standard project flood, which has an estimated peak discharge of 9,700 cubic feet per second, would inundate 4,400 acres in the alluvial area of San Lorenzo Creek. A flood of 7,000 cubic feet per second would inundate an estimated 4,160 acres. Data on flour floods for which the flood plains have been determined from field interviews are indicated below:

Discharge at
Bridge St.
(cubic feet
per second

Area inun-
dated (acres)

December 19314,8001,915
December 19372,8001,750
February 19403,8001,620
December 19503,0001,960

The apparent lack of correlation between discharge and area inundated for the above floods is due, principally, to change in channel conditions during the period of record, and, partly, to location of levee breaks. The data point up the fact, also, that the channel capacity has markedly deteriorated during the past several decades, since about the same area is now inundated by floods of much smaller magnitudes.

30. Type and value of improvements in flood plain. -- The flood plain upstream from Lorenzo Avenue is predominantly residential with some business establishments in the vicinity of East 14th Street. From Lorenzo Avenue to the bay, a large residential section is now under construction in what has been agricultural land. Other improvements in this area are principally agricultural with a scattering of industrial developments. Three through highways and three railroads cross the flood plain in the north-south direction. Roads and utilities to serve the needs of the community crisscross the flooded area.

31. The value of property in the flood plain was estimated on the basis of prevailing market prices for residential, commercial, and agricultural properties and the cost of replacing damaged transportation and utility facilities under present economic conditions. Agricultural products of the flood plain consist of nursery products, truck, tree, and vine crops, and some field crops. The 1950 value of these products was estimated from information obtained from representatives of local agricultural associations and buyers for food-processing plants. The value of improvements in the area that would be inundated by the standard project flood is estimate at $53,420,000, including the following:

Type of property:Value
Residential and business ....................$16,100,000
Agricultural ......................................1,100,000
Transportation facilities .....................6,000,000
Channel improvements and utilities .....220,000


32. Effect of floods on property values. -- The flooding of residential and business property has not been frequent in recent years and there has therefore been no noticeable depression of property values in this expanding area because of its vulnerability to flooding. The agricultural areas below Lorenzo Avenue are subject to flooding at frequent intervals; however, the trend from agricultural to urban use has caused the value of these lands to appreciate despite the probability of inundation under present conditions. There are some areas below Lorenzo Avenue where urbanization is being restricted until property remedial measures are first undertaken to alleviate the flood hazard.


33. Flood-damage surveys. -- Surveys were made of damages resulting from the floods of 1940 and subsequent years. The data obtained are compiled in appendix III. (Not printed.) Insofar as possible, informaiton on floods prior to 1940 were also collected during the course of these surveys. The greatest damages sustained in recent years were those resulting from the February 1940 flood. The estimated damages for that flood, based on conditions existing at the time of its occurrence, but on prices adjusted to 1950 levels, were $103,400. Under present conditions of development, a flood of similar magnitude would cause damages in the estimated amount of $150,000. The estimated damages from a flood equal to that of February 1925, the estimated maximum since 1905, is $2,400,000.

34. In the absence of major floods of recent occurrence, it was necessary to establish overflow areas for various discharges by hydraulic computations and to determine therefrom the lands and improvements which would be inundated. From these data, it was possible to correlate the extent of flooding with damage and corresponding flood magnitude and to determine average annual damage under existing physical conditions.

35. Flood damages. -- Floods in San Lorenzo Creek cause damage by inundation and bank erosion. Due to the relatively low flood-carrying capacity of the stream in its lower reaches, the areas most freuently flooded are those downstream from Lorenzo Avenue. The flood problem in this area is aggravated by the indefinite, inadequate outlet to the bay. Prior to 1949, most of the land in these areas was used for agricultural purposes, and the damages sustained included loss of topsoil by erosion, loss of the winter crops growing at the time the floods occurred, losses due to delays in spring planting, loss of hothouse crops due to interruption of heating, and the covering of agricultural land with flood-borne debris. In recent years, significant portions of the land subject to inundation damages have been utilized for residential building sites. Future floods in those areas will therefore cause damages which are typical of urban areas, including damages to residential buildings and their contents such as home furnishings, decorations, clothing, stored goods, and appliances; damages to landscaped ground; and damages from inundation of business establishments. From the estimate of the extent of inundation by floods of various magnitudes mentioned above and from consideration of the damages that such inundation would produce with the existing state of development, it was possible to estimate the damage to be expected from floods of various magnitudes.

36. The creek channel from Foothill Boulevard to B Street has an adequate carrying capacity for most floods, but extensive damages from bank erosion occur in this reach. This type of damage, which is largely concentrated at the several sharp bends, is particularly serious where houses are built very near to eroding banks and the loss of a small parcel of land may result in the loss of a house as well. Property owners in this area have made considerable effort to protect their property by installing various types of local protection works. These include concrete and timber walls, brush mats and fills, wire and poles, and riprapping with rock and scrap material.

37. Average annual direct flood damage.-- Damages from inundation and from bank erosion were correlated with flood magnitude and, through the relatlonship between flood magnitude and frequency of occurrence developed in appendix II [not printed], a graphical relationship between flood damages and the frequency of their occurrence was obtained from these relationships, the average annual direct damage from inundatlon was estimated at $147,500, and the average annual damage from bank erosion was estimated at $21,500. The average annual bank-protection expense was estimated, on the basis of annual charges for interest and amortization of the estimated present value of those improvements and the annual cost of their maintenance, replacement, and repair of damages resulting from floods, to be $8,600.

38. Indirect damages.-- Even during extreme floods, the overflow in the flood plain of San Lorenzo Creek is relatively shallow. Therefore losses from evacuation and reoccupation and lost labor opportunity are not great. As the area is served by several highways and three interconnected rail routes, traffic interrupted on one road or rail line can be diverted to another. Losses of income in the agricultural regions are due primarily to the delays in planting of crops. This frequently results in the crops growing under less favorable weather conditions and maturing at a time when prices are not too favorable for marketing. Indirect damages were estimated for the standard project flood and were found to be equal to 2 percent of the direct damage from such a flood. The average annual indirect damage was therefore estimated to be 2 percent of the direct damage from inundation as estimated above, or $2,900.

39. Intangible flood damages.-- Investigations of the severity of floods whIch have occurred on San Lorenzo Creek have revealed no loss of life attributable to high water. The retreating waters leave unsightly and unsanitary debris on the flood plain, but the litter is removed before it becomes a menace to health. Water from the shallow wells is not used for domestic consumption, and pollution of these wells, therefore, has no effect on the water supply. From all data available, the intangible damage appears to be negligible.

40. Future damages.-- As indicated in previous paragraphs, the use of the lands encompassed within the flood plain of San Lorenzo Creek has changed from predominantly agricultural to urban. From the present trend, it is considered reasonable to assume that all land in the flood plain except the area downstream from the lower Southern Pacific Railroad track will be completely urbanized by 1970 if an acceptable degree of protection from future floods is provided. It is not anticipated that appreciable additional home building will occur in the area without such protection, however, and it is therefore believed that the damages estimated above constitute an acceptable estimate of future damages under existing physical conditions.

41. Summary of damages.-- Average annual flood damages in the drainage basin of San Lorenzo Creek, as developed in the preceding paragraphs, are estimated to total $180,500. This evaluation is based upon long-term projected price levels recommended by the Federal Inter-Agency River Basin Committee and is summarized below:

                                          Estimated average
Type of damage:                           annual damages

     Direct --------------------------------- $147,500
     Indirect -------------------------------    2,900
  Bank erosion ------------------------------   21,500
  Bank protection expense -------------------    8,600

  Total -------------------------------------  180,500


42. The Corps of Engineers has no project for the improvement of San Lorenzo Creek for flood control. At the request of local interests, however, Federal funds totaling approximately $22,600 were expended by the Corps for emergency repairs to local levees along San Lorenzo Creek in 1951 and 1952. Weakened portions of the levees below Hesperian Boulevard were raised or strengthened as required to restore them to their condition before the floods of December 1950 and January 1952.

43. Improvements by other agencies.-- Levees have been constructed between Hesperian Boulevard and San Francisco Bay by local interests over a period of years, but no data on these works prior to 1933 are available. In that year, an emergency work project sponsored by the SERA for reconstructing levees, building new levees, and channel clearing, was carried out at a cost of $78,880. As local interests downstream from the Southern Pacific coastline bridge opposed completion of the channel, this construction was stopped at the bridge, leaving the creek with no well-defined outlet to the bay. The improvement has deteriorated through lack of maintenance, and the channel capacity has been reduced by siltation and by the growth of trees and brush. In addition to the levee and channel works constructed by the SERA, several short reaches under bridges and around sharp bends have been protected by concrete revetments constructed by the county of Alameda. These protective works were well designed, properly constructed, and have suffered little damage from erosion.

44. Improvements by private interests.-- Individuals and corporations owning lands adjacent to the creek have constructed local bank protection works of various types as mentioned in the preceding paragraphs. These works have been constructed in accordance with the design and materials the property owner believed would best alleviate the erosion problem. Most of these revetments have failed, or required excessive maintenance, and will not be suitable for inclusion in the final flood- and erosion-control plan. Their replacement has been estimated at $220,000.


45. Public hearing.-- A public hearing was held by the district engineer at Hayward on April 23, 1946. This hearing considered floods, erosion, property values, water conservation, recreation, and ground water. A transcript of the hearing is on file in the offices of reporting and reviewing officials. Forty-five persons were in attendance, including local officials, business representatives, property owners, State and county officials, and representatives of Federal agencies, including the Bureau of Reclamation, Fish and Wildlife Service, Forest Service, Department of Agriculture, and Soil Conservation Service.

46. Improvements desired.-- No definite plans of improvement were proposed at the public hearing. Data were presented, however, on the effects of inundation and erosion and the belief was expressed that a solution to the problem could be found in one of the following methods or in a combination of them:

(a) Enlargement, realinement, and lining of the channel to provide a clear passage to the bay in the lower reaches.

(b) Construction of training walls, bulkheads, and revetments at critical points, and realinement, where possible, in the upper reaches.

(c) Elimination of debris accumulations by clearing and snagging operations.

(d) Construction of multiple-purpose reservoirs for control of excess flows from headwaters.

47. Justification advanced for improvements desired.-- Proponents of the channel improvements were of the opinion that such works would be justified by the benefits resulting from prevention of damages to urban and rural properties by inundation and from elimination of loss of land and improvements by erosion. Proponents of a multiple-purpose reservoir included the foregoing benefits in their justification analysis and also indicated that additional justification may be forthcoming from benefits resulting from the recreational advantages provided and from the increased ground-water supply and impounded water for domestic and industrial use.

Continued (Part 2)