TABLE 6.-Merced County agricultural statistics-Gross value of principal crops Tr &S Crop Livestock, dairy and poultry products. Grapes Alfalfa Grain Cotton.. Miscellaneous. Total.... 1929 1935-36 1938-39. Year 1934 $5, 233,000 1,025,000 2,550,000 820,000 770,000 730,000 358,000 12,763,000 Year Number Average number of estab- 35 42 TABLE 7.-Merced County manufacturing statistics 16. The principal industrial plants in the area consist of a cement plant, a planing mill, and plants which pack or process fruits and vegetables. Manufacturing statistics from the United States Biennial Census of Manufactures for 1929, 1935, and 1937 are summarized in table 7. Present assessed value of area flooded, 1935-38. 1 Data not available. wages earners 1935 865 779 1, 165 $6,560,000 2,230,000 3,500,000 Total assessed valuation $52,799,000 720,000 540,000 884,000 640,000 968,000 446,000 272,000 15,586,000 21,393,000 Yearly wages 1938 $9,858,000 1,370,000 5,000,000 968,000 $1,449, 440 TABLE 8.-Tax and assessment data, Merced County t to floodi es not mo chann 17. Property valuations. Assessed valuations of rural land in the $1,055,000 1, 582, 000 $5,385, 615 Cost of mate- energy 1939 Assessed Taxes paid taxes from $8,291,000 Cro 3,236,000 1,193,000 1,003,000 2,656,000 $960,000 1,121,000 1,251,000 1,487,000 21,251,000 on, T ern Pa con 4 Main $3,899.000 12 372 000 $8,582 568 193 County County mprove usage. 18. Transportation facilities.-The main line of the Atchison, Topeka & Santa Fe Railroad, the main valley line and Oakdale Branch line of the Southern Pacific Railroad, and the Yosemite Valley Railroad from Merced to Yosemite Valley, serve the area. All of these railroads cross the area subject to flooding by this group of streams. Table 9 gives approximate mileage of railway lines within the area under consideration. There are no navigation facilities. Atchison, Topeka & Santa Fe, main line..... Main Branch Yosemite Valley, main line.... Total Total within flooded area, 1935-38- Merced County TABLE 9.-Railway mileage, Merced County 19. Golden State Highway (US 99) connects the area with Sacramento and San Francisco on the north and with Fresno, Bakersfield, and Los Angeles on the south. Numerous State highways and county roads, the majority of which are improved, reach all developed portions of the immediate area. Table 10 gives the mileage of each class of road and highway in Merced County, the mileage of these flooded during 1935-38, and the mileage subject to flooding by the various creeks in the county. TABLE 10.-Highway mileage United Canal. Bear. W Miles. Owens. Mariposa Total Creek Includes 483.1 miles of miscellaneous roads not maintained by county. Railroad 688881 20. Bridges. There are 180 bridges across the streams and overflow channels in the area, including those for railroads, State highways, and county roads. No existing bridges are affected by proposed plans of improvements. Table 11 gives a summary of bridges by stream and usage. TABLE 11.-Summary of bridges 13 13 County highways and miscellaneous roads 11 1 1,985.7 93 302 7 59 State highway 200300 State and 6 11.7 1 4 9 32 126 12 37 89 IV. HYDROLOGY 21. General.-Practically all flood flows in the area have occurred 13 during the period November to April, inclusive. These winter floods are caused by heavy rainfall on the watersheds of the various creeks and are characterized by high peak flows of short duration and small volume. Floo 22. Rainfall.-Storms producing rainfall in the area usually come wi from the northwest, but occasionally from the southwest. While they frequent northwesterly storms are responsible for a large portion of thi the winter precipitation, they are rarely of sufficient intensity to caused of great peak floods. The less frequent southwesterly storms, after de f crossing the coast ranges, still carry sufficient moisture to produce anne prolonged intense precipitation over these watersheds. Normal seafood sonal precipitation on the valley floor is only about 11 inches and its aty, e contribution to stream flow is negligible. Normal seasonal precipitaals tion in the drainage basins ranges from about 13 inches in the foot-food hills to a maximum of about 35 inches at the higher elevations. Table of 18 12 gives the seasonal distribution of precipitation at Merced and that Mariposa which are representative of valley and foothill drainage areas, respectively. Data relative to precipitation stations and records are given in table 1, appendix III and tables 13 to 18, appendix IV.' 0 inclu hav 1 TABLE 12.--Seasonal distribution of precipitation-Percent of total mean seasonal Merced.. Station July Aug. Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June (1) 1 Trace. precipitation Normal seasonal precipi tation Fears. table 23. Snowfall. Because of the relatively low mountains, no snow pack is formed. Storms which bring heavy rainfall in the lower area sometimes deposit a small amount of snow at the higher elevations of the watersheds. Such snow has little, if any, influence on floods. T s Creek 24. Run-off-Flow of the streams is intermittent throughout the year. There is no mountain storage and therefore no natural flow during the summer period. Records of run-off are available for short periods during 1923, 1936, 1937, 1938, 1939, and 1940. These run-off records have been supplemented by estimates of run-off on each stream to cover the entire period from 1901 to 1940. These estimates have been based on rainfall records, observed rainfall run-off relations, and observed hydrographs. Available data were used to estimate the worst possible flood which could result from the occurrence of the worst possible single storm or succession of storms combined with the most unfavorable ground conditions. Data are not available to determine the frequency of such a storm. However, its occurrence would be infrequent and it is not considered practicable to provide protection 1 Not printed. Sh. Sind de ran appro enclo Set print against a recurrence of floods greater than those occurring within the period of hydrological records. (Further details on run-off are given in table 13 of this report and appendix III).' 25. Floods result from intense rainfall in the foothills and mountains during winter months and are flashy and of relatively small volume. Normally, the creeks rise from low flow to peak stage and return to low flow within a period of 24 hours or less. A series of storms may cause sa period of sustained abnormal flows. Since irrigation diversions are not made from the creeks during the flood season, it is necessary that the channels carry flood waters through the area and discharge them onto flood plains in the lower valley. Because of inadequate channel capacity, either from tule growth induced by use of channels as irrigation canals and drains, or from excessive flow, this area has suffered from many floods since it was first settled. Residents have stated that the flood of 1862 is the maximum historical flood. Another severe flood was that of 1911, and recent floods occurred each year from 1935 to 1938, inclusive, and in 1940. Based on hydrological studies, damaging floods have occurred in 16 of the past 40 years or at the rate of 40 in 100 years. Tables 13 to 18, appendix IV, list most of the past floods, and table 13 of this report gives the measured discharge of maximum winter floods that have occurred during the short period of run-off records. (Further run-off data are given in appendix III).' FOC TABLE 13.-Measured winter peak flood flows V. FLOOD CHARACTERISTICS Mariposa Creek. No measurements on Burns, Black Rascal, Canal, and Fahrens Creeks. Daily mean flow Cubic feet per second 6, 400 690 520 240 4, 280 2,300 Instantaneous peak flow Cubic feet per second 6, 150 9, 800 9,750 5,600 1,500 1,680 500 600 930 350 6,600 7,900 7,800 26. Since the present channels are not uniformly maintained or developed, the capacities vary for the different reaches, resulting in a wide range of discharges at which overflow occurs. Table 14 gives the approximate range of discharges above which overflow will occur under present conditions and estimated peak flows for each creek. (See enclosure 1.) 'Not printed. TABLE 14.-Present channel capacities and estimated peak flows Length of valley area Canal Black Rascal. Bear (below Burns). Burns. Miles' Owens Mariposa Creek Miles 14 4 11 30 1 13 18 19 Capacity of channel Cubic feet per second 100-500 200-500 5,000-8,000 6,000 1,000 200-800 1,000-5,000 of th ructio ESIVE C reme Consi ma they 2,000 3.000 US 11 Estimated maximum peak flow 1900-40 Cubic feet per 4,000 15,500 dama 9,000 ttribu 1,000 3,100 wa 15,000 nd wat Recently cleaned and enlarged to 1,000 cubic feet per second capacity, except 21⁄2 miles west of Southern lely e Amo Pacific R. R. -35 flo obtain VI. EXTENT AND CHARACTER OF FLOODED AREA 27. The total area subject to flooding by the streams under con-ich q sideration comprises about 136,000 acres, much of which is highly ages, a developed agricultural land. About 50,000 acres of this flood plain of th is along the lower reaches of the creeks, where the poor quality of the en in land and the cost of protection from floods limits development to pasturage. While the area subject to flooding is only about one-ninth of the total area of Merced County, it includes a substantial portion of the developed area within Merced irrigation district, as shown by enclosure 2. The flooded area is influenced by irrigation canals, roads, levees, and channel conditions which frequently change and rek which make the flood area extremely irregular and spotted. These features make it difficult to define the extent of flooding which would result from a recurrence of any specific flood. During the 1935-38 flood seasons, 56,000 acres were flooded. Tables 3 to 11, inclusive, indicate the development within the general flood area. Practically all of the 49,000 people of the county are affected directly or indirectly by flooding within this area. Rascal VII. FLOOD DAMAGE 28. General. In order to obtain flood damage data for the streams in Merced County, a survey was made of the damage caused by the floods of 1935-38. In this survey interviews were made and questionnaires distributed and collected for the major portion of the area subject to damage. The documented damages thus obtained are detailed in appendix IV.1 29. Type of damage. In the upper valley area, winter floods affect rich agricultural lands adjacent to stream channels. In the lower areas, south and west of Merced, flooding occurs over many sections of pastured land. Extensive damage to grain, alfalfa, orchards, and other crops; to roads and bridges; to irrigation works; to urban prop dual erties; and to railroads has resulted from past floods. Routing of traffic around the flooded area was estimated to have cost the traveling public at least $20,000 during the 1936 flood. Several serious acci dents occurred during and because of this flood. They resulted in 1 Not printed. ading cu Are flod |