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27. Historical floods.--According to local residents and newspaper accounts, floods of major proportions occurred on the Pajaro River in 1852, 1862, and 1890. The available information is too indefinite to permit estimation of stages or discharges during these floods. Hydrologic studies of the period from 1907 to 1941 were used to estimate the maximum peak discharge at the Chittenden gage for each season of the 34-year period. This series, presented in table X, is considered the most reliable information on past flood flows available, and was used for the determination of flood frequencies in the lower Pajaro River. Estimated maximum peak discharges for a 33-year period at the Uvas Creek gage, and for a 35-year period on the San Benito River near Willow Creek School, are presented in tables XI and XII, respectively.
TABLE X.-Estimated maximum peak discharges, Pajaro River near Chitenka?
Table XI.- Estimated maximum peak discharges, Uvas Creck near Morgan
af urba the
1 Gaged. Table XII.-Estimated maximum peak discharges, San Benito River near Wine
Creek School (1906-41)
28. Probable frequency of occurrence of floods. The probable frequencies of occurrence of flood peaks of different magnitudes, expressed in percent of time, at the Chittenden gage on the Pajaro River, at the Uvas Creek gage, and at the Willow Creek School gage on the San Benito River were determined from the data shown in tables X - to XII. These frequencies have been reexpressed as the probable number of seasons in which each peak will be equaled or exceeded in a 50-year period, the usual economic life of most flood-control works.
29. The probable number of times that any flood peak will be equaled or exceeded in 100 years or seasons is used throughout this report to designate its magnitude and is expressed as a percent chance of occurrence. For instance, the seasonal flood peak expected to be equaled or exceeded, on the average, only once in 50 years, or twice in 100 years, is termed the 2-percent-chance flood, and is found on the frequency curves presented in appendix III' at the abscissa marked 2 on the percent-of-time scale. Otherwise expressed, there is 1 chance in 50 or 2 chances in 100 that a flood of this magnitude, or one greater, will occur in any particular season. The 2-percent-chance flood peak at the Chittenden gaging station, taken from the curve, is 19,000 cubic feet per second, as compared with a peak discharge of 11,100 cubic feet per second, measured during the flood of April 1941, and a discharge of 16,200 cubic feet per second in February 1938, estimated from high water marks.
EXTENT AND CHARACTER OF FLOODED AREAS
30. Ertent of flooding.-The areas inundated during recent floods, for which damage appraisals were made, are shown on enclosure 2. Also shown and designated as the "Possible Flood Plain" is the estimated extent of inundation which would occur under the most severe storm conditions believed to be possible. During such a flood, the entire area limited by the dotted lines delineating this food plain would be inundated with negligible exceptions. From this map it may be seen that there are two principal flood plain areas, one in South Santa Clara Valley and one in Pajaro Valley. The acreages of urban properties, cropland, and other lands in the flood plain for the various floods considered in the damage survey are presented in table XIII.
* Not printed.
TABLE XIII.-Extent of inundation in major damage areas for recent floods e
Cubic feet per
Feb. 1937 | 12,500 Pajaro River from mouth to Chittenden gage.- Feb. 1938
16, 200 San Benito River.
9, 900 Pajaro River above mouth of San Benito River-includes
Uvas Creek gage. - Feb. 1940
8, 600 lower Llagas Creek and San
5,700 Felipe Lake areas.
(Dec. 1937 Uvas-Carnadero Creek..
(Feb. 1940 5, 700 Llagas Creek above vicinity
Dec. 1937 8, 600 of mile 2.
Feb. 1940 5, 700
Inches San Benito River below mile (San Benito River
Feb. 1938 1.89
near Willow 23.
n Creek school.
Feb. 1940 0. 57 Tres Pinos Creek below mile
(Feb. 1938 1,89 18.
(Feb. 1940 0. 57
Th 31. Character of flooded areas.- The flood plains are predominantly stina agricultural but include some urban developments and population for es centers. Both highway and railway facilities enter or cross them tefiant various points. From the mouth of the Pajaro River to the San Benito is River, the flood plain covers rich bottom land and urban parts of a firem sonville. The present levee system leaves Watsonville and the urban the area across the river from Watsonville vulnerable to the larger flood. The flood plain along the Pajaro River above the mouth of the Sun Benito River, including the lower Llagas Creek and San Felipe Lake 1900 areas, contains no urban developments and is used chiefly for pasture doader and for hay and grain crops. Uvas, Carnadero, and Llagas (abore specif mile 2) Creeks cause flooding of areas planted to orchards and vine de ser yards in the upper reaches and to row crops in the lower reacheol the Parts of the town of Gilroy are subject to inundation by the larger apprai floods. Severe bank cutting occurs along the San Benito River and abere Tres Pinos Creek, where valuable orchard, row-crop, and feed-cro lands are eroded. Some overbank flooding occurs along the lower miles of Tres Pinos Creek.
32. Value and productivity of the flood plains.-The estimated value of all privately owned lands and improvements thereon in the major flood-damage areas is $12,700,000, of which $7,700,000 representa urban property. The annual gross value of the agricultural products of the flood plain areas is estimated at $2,120,000 with normal yields and market conditions. The value and productivity of the several major flood-damage areas in the drainage basin are set forth in table II of appendix IV 1 in some detail.
33. Flood hazards to life, health, and property.—There is little danger to life from floods in the Pajaro River basin, and no serious --epidemics have been attributed to such floods. Throughout Pajaro Valley and in the Uvas-Carnadero, Llagas, and San Felipe Lake flood areas, damage is primarily due to inundation and is chiefly crop destruction and impairment of land and improvements. Along the UvasCarnadero Creek channel, bank erosion causes considerable loss of croplands and improvements adjacent to the stream.
Urban properties in Watsonville, Pajaro,' Watsonville Junction, and Gilroy, though partially protected by levees, are subject to flooding. Along the San Benito River and Tres Pinos Creek, most of the damage is due to bank cutting rather than to inundation. In all of these areas, there is the hazard of flood damage to bridges, utilities, and channel works.
34. Field surveys.--Flood damage investigations of agricultural and urban properties in the Pajaro River Basin were made under a cooperative arrangement with the Bureau of Agricultural Economics of the United States Department of Agriculture. Damage surveys relative to highways, railways, public utilities, public buildings, and other flood plain developments of similar character were made by district office personnel who consulted all available sources of information. The available records of flood damages to this type of property and estimates by responsible officials provide a reasonably complete basis for estimating the average annual damage. Statements as to the effect of each of the specified floods in the urban and suburban areas of Watsonville, Pajaro, Watsonville Junction, and Gilroy were secured from individual owners, renters, or operators representing 25 percent of the residences and retail establishments in the flooded areas. The properties were chosen at random, and the reported damages were expanded to obtain the total damage in each flood. Statements for 100 percent of the industrial and wholesale establishments in the flooded areas were secured. Statements as to the effect of each of the specified floods were obtained from the farm owners or operators in the several major flood plain areas representing from 27 to 100 percent of the farm acreage damaged. The methods used for flood damage appraisal are fully explained in appendix IV.? The results of the above-described surveys are summarized in table XIV.
? The name "Pajaro" as used in this report refers to the small urban area on the south bank of the Pajaro River between Watson ville and Watsonville Junction. (See enclosure 2) Not printed.