TABLE IX. Peak discharges measured during recent damaging floods 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, (1907-41) 1907-8. 1908-9. 1909-10. 1910-11. 1911-12. 1912-13. 1913-14. 1914-15 1915-16 1916-17. 1917-18. 1918-19. 1919-20. 1920-21. 1921-22. 1922-23 1923-24 1924-25 1908-9. 1909-10. 1910-11. 1911-12. 1912-13. 1913-14. 1914-15. 1915-16. 1916-17. 1917-18. 1918-19. 1919-20. 1920-21. 1921-22. 1922-23 1923-24 1924-25. 1906-7. 1907-8. 1908-9. 1909-10. 1910-11. 1911-12. 1912-13. 1913-14 1914-15. 1915-16. 1916-17. 1917-18. 1918-19. 1919-20. Season 1920-21 1921-22 1922-23 1923-24. 1 Gaged. Season Day Season Jan. 25 Jan. 21 Dec. 9 Mar. 7 Apr. 11 Jan. 15 Jan. 25 Feb. 9 Jan. 17 Feb. 22 Mar. 19 Feb. 11 Mar. 21 Jan. 18 Feb. 11 Dec. 13 Mar. 26 Feb. 12 Day Jan. 20 Mar. 12 Jan. 15 Jan. 1 Feb. 8 Jan. 3 Feb. 21 Mar. 19 Feb. 10 Apr. 15 Jan. 17 Feb. 9 Dec. 12 Jan. 27 Day Jan. 9 Jan. 27 Dec. 9 ? Gaged. 1 Estimated from gagings at Watsonville. $ 19 TABLE XI.-Estimated maximum peak discharges, Uvas Creek near Morgan Hill (1908-41) Mar. 7 Apr. 10 Jan. 17 Jan. 25 Feb. 9 Jan. 17 Feb. 22 Feb. 22 Feb. 11 Mar. 21 Jan. 18 Feb. 11 Peak dis- Jan. 24 Peak dis- (cubic feet 10,900 1.400 6,900 3,300 12, 600 2, 100 14.000 5,800 6.000 2.400 3,600 300 500 600 1925-26. 1926-27 1927-28. 1928-29. 1929-30. 1930-31. 1931-32. 1932-33. 1933-34. 1934-35. 1935-36. 1936-37. 4,000 1,300 1937-38. 1938-39. 1939-40. 1940-41. 1924-25. 1926-27. 1927-28. 1928-29. 1929-30. 1930-31 1931-32 1932-33. 1933-34. 1934-35 Pajaro River near Chittenden 2 que II he San 70 1 Gaged. Wibe TABLE XII.-Estimated maximum peak discharges, San Benito River near 1935-36. 1937-38. Season 1938-39. 1939-40. 1940-41 Season Day Feb. 13 Mar. 24 Season Feb. 4 Jan. 5 Dec. 28 Jan. 29 Jan. 1 (Feb. 6 Feb. 12 Mar. 10 Feb. 28 Apr. 5 Day Feb. 2 Jan. 29 Dec. 12 Mar. 6 Feb. 13 Dec. 10 Mar. 9 Feb. 27 Apr. 4 Peak chaz (cubic t per scont Day Jan. 1 Apr. 8 Feb. 22 Feb. 11 Mar. 10 Apr. 4 16.00 he 10 1000 mark #10 Peak 1908 chara (cubiert per seond 1300 1800 1800 Peak chere per seat Feb. 16 Feb. 4 Feb. 4 Mar. 5 Jan. 8 Dec. 28 Jan. 23 (30 $50 1.30 1.500 29 મૂળ નાય 130 2.30 of o 60 130 ca 20 1300 .00 14.30 estima 1000 Severe 1100 100 1100 14.30 Would 1830 20 J 1000 30. For wh May b South of arbe 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.. 1 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. Extent 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 flood 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. 1 Not printed. 96893-44-4 TABLE XIII.-Extent of inundation in major damage areas for recent floods Damage area (Feb. 1937 Pajaro River from mouth to Chittenden gage. Feb. 1938 River. Feb. 1940 Pajaro River above mouth of Llagas Creek above vicinity Damage-discharge Dec. 1937 Uvas Creek gage.Feb. 1940 Tres Pinos Creek below mile do. do.. San Benito River below mile San Benito River 23. Dec. 1937 Feb. 1940 Dec. 1937 (Feb. 1940 near Willow do.. Feb. 1938 Dis charge or run- Urban ties Cubic feet per second 12, 500 16, 200 9,900 8,600 5,700 8,600 5,700 8,600 5,700 Inches 1.89 0.57 Area inundated in acres 1.89 0.57 200 470 0 0 0 160 0 0 0 0 0 jar 1,260 40 460 The t Goode 31. Character of flooded areas.-The flood plains are predominant Toe 800 6.000 1878 2.30 40 30 DER 32. Value and productivity of the flood plains.-The estimated val of all privately owned lands and improvements thereon in the major flood-damage areas is $12,700,000, of which $7,700,000 represents urban property. The annual gross value of the agricultural produc of the flood plain areas is estimated at $2,120,000 with normal viels and market conditions. The value and productivity of the sever major flood-damage areas in the drainage basin are set forth in table II of appendix IV in some detail. 1.Not printed. FLOOD DAMAGES 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 Watsonville and Watsonville Junction. (See enclosure 2.) 2 Not printed. |