bank-protection works, channel enlargement, and installation of pumping plants on the Pajaro River in the vicinity of Watsonville. (6) Construction of jetties and other works at the river mouth. (b) Recommended for further study. (1) Reservoir control, Uvas and Pacheco Creeks. (2) Watershed erosion. 44. Reasons advanced in justification for improvements desired include: Prevention of blocking of military roads by flood waters; a reduction of $209,600 in annual flood damage; the saving of $172,400 per year in pump operation costs due to raising the water table in certain areas; an increase of $1,400,000 per year in crop values; a $6,490,000 increase in land value due to better drainage and lowering of the water table; and the prevention of a capital loss of $6,000,000. It was also stated that soil erosion would decrease, Watsonville business would increase, and all transportation would benefit. All of these claimed benefits are presented in tabular form in appendix IX,' where they are broken down to show the areas most benefitted. 45. Cooperation offered by local interests.-A prepared statement of the Four County Committee, representing the boards of supervisors of the counties in which the drainage basin is situated, and the city of Watsonville, was read at the public hearing. The statement contained the following: Subsequent to the adoption of a definite program or plan, the directors will submit to voters of their respective districts for appropriate action the matter of financing their shares of the costs as required by law for cooperative action under the following acts: Public, 738, Seventy-fourth Congress. Responsible local agencies will (a) provide without cost to the United States all lands, easements, and rights-of-way necessary for the construction of a project, with certain exceptions, (b) hold and save the United States free from damages due to the construction works, and, (c) maintain and operate all of the works after completion in accordance with regulations prescribed by the Secretary of War. 46. Willingness and ability of local interests to fulfill requirements for cooperation. The following statement by the Four County Committee was read at the public hearing: The duly constituted districts, through action of their respective boards of directors, possess the rights of eminent domain, and thereby can acquire necessary rights-of-way and easements. They also have the authority for raising required funds. It was proposed at the public hearing that local interests contribute $155,000 of the estimated $718,000 cost of the immediate program suggested by them. FIELD SURVEYS 47. Topography and hydrography.-River cross sections and profiles were secured for use in flood routing. A detailed topographic survey was made of the Sargent-San Felipe Lake area up to contour elevation 165 feet. Aerial photographs in strip flights were used to supplement instrumental surveys. 48. Soil tests were made in the field and laboratory to determine the suitability of materials available for levee construction in the Pajaro Valley and to determine the stability of existing levees. 1 Not printed. 49. Stream-gaging operations and rainfall measurements.-A regular program of stream-flow measurements was carried out by distret office personnel during the 1939-40 and 1940-41 seasons, partly in cooperation with the United States Geological Survey. A numbere precipitation stations were installed and operated during these two seasons. Appendix III contains descriptions of the locations of stream flow and rain gaging stations and presents the data securedly from their operation. 50. Ground-water-level measurements were made in a number of wells during the 1939-40 season. Water conservation investigations ten and reports on portions of the Pajaro River drainage basin have been made from time to time by various organizations and individuak. These reports were studied and some of the investigations were extended with particular attention to the relation between water conservation and flood control problems. Appendix V summarizes these investigations. 51. Field flood damage surveys covering agricultural and urban private properties were made by the Department of Agriculture as a part of the flood control survey of the Pajaro River Basin by that o Department. Damages to public utilities, public roads and bridges were canvassed by Engineer Department personnel as more fully described in appendix IV.1 WGS FLOOD PROBLEMS 52. Drainage problems are important in several flood plain areas, dzes especially along lower Llagas Creek and upper Pajaro River. There abl are also some poorly drained areas in the Pajaro Valley in the vicinity of Watsonville. These problems were investigated sufficiently to determine their relation to plans of improvement for flood control. moun 53. Examination by the district engineer.-The drainage basin was inspected and the sites of the principal projects discussed herein were examined by the district engineer. 55. In addition to these principal flood-plain areas, numeros small depressions, swales, and ponds along minor tributaries and elsewhere at isolated locations, are inundated during severe storms. Some of this inundation is caused by poor natural drainage. Some small drainage areas are so scattered that no practical purpose world 1 Not printed. 59.7 54. Inundated areas.-Floodwaters cause extensive inundation two separate areas. These are in the South Santa Clara and Pajam Valleys, as indicated on the flood-plain map (inclosure 2). The former is made up of the flood plains of Llagas and Carnadero Creeks which, during large floods, merge with the great bottom-land ske area which extends westward from San Felipe Lake and Tequisquia Slough to the vicinity of Sargent. This lake area is flooded by d charge from Pacheco and Santa Ana Creeks and the other tributaries to Tequisquita Slough, as well as by run-off from Llagas and Carnade Creeks. Except for comparatively small areas near the mouth d Tres Pinos Creek, the San Benito River system has no flood pla outside its natural wash, but its banks are subject to severe erosion. In the upper reaches of the river, such erosion is not economical important. cept he oth the ing b Benito ough The soil provi The area he Hoo Beneficia be served by attempting to consider them in detail herein. Many of these, lying in the upper watershed, have been considered as parts of special problem areas in the current survey of the Pajaro by the Department of Agriculture. 56. Variations in flood flows.--Although the basin lies in the region determined by the United States Weather Bureau to be meteorologically homogeneous in which floods are caused by storms of wide extent, storm run-off is influenced, nevertheless, by great variations in the infiltration rates and the trajectory of the storm centers to such an extent that great dissimilarity in discharge results from storms which are nearly the same in amount and intensity of rainfall. Conditions are generally most favorable for floods in the San Benito River and Tres Pinos Creek when the storm center approaches the coast along, or south of, the thirtieth parallel of latitude, the resulting surface winds being generally south or southeast. When the storm center approaches from, or has moved to, positions farther north, the surface winds are generally southwest, and the moisture-laden maritime air sweeps directly from the ocean into the drainage basin through Monterey Bay. These conditions are most favorable for producing floods in Llagas, Uvas-Carnadero, Pacheco and Corralitos Creeks. Under these conditions, however, lighter rainfall occurs in the San Benito River Basin as the orographic rainfall during passage over the ridges of the neighboring Salinas River Basin has reduced the precipitable moisture in the air mass considerably. 57. River regimen with respect to flood flows.-Flood discharges in all the tributaries of the Pajaro River are flashy. Those in the San Benito-Tres Pinos system are heavily silt-laden. A considerable amount of silt is carried, also, by floodwaters in the upper Llagas, Uvas-Carnadero and Pacheco Creeks, most of which is deposited in reaches well upstream from the confluence of Pajaro and San Benito Rivers. As the tributary streams approach the San Felipe LakeSargent bottoms, their channel cross-section areas and stream slopes tend to diminish. 58. The bottom lands from Tequisquita Slough through San Felipe Lake to Sargent constitute a natural flood reservoir. The discharge from this area is impeded by flat channel gradients and, to some extent, by dense tree growth in the stream and by inadequate bridge clearances. Here, a large lake is formed during severe storms by the excess of inflow over outflow. 59. The resulting flood regulation is sufficient to prevent damaging flood peaks in the lower Pajaro River during ordinary winter seasons except when the San Benito River is in flood simultaneously with the other tributaries. The duration of moderately heavy flows in the lower stream is increased by the regulating lake, the effect being beneficial in that the sustained discharge removes deposits of sand brought into the Pajaro River channel by flood flows in San Benito River. The protracted inundation between Tequisquita slough and Sargent also tends to leach out alkali salts which exist in the soils of the bottom lands south of the Pajaro River, thereby improving the pasturage in this portion of the lake area. However, the area north of the river is cultivated to row crops which are frequently and seriously damaged by flooding. In sum, the formation of the flood lake is detrimental to the area it covers, although it is beneficial to downstream areas. 60. Flood peaks in the lower San Benito River are reduced on-ch siderably by channel storage in the river and by percolation into the ntild stream bed. The combined effect of these features is large. For ring example, it is estimated that the storm of February 1938 produced a peak discharge of 20,000 cubic feet per second just below the on-ajaro fluence of Tres Pinos Creek and the San Benito River and that his int peak was reduced to about 12,000 cubic feet per second just above the confluence of the San Benito and Pajaro Rivers. The correspondag iel maximum flood peak at Chittenden is estimated from high-water onts marks to have been 16,200 cubic feet per second. Holliste F 61. It is apparent, therefore, that Pajaro Valley enjoys considerable loca flood protection from the regulating influences in the natural regimen ther of the river and its tributaries. Nevertheless, a considerable flood plain exists on both sides of the river in the valley. A substanta sused degree of protection is now afforded Watsonville and the urban and par agricultural areas on the left bank of the river opposite Watsonvile he che by levees constructed during 1939 and 1940, but, even under these are improved conditions, the average annual flood damage in Pajo terial Valley exceeds that of all other flood plain areas in the watershed.rainage Flooding in the valley is not greatly influenced by tides or sand bar formations in Monteray Bay at the mouth of the river. orks to 62. Related problems. Water conservation and drainage are clos 64. C related to the flood problems throughout the watershed. The im diate need for additional water for irrigation, centering in Souths Santa Clara Valley, and especially in the vicinity of Hollister, as been relieved somewhat by the comparatively wet seasons since 1955. The need for improved drainage in South Santa Clara Valley is most acute along the lower Llagas and the upper Pajaro River channels above State Highway 25. Effective drainage of this area is party dependent upon improvement of the Pajaro River channel from that vicinity to its confluence with the San Benito River. There are also several poorly drained areas in Pajaro Valley. In some places,pro exposure to flooding with the consequently increased maintenare cost of ditch systems militates against construction of suitable drainage systems. There is no serious water supply problem in Pajaro Valley. 63. Summary of flood and related problems. The major water prov lems of the Pajaro River may be summarized as follows: 10 Min ide Ma ley Pos 66. T (a P (a) Carnadero Creek-Llagas Creek-San Felipe Lake region.—The principal flood damage in the upper reaches of Carnadero and Llagas Creeks is caused by overbank flow, the floodwaters seeking and following low swales and depressions between normal channels (see inclosure 2). Additional water supplies are desired by local interests along upper Llagas and Carnadero Creeks and also along Pache Creek and other tributaries to San Felipe Lake. The lower reaches of these streams are in the area covered by the flood lake previous described. Here flood control by levee works would entail the co struction of extensive local drainage systems, including pumpg plants of large capacity. (b) San Benito River-Tres Pinos Creek region.-The principal feed damage in this area is bank erosion. Inundation is confined to the river wash in all but a few places (see inclosure 2). Great quantites of silt carried in the flood discharges of these streams are deposited in 1 Not printed. eco tudies wh buth Sa 65. T ly on ruction (b E be a elow G roveme Extensiv anderin hallow Not p he channel of the Pajaro River downstream, reducing its capacity until discharge from the upper Pajaro River clears the channel again. During the summer months, these sand bar deposits at the mouth of he San Benito River act as a control upon the water level in the Pajaro River immediately upstream, increasing the amount of percoation to the ground water table, a feature regarded favorably by water conservation interests along this reach of the river. The natural egimen of the San Benito River effects considerable regulation for joints downstream from its confluence with the Pajaro River. In the Hollister and San Juan Bautista areas, the water conservation desired by local interests leads them to favor reservoir construction over other methods of flood control. (c) Pajaro Valley. The principal flood damage in Pajaro Valley is caused by overbank flow (see inclosure 2). The channel gradient is comparatively flat, especially from Watsonville to the river mouth. The channel cross sections are relatively large and the stream velocities are about 6 feet per second during flood flows. Ocean tides do not materially affect flooding in the lower Pajaro Valley. There are local drainage problems, but no serious water supply problem. PLAN OF IMPROVEMENT 64. Complete flood protection versus partial flood protection.—Investigations made during this survey have shown that construction of works to effect complete control of flood flows in the Pajaro River is not economically feasible under present conditions in the basin. Studies have clearly demonstrated that the only areas in the watershed where the cost of effective flood control works is commensurate with the benefits are in Pajaro Valley below river mile 11.8 and in South Santa Clara Valley in the vicinity of Gilroy. 65. The broad considerations governing the selection of any plan of improvement for flood control are that it must provide or require: (a) Minimum disturbance of the natural regimen of the river. (b) Maximum effectiveness in the areas where damages are concentrated. (c) Minimum interference with existing development and facilities. (d) Maximum opportunity for collateral benefits and multipurpose use of improvements. (e) Positive action, the effects of which can be evaluated. 66. Types of improvements considered. (a) Reservoirs.-The basin was reconnoitered for sites for reservoirs which would effectively reduce flood peaks in the damaged areas. Only one advantageous site was found, and although reservoir construction was not found to be economically feasible, this development is discussed in more detail in a later paragraph. (b) Bypass floodways.-The only area in which this method appears to be adapted to flood control is in the Carnadero-Llagas Creek area, below Gilroy. Investigations showed, however, that a plan of improvement employing this method of control is not practicable without extensive supplemental highway and railway bridge modifications, rendering the project economically unfeasible. (c) Spreading works. The only area available for spreading works is in the lower portion of the San Benito River where the wide, shallow channel now is effective in reducing the peak discharge of 1Not printed. |