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floods by channel storage and percolation. Land having sufficient it area and low productive value with high percolating qualities is not available in the vicinity. This method of flood control, therefore, is not practicable.
(d) Evacuation of lands and property now subject to flood damage is not economically sound as the value of such property exceeds the
69 capitalized average annual flood damage. (e) Stream clearing.-The prospective benefits to be expected from
pere this type of project without supplementary channel works are too uncertain to warrant detailed planning of channel clearing alone is : River definite project.
(f) Channel improvement by means of levees and bank protection works offers the most direct, economical, and satisfactory means of climinating the damages now caused by floodwaters in Pajaro l'adey and in south Santa Clara Valley in the vicinity of Gilroy Chanel clearing within the limits of the projects is considered a part of all levee projects. It was found that jetty construction is not a necesur
Рpris feature of the proposed channel works in Pajaro Valley (see appendisch VI,' par. 19).
heter 67. Most practicable plan of improvement, Pajaro Valley.-- The cus struction of levees and appurtenant works from the mouth to nier mile 11.8 on the right bank and river mile 10.6 on the left bank Is found to be the most practicable plan of improvement for Pajaro Valley at this time, and is designated (see enclosure 3) as project A. Corralitos Creek also would be leveed on both banks for a distance of about 1 mile. Field and laboratory investigations have shown that the recently constructed levees on both banks of the Pajaro River are structurally sound and could be incorporated in this plan of
whefin improvement (project A). This project would include bank protection works composed of steel jacks and rock riprap at all bends and other critical places, the modification of several bridges, and the provision for drainage of local areas adjacent to the works. Details of this plan are shown on enclosure 3, sheets 1-12,' and the varpus specific items of work are described in appendix VI."
68. The channel is designed to safely discharge 19,000 cubic fet per second above the mouth of Corralitos Creek and 22,000 cubic feet per second from this point to the end of the project near Montres Bay, with 3 feet of freeboard. These quantities correspond to the 2-percent-chance flood as estimated in appendix III. It is desirable
, however, to protect the urban areas against larger floods. There are no reliable records of extremely large past floods such as that of 100% upon which to base the channel design. However, the freebeard allowance referred to above could be safely encroached upon important urban areas where ample labor is available for adequate emergency flood fighting. The channel capacity above Corralios Creek would be increased by 1,900 cubic feet per second if the frit board were allowed to be reduced to 2 feet and by 4,000 cubic fei per second if the freeboard were reduced to 1 foot. The sponding discharges at the Chittenden gaging station would be 2017 cubic feet per second and 23,000 cubic feet per second, respectira probali. These values are considerably greater than the probable magnitude
of the 1e 7 Fere for
Pree, a protect - Perce
be linn chance
1 Not printed.
of the 1-percent-chance flood, as indicated by the frequency studies previously described (about 19,700 cubic feet per second). The design channel capacity is accordingly established as the 2-percentchance flood in the agricultural areas and the 1-percent-chance flood in the urban areas.
69. An alternate plan of improvement, Pajaro Valley, designated as project B and shown on sheet 1 of inclosure 3, would extend from approximately river mile 5.25 upstream to river mile 8.6 on the left bank and river mile 6.5 on the right. On the right bank, the Pajaro River levee would extend up Corralitos Creek, a distance of approximately 1 mile. The existing levees would be incorporated into this project in the same manner as for project A. The purpose of studying this alternative project is to determine the economic justification for providing adequate flood protection for the highly developed urban areas in Watsonville, Pajaro, and Watsonville Junction, which are now partially protected by existing levees. Essentially, this project represents a minimum improvement of the existing works in the river reach where food damages are most highly concentrated. Except for the terminal arrangements and the omission of added freeboard to the existing levees, the work contemplated under this plan is similar to that under project A for the same reaches and river banks. The design channel capacity would be the same as for project A below Corralitos Creek.
70.- Plan of improvement, South Santa Clara Valley.- Modification of the existing levee on the left bank of Carnadero Creek from river mile 7.70 to river mile 9.25 and the construction of a short tie levee were found to be the only project in the upper watershed where the benefits were commensurate with the costs. Details of the modification of the existing levee, together with the construction of the new levee, are shown on inclosure 3, sheet 13. This project would afford protection to the city of Gilroy from all floods up to the limit of the 1-percent-chance flood as determined from the flood-frequency studies presented in appendix III.'
71. Estimates of discharge on Carnadero Creek were made by correlating with Uvas Creek, one of its tributaries (see appendix III). Since 1908, this area has experienced only one flood greater than the 2-percent-chance flood as determined from the flood-frequency curve (Uvas Creek) developed in appendix III. No historical floods greater than those estimated in the annual series are definitely known. Since the city of Gilroy is a highly developed urban area, the limit of discharge for design purposes was set as the l-percentchance flood in a manner similar to that discussed above for the Pajaro River levee.
ESTIMATES OF FIRST COST AND ANNUAL CHARGES
72. Cost influences.-The estimates of cost herein are based on normal conditions for construction work in the general area in which the Pajaro River is situated. No attempt is made to appraise the probable influence of present war activities on future construction
1 Not printed,
costs. Contingency allowances cover only the usually uncertain field conditions met with during construction.
73. Conditions influencing annual charges and maintenance costs Amortization charges are based on an assumed economic life of 3) years for the various new projects discussed herein. Replacement of some bank-protection works is expected within this period and is provided for in the annual maintenance charges. Construction of the Pajaro River levee project A would be completed within 2 years and the Carnadero Creek levee project would be completed within 1 year. Maintenance costs on 4.25 miles of existing levee near Watsirville are estimated at $1,200 per year. Local interests now maintaan these existing levees. Interest rates are assumed at 3 percent and 4 percent annually for Federal and non-Federal investments, respietively. Cost allocations are discussed in paragraph 84.
74. Project first cost estimates and cost of cristing projects. The project first cost of Pajaro Valley levee project (plan A) is estimated at $741,000 and the alternate (plan B) at $301,000. The cost of the existing levee project at Watsonville and vicinity was $316,000. The project first cost of Carnadero Creek levee near Gilroy is estimated at $65,000, and the cost of the existing levee there was $34,000. L all cases the project costs for new work are added costs, over and a bore the costs of existing works. Detailed estimates are contained in appendix VII ' and total costs of Federal and non-Federal first costs are summarized as follows: (a) Pajaro Valley le vee project, plan A: (1) Federal first costs: (a) Levee construction: 1. Clearing right-of-way
(b) Southern Pacific Co. railroad bridge change
and incidental items.
Total Federal first cost, plan A. (2) Non-Federal first costs; (a) Provide easements for levee construction and maintenance.
plants, irrigation pipe lines, wells, sumps,
pile trestle approach with paved ramp
pave channel transition and construct
Total non-Federal cost, plan A.
Total project first costs, plan A.
1 Not printed.
(6) Pajaro Valley alternate levee project, plan B:
1. Clearing right-of-way
8, 800 10, 500
(6) Southern Pacific Co, railroad bridge change
and incidental items.
Total Federal first cost, plan B.
channel transition and construct steel
Total non-Federal first cost, plan B.
Total project first costs, plan B..
301, 000 (c) Existing 4.25 miles of levee project in Watsonville and
95, 810 Expenditures to Mar. 31, 1941.
1230, 600 (2) Right bank, 1.45 miles of levee in Watsonville,
Santa Cruz County:
3, 800 Expenditures to 1939.---
85, 400 Total cost of existing levee in Watsonville and vicinity --
316, 000 (d) Carnadero Creek levee project near Gilroy: (1) Federal first costs: (a) Levee construction: 1. Clearing right-of-way.
$400 2. Earthwork.
10, 200 3. Bank-protection work.
Total Federal first cost.---
Total non-Federal cost.
Total project first costs, Carnadero Creek levee im
In addition, unexpended funds of $98,937 are held under notice of indefinite suspension of work effective Mar. 31, 1941.
(e) Existing 1.04 miles of levee project on Carnadero Creek near
$34, W 75. Estimates of annual charges.--The total added annual charris for Pajaro Valley plan A are estimated at $35,000 and for plan Bit $13,650. The annual charge of the existing levee project at Watsuville and vicinity is $14,870. The total added annual charge of the Carnadero Creek project is estimated at $2,570 and the annual charge for the existing levee project is $2,180. The detailed estimate of maintenance charges is given in appendix VII. The allocations of first costs and annual charges are set forth in paragraph 84 and the estimates of annual charges are developed as follows: (a) Pajaro Valley levee project, plan A: (1) Federal first cost for new work items, as set forth in par. 74a (1)
$497, 700 Interest at 3 percent during one-half of approxi
mate 2-year construction period...
(2) Federal annual charges:
(a)Interest at 3 percent on $512,000.-
Net Federal annual charge for plan A.
and improvements, as set forth in par. 74a (2) - $243, 300
(4) Non-Federal annual charges:
(a) Interest at 4 percent on $253,500.-
and operation cost of plan A in con
junction with existing works ---
4.25 miles of existing levee-----
(d) Net non-Federal annual charge for plan A..
Total added annual charge for plan A.--(6) Pajaro Valley alternate plan B: (1) Federal first cost or net investment for new work
items, as set forth in par. 746 (1)-
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