various local authorities were consulted. The enhancement attribut- 88. Summary of average annual benefits.-The estimated average annual benefits, which may be credited to adequate flood control improvements for prevention of flood damages and for contingent returns during their effective life, are $41,650 in the Trinidad urban area and $39,021 in the agricultural area. Classification of the estimated benefits is shown in table No. 11. TABLE NO. 11. Summary of average annual benefits, Purgatoire River flood plain,1 URBAN AREA (CITY OF TRINIDAD) Residential, commercial, industrial, municipal, railroads, utilities, com- Total urban area. AGRICULTURAL AREA Crops, lands, and improvements.. Permanent structures_. Flood-water detention... Less than bankfull stage (reduction of operating costs). Total agricultural area. Total urban and rural benefits for entire flood plain... ! For floods of probable occurrence equaled or exceeded per hundred years. 'Not printed. Annual benefits $29, 650 12, 000 41, 650 834 1, 611 4, 561 27, 700 4,000 315 39, 021 80, 671 89. Project justification.-The benefits which might accrue from prevention of floods for any project or combination of projects, in the area under consideration, for floods of probable occurrence equaled or exceeded per 100 years are given in the preceding paragraphs. 90. The average annual benefits accruing to a dual-purpose reservoir at the Sopris Dam site, which would provide flood protection for a probable flood occurrence of once in 100 years, would be $80,671. This amount includes the total average annual benefits in the urban area of Trinidad and the agricultural area from Sopris Dam site to the head of the canyon section as given in Summary of Average Annual Benefits, table No. 11. The annual charges for she project are $176,000. (See appendix D.) The ratio of average annual benefits to annual cost is 0.46, and the project is not economically justified. 91. The average annual benefits accruing to the dual-purpose reservoir, which would provide the greatest ratio of benefits to costs, were evaluated by assuming that the total benefits would include those irrigation benefits by flood detention and the prevention of flood damages up to and including probable flood discharges of 19,700 cubic feet per second. The average annual benefits were $31,425. The annual charges for the project are $61,600. (See appendix D.) 1 It is apparent that with a ratio of benefits to costs of 0.51, the project is not feasible at this time. 92. A reservoir at the Sopris site for flood control only would not be feasible. This is evident from the low economic ratio for the dualpurpose reservoir which would control a probable flood occurrence of once in 100 years. Of the benefits attributable to the dual-purpose reservoir, 39.3 percent are irrigation benefits. To control a flood with a probable occurrence of once in 100 years would require a reservoir capacity of about 28,400 acre-feet for the dual-purpose structure. Only 5,200 acre-feet of this total are allotted for irrigation detention. It is therefore obvious that the capacity for a flood control reservoir would not be substantially smaller than for a dual-purpose structure. Also such a structure would have a small ratio of benefit to cost. 93. Investigations for several channel designs through the urban area of Trinidad indicate that none of the designs would be economically justified. The economic limits of the various designs are given on chart No. 1,' appendix A. It will be noted that the break in the annual charges curve indicates a change in type of channel. 94. It is evident that a combination of a reservoir for flood control only or a dual-purpose reservoir at the Sopris site and a channel improvement through Trinidad would not be feasible. The most economical dual-purpose structure at the Sopris site is not feasible, and the improved channel through Trinidad with the greatest ratio of benefits to cost is not economically justified. Therefore, a combination of the two structures for prevention of flood damages is not feasible. 95. An inspection of chart No. 1,' appendix A, indicates that a channel improvement through Trinidad with a design capacity of 45,000 cubic feet per second (described in par. 78) would have a ratio. of benefits to cost of slightly less than 1: 1. Therefore, construction of this project is not economically justified, but, considering the national economic importance of the railroad facilities and the welfare of the city of Trinidad, the project is warranted. 1 Not printed. from aphs. reser 0,671. 96. Flood channel delineation.-The recommended plan of improvets, m ment is described in paragraph 78. With this type of improvement aled there is little likelihood of any encroachments by future residential, commercial, or industrial construction that would in any way constrict the flood channel. In the event that the Chestnut Street Bridge is on for replaced, the structure should be located so that the center line of the structure will be approximately on the center line of any future channel developments in that particular reach. The clear span should be adequate to provide for the probable channel widths, and any piers in the channel should not give a ratio of channel constriction greater than that of the Colorado & Southern Railway Bridge, just downstream from Chestnut Street. Any future channel improvements within the city of Trinidad should be in conformity with the recommended plan of improvement in order to preserve its integrity. urban -ite to nnual $176 its to 97. Discussion.-Dissimilar type storms in the Purgatoire River drainage area result in flood waves of comparable peak discharge, but vary greatly in volume of run-off. The maximum flood of record in the area under consideration occurred in September 1904, and a flood of this magnitude would have a probable occurrence of about once in 100 years. e res costs, those flood 9.700 425. D.)1 oject not Jual rence pose with Ivoir ture. tion. voir ure. ban omi n on nual trol nel eco the fits of t a - of atio tion the are 98. In the area under consideration from Sopris Dam site (mile 163.3) to the head of the canyon section (mile 122), 5,000 acres are in the probable overflow area. Of this area, 370 acres are in the urban area of Trinidad, 1,050 acres are cultivated, 400 acres are irrigated pasture, 730 acres are grazing land and the remainder is waste. The urban area of Trinidad contains railroad tracks and yards, residences, city parks, business and industrial establishments, highway and railroad bridges, streets, sewers, water and gas mains, wire lines, and other improvements of an urban nature. Both railroad and city property in the urban area are particularly susceptible to damage. In the agricultural flood plain, cultivated acreage and fences sustain the major flood damages to lands and improvements. Seven low concrete and several brush and rock diversion dams, headgate structures, and canals are partially or wholly destroyed during floods. The direct and indirect damages for the flood of April 1942, which affords the most logical basis for reducing past flood damages to present conditions, were $530,200. 99. Local interests' plan for four regulatory reservoirs and one irrigation storage reservoir would benefit the valley by conservation of floodwaters. The small reservoir storage capacity of these structures would not provide dependable flood control for the drainage areas above the dam sites. The total benefits which would accrue from the execution of the plan would not be great enough to warrant the construction of the five reservoirs suggested by local interests. 100. The construction of the John Martin Dam and Reservoir (authorized by the Flood Control Act of 1936, as amended by the Flood Control Act of 1938) is now under way at a site on Arkansas River, immediately below the mouth of Purgatoire River, and will provide a reservoir of sufficient capacity to control completely all stream flows at its site, consequently, there is no immediate need for reservoirs in the lower Purgatoire River Basin to provide supplementary water supply in Arkansas River. The "308" studies have indicated, and the present studies confirm, that the benefits to be gained in the lower Purgatoire River Basin from the development of supplemen tary water supply at the Mammoth, I. S., or Purgatoire No. 2 sites are negligible and that developments at these sites would have value only in the Arkansas River Basin. They may be of value in the future as possible replacement for storage capacity in John Martin Reservoir that may be lost by silt accumulation. 101. In a flood plain such as that of Purgatoire River, in which the overflow limits are narrow, the area benefited by channel rectification, levees, or bank stabilization is small. The costs per unit length of channel exceed the benefits which would accrue from such flood-control works. 102. The erratic stream flow of Purgatoire River and demand for all available water for other purposes precludes the development of There is no need for further munichydroelectric power in this area. ipal water supply in the basin as existing reservoirs meet the present needs. There is no stream pollution problem in Purgatoire River under present conditions. 103. The dual-purpose reservoir, plan 1, at the Sopris site, which was designed to control a flood of probable occurrence of once in 100 years, would control all experienced floods and provide additional water for irrigation which is urgently needed by agricultural areas in the valley. The reservoir would provide complete protection from small volume floods, but those of sufficient magnitude to produce a sustained outlet discharge would damage irrigation structures and cause bank caving. Even though a dual-purpose structure would more adequately serve local interests in the valley, it is not economically justified at this time as the ratio of annual benefits to annual charges is 0.46. 104. The dual-purpose reservoir, plan 2, with spillway crest at elevation 6,195 was investigated in the light of data on irrigation benefits furnished by the Bureau of Reclamation and the results of the study were discussed with representatives of that agency. It was found that a large portion of probable irrigation benefits obtainable, and some flood-control benefits through detention of small volume floods, would be provided. The flood-control benefits would be small. The ratio of total benefits to costs being only 0.51 shows that a development of this type is not economically justified. 105. A reservoir at the Sopris site, for flood control only, which would control the maximum flood of record would require a capacity not much smaller than the dual-purpose reservoir controlling a flood of the same magnitude. Therefore, it is evident from the small economic ratio (0.46) for the dual-purpose reservoir that a reservoir for flood control only is not economically justified. 106. It was considered that a channel improvement through the urban area of Trinidad should provide flood protection for the maximum flood of record, if economically justified, so as not to induce a false sense of security among inhabitants in the area. The most economical degree of improvement is indicated on chart No. 1,1 appendix A. This degree is provided by the recommended plan of improvement. The economic comparison of other plans considered is also shown on chart No. 1.1 107. The recommended plan of improvement will safely pass a flood discharge of 45,000 cubic feet per second which is equal in magnitude This chart is not printed. th to the maximum flood of record, and also equal to the probable maxi- 108. The average annual tangible benefits accruing to the recommended plan of improvement amount to $41,650. While the annual benefits closely approximate the annual costs ($42,200), the economic ratio is slightly less than 1 to 1 and conclusive economic justification is lacking. The intangible benefits accruing to the recommended plan of improvement would be large. Among such benefits, on which monetary values cannot be placed, are the preservation of nationally important transportation facilities, both rail and highway, and prevention of service interruptions affecting national commerce. Also included are the more local benefits of prevention of possible loss of life, exposure, and human suffering due to flood destruction, and general impairment of community welfare. Therefore, in consideration of both tangible and intangible benefits accruing to the recommended plan of improvement, in comparison with its estimated annual costs, its construction is warranted. 111. The recommended plan of improvement would provide flood protection to the urban area of Trinidad only. It would provide no additional divertable water to the irrigation systems. However, construction of the recommended plan of improvement will in no manner interfere with construction of irrigation storage reservoirs anywhere in the watershed when such construction may be found desirable. 112. Because of the local nature of the tangible benefits, responsible local interests should give assurances satisfactory to the Secretary of War that they will: (a) Provide without cost to the United States all lands, easements, and rights-of-way necessary for the construction of the project; (b) hold and save the United States free from damages due to the construction of the works; (c) provide the costs of public utility and bridge revisions, relocations, or replacements; (d) provide next 1. 109. A dual-purpose reservoir at the Sopris site in combination with an improved channel through Trinidad would most adequately serve the Purgatoire River Valley. Local interests have stated that flood protection in the urban area of Trinidad and the agricultural areas along the main river and tributary streams is desired. They have stated, also, that conservation of floodwaters above the ability of the irrigation systems to divert these flows is desired. The dualpurpose-reservoir under plan 2 would provide conservation of water for irrigation use and some flood protection in agricultural areas while the improved channel through Trinidad would provide complete flood protection in the urban area. This combination is shown by analyses to provide the most economical accomplishment of the desires of local interests. Since neither item of the combination shows a favorable ratio of benefits to costs the combination is not economically justified at this time. 110. Summarizing all plans submitted by local interests, multiplepurpose projects which would most adequately serve the needs in Purgatoire Valley, and channel improvements through Trinidad for protection of the major damage center, it is found that the recommended plan of improvement would provide the greatest returns on the investment. |