studies for the structure providing the most economic degree of protection at Sopris Dam site and for a channel improvement through Trinidad, that a combination of these structures is not feasible at this time. (See project justification, par. 94.) 78. Plan of improvement recommended. The most feasible plan for the protection of Trinidad, where the largest concentration of losses occurs, provides a channel improvement within the city of Trinidad for the protection of the urban areas which are overflowed by floods from Purgatoire River. In order not to induce a sense of false security in people in the flood plain, it was considered that the structure should provide flood protection equal to that of the maximum flood of record, if economically justified. To obtain the most economical design which would provide the greatest degree of economic protection, various channel sections and types were considered. Limiting channel widths and velocities governed the design of the recommended plan of improvement. For peak discharges up to and including a flood with a probable occurrence of once in 100 years, 45,000 cubie feet per second, and the maximum of record, a channel with an unpaved invert would not produce excessive velocities. To provide for discharges above 45,000 cubic feet per second would necessitate a wider unpaved channel section to control sufficiently the velocities. A wider channel section would involve expensive construction of several bridges and removal of commercial and industrial property. A channel with a paved invert which would control floods above 45,000 cubic feet per second, although of lesser width than an unpaved section, would require a more expensive design to control the resulting high velocities. The recommended improvement would consist of two typical sections and levee construction above and below these sections. One typical section would be an unpaved rectangular channel and reinforced concrete cantilever walls, with back-up levees having riprapped slopes capable of passing a peak discharge of 45,000 cubic feet per second with a 2-foot freeboard. The other typical section would be an unpaved trapezoidal channel with riprapped side slopes. Trapezoidal reaches would be constructed above and below the cantilever wall section. Upstream from the trapezoidal channel, levees with riprapped side slopes are provided to collect the flow into the channel proper. Below the downstream trapezoidal section, a levee with riprapped side slopes would be constructed on the right bank to confine the flow below Linden Avenue. Two drop structures are provided to obtain the most economical channel design. A two span deck girder bridge, with 150 feet between abutments, would be constructed at Commercial Street to replace the present two span concrete arch structure. The intake of the Baca irrigation ditch would be moved about 2,450 feet upstream and would be constructed as an integral part of the drop structure above Animas Street. A concrete conduit from the intake would carry the water diverted to the present ditch at Chestnut Street. Concrete channel weirs would be constructed at critical locations to control the channel slopes to the desired grade. The construction of the improvement would entail special construction provisions at the abutments and piers of some of the existing bridges. The plan and cultural details are shown on map No. 9, appendix A. The plan and sections of the recommended improvement are shown on map No. 10,' appendix A, and profiles are shown on map No. 11,' appendix A. Structural details are shown on map No. 12,' appendix A. The engineering features are given in table No. 9. TABLE No. 9.-Engineering features of recommended channel improvement at Trinidad, Colo. 79. Estimate of first costs and annual charges.-The estimated cost of the recommended improvement, including overhead and contingencies is $1,034,500. The estimated annual charges are $42,200. Estimate details are shown in table No. 10. This map is not printed. TABLE NO 10-Estimated first costs and annual charges for recommended plan of improvement at Trinidad [45,000 cubic feet per second] Cost of channel: Designation Clearing, diversion and care Walls. Footings and slabs Riprap, handplaced. Riprap, dumped.. Black steel pipe, 2 inch. Total cost of channel. Special construction provisions: Local drainage Bridge alterations. Relocation of Baca diversion. Total cost of special construction provisions. Cost of Commercial Street Bridge: Total net cost. Cost of lands and damages: Lands and severances. Other damages... Total cost of lands and damages. Total net cost of project Overhead engineering and contingencies (25 percent). Total estimated cost of project.. NOTE: L. S., indicates lump sum Non-Federal investment: L. S.. Cubic yard Commercial St. Bridge. Unit Cubic yard. Cubic yard. Lineal feet. Lineal feet.. Square feet. L. S L. S. L. S. L. S. ANNUAL CHARGES L. S. Federal investment: Trinidad Channel improvement_ Net Federal investment.. Local drainage. Lands and severances.. Other damages Interest during construction (period less than 1 year). Net non-Federal investment.. Total cost of project- Quantity 10,400 670 254,000 202,000 128,000 14,000 7,600 9, 100 25, 600 5,600 5,000 2,520 5,760 Federal annual charges: 3 percent of Federal investment.. Total Federal annual charges. _ TABLE NO 10-Estimated first costs and annual charges for recommended plan of improvement at Trinidad—Continued ANNUAL CHARGES-Continued 1. Urban area. 2. Agricultural area. 80. Estimated average annual benefits-general.-Principal flood damage areas in the Purgatoire River Basin have been shown to be in the Trinidad urban area and in the agricultural area along Purgatoire River from Sopris dam site (mile 163.3) to the head of the canyon section (mile 122). (See par. 38.) The estimated average annual benefits have been determined for the urban area in Trinidad and the agricultural area from the Sopris dam site to the head of the canyon section. These include the benefits accruing from prevention of tangible recurrent flood damages as follows: (a) Crops, lands, and improvements. $5, 100 800 900 100 3. Irrigation structures. 4. Land enhancement. 6, 900 42, 200 5. Irrigation (flood water detention). 6. Irrigation (maintenance less than bankfull stages). Although the magnitudes of past disparate floods form the basis for the estimates of losses, the prevention of damages assigned for floods of a given magnitude are those which would occur under present conditions of development. Equal peak discharges in Purgatoire River will produce varying amounts of damage. This anomaly is explained when the great difference in volume of run-off resulting from dissimilar storms is considered. Damages are relatively small that result from floods produced by local storms, like that of July 1925 which resulted in the third highest peak discharge of record. The run-off in these floods is characterized by a sharp peak and small volume. Floods produced by storms similar to those of September 1904 and April 1942 remain at high stages for comparatively long periods. Consequently, the damages resulting from wave trains due to the present channel alinement are much greater than for small volume floods of comparable peak discharge. Therefore, the type of flood has also been considered in determining the average annual benefits from prevention of flood damages. The benefits accruing in the area under consideration are briefly discussed in the following paragraphs and are fully covered in appendix C.1 81. Average annual benefits-Trinidad. The recurrent tangible flood losses at Trinidad have consisted principally of damages to residential, business, industrial, municipal, and public utility properties. The average annual prevention of flood damages in the city 1 Not printed. was estimated by a flood occurrence-damage analysis. Stagedamage curves for disparate floods were constructed and were based on damage survey data. A summation correlation of stage-damage and stage occurrence for the dissimilar floods gave an indicated average of annual benefits of $29,650. 82. Average annual benefits-agricultural-crops, lands, and improvements.-Losses resulting from caving banks have not been considered as a preventable loss. The losses to the agricultural lands and improvements result principally from damage to crops; destruction of fences; deposition of sand, gravel, and debris; and erosion of top soil. The average annual prevention of flood damages has been based on the assumption that the chronological occurrence and also the magnitude of future floods will be similar to that of past floods of record above 15,000 cubic feet per second (bankfull stage). Floods of less than 15,000 cubic feet per second produce no flood damage under present channel conditions. Consideration has been given to the type of flood and the time of year in which the flood occurred, in determining the average annual benefits to crops, lands, and improvemeats which were computed as $834. 83. Average annual benefits-transportation facilities, agricultural area. The flood damages to transportation facilities in the agricultural area of the flood plain, which does not include the urban area of Trinidad, have been erosion of highway embankments due to overflow, bridges, and railway property. The average annual benefits, which were estimated in analysis similar to those for Trinidad, were computed as $1,611. 84. Average annual benefits irrigation structures.-The average annual benefits for irrigation structures in the agricultural area give consideration only to those damages occurring to permanent irrigation structures, for floods exceeding bankfull stage, or 15,000 cubic feet per second. It is considered that all benefits accruing for discharges of less than 15,000 cubic feet per second will be irrigation benefits. The average annual benefits were estimated in a similar manner to those for Trinidad. The estimated average annual benefits computed were $4,561. 85. Irrigation benefits-flood detention. The United States Department of the Interior, Bureau of Reclamation, has estimated that the benefits derived from a supplemental supply of water (water saved for irrigators) made available to lands with annual shortages by operation of Sopris Reservoir, reducing flood flows to a maximum of 700 cubic feet per second, would be $5 per acre-foot. It was also estimated that approximately 5,540 acre-feet per year, with a detention pool of 5,200 acre-feet, will be made available to about 15,000 acres of land having an annual shortage. The average annual benefits would be $27,700. 86. Irrigation benefits- less than bankfull stages.-A reservoir at the Sopris site would, by regulating those channel flows which now damage or destroy the minor brush and rock-irrigation dams, reduce the annual maintenance for those works, thus resulting in a further benefit to water users. The estimated annual value of this benefit is $4,000. 87. Benefits-enhancement of values. The area subject to flood damage was investigated for possible enhancement of values which might result from flood protection. An estimate of the probable enhancement was based on field investigation and interviews in which |