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- 14 - r All but one of the floodwater-retarding structures are designated as class "a" (as defined in Engineering Hemorandum SCS-27 , Page I, dated Harch 14, 1958), The exception is designated as a class "c" structure because of its location above the com~unity of Parker, Colorado, ;; ~ ~ ~ , ~ The class "aN structures as planned will control the runoff from 92,62 square miles of area fronl a 25-year frequency stonn, The cluss "c" structure is planned to control an area of 0,68 square mile and runoff fram a 100-year frequency storm. Floodwater storage capacities are based either on containment of a runoff volume produced by a 25-year frequency storm of 2,40 inches of rainfall occurring in a six-hour period or a 100-year frequency storm of 3,04 inches of rainfall occurring in a six-hour period. ~ t J i l The 24 structures will have a combined storage capacity, including sediment storage, of 4,470 acre-feet. All structures are to be rolled earthfill type with ungated principal spillways of reinforced concrete pipe and vegetated emergency spillways, The floodwater diversion structure will consist of a compacted earthfill dike and an unlined channel approximately 3,000 feet in length. It will divert the runoff fram a small unreservoired drainage of 0,54 square mile into floodwater-retarding structure FP-E2 at a maximum rate of 230 cubic feet per second. ;: G ,. ~ I ~ [: ~~ The locations of the structures are shown on Figure 2 (Structure Location Hap), The costs and pertinent physical data are shown in Tables I, 2, 3, and 3 a. A general pIan and a cross section of a typical floodwater-retarding structure are shown on Figure 3, BENEFITS FROM HaRKS OF ll1PROVEHENT I It is estimated that the average annual damages will be reduced from a total of ;;;88,393 to ~37,109 with the cOlnpleted program (Table 7), This amounts to a reduction in annual damages of ~51,284 or 58,0 per cent. Of this reduction,3.16 per cent is credited to the effects of the land treatment measures and 54.4 per cent to the structural measures. Included in the above program benefits are benefits from restoration to former productivity, amounting to $1,453 annually (Table 7). In addition it is estimated that ~S07 of annual benefits will accrue to land treatment measures for flood prevention (fire protection and control, F The watershed program is expected to make a substantial reduction in damages and area flooded of storms with a recurrence interval less than 25 years,