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. ( . .. ( .. .~ L - 17 - BENEFITS FRCM vl0RKS OF IMPROVEMENl' The floodwater-retarding structure IW-1 will provide a high level of. protection to the icinds and .improvemEmts in the area subject to da"'nage~ The peak discharge. runoff from the 100-year frequency stonn from the unreservoired. area above the High Line Canal plus the principal spillway discharge fram the structure will not exceed the flood channel capacity through Grand Junction. However, occasionally runoff fram the uncontrolled area below the High Line Canal may combine with the above flows and reduce the above benefits by causinq minor overbcu'lk flooding along the Indian Wash channel and backwater ponding and sediment damage along portions of North Avenue beginning about the fifty-year frequency storm. Minor damage fram overland flows and local runoff may occur in ponding areas in the commercial area along the lower Highway 6-24 and the railroad grades at about the twenty- year frequency stonn. These areas of remaining damages are sho\offi on the Flood Damage Map, Figure 1. With the project installed a recurrence of the stonn of June 6, 1958, would produce no damages. Flooding of floodplain lands will be reduced to that caused by local runoff, with the area flooded and the frequency of flooding greatly reduced. The floodplain area subject to flooding will be reduced from 2,500 acres to 450 acres with the project installed. On the basis of 2,050 acres benefited, the cost of the floodwater- retarding structure per acre benefited is approximately $190. The land use in the floodplain area is about 60 percent urban and commercial and .40 percent suburban, of wh~~ the suburban area is estimated to be about 65 percent agricultural and 35 percent miscellaneous. The cropping pattern used for the composite acre for the irrigated lands are: Crop Alfalfa Corn Irrigated Pasture Small Grain Truck Crop (Tomatoes) Orchard (Peaches) Percent 35.5 28~1 12~5 14.7 2.2 7.0 Total 100.0