FLOOD06660 - Laserfiche WebLink

Home Browse Search

- 27 - !t it' .' "'\ ~ ~ ~' ( ~' Standard structural design and cost estimates were made. Embankment slopes of 3:1 and 2:1 were used for structures under thirty feet in height. For structures over thirty feet in height, embankment slopes of 3t:l and 2:1 were used. Cost estimates were based 0'1 current Colorado Pilot Watershed and Public Law 566 contract costs. I The geologic and topographic conditions and "cut" and "fill" relationships are suitable for earth spillways. The emergency spillway discharges will be confined with compacted, moisture- controlled, earth-wing dikes. i I l I , Twenty-three of the 24 proposed floodwater-retarding structures are classifi2d as class .'a" structures. Structure FP-Pl is classifiec'. as a class "c" structure. These structure classifications are based on an interpretation of definitions in the Engineering Memorandum SCS No. 27. The sediment storage capacity in each of the twenty-four structures is provided to contain the estimated accumulation at each site for the next 50 years. The floodwater-retarding capacity for the class "a" structures was based on the runoff from a 25-year, six-hour storm for those structures emptying in 48 hours or less and a 25-year, 24-hour storm for those structures emptying in a longer period of time. The floodwater-retarding capacity for the class "d' structure l~S based on the runoff from a 100-ye~r, si;:-hour stome, r:- t. The principal spillways 1'Jill be reinforced concrete pipe and will be equipped with an ungated, reinforced concrete riser to the height of the expected level of sediment deposition at the upstream face of the dam as shown on Figure 3. Release rates of these spillways conform to Colorado State policies with the exception of structures in the East Cherry Creek subwatershed where special permission was obtained for prolonged maximum release rates in order to reduce overbank flooding in downstream areas, i I ~. ~ Criteria for the emergency spillways is in accordance with Soil Conservation Service Engineering Memorandum No. 27 and 31. (revised), Advisory Notice W-428, and Section 3.21 of the Soil Conservation Service National Engineering Handbook, Section 4, Supplement A. ~ For the class "au structures, routing of the 25-year frequency, 24-hour duration rainstorm for East Cherry Creek Watershed and six- hour duration rainstorm for the remaining structures determined the minimum crest elevation of the emergency spillways. The spillway widths were determined by using criteria set up in Engineering Memorandum SCS 31 (revised). The maximum capacities were determined by using the width found and a depth of flow (Hp) equal to the difference in elevation between the crest of the emergency spillways and the top of the dams, These maximum spillway capacities are equal to or exceed the required freeboard hydrograph peaks.