Feasibility Report of Proposed Watershed Program: East Plum Creek Watershed

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Eva Unit II -- S a N a m Gu lches Soveral alternati.v& sit wese Studied on Gulch; However it was found impossible to give the desired. pro: ecti.on to 1-:he Dian of Castle Roca_ except by placing the 3t:+. acture site EP-3 approximately one and one half miles above to7,n where it wonl4. reservoir ]..!-,.SO square. miles of drainage area, In addition, a f' ood ^rut: etc- etas- -3i.ng si ructure No_ EP-4 is needed. on Neirmen Gulch to reservoir 1.48 square miles of drainage area, The combined storage capacity of struci:ude sit:?s EP-3 and EP A ire 1,586 acre-2--et, at an estimated. •:_otal cost of , >1.50., 687 or the average anneal cost of 37,383 (See Tablos 2 and 6) , `t'he'se structures will have to be high hazard and i:;ould reservoir 87 ,,e:r cent of the drainage area above the t(mrn of Castle Rock, Colorado, Agricultural floodwater and sediment damages on Sell_c,rs G:aleh are confined to the upper part of the watervhed and the channel has becc,ie entrenched so far upstream that no possibilities exist 2or an economically feasible control under the watershed program, As a result, the only agr;icaltural floodwater damages which can be, controlled are to livestock and buildings primarily on - Nemmen Gulch which are estimatted to be $102 average annual damage, Floodwater damages to the 'totm. of Castle Rock, Colorado, roads, bridges, railroads, and residential or commercial property were calculated by estimating the damage from a 100 –yaar Frew -ency storm (1 per cent chance of occurrence) and from a 65 per cent chance of oc;currezce storm, It was .then assumed that the stoiras of intermediate fra quencies would have a noa'mal distribution of damages bet:vreen these two calculated frequencies„ The one per cent chance of occ -rence flood is estimated to have peal-, flow of 3,050 cubic feat por. second and to cause ;339,500 in damages. The 65 per cent chance of occurrence st:oi r. is eat:imated to cause 1$500 in damages. When these figures were converted t:o average annual damages they gave a total of x$2,960 average annual damages, In addition to these direct floodater damages there are inclirect damages Aiuh were evaluated by taking 15 per cent of the dire -A loodwater damages, Total average annual damages from Sellers and Iaemraen Gulches are estimated to be x$3,023 and the average a_raual monetary benefits at; $2,360 for a damage reduction of 73 pex cant, The ratio of average annual benefits (,$.2,360) to the average annual cost of structural. measures ($7,383) is ,.32 to 1000,