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Evaluati on Unit III - Cau'O'bina-1 and grade <br />S Structures <br />The three combination structures a grade <br />stabilization s`L sitar selected for evaluation were 'chose believed <br />- aicall y f easilble,, The p <br />chance of being ocona. I.- --- a <br />to have the most: <br />purpose of thesci structures i3 grade stabilizat-ion and. sediment control.. <br />They will provide only a small amount of f lood -Iiat -or damage reduction, <br />s ince,. ince, the 3,65 square miles of area reservoired is only 6..4 par cent <br />of the area above Larkspur and the area reservoired by structure site <br />EP-5 is rereservo-ired by site number EP•1. Since none of the evaluated <br />combination floodwater-retarding and grade stabilization zation structure sit <br />approach a favorable benefit•cost ratio; it is bolieved. that - there are <br />o <br />i <br />T feasible sites for this type of strucil- i <br />Lre in the ratershed <br />under present criteria for evaivations, The pr_JI.ary bencfits of each <br />of these structures accrue to one indiv`dual and they would therefore <br />not meet the requirements of Public Law 566, 41 <br />Cook Creek Site.- The co: floodwater- retarding and grade <br />stabilization stxuctuxe EP-5 is located on a branch of Cook Creah in the <br />South Part of Sec ion 16, Townahip 10 S•, Range 67 W., and would <br />reser-oir the runoff from 1.36 square miles of drainage area,. This <br />structure site would protect an estimated 10 acres of good s-&-irrigated <br />pasture from total loss due to channel entrenchment and an additional <br />L. - LI: of the water table The <br />65 -acres from partial loss due to draining o <br />calculated average annual monetary benefit lxcrt preventing this channel.. <br />trenching is $267,. A.en this is compared. with the expected average <br />annual cost of the structures $1,043 it gives a benefit -cost ratio of <br />24 to 1,00„ <br />Carpen Cruek Site The combination and <br />grade stabilization structure site RP-6 with a. drainage area of 1.03 <br />square miles is located on a side drainage of Carpenter Creek that comes <br />in just below structure site EP-2� It is estimated that this structure <br />site will prevent the to'cal loss of five acres and the partial loss of <br />15 acres of meadow hayland as a result of channel entrenchment and the <br />su-bsequent loss of the i% i:able, It could also reduce sediment <br />deposition on five acres of meadow hayland below the structure which <br />, All be a total loss during the evaluation period iryithout the structure, <br />The estimated average annual monetary benefit to structure Bite <br />EP-6 is X294 and the average annual cost of the structur-. is <br />estimated at $1,218 which gives a benefit -cost ratio of .24 to 15.00 <br />