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I I I I I I I I I I I I I I I I I I I iv I I I I I I I I I I I I I I I I I I I Coope7' SLough F7'om the confLuence with BoxeLde7' C7'eek to State Highway 14, the shaLLow !,Looding east of the channeL is eLiminated by the storoge in the upst7'eam detention ponds. The 7'ecommended imp7'ovements downst7'eam of Highway 14 7'equi7'e a channeL with a design fLow of 300 cfs. The existing channeL shouLd be enLapged and a new channeL wiLL paroLLeL the Lake CanaL. A concpete Lined channeL win be 7'equiped f7'om Highway 14 to about 1400 feet downst7'eam. The 7'emaining p07'tion of the channeL wiLL be g7'ass-Lined. A new c7'ossing at She7'py Road and cpos sing unde7' the Lake CanaL neap the confLuence with BO$eLde7' C7'eek a7'e 7'equi7'ed. F7'om State Highway 14 to a haLf miLe upst7'eam of Vine Dr>ive, the fLood- pLain is shaLLow, divided and wide with nume7'OUS ponds f07'ming upstpeam of channeL c7'ossings. The 7'ecommended imp7'ovement pLan is to constpuct detention ponds upst7'eam of vine D7'ive, CoLorodo and Southem RaiZ7'oad and Highway 14. The combined stopage voLume fo7' the 100-yea7' 7'eCUP7'ence inte7'vaL fLood fo7' deveLoped conditions is app7'o$imateLy 500 aC7'e-feet (ac-ft). Of this voLume, 190 ac-!'t is Located in 2 ponds upstpeam of Highway 14 and 108 ac- ft is p7'O- posed upst7'eam of colorodo and Southem RaiZpoad. The 7'erraining 205 ac-ft of 7'equi7'ed storoge volume would be above vine Dr>ive and may be buiLt as develop- ment occu7's. A rraximum discha7'ge of 300 cfs at Vine Dr>ive is anowed. Culve7'ts at the roil7'oad and Vine D7'ive c7'ossings would be enlapged. The existing culve7'ts at Highway 14 a7'e adequate. BO$elde7' C7'eek Fpom the Cache la Poud7'e Rive7' confluence to ppospect St7'eet, the Boxelde7' C7'eek channel will 7'etain its naturol state. "7'oposed access mads paroLleling the cpeek fop the sludge application apea will be used to confine the floods within the channel. Fpom FPospect St7'eet to the Coopep SLough confluence, a gross-lined channeL with a capacity of 2,000 cfs is 7'ecommended. The bpidge at P7'ospect St7'eet shouLd be enLa7'ged to this capacity. The ppi- vate d7'ive c7'ossings should be 7'eplaced by paved dip cpossings to pass the ten-yea7' flood (1200 cfs). Fpom the Coope7' Slough confluence to State Highway 14, the floodplain is na7'7'OW because the flow is confined to the channeL vaney. At Vine D7'ive, the dip in the 7'oad cpossing acts as a wei7' and the capacity of the b7'idge at Highway 14 is adequate. Limited fLood damage OCCU7'S along this peach and consideped channelization and enLapged openings at channeL c7'ossing but '~th detent~~ stopage to 7'educe peak flows. The anaLysis l<l::ts pepfopmed on a peach-bY-7'each basis. The 7'esults a7'e p7'esented as be~efit cost rotios and net benefits f07'each 7'each fo7' each imp7'ovement pLan. The mnge of the benefit cost rotio was fpom 0;2 to 3.7. The 0.2 7'atio was along a p07'tion of BO$eldep C7'eek whe7'e the7'e is too little existing darragepotential to economicaLLy justify the cost of imppovements. The benefit cost mtios fo7' Coope7' Slough ronged f7'om 1..3 to 3.6. The averoge benefit cost patio fo7' AltePYllJ.tive 2 was 2.1 as compaped to 2.7 fo7' Alte7'native 3. These mtios diffe7' f7'om the ronge of rotios because aLL benefits and costs a7'e conside7'ed to aJJ~ct the enti7'e basin whe7'eas on a 7'each by 7'each basis only cost and benefits fo7' each 7'each a7'e consideped. Total costs fo7' AltePYllJ.tives 2 and 3 a7'e $5,518,000 and $4,218,000, pespectively, fo7' the 100-yea7' design flow. Total annual cost fo7' Alte7'native 2 is $430,240 and $330,480 fo7' AltePYllJ.tive 3. The annual costs a7'e based on a 50-yea7' life and a discount rote of 7 3/8 pe7'cent. The discount rote is f7'om federoL guideLines. Plan F07'mulation Alte7'native plans consideped floodplain 7'egulation, channel imp7'ovements and/o7' detention sto7'age. Design flows based on the 10-, 50- and 100-yeap 7'eCUP7'ence inte7'val flood events we7'e conside7'ed fo7' channel imp7'ovements. The puppose of this section is to 7'ecommend a final imp7'ovement plan based on the ppevious anaLysis. The suggested imp7'ovements a7'e not binding on the local communities but a7'e 7'ecowoonded, wouLd imp7'ove d7'ainageway pLanning and a7'e economical fo7' the entipe basin. AltePYllJ.tive 3 is the suggested final plan and 7'equi7'es a combination of channels, bpidge c7'ossings and detention ponds. The 7'equi7'ed imp7'ovements ape summx7'ized fo7' Coope7' SLough and BO$elde7' C7'eek. ALL imp7'ovements ape based on the 100-yea7' 7'eCU7'7'ence inte~ vaL flood. , v