FLOOD03098 - Laserfiche WebLink

Home Browse Search

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 Coopero SLough Froom the confLuence with BoxeLdero Croeek to State Highway 14, the shaLLow !,Looding east of the channeL is eLiminated by the storoge in the upstroeam detention ponds. The roecommended improovements downstroeam of Highway 14 roequiroe 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 roequiped froom Highway 14 to about 1400 feet downstroeam. The roemaining porotion of the channeL wiLL be groass-Lined. A new croossing at Sheropy Road and cpos sing undero the Lake CanaL neap the confLuence with BO$eLdero Croeek aroe pequiped. Fpom State Highway 14 to a haLf miLe upstpeam of Vine D1'ive, the fLood- pLain is shaLLow, divided and wide with numeroous ponds fopming upstpeam of channeL cpossings. The roe commended improovement pLan is to constpuct detention ponds upstpeam of vine Dpive, CoLorodo and Southem RaiZpoad and Highway 14. The combined stopage voLume fop the 100-yeap pecuproence intepvaL fLood fop deveLoped conditions is apppo$imateLy 500 acpe-feet (ac-ft). Of this voLume, 190 ac-!'t is Located in 2 ponds upstpeam of Highway 14 and 108 ac- ft is ppo- posed upstpeam of colorodo and Southem RaiZpoad. The roerraining 205 ac-ft of roequiPed storoge volume would be above vine D1'ive and may be buiLt as develop- ment occups. A rraximum discharoge of 300 cfs at Vine D1'ive is anowed. Culverots at the roilrooad and Vine Dpive cpos sings would be enlapged. The existing culverots at Highway 14 ape adequate. BO$eldero Croeek Fpom the Cache la Poudroe Rivep confluence to ppospect Stroeet, the Boxeldero Cpeek channel will roetain its naturol state. "poposed access mads paroLleling the cpeek fop the sludge application apea will be used to confine the floods within the channel. Fr'om FPospect Stroeet to the Coopep SLough confluence, a gross-lined channeL with a capacity of 2,000 cfs is Pecommended. The bpidge at ppospect Stpeet shouLd be enLaroged to this capacity. The ppi- vate dpive cpos sings should be Pep laced by paved dip cpossings to pass the ten-yearo flood (1200 cfs). Fpom the Cooper' Slough confluence to State Highway 14, the floodplain is naroroow because the flow is confined to the channeL vaney. At Vine Dpive, the dip in the rooad cpossing acts as a weip and the capacity of the broidge at Highway 14 is adequate. Limited fLood damage occups along this peach and consideped channelization and enLapged openings at channeL cpos sing but '~th detent~~ stopage to roeduce peak flows. The anaLysis l<l::ts pepfopmed on a peach-by-roeach basis. The Pesults ape ppesented as be~efit cost rotios and net benefits foro each roeach fop each improovement pLan. The pange of the benefit cost rotio was fpom 0;2 to 3.7. The 0.2 roatio was along a porotion of BO$eldep Cpeek whepe theroe is too little existing darragepotential to economicaLLy justify the cost of imppovements. The benefit cost patios fop Coopep Slough ronged fpom 1..3 to 3.6. The averoge benefit cost patio fop AltePYllJ.tive 2 was 2.1 as compaped to 2.7 fop Alter>native 3. These patios differo fpom the ronge of rotios because aLL benefits and costs aroe consideped to aJJ~ct the entipe basin whepeas on a roeach by roeach basis only cost and benefits foro each roeach aroe consideped. Total costs foro AltePYllJ.tives 2 and 3 aroe $5,518,000 and $4,218,000, pespectively, foro the 100-yearo design flow. Total annual cost foro Alter>native 2 is $430,240 and $330,480 foro AltePYllJ.tive 3. The annual costs aroe based on a 50-yeap life and a discount rote of 7 3/8 perocent. The discount rote is froom federoL guideLines. Plan Foromulation Alter>native plans consideped floodplain roegulation, channel improovements and/oro detention storoage. Design flows based on the 10-, 50- and 100-yeap roecuproence interoval flood events weroe consideroed foro channel improovements. The puppose of this section is to roe commend a final improovement plan based on the ppevious anaLysis. The suggested improovements aroe not binding on the local communities but aroe roecowoonded, wouLd imppove droainageway pLanning and aroe economical foro the entipe basin. AltePYllJ.tive 3 is the suggested final plan and roequiroes a combination of channels, bpidge croossings and detention ponds. The roequiped improovements aroe summxroized foro Coopero SLough and BO$eldero Croeek. ALL improovements aroe based on the 100-yearo roecuroroence inte~ vaL flood. , v