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<br />111-3 <br /> <br />This alternate Involves constructing a !Jeneral Iy geometric <br />channel, usually trapezoidal, and oth"r strll<;tural facilities such <br />as bridges, road embankments, culverts, Irrl!Jatlol1 dItch diversion <br />structures, and slope protection that are cc>rnpatrble with the chosen <br />design frequency. The side slope~; aro~ generllliy grass, although <br />they may at times be concrete, rlprap, 01- gClbions. Special provi- <br />sions are usually made for transporting low flows, which, for Big <br />Dry Creek, amount to Irrigation releases from 250 cfs to 400 cfs. <br />The low flow channel is sometimes constructe,d of riprap or concrete. <br /> <br />The channel is generally located near ':he natural thalweg, <br />although actual routing would take Into account artificial barriers <br />such as sewage treatment plants, businesses" homes, and roads. <br />Bridges and culverts would be sized in accordance with the require- <br />ments of the Drainage Criteria Manual relative to the projected <br />future street use. Where they Join the main channel, tributaries <br />to Big Dry Creek will necessitate specIal hydraulic considerations. <br /> <br />4. Closed Conduit. This alternate consists of constructing an under- <br />ground conduit. To keep construction costs withIn bounds, it Is <br />des i rab 1 e to des I gn the condu it for hi gh-ve 'I oc I ty f 1 O"'S; howeve r, <br />the "maximum velocity" approach causes serious hydraul ic problems <br />which requi re that the conduit alignment be as straight as possible <br />and that Influent flows from tributaries be gIven special hydraulic <br />consideration (often model testln!j), Where 1,0ssIble, the changes <br />In channel alignment and Influent flo<II's wou'l d be made by de- <br />accelerating the flood waters In l'ondlng are,as and then again <br />acceleratIng the flood waters Into th€, new al ignment. The pondlng <br />areas would also serve to lessen the higher peak flows to the <br />downstream portions of Big Dry Creek that resul t from decreased <br />channel storage. <br /> <br />Although this alternate appears to be lJi1suItable because of <br />high construction costs, complicated hydrau:ics (especially at <br />tributarIes to BIg Dry Creek), difficulties with IrrigatIon <br />releases, and general overall insensitivity to overloading and <br />to multi-use, this alternate was investigated to provIde compara- <br />tive Information. <br /> <br />5. Pondlnq. In many Instances, planned pondln!1 can have a profound <br />effect on the costs of drainage works. Each of the prevlously- <br />listed alternates can be benefitted by the reduction of peak flows, <br />resulting in lower structural costs and prob.ably reduced annual <br />damages. <br /> <br />On Big Dry Creek there Is considerable ,.atee storage in the <br />over-bank areas and behind certain embankments. This pondIng Is <br />accounted for In the method used for determining flood flow peaks, <br />the unit hydrograph. Any alternate whIch reduces the pondlng as <br />