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111-3 This alternate involves constructing a !Jeneral Iy geometric channel, usually trapezoidal, and other strll<;tural facilities such as bridges, road embankments, culverts, irri!Jatlon ditch diversion structures, and slope protection that are cc>rnpatrble with the chosen design frequency. The side slope~; aro~ generally grass, although they may at times be concrete, rlprap, 01- gClblons. Special provi- sions are usually made for transporting low flows, which, for Big Dry Creek, amount to Irrigation releases from 250 cfs to 400 cfs. The low flow channel Is sometimes constructe,d of rlprap or concrete. The channel is generally located near .,he natural thalweg, although actual routing would take Into account artIficial barriers such as sewage treatment plants, businesses" homes, and roads. Bridges and culverts would be sized in accordance with the require- ments of the Drainage Criteria Manual relative to the projected future street use, Where they Join the main channel, tributaries to Big Dry Creek will necessitate special hydraulic consideratIons. 4. Closed Conduit. This alternate consists of constructing an under- ground conduit. To keep construction costs within bounds, it Is des I rab 1 e to des I gn the condu it for hi gh-ve.1 oc i ty f 1 o",s; howeve r, the "maximum velocity" approach causes serious hydraul ic problems which requi re that the conduit alignment be as straight as possible and that Influent flows from tributaries be given special hydraulic consideration (often model testin!j). Where I,osslble, the changes In channel alignment and Influent flo<II's wou'l d be made by de- accelerating the flood waters In pondlng are,as and then again acceleratIng the flood waters Into th€, new al ignment. The pondlng areas would also serve to lessen the higher peak flows to the downstream portions of Big Dry Creek that resul t from decreased channel storage. Although this alternate appears to be lJi1sultable because of high construction costs, complicated hydrau:ics (especially at tributaries to Big Dry Creek), difficulties with IrrigatIon releases, and general overall Insensitivity to overloading and to multi-use, this alternate was investigated to provide compara- tive information. 5. Ponding. In many Instances, planned pondln!1 can have a profound effect on the costs of drainage works. Each of the previously- listed alternates can be benefitted by the reduction of peak flows, resulting In lower structural costs and prob.ably reduced annual damages. On Big Dry Creek there Is considerable ,.atee storage in the over-bank areas and behind certain embankments. This ponding Is accounted for In the method used for determining flood flow peaks, the unit hydrograph. Any alternate which reduces the ponding as