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3. FLOOD WARNING; 3.1 Definition of' ,Flood Warn).ng Flood wclrning time is defined as the time between when water reaches the upper alarm stage at Stl?t"llng and when water reaches the invl?rt elevation of the Sighway 55 closure structure at Cr.-o':lk, Flood warning 'time involves two compc,nents. The first is the flood routIng time required to transport a given discharqe from Ster,Ling to Crook. The sec;Ond component is the time for the discharge at Crook to inc;tease to the value necessary to caUSE~ water to enter Crook tlirough the closure structure opening. The HEC~l results indicate that once a flood wave reachel~ a point downstream, wat;er levels will rise quickly. The rising limb of the hydrographs are quite steep. The second componen1: to the flood warning time mentioned above is not very siqn:lfic:ant in the amount of t:i,me involved. 3.2 Flood Warning l'i.mes Flood warninq times were computed from sterling to the closure structurE! at: Crook. Table 4 lists flood warning times developed for the ,fiighway 5!5 closure structure. These are based on hypol:h,~t leal modeled floods. ,i\cppendix A contains warning times that were calCulated for other type of flood events. TAaLE 4. FLOOD WARNING TIMES WARNING GJI,GE 10-YEAR 5Q,-YEAR 100-YEAR 50Q-YEAR Highway 6 (sterling gagE!) * n/a 20.0 18.4 13.5 * Flood '-'a'l:ers never reach height Of 3711. 8 feet, at the Highloray 55 closure structure for a 10-year flood. The m.u:imum water level a 'I: Crook occurred approximately 23 hours after the sounding of the upper alalrm in Sterling. The above flcfod warning times were calculated using a calibrated HEC-1 mc)del and a setting for the upper alarm float at Sterling co;rr'~spcmding to a discharge o:f 18,000 cfs. The critical threshold discharge at Crook, when water levels reach the height of the Highway 55 closuJie structure, is approximately 30,000 cfs. This assume. that the invert elevation for t:hE! lst:ructure w).ll be at an elevation of 3711.8 feet, msl, which cOl:responds to the elevation of the highway centerline at the site of the proposed levee crossing. The recommended se'tting of 18,000 cfs for thE~' upper alarm float provides an additional margin of safety. For the historic period of record 1920 to 1990, there have been 4 flood events when the discharge exceeded 18,000 cfs. In all but one of 8