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<br />The above adjustment factors apply to existing and future development
<br />resulting in area-weighted runoff coefficients which vary from 0.40 to 0.55.
<br />The factors do not apply to future development that would create large imoer-
<br />vious areas, or shorten the time of concentration appreciably.
<br />
<br />Oischarqe Profiles
<br />Tables 3 and 4 summarize discharge profiles computed for several locations
<br />along the pioneer Ditch for the Sand CreeK' watersheds and Pawnee Overflows.
<br />The reader is referred to Figure 8 for the location of each reach. The peak
<br />discharges summarized in Tables 3 and 4 are for existing conditions. Oata
<br />furniShed by the City Planner indicate that a major development surge is not
<br />likely in tMSterling area for the next 20 years. Accordingly, thedis-
<br />charge profiles also reflect flooding potential for the forseeable future.
<br />The cumulative areas of developed and undeveloped watersheds area lsoshown
<br />for comparison.
<br />The discharges in Table 3 are estimates of imported water fronl the
<br />Pawnee overflow which greatly exceeds the amount of water genera tedfrom
<br />theupstre.lmarea tributary to the Pioneer D1tch between Riversid eCemetery
<br />and Hignway 14. An examination of the flOOd history has shown that the
<br />time to peak for ~awnee Creek at U.S. Highway 6 was 12 hours during the 1965
<br />flOOd (Matthai, 1969 a~d Ster1i~ Jou~~al ,~d'locate' J~ne 15, 1965). An
<br />additional 3 or more hours lapsed before flooc waters reached the ~awnee
<br />Overflow. Giv~n tMs lag ti:;,c for the larger f:ocd events, it is seen that
<br />peak flows from watersheds tributary to the Pioneer Oitch are likely to have
<br />receded prior to the occurrence of Pawnee Creek overf1ows. Gjventhis
<br />assumption, and independent occurrence of Pawnee and Sa~d Creek floods,lOO-
<br />year flood peaks along the ?ioneer Ditch vary from 5,000 c.f.s. between the
<br />overflew and Highway 14; 5,352c.f.s. at the Sand Creek confluence; 6,483
<br />c.f.s. at North 7th Avenue; and 6,8S0c.f.s. at Highway 138. Again, these
<br />oeaKS aDove are generateo as the result or a tlood occurrence on the Sand
<br />Creek 'Hatersheds tributary to ~iQneer ditCh. They do not included Pa'rl!lee
<br />overflows. For emergency olanning. it should ce assumed that a Sand Creek
<br />floOd will oeakwithin about 4 hours from the onset of neavy rainfal 1. This
<br />com;'Jaresw:tha lag ti.,e Of approxirr.ately 15hOI)rs fora PawneeQv erflel'!
<br />flOOd 1:0 peak at Steriing.
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