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, I I I I I I I I 1 I I I 1 I j I J ~ SYNOPSIS This report summarizes II comprehensive flood plain information and drainage plan for the Sterling area (Leaf dnd Resource Consultants. Inc., 1983). The comprehensive report was prepared for the Cay of Sterling and Colorado Water Conservation 80ard to serve as a guide for solving regional flood1ngandlocal drainageprotl1ems. Includedarehydrologic,hydfilulic. and economic analyses which define flOOd hazard areas, damages. deficiencies in the existing drainage system, dnd approximate costs for regional flood control and drainage improvement. Thissulll]1dryreportcontainsinformation necessary for regulating the flood plain in accordance with the National Flood In$urance requirements. Reoionall!.ood_~ The City of Sterling lies at the business end of the 3D-square mile Sand Creek watershed. !nJune, 1958, several thousand cubic feet of water per second from intense rainfall on Sand Creek, flowed through Pioneer Park. This flood inundated the City along Pioneer Ditch from Highway 14 to Highway 133. 3utSand Creek is not the only flood threat. In1965, floodwaters pouring through the so-called "Pawnee Overflow" at Riverside Cemeteryr eached St..rling, wcl ~l~o put much of tll(> ~rf>~ h,,]ow Pioneer Ditch under water_ Some of the overflow from this flood also reaclled the downtown area. This report does not consider flooding from the South Platte River. Flooding on the South Platte is descrihed in detail in a 1978 U,S. A~, Corps of Engineers report. Natural drainage in the Sterling area is hindered by Highways 6,14,and 138, and tile Burlington and Union Pacific railroads. Theirembankments divert runoff out of natural channels, while grossly under sized bridges and culverts severely restrict flood flows. Hence, much of the exist1ng flood hazard in Sterling would be reduced simply by replacing inadequate crossings with larger hydraulic structures. The most severe bottlllllecks are Highway 6/Union Pacific Railroad at Pawnee Creek south of Sterling, Burl ington Northern Rail road at Pioneer Ditch, Highway 14 <It I'iu"e~r Di t<.;h, and Highway 138/Burlington Northern/Union Pacific Railroad at Pioneer Ditch. I I I I I I I I I I I I I I I I I I I Flood Magnitude Ahydrclogic analysis was made to determine peak flOlirates for va rious storm frequencies at several locations in town for both Sand Creek floods and Pawnee overfl ow~. Syntheti c hydrographs were computed for 5-,10- ,25- ,50-,100-, and 500-year frequency storms, Floodpeaksweredeterminedassumingthe simultaneous occurrence ofa lO-year flood in the South Platte Riv er. F1oodWaterSurfaceElevations Peak flows from the hydrologic analysis were used as inputs to the U.S. Army Corps of Engineers' HEC-2watersurface profile computer mod el. This model provided estimates of the water surface elevations for each of the 5-through 500-year flood events. The water surface profiles were transferred to topographic maps to establish the 5- and 100-year flood plain boundaries 1n the cornprehensive report. This report shows only the lOO-year flood plain boundaries. ii