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EXECUTIVE SUMMARY This report presents ~ comprehensive flood plain information and drainage plan for the Sterl ing are~. The report was prepared for the City of Sterling ~nd the Color~do ~ater Conservation Soard to serve as a guide for 501 vi ng regional fl oodi ng and 1 oc~ 1 drai nage llrob 1 ems. I nc 1 uded are hydrologic, hydraulic, and economic analy,es which define flood hazard areas. damages. deficiencies in the existing dr~inage system, and approx- imate costs for regional flood control and drainage improvement. Flood ~ater Surface Elevations Peak flows from the hydrologic analy,is were used ~s inputs to the U.S. Army Corps of Engineers' HEC-2 water surface profile computer model. This model provided estimates of the water surface elevations for each of the 5- through SOO-year flood events. The water surface profiles were transferred to topographic maps to establi>h the 5-and 100-ye ar flood plain boundaries. Reaional Floods The City of Sterling lies at the business end of the 3D-square mile Sand Creek watershed. InJune.lg68.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 H1ghway 138. But Sand Creek is not the only flO<ldthreat. 1n1965, floodwaters pouring through the so-called "Pawnee Overflow" at Riverside Cemetery reached Sterling, and also put much of the are~ below Pioneer Ditch under water. Some Of the overflow from this flood also reaChed the downtown area. This report does not consider flooding from the South Platte River. Flooding on theSoutll Platte is described indetai1 ina 1'178 U.S.Army. Corps of Engineer report. N~tural drainage in the Sterl ing area is hindered by Highw~ys 5,14, and 138, and the Burlington Jnd Union Pacific railroads. Their e~bank- ments divert runoff out of natural channels, while grossly ~nder sized bridges and culverts severely restrict flood flows. Hence. much of tile e~isting flood hazard in Sterling would be reduced si~ly by replacing inadequate crossings with larger hydraulic structures. The most severe bottlenecks are Highway 6/Vnion Pacific Railroad at Pawnee Creek south of Ster1 ing. Burl ington Northern ~ailraad at Pioneer Di tell. Kighway 14 at Pioneer Ditch. and Highway 138/Burlington Northern/Union ~acific ~ailroad at Pioneer Ditch. Oamaqes Damages in the stUdy area are considerable, varying from nearly S 550.000.00 for a 10-year flood to more th~n S 10,000.000.00 for ~ 100- yearfloo<::. These damages translate into an average annual cost to the citi~ens of Logan County and the City of Sterling of appro~imately $ 450.000.00 per yeJr. These costs can be expected to increase in pro- portion to the amount of future development in the flood-hazard area. Reqional Flood Control Annual flOOd damage costs of the magnitude cited above justify consideraticn of structural ",e~ns to control ftit~re floods. T>la ,01 utions appear to be feasible at this time: 1. A dike across the Pawnee Overflow near Riverside Cemetery, and 2 A. small flood control dam a~d reservoir ~t nioreer Park. Flood Maqnitude We made a hydrologic analysis ~o determine peak flow rates for various >to~ frequenCies at several locations in tcwn for beth Sand Cree~ f~ood, and Pawnee overflows. Syntnetic nydrographs were compvted for 5-,10-.25-,50-,100-. and 50Q-year frequency s,orm~. rlood peaks were determined ~s~vming the simu1taneouSoccurrenceofa lO-yearflood in tnf'Soutl'i PlatteRiv er. Cemetery Dike As discussed above, the major reason for overflows from Pawnee Creek 1s the lack of sufficient wate~'ay area in the bridges and culverts beneath Highway 6 and rail road south of town. If tnese ,tr~ctures were enlarged, overflows in the vicinity of Riverside ~emetJry would be reduc~d. Thus we r~commend thdt the City of Sterling, the Jivision of Highways. the Union ~acific railrOad, and Logan County collectively develop d comprehensive solution to this problem. For purposes of this study. ~e assumed that a dike was const~Jcted across the Pawnee overflowwitl1out any enlargement of drainJge struc tvr/!, on Pa>lnee Creek. S"ch a structure '~ould require Nisir.g Hig.~way 6 edS t of ~he cerr.ecery dbOut 4 feet. 7he:li ke '~ould then fo 1i ow che south and '~est boundaries of the cernetery to the county road. then proceed west by )'aging the road for a d1>tance of about 4.200 feet. ~stir;;ated ii