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EXECUTIVE SU~A~Y This report presents a comprehensi~e flood plain information and drainage plan for the Sterling area. The report was prepared for the City of Sterling and the Colorado Water Conser~ation Board to serve as a guide for solving regional flooding and local drainage problems . Included are hydrologic, hydraulic, and economic analyses which define flood hazard areas, damages. deficiencies in the existing drainage system, and approx. imatecostsforregional f1oodcontrolanddrainageimprovement. Flood Water Surface Elevations Peak flows from ~he hydrologic analysis were used as inputs to the U.S. Army Corps of Engineers' HEC-2 water surface profile co~puter model. This model providedestill'.ates of the water surface elevations f or eachofthe5-through500-yearfloodevents. Thewatersurfaceprofi1e5 were transferred to topographic map;; to establlsh the S-and lOO-y ear flood plain boundaries. Reqional Floods The City of Sterling lies at the business end of the 3D-square mile Sand Creek watershed. In June. 1968. several tMousandcubicfeetofwater 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 138. But Sand Creek 15 not the only flood threat. In 1965. flOOd waters pouring through the so-called "Pawnee Overflow" at Riverside Cemetery reacned Sterling. and also put much of tne area below Pioneer Ditch under water. Sow.e of the o~erflow from this flood also reached the downtown area. This report does not consider flooding from the South Platte Ri~er. Flooding on the South Platte is described in detail in a 1978 U.S.Army, Corps of Engineer report. ~atural drainage in the Sterling area is hindered by Highwaj> 6.14, and l38,and the8urlington Jnd Union Pacific railroads. Their embank- ments divert runoff out of natural channels. white grossly under sized bridges and cul~erts severely restrict flood flows. Hence. much of the existing flood hazard in Sterling would be reduced simply by replacing inadequate crossings with larger hydraulic structures. The most severe bottlenecks are Highway 6JUninn Pacific Railroad at Pawnee Creek south of Sterling, Burlington ~orthern Railroad at Pioneer Ditch, Highwaj 14 at Pioneer Ditch, and Highway 138!Burlin~ton ~ortnern!Union Pacific ~ailroad at Pioneer Ditch. Damages Damages in the studJ area are considerable. ~aryin9 from nearly $650.000.00foralO-yearfloodtomorethan$10,OOO,QOO.00foral00- year flood. These damages translate into <In a~erage annual cost to the citizens of Logan County and the City of Sterling of approximately S 450,000.00 per year. ihese costs can be expected to increase in pro- portion to the amount of future de~elopment in the flood. hazard area. Reuional Flood Control Annual flood d~ge costs of the magni:ude cited abo~e justify consideration of structural "'eans to control fut:.irefloods. T'IIosolutions appear to ~e feasible at this time: 1. A dike across the Pawnee Overflow near Riverside Cemetery, and 2 As",all f1oodcontrol dam and r'i!ser'loir at Pioneer Park. i Cemetery Dike As discussed abo~e, the major reason for overflows from Paw~ee Creek is the lack of Sufficient waterway area in the bridges and cul~erts beneath Highway 6 and railroa~ south of to'~n. If these structures were enlarged, overflows in the ~icinity of Riverside Cemetory would be recuced. Thus we recommend that the City of Sterling, the Division of Highways, the Union Pacific railroad, and Logan County collectively develop a comprehensi'le solution to this problem. For pur~oses of this study, we assumed that a dike was constructed across the Pawnee overflow without any enlargement of drainage structures on Pawnee Creek. Such a structure would require raising Highway 6 east of the ceO'.otery about 4 feet. The dike '~ould then follow the SOuth and west boundaries of the cemetery to the county rOde. ~hen proceed westby raising theroa<.l for a distance of about 4.200 fee:. Estir.: ated ii Flood Maqnitude We made a hydrologic analysis to determine peak flow rates for various storm frequencies at se~eral locations in tCwn for both Sand Creek floods and Pawnee overflows. Synthetic hydrographs were computed for 5-,10.,25-,50-,100-. and SOD-year f~equency storms. Flood "eaks were determined assuming the simu1taneous occurrence ofa ]D-year flood in the South Platte River.