Rainfall-Runoff Relationships and Water-Quality Assessment of Coon Creek Watershed, Anoka County, Minnesota

Rainfall-runoff relationships and results of water-quality analyses were studied to develop an understanding of flooding problems and to assess present and potential water-quality problems in the 96.9-square-mile Coon Creek watershed, Anoka County, Minnesota. Rainfall, runoff, and water-quality data were collected from March 1979 to November 1980 at five continuously recording streamflow sites, seven crest-stage sites, and three continuously recording rainfall sites. During the study, eight storms occurred with sufficient rainfall to produce measurable runoff at most of the gages in the basin. The resulting hydrographs show, as expected, higher unit peaks, shorter times to peak, and shorter durations of high flows in streams draining urban areas than in streams draining rural areas. Constrictive culverts and bridges at roadways resulted in attenuation of hydrograph peaks in urban areas. Runoff amounts were nearly the same in all the subareas for storms with uniformly distributed rainfall. The greatest recorded rainfall during this study was 3.95 inches on August 7, 1980. The basin-weighted rainfall for that date was 3.56 inches, which resulted in the greatest observed peak flow for Coon Creek at Coon Rapids Boulevard of 185 cubic feet per second. The eight storms produced eight hydrographs suitable for model simulation of Sand Creek, seven hydrographs for simulation of Coon Creek, and four hydrographs for simulation'of County Ditch 58. The U.S. Army Corps of Engineers HEC-01, Flood Hydrograph Package computer model was used with the parameter optimization routine to develop parameter values to closely match observed hydrographs. A multiple-linear-regression technique was used to develop linear equations for relating HEC-1 parameters to variations in rainfall and antecedent moisture. Tftiis procedure resulted in generalized models of the three principal subareas that reasonably simulated 10 of the 19 observed hydrographs. Water-quality characteristics were determined based on 14 water samples from 4 sites and 1 bottom-mate rial sample from each site. Results of the analyses indicated that streams draining urban areas carry the highest concentrations of most constituents sampled. Sand Creek at Xeon Boulevard, which drains the most urbanized area, had the highest mean concentration of metals, chloride, dissolved solids, and suspended sediment. Concentrations of total phosphorus ranged from 0.04 to 0.43 milligram per liter at the rural sites on County Ditch 58 at Andover Boulevard and Coon Creek at Raddison Road. Average phosphorus concentrations at the rural sites are comparable to concentrations at the urban sites.