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. . . Continuous Hydrologic and Hydraulic Modeling for Flood Plain Mapping along the West Branch DuPage River, Dupage County, D1inois Frank Lan. Steve Rogers and John Sikora Woodward-Clyde Consultants, Denver. Colorado Jeff Dailey DuPage County Department of Environmental Concerns Introduction A new approach for estimating floodplain limits using continuous-simulation modeling and statistical analysis on the simulated peak stages was performed for the West Branch DuPage River. FEQ. a one- dimensional unsteady flow hydraulic model with flood routing capabilities is coupled with a continuous- simulation hydrologic model to compute flood hydrographs and river stages. The model was constructed using a large amount of field surveyed data consisting of river and flood plain cross sections and hydraulic structure data, The river network was divided into computational branches connected by representations for hydraulic structures, such as bridges. culverts. reservoirs. spillways. etc. The main stem model has approximately 100 branches, 20 reservoirs. 5 braided sections of channel. 3 in- stream dams and 55 bridges. culverts. and other hydraulic structures. The watershed hydrology was simulated using HSPF (EPA 1993) and was developed using land use data. soil type information. and precipitatiou and evaporation records for the area. The precipitatIOn data was obtained from six rain gages with approximately 50 years of record, The hydraulic model was calibrated to USGS continuous recording stream gauge records and high water marks along the river for recent large storm events, The calibrated model was verified by comparing simulation results with recorded peak discharges, stages. and volumes for a 50-year continuous record period through to 199.\. The calibrated model will ultimately be used to determine flood levels and delineate the floodplain for the West Branch DuPage River watershed based on a peak-to-volume statistical analysis which utilizes the simulated peak stages and flow volumes to estimate peak stage exceedence probabilities. In addition. the model has been used and will continue to be used for evaluating strategies for flood plain management and flood hazard reduction within the basin. The West Branch DuPage River flows through a developing area of DuPage County, lllinois. a suburban area approximately 80 km (50 miles) west of Chicago. The West Branch DuPage River has a drainage area of 328 km' (126.7 mi'), a main channel length of 51.2 km (32.0 miles), and an average channel slope of O.O()~;;1. The \\atcrshcd is prcdomiJl:1Iltly rcsidcnti:-ll \Vith some light industry, business, rO;lds and open space. The watershed has approximately 66 km' (25,5 mi') of impervious area. The river is prone to flooding due to its very flat slope and the large amount of development which has occurred in the