Urban storm-runoff modelling; Madison, Wisconsin

Abstract

The Illinois Urban Drainage Area Simulator was used to analyze the effects that (1) physical changes to storm-sewer conduits, and (2) increased runoff detention and infiltration would have on storm runoff in four urban basins in Madison, Wisconsin. The model was calibrated using monitoring data for the four basins collected over a 1-year period. A brief evaluation was made of a modified version of the model that simulates quality of urban runoff. Additional monitoring and computer analysis are necessary to calibrate the water-quality portion of the model before it can be used as a management tool in Madison. This study was done in cooperation with the Dane County Regional Planning Commission (DCRPC). Tables presenting results of various storm-water-management options are included. Some notable simulation results were that a 25 percent storm-sewer slope reduction yielded only a 3 percent peak-discharge reduction, and increasing storm-sewer roughness by increasing Manning's "n" from 0.013 to 0.0^0 decreased peak discharge about 10 to 20 percent. Detention of 10 percent of runoff throughout each basin yielded peak-discharge reductions of about 10 to 20 percent. Infiltration of all parking-lot runoff reduced peak discharges 5 to 2h percent. Peak discharges were reduced by 71 to 88 percent by substituting porous pavement for conventional pavement. Draining 90 percent of the residential rooftops onto lawns instead of driveways reduced peak discharge from 7 to 31 percent. Runoff-volume reduction was similarly reduced for the induced infiltration simulations. Storage requirements for hypothetical storm-water-treatment plants ranged from 2.6 to 29 acre-feet for the smallest and largest basins, respectively, with a treatment capacity of 25 cubic feet per second. A brief inconclusive evaluation of the water-quality subroutines of the model was made. Close agreement was noted between observed and simulated loads for nitrates, organic nitrogen, total phosphate, and total solids. Ammonia nitrogen and orthophosphate computed by the model ranged 7 to 11 times greater than the observed loads. Observed loads are doubtful because of the sparsity of water-quality data.