Washington Street West watershed - City of Champaign

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Draft ReportWest Washington Street Watershed Master Plan5.0 EXISTING CONDITIONS HYDROLOGIC/HYDRAULIC ANALYSIS:KEY FINDINGS5.1 INTRODUCTION TO MODELING TECHNIQUES5.1.1 IntroductionThe hydrologic/hydraulic modeling program XP-SWMM 2009, version 11.1, was used toestimate peak flow rates and determine the hydraulic capacity of the West WashingtonStreet trunk sewer. XP-SWMM is a physically-based storm event simulation modelcapable of simulating runoff from various land uses and soil types, combining sub-basinhydrographs, and routing flow through storage (detention ponds, natural low areas) andconveyance elements (channel or sewers).XP-SWMM integrates the hydrologic analysis with the hydraulic analysis, so anystormwater storage resulting from detention ponds or surface flooding is calculated andpeak flows are adjusted accordingly. Flow data used for the hydraulic analysis consistedof unsteady flow data computed using the hydrologic analysis tool of XP-SWMM. Peakflows from the hydrologic analysis were used to compute a hydraulic grade line (HGL)for each section of sewer pipe and open drainage channel.An Existing Conditions XP-SWMM model was developed to simulate the watershedunder existing land use conditions. The key findings of the Existing Conditions XP-SWMM model are discussed in this chapter. Chapter 6 includes discussion of ProposedConditions modeling.5.1.2 Hydraulics – Trunk Sewer ParametersTrunk storm sewers were identified from the City’s storm sewer map and confirmed bysurvey data. Sewer survey data collected included manhole rim elevations, pipe sizes,pipe connectivity, and invert elevations, so that accurate information could be used whenthe XP-SWMM existing conditions model was developed. The drainage areas weredelineated based on the sewer network and contours so that portions of the network couldbe assigned local drainage areas that would be conveyed to the trunk sewer. Thecomputer model has each drainage area assigned to the appropriate node along the WestWashington Street trunk sewers to simulate the flow conditions within the watershed.The roadways and surfaces along the West Washington Street corridor were also modeledas a link or node to provide conveyance and storage potential based on the existingtopography. The roadway cross sections were approximated as shallow channel crosssections based on the survey information and the 2-foot contours (2008). This was donein order to more accurately model the larger precipitation events, allowing the flow,which surcharges the West Washington Street trunk sewer, to flow downstream via theroadway or surface channel.5.1.3 Hydrology – Subbasin ParametersThe 408-acre watershed was subdivided into 30 individual subbasins to quantify thestormwater runoff contribution from individual portions of the watershed (see Figure 5-1). Subbasin delineation was performed using 2-foot contours (2008) provided by theSeptember 20095-1Clark Dietz, Inc.
Draft ReportWest Washington Street Watershed Master PlanChampaign County GIS Consortium and by determining the locations of storm sewersusing field survey and the City storm sewer map. Storm sewers that cross the watershedboundaries and appear to be sloped away from the West Washington Street watershedwere not evaluated or modeled as part of this study, but were assumed to be appropriatelysized and sloped to adequately convey stormwater runoff away from the WestWashington Street watershed. A few areas along the sewershed boundary had groundslopes toward the West Washington Street watershed, but sewers tributary to anotherstorm sewer. In these instances, the drainage area was added to the West WashingtonStreet watershed to more accurately model peak flows associated with large storm eventsthat would exceed sewer capacity.Figure 5-1West Washington Street Watershed Subbasin MapSoil types, provided in shapefile format by the City of Champaign, were used to assignrunoff variables for the surface conditions of each local drainage area. The Green Amptmethod was used to determine flood routing and infiltration conditions throughout thesewershed. The soil types within the watershed are a combination of silt loam, silty clay,and loam, which were used in determining the Green Ampt flood routing and infiltrationparameters. Based on these soil types, the average capillary suction was between 8 and12 inches, the initial moisture deficit was between 0.25 and 0.32, and the saturatedhydraulic conductivity was between 0.2 and 0.6 in/hr.September 20095-2Clark Dietz, Inc.
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Washington Street West watershed - City of Champaign

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