Washington Street West watershed - City of Champaign

Draft ReportWest Washington Street Watershed Master Plan5.3.2 Undersized Local Storm SewerTraditionally, minimum storm sewer diameters are 12 and 15-inch sewers, withoccasional instances of 10-inch storm sewers when applicable. These diameters allow alarge enough cross sectional area and provide enough flow capacity to limit debrisbuildup or restricting flow from surface inlets. Based on the storm sewer informationprovided by the City, many storm sewers in this watershed have diameters of 4, 6 and 8-inches, often located at storm sewer inlets but also scattered throughout the watershed aslocal drainage sewers. These smaller diameter sewers require additional maintenance andmonitoring so that blocked pipes can immediately be cleared before a storm event occurs.The additional frequency of pipe blockage with smaller diameter sewers often limits theireffectiveness and drainage areas are often better served with a slightly larger diameterstorm sewer. The problem with simply increasing local sewer sizes in this watershed isthat downstream hydraulic restrictions are limiting the flow capacity, most likely causinga greater impact on flooding conditions than the smaller diameter storm sewers. For thetime being, the City should commit a more extensive sewer maintenance program aimedat ensuring these smaller diameter sewers operate as intended. One of the long termgoals of this study should be to design any alternative assuming that in the future the Citywould replace all small diameter sewers with at least 10-inch diameter sewers. Thisassumption will increase flows to the trunk storm sewers and likely operate moreefficiently throughout the entire lifetime of the proposed improvements.Figure 5-3Example Location of Undersized Local Storm SewersSeptember 20095-8Clark Dietz, Inc.