PREVENTING INCREASED INFILTRATION/INFLOW FROM RESIDENTIAL STORM WATER BEST MANAGEMENT PRACTICES

This paper discusses design criteria and guidelines for the placement of three types of residential storm water best management practices: rain gardens, downspout extenders, and rain barrels. Guidelines are needed to prevent unintended increases in private property infiltration and inflow (I/I). These guidelines specifically focus on soils and construction practices typical of the Milwaukee, Wisconsin area.The rain garden investigations were designed to determine the recommended distance between a raingarden and a house's sewer lateral. The field experiments consisted of a series of 100 square foot simulated rain gardens placed at increasing distances from the sewer lateral. Rising groundwater levels or soil moisture content at the sewer lateral during a test indicated the potential forincreased I/I. As a result of the field experiments, the investigators recommend placing a rain garden at least 10 feet away from the sewer lateral or house foundation in the silty clay soils typical of the Milwaukee area. Placing test rain gardens at this distance resulted in no impact on soil moisture at the sewer lateral. If groundwater mounding occurs, it is unlikely that any increase in infiltration into leaky laterals will occur within the critical time frame for sanitary sewer overflows.The downspout investigations were designed to determine the recommended length of a downspout extender. The field experiments consisted of simulating rainfall discharge through downspout extenders of different lengths. Increased flow in the house's foundation drain indicated the potential for increased I/I. Downspout extenders that discharged at least 5-feet away from the house foundation resulted in no increase in drainage to the house foundation. As a result of the field experiments, the investigators recommend using a minimum of 5-foot-long downspout extenders in the siltyclay soils typical of the Milwaukee area. Ten-foot-long extenders are preferred.The rain barrel investigations were designed to determine guidelines for draining rain barrels into planting beds adjacent to house foundations. The field experiments consisted of simulating rain barrel discharge through weeping hoses of different lengths while monitoring the discharge through the foundation drain. Increased flow in the house's foundation drain indicated the potential for increased I/I. The rain barrel field experiments show that using a 25-foot-long weeping hose placed 2.5-feet from the house does not increase the foundation discharge. A weeping hose may be placed anywhere along a house without running the risk of the water infiltrating into the foundation drain, as long as it discharges onto relatively dry soil consisting of sandy loam or finer material.