DEMONSTRATION OF DOWNSPOUT DISCONNECTION EFFECTIVENESS

The Detroit Water and Sewerage Department (DWSD) has undertaken a comprehensive program to control Combined Sewer Overflows (CSOs) from the City of Detroit wastewater collection system. A primary component of this program has been the development of a Long Term CSO Control Plan in 1996. The Long Term CSO Control Plan identified rainwater control as a primary CSO control strategy. One of the primary rainwater control measures identified was residential downspout disconnection. A pilot program was implemented to: verify the effectiveness of the downspout disconnection in reducing stormwater inflow and resulting CSO; and, assess public acceptance.The pilot program was conducted as a voluntary program in four areas in the City of Detroit. Side by side test pilot sites were identified in each area. Both test pilot sites were monitored for approximately 2 years to obtain baseline data. Downspout disconnection was performed after collecting baseline data in one test site, while the other test site served as a hydrological control. Both test sites were then maintained for an additional 6 months to a year to determine the effectiveness of the downspout disconnection. The acreage of the pilot sites ranged from 18 to 40 acres. Residents were asked to participate in the program and provide rights of entry to allow contractors to disconnect the downspouts. A public involvement initiative was performed to inform the residents about the pilot program, increase participation, and assess public acceptance. The total number of downspout disconnected by the pilot program was 577. The disconnection cost per downspout ranged from 243 to 278.The downspout disconnection pilot program was well received and the public's perception was very positive. Household participation in the pilot program ranged between 61% to 74%, and resulted in 50% to 83% of the downspouts disconnected. Other than the voluntary nature of the pilot program, not all downspouts were viable for disconnection for reasons such as the presence of cracks in basement walls, backyards that are not properly graded, and/or no lawn area available.The pilot project results demonstrate that residential downspout disconnection will reduce the percentage of the Directly Connected Impervious Areas (DCIA) between 40% to 44% when applied city-wide. The impact of residential downspout disconnection on the total wet weather flow and annual volume of CSO from the City of Detroit collection system was evaluated using the Greater Detroit Regional Sewer System (GDRSS) Model. The DCIA values for the tributary areas in the GDRSS Model were reduced to reflect the observed reduction obtained from the pilot project. Continuous GDRSS Model simulations using 36-years of historical rainfall and snowmelt data indicated that downspouts disconnection reduced the annual volume of CSO by approximately 2 billion gallons.