What Site Characteristics Explain Variability of Peak Footing Drain Flows?
Abstract:Residents of the City of Ann Arbor, Michigan have experienced periodic basement backup problems dating back to at least 1968. After a significant rain event in 1998 that caused basement backups in numerous homes throughout the city, Ann Arbor formed a task force to develop a plan to help prevent future basement backups and sanitary sewer overflows (SSOs). The consulting team worked with the task force and determined that the sanitary sewer capacity problems sanitary sewers were primarily caused by rainfall derived inflow/infiltration (RDII) collected by foundation footing drains located around residential structures. Protection of the environment, public health, and property drove the analysis of alternative solutions and footing drain disconnection (FDD) from the sanitary sewers was selected for city-wide application.
The FDD program was enabled through a city ordinance and the city signed an administrative consent order (ACO) to address their SSO discharges with the State of Michigan Department of Environmental Quality (MDEQ). The ACO included an estimate that I/I from residential footing drains during large rain events would range from 3 to 5 gallons per minute (GPM). The modeling analysis showed that during the 25-year, 24-hour precipitation event, which is approximately 3.6 inches for Ann Arbor, basement backups would take place. This matched the experience of storms that caused water in basement problems. The ACO required follow-up monitoring to validate this assumption of footing drain flows. The FDD program has progressed in phases, working in the areas most frequently affected by basement backups and SSOs first followed by work in other areas to reduce RDII to transport and treat. This FDD solution typically requires the construction of a sump pump system in the basements where the FDD is performed, except where gravity discharges can be accommodated.
Following the disconnection of the footing drains from the sanitary sewer system and the installation of the sump pump system, selected systems were monitored to measure the peak flows generated by these installed sump pumps. Monitoring has taken place from the onset of the program in 2002. The number of monitoring sites has been increased during the different phases of the project to the 45 properties that are now monitored. A total of 60 different properties have been monitored. Nearly 1,300 properties have had FDD construction completed under this program within the City of Ann Arbor's 34 square mile sanitary sewer system upstream of the 29.5 MGD waste water treatment plant
Over the last 6 years, the city of Ann Arbor FDD Program has collected a significant amount of data on individual building footing drain RDII rates. This paper provides an overview of the FDD program, the development of the initial FDD flow estimates, as well as providing a review of the monitoring data collected that has been used to validate the initial flow estimates. The extensive monitoring data has been compared to several external factors thought to impact the rate of generation to determine the validity and influence of these different factors on the individual building footing drain flow.
This paper explores the characteristics of properties that can be used to predict the response rates from different footing drain systems, including how flows are affected by seasonal variation and antecedent precipitation. Most of the data included in this evaluation is from monitoring performed in the City of Ann Arbor.
Furthermore, this paper develops a correlation of the peak responses at individual monitoring sites based on the following factors:
Relative elevation and grade in areas surrounding the of building
Soil type, permeability and hydrologic group
Basement area and perimeter
Age of home
Each of these characteristics were examined and compared to the individual peak footing drain responses to determine correlations to each of these factors. These correlations can be used to predict the expected flows from groups of homes, and neighborhoods. The ability to predict what sites will produce high footing drain flows would be of great use in deciding whether to engage in a footing drain disconnection program or where to target a footing drain disconnection program to most efficiently remove inflow and infiltration to the sanitary sewer system.
Document Type: Research Article
Publication date: January 1, 2009
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