CAC members: At the August CAC meeting, Jim asked a question about stormwater flooding and it's possible impact on the sanitary sewer capacity. Below is an expanded response.

A. [CITY]

Footing drain flow is generated from an area near the house where stormwater runoff and ground water are captured and drained to the footing drain.  For stormwater runoff, this is generally a small area directly around the house, perhaps on the order of 5-20 feet from the foundation, depending on the grading and slopes around the house. This is why proper grading and extension of roof downspouts away from the house is so important. 

 

There may be some homes where, in a large enough storm, surface flooding reaches the contributing area of a house that generates footing drain flows. However, it is not uncommon for backyard drainage from a number of homes to contribute to footing drain flows at the home located at the lowest elevation if this water cannot flow to the street and enter the stormwater system. In those locations, preventing stormwater from reaching the footing drain contributing area through better grading around these types of homes would reduce the flows to the footing drains during an extensive surface flooding event.  However, the number of houses that occasionally experience this condition are small compared to the large number of houses with footing drains. The majority of footing drain flows are generated from houses without extensive surface flooding issues, where the footing drain flow is generated from the regular house contributing area. 

 

The impact of improving the stormwater system on reducing sanitary sewer flows has not been studied in detail by the City.  However, we do have some anecdotal observations that we may draw conclusions from. Prior to the FDD program, there were several areas around the City that experienced sanitary basement backups without extensive reports of surface flooding occurring (such as the Orchard Hills and Bromley areas).  The significant footing drain flows in these areas were generated from the normal stormwater runoff captured from around the house. Now that FDDs have been competed in these target areas, the risk of sanitary basement backups has been greatly reduced, to the point where additional FDDs are not recommended. Additionally, on June 27, 2013, the City experienced a very large storm that produced extensive stormwater surface flooding, and there were very few reports of sanitary basement backups in the City.  

 

Another contributor of flow into the sanitary sewer system is surface flooding in the streets above the sanitary manholes through holes that are used for removing these manholes.  This source of additional flow into the sanitary sewer system was identified several years ago, and the City has installed plugs in many of these manhole holes in locations where flooding has been observed or the official City floodplain map showed flooding should occur for large storms to very cost effectively remove this source of wet weather flow in the sanitary sewer system.  An ongoing study of the stormwater system is identifying additional localized surface flooding locations where these manhole holes should be plugged.  The City is also installing better sealing manholes and solid manhole covers in areas where they are needed as street reconstruction or resurfacing is being performed. 

 

These observations suggest that the footing drain flow generated from surface flooding that approaches the house is not a significant source of wet weather flows into the sanitary sewer system. That is not to say that these surface flooding issues are not important - it simply means that the main driver for addressing them should be to address the surface flooding problems, and not because of excessive flows in the sanitary sewer system.

 

While the reduction of surface flooding in some areas may cause a small reduction in the flows into the sanitary sewer system, the flow evaluation and modeling performed has shown that sufficient flows have been removed, and that additional FDDs are not necessary in these areas.