Allocating RDII Using Footing Drain Flow and Other Information
Rainfall dependent inflow and infiltration (RDII) can be modeled in different ways using the United States Environmental Protection Agency Stormwater Management Model (SWMM). This includes using the RDII shape and volume routines in the SWMM RUNOFF module; using typical combined area parameters for a reduced sub-basin area to produce an inflow response; and using baseflow factors in EXTRAN or the groundwater routine in RUNOFF to produce an infiltration response. Regardless of methods used to model RDII, quantifying RDII can be performed by an even wider variety of methods, typically tailored to local conditions as no one method is universally applicable (Shultz et al.).
For this chapter, the RDII shape and volume routines in RUNOFF are used. Total wet weather response is allocated based on understanding of connected footing drains tributary to the sanitary collection system and supporting sewer metering and footing drain flow monitoring data. Footing drain contribution to RDII is important to the local conditions of the Ann Arbor sanitary collection system (Sanitary Sewer Overflow Study Task Force; Sherman et al.; Stonehouse et al.; Zawacki et al.). In Ann Arbor, an extensive Footing Drain Disconnection (FDD) program is currently underway. The citywide model’s hydrology required characterization that could be used to prioritize FDD work as well as allow ready updates to the model hydrology as work proceeds.
This chapter provides some background regarding the footing drain monitoring information. This information discusses seasonal variation in response volume while exhibiting little seasonal differences for peak flow; thus, indicating that inflow responses are fairly consistent through the year and the infiltration response is what predominantly undergoes seasonal change. Also, discussed is a site characterization index (SCI) that may be used to assess likelihood of a range of footing drain responses based on a simple site assessment. Applicability of this index to other areas of the city is planned for future investigation.
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