Comparing Rainfall Dependent Inflow and Infiltration Simulation Methods
Rainfall dependent inflow and infiltration (RDII) is a significant, though undesirable, component of the urban wet-weather water budget in many sanitary sewer systems. Costs and environmental damage attributable to RDII are significant. Costs may be accrued through increased treatment and conveyance costs, increased maintenance costs, and sanitary sewer overflows (SSOs). To reduce these costs and mitigate environmental damage, engineered solutions require estimations of the long-term characteristics of the RDII response to wet weather. This in turn requires estimation of the performance of the existing collection and treatment system, as well as the expected performance of various possible solutions.
Due to the complex number of available pathways that RDII may enter a sanitary sewer, RDII is one of the most difficult components of the urban wet- weather water budget to estimate. RDII observations typically indicate response times that may range from several minutes to several days or weeks. This is confounded by regional and/or seasonal groundwater trends that influence RDII response. Various tools have been applied to estimating this special hydrologic response, including the rational method and several unit- hydrograph methods. This chapter provides an overview of available RDII estimation methods and highlights results from a relatively new physically based conceptual method first introduced by Kadota and Djebbar (1998). Kadota and Djebbar (1998) reported on modifications to the USEPA SWMM RUNOFF model that include the response of a conceptual non-linear reservoir to changes in groundwater elevations resulting from permeable-area infiltration.
The model has recently been applied to a sanitary collection system in Vallejo California. Initial results indicate the modified RUNOFF model is very good at computing RDII, including the delayed, seasonally varying groundwater components of RDII. It is recommended that the USEPA SWMM model be updated with the revisions made by Kadota and Djebbar (1998).
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