Note: This section is an adaptation from the Storm Water Management Model Reference Manual Volume I – Hydrology By: Lewis Rossman U.S. This chapter describes how these RDII flows are computed from the precipitation records supplied to a SWMM data set. RDII calculations were added to version 4 of SWMM by C. RDII flow is added onto the other inflow categories (such as dry weather sanitary flow, overland runoff, and groundwater interflow) during each time step of a simulation. It is computed independently of the surface runoff, infiltration, snowmelt and groundwater processes described in previous chapters of this manual. SWMM treats RDII as a separate category of external inflows that enters the conveyance system at specific user-designated nodes. It can also cause significant flow increases to wastewater treatment plants resulting in hydraulic overloading and disruption of plant processes. RDII can be a significant cause of sanitary sewer overflows (SSOs) of untreated wastewater into basements, streets and other properties, as well as receiving streams. as well as "infiltration" of subsurface water through cracked pipes, leaky joints, poor manhole connections, etc. Rainfall dependent (or rainfall-derived) inflow and infiltration (RDII) are stormwater flows that enter sanitary or combined sewers due to "inflow" from direct connections of downspouts, sump pumps, foundation drains, etc. Rainfall Dependent Inflow and Infiltration from the EPA SWMM 5 Hydrology Manual
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