What Is RDII in Sewer Systems?

RDII stands for Rainfall Dependent Infiltration and Inflow. It refers to stormwater that enters the sanitary sewer system during and after rainfall events through defects in the collection system infrastructure. RDII is the primary cause of wet weather capacity problems in sanitary sewers, including sanitary sewer overflows (SSOs), treatment plant surcharging, and basement flooding.

What Causes RDII?

RDII has two distinct components that enter the sewer through different mechanisms and require different rehabilitation approaches:

Infiltration is groundwater that enters the sewer through deteriorated pipe joints, cracked pipe walls, and failed service laterals. When rainfall raises the groundwater table, this water is pushed into the sewer through every defect below the water table. Infiltration produces a slow, sustained flow increase that builds over hours to days after rainfall and can persist for days after rain stops as the water table slowly recedes.

Inflow is surface stormwater that enters the sewer directly through illicit connections (such as roof drains or area drains connected to the sanitary sewer), damaged manhole covers that allow surface water to pour in, cross-connected storm drains, and foundation drains that are improperly connected to the sanitary system. Inflow produces a rapid flow response that closely tracks rainfall intensity — flow rises quickly when rain starts and drops quickly when rain stops.

Why the Distinction Matters

Infiltration is addressed through structural rehabilitation: pipe lining (CIPP), joint sealing, manhole rehabilitation, and lateral replacement. Inflow is addressed by locating and disconnecting illicit connections, repairing manhole structures, replacing damaged covers, and redirecting improperly connected drains.

Without RDII analysis that separates these components, a municipality might spend millions rehabilitating pipe when the actual problem is illicit inflow connections — or vice versa. Accurate RDII quantification by sub-basin directs rehabilitation investment to the locations and mechanisms that produce the greatest measurable reduction in SSO risk.

How Engineers Quantify RDII — The RTK Method

The industry-standard method for quantifying RDII is the EPA RTK unit hydrograph approach, which is built into EPA SWMM and other hydraulic modeling platforms. Engineers calibrate three parameters for each monitored sub-basin:

R (Volume fraction) — the fraction of total rainfall volume that enters the sewer as RDII. Higher R values indicate more stormwater is entering the system. Typical values range from 0.01 (1% of rainfall) to 0.15 (15% of rainfall), with values above 0.05 generally indicating significant defects.

T (Time to peak) — the time from the center of a rainfall event to the peak of the RDII flow response. Short T values indicate inflow-dominated response; longer T values indicate infiltration-dominated response.

K (Recession ratio) — describes how quickly RDII flow decreases after the peak. This parameter captures the "tail" of the RDII response and is important for modeling how long elevated flows persist after rain stops.

These three parameters are calibrated by matching modeled flow hydrographs to observed flow monitoring data during multiple storm events. The calibrated R, T, and K values become the direct inputs for hydraulic model wet weather simulation.

Why RDII Analysis Matters for Municipalities

EPA consent decrees require municipalities to quantify RDII as part of corrective action planning. RDII data identifies which sub-basins contribute the most stormwater to the sanitary sewer, allowing rehabilitation investment to target the highest-impact areas first. Without RDII data, municipalities cannot justify capital improvement programs, demonstrate consent decree compliance, or prioritize SSO reduction efforts based on engineering evidence.

Need this service? Learn about our rdii analysis services or request a quote.