This is part 2 of Mike Homenuke’s series on Inflow and Infiltration. For part 1 click here.
Lets take a closer look at the components that comprise I&I:
Groundwater Infiltration (GWI)
These rates in the Pacific Northwest typically occur in the 3,000 – 5,000 L/ha/d range. High GWI can be found in:
- Catchments with poorly draining soils
- Deep sewers
- Structural decay
- Ungasketed joints
Achieving complete I&I reduction in these catchments will likely require an intense focus on rehabilitation of the lower portions of the system.
Stormwater Inflow (SWI)
SWI usually results from:
- Directly connected roof leaders
- Foundation drains
- Sump pumps
- Catch basins
- Defective sewer access points including manhole frames and covers, cleanouts and inspection chambers
SWI is indicated in flow monitoring records as sharp responses to rainfall events during summer months, but is nearly impossible to separate from Rainfall-Induced Infiltration (RII) during winter months. This can be quantified at a return period using the I&I Envelope Method.
Rainfall-Induced Infiltration (RII)
RII enters the sewer system through:
- Leaking joints
- Structural defects in pipes and manholes
Service connections are typically buried at shallow depths and are easily damaged. Additionally, brick risers and other component interfaces in manholes are not typically sealed. This allows for rapid infiltration of groundwater to occur.
Infiltration in pipes and lower portions of manholes occurs more slowly, but can be significant during longer wet weather events.
It’s difficult to visually detect RII Because CCTV inspections are usually conducted during dry weather to prevent submergence of the camera. RII becomes fully developed when soils surrounding the sanitary sewer become saturated. This is also determined using the I&I Envelope Method, indicated by flow events with the highest ratio of Rainfall-Dependent Inflow & Infiltration (RDII) to rainfall.