In the San Francisco Bay Area, inflow and infiltration (I&I) problems are epic, with wet-weather peaking factors in some system being 12X or more of dry-weather flows. Operators in such systems are under huge pressure to reduce I&I ASAP, and with limited resources.

One of the best ways to flatten out the highest peaks in wet weather flow comes from considering the differences between inflow and infiltration.

Take a look at the hydrograph above, which displays the flow from a monitor in a north bay manhole.

It shows massive inflow in conjunction with storms of over 3/4 of an inch but almost no impact from smaller storms. The data tells us that upstream of this manhole there is probably a manhole or manholes with huge amounts of inflow—but only when there is enough rain for ponding to occur.

When we look upstream of that manhole (shown in red) we do see a handful of manholes in a marshy area. It is almost certain that one or more of these manholes are the source of the inflow and the district can reduce hundreds or thousands of gallons of I&I during its heaviest storms just by sealing the tops of a few manholes. 

Prioritizing Inflow

While infiltration can add more water to a system over time than inflow, prioritizing the finding and fixing of inflow is more productive because:

1. Rain-driven SSOs are generally caused by massive spikes in flow and these spikes are a result of inflow not infiltration. Fixing inflow is the fastest way to flatten spikes.
2. Inflow repairs are often faster and less inexpensive than infiltration repairs because inflow occurs at the surface—do digging required. It can often be fixed by simply raising or sealing and locking manholes, capping open cleanouts, and removing roof drain connections to the collection system.

The BasinIQ system from RH Borden uses a combination of manhole modeling and synchronized level monitoring to calculate flow and identify sources of inflow and infiltration. Click here to begin addressing your I&I issues today.