Key Takeaways

• Inflow enters a sewer system through surface defects like open manholes, cleanouts, and roof drain connections
• Infiltration seeps in underground through cracked pipes, failed joints, and deteriorated manholes
• Inflow drives the sharpest wet-weather flow spikes and is usually the faster, cheaper fix
• Infiltration adds more total volume over time but is harder and more expensive to repair since it requires digging
• A hydrograph showing sharp spikes only during heavy storms is a strong signal of inflow, not infiltration
• RH Borden’s BASINiQ system combines manhole modeling and synchronized level and precipitation monitoring to pinpoint both inflow and infiltration sources

Inflow enters through surface openings like manholes, while infiltration seeps through cracked pipes underground; fixing inflow first is usually faster, cheaper, and flattens wet-weather flow spikes fastest.

In the San Francisco Bay Area, inflow and infiltration (I&I) problems are epic, with wet-weather peaking factors in some systems 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 expensive than infiltration repairs because inflow occurs at the surface, no 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.

What Is Infiltration?

Where It Comes From

Infiltration is groundwater that seeps into a sanitary sewer through structural defects that already exist below ground: cracked pipe walls, failed joints between pipe segments, deteriorated manhole benches, and root intrusion. Unlike inflow, infiltration doesn’t need a storm to show up. It’s often present as a steady, elevated base flow, worse when the water table is high, but rarely absent entirely.

Why It’s Harder to Fix

Because infiltration defects sit underground, repairing them usually means excavation, pipe lining, or joint sealing work that requires locating the pipe, digging or accessing it, and doing structural repair. That’s a fundamentally more expensive and slower process than sealing a manhole lid at the surface.

What Is Inflow?

Where It Comes From

Inflow is stormwater or surface water entering the system through direct, visible openings: manhole covers with perforations, cleanouts left uncapped, catch basins cross-connected to the sanitary line, and downspouts illegally tied into the collection system. It shows up fast during storms and disappears just as fast once the rain stops.

Why It’s the Better First Target

Since inflow sources sit at the surface, they’re often visible without excavation, and fixes like sealing a manhole lid, capping a cleanout, or disconnecting a downspout can sometimes be completed in a single visit. That’s a meaningfully different cost and timeline than a buried pipe repair.

How to Tell Which One You’re Dealing With

Reading a Hydrograph

The case above is a good example of this in practice: a hydrograph that spikes sharply during storms and then immediately disappears, points strongly toward inflow rather than infiltration. Infiltration tends to show up as a slower, more gradual rise and fall that tracks with groundwater levels rather than storm intensity.

Field Verification Methods

A hydrograph is a strong first clue, but utilities typically confirm the source with a follow-up method: smoke testing to visually spot surface openings, visual inspection of a manhole,, or CCTV inspection to confirm a cracked joint underground. Acoustic assessment can also help narrow down which pipe segments are worth a closer look before committing to an excavation.

The Cost Difference: Why Inflow Fixes Pay Off Faster

A sealed manhole lid or capped cleanout can cost a small fraction of what a buried pipe repair runs, and the labor is measured in hours rather than days. That’s why most collection system managers, when working with a limited annual I&I budget, get more flow reduction per dollar by repairing inflow sources first, even though infiltration may be adding more total volume to the system across a full year.

How RH Borden Locates Both Inflow and Infiltration Sources

Finding the exact manholes and pipe segments responsible for I&I is the hard part, which is where RH Borden’s BasinIQ system comes in. By combining manhole modeling with synchronized level monitoring across a basin, BASINiQ narrows a citywide problem down to the specific structures worth fixing first, the same kind of analysis behind the north bay hydrograph example above.

Getting Ahead of Your I&I Problem Before the Next Storm

Recognizing the difference between inflow and infiltration isn’t just an academic distinction, it’s the difference between a fast, affordable fix and a slow, expensive one. Start with inflow, since it’s usually the quicker win, but don’t ignore infiltration’s slower, steadier contribution to your system’s total flow.

RH Borden helps municipalities pinpoint exactly where their I&I is coming from so budget goes toward the manholes and pipe segments that actually move the needle. Ready to start addressing your I&I issues today? Visit rhborden.com or call 385-510-3223 to see how BASINiQ can map your system’s inflow and infiltration sources.

Frequently Asked Questions

Can a system have both inflow and infiltration problems at the same time?

Yes, and most systems do. They usually show up differently on a hydrograph though, inflow as sharp storm-driven spikes, infiltration as a slower, steadier base flow increase.

How quickly can inflow repairs typically be completed once a source is found?

Simple fixes like sealing a manhole lid or capping a cleanout can often be completed in a single site visit, though more complex sources like cross-connected catch basins may take longer to trace and fix.

Does fixing infiltration ever make more sense to prioritize first?

In some systems, yes, particularly where infiltration is severe enough to threaten pipe capacity. But for most utilities working within a limited annual budget, inflow offers a faster return.

What’s the risk of not addressing inflow before the next major storm?

Unaddressed inflow sources can drive sanitary sewer overflows during heavy rain, which carries public health, environmental, and regulatory consequences well beyond the cost of the repair itself.

How does RH Borden’s BASINiQ system pinpoint these sources?

BASINiQ combines manhole modeling with synchronized level monitoring across a sewer basin, allowing engineers to calculate flow at each structure and narrow down exactly which manholes are contributing the most I&I.