Sewers, drainage and stormwater

Why does a manhole surcharge?

• Surcharge is a symptom, not a fault. It says the downstream pipe cannot take what is arriving.
• Blockage surcharge builds in dry weather. Level climbs steadily with no rainfall to explain it.
• Capacity surcharge tracks the rain. Level rises and falls with the storm and recovers on its own.
• The shaft is the warning. Level in the chamber rises well before anything reaches the cover or the street.

The four things that cause it

A blockage downstream. Fat, oil and grease bind with wet wipes and rag to form a restriction that grows over days or weeks. Flow backs up behind it and the nearest chamber upstream fills first. This is the most common cause on commercial and catering sites.

Not enough capacity for the flow arriving. During heavy rain a combined sewer can simply receive more than it was sized to carry. Infiltration and inflow make this worse by using up headroom with clean water before the storm even starts.

A pumping station that has stopped keeping up. If a pump has failed, a float has stuck or a rising main is partially blocked, the wet well fills and the backing up propagates into the chambers upstream.

A structural or tidal restriction. A collapsed section, displaced joint, intruding root mass or a high receiving water level can all throttle the outgoing pipe.

How the level trace tells you which one it is

This is the practical value of putting a level sensor in the chamber rather than relying on a report from the street. The shape of the curve identifies the cause.

A blockage produces a slow, monotonic climb with no rainfall to account for it, often stepping up a little each day as the restriction grows. It does not recover on its own. Catching this curve early is the difference between a planned jetting visit and an escape into a car park.

A capacity event produces a sharp rise that correlates with rainfall and then falls away as the storm passes. It recovers without intervention. If it recurs at modest rainfall totals, the network has an I&I problem rather than a design problem.

A pumping station fault usually shows as a rise upstream that coincides with a change in the station’s own cycling pattern, which is why level in the chamber and pump run data are more useful read together than apart.

Why waiting for the cover to lift is the expensive option

By the time surcharge is visible at ground level the sewer has already been full for some time, and the consequences have usually already started. Foul water backing up reaches basements, lower ground floors and gullies before it reaches the street.

Surcharge that reaches a watercourse becomes a pollution incident with a reporting obligation attached. Surcharge that reaches a building becomes a contamination clean-up and an insurance claim. Neither is proportionate to the cost of catching a rising level trace a week earlier.

A continuous level sensor in a chamber turns this into a threshold alarm. Set a level that corresponds to a restriction rather than to normal wet weather operation, route it to whoever can act, and the failure mode changes from an incident into a work order.