Catchpit — StormWater Solutions

A catchpit is an access chamber built with a sump below the level of the outgoing pipe invert, positioned on a drainage run so that silt and grit settle out before they reach downstream storage, treatment devices or an outfall. The sump is the defining feature. A chamber without one, however deep, is an inspection chamber or a manhole, not a catchpit.

How a catchpit works

Flow entering the chamber slows as the cross-section widens, and the heavier fraction of the suspended load falls into the sump rather than being carried onward. The outgoing pipe is set above the sump floor, so the accumulated material is held below the flow line and does not remobilise in normal conditions. Typical sump depths on UK highway and development work are in the range of 300 to 600 mm below invert, sized against the expected sediment load and the intended emptying interval rather than a standard figure. Efficiency falls sharply as velocity rises, so a catchpit on a steep run captures far less than the same chamber on a flat one.

Where to place catchpits

Catchpits belong immediately upstream of anything that is expensive or impossible to clean. That means before an attenuation tank or crate field, before an infiltration system such as a soakaway, before a treatment device, and at the last access point before an outfall to a watercourse. On highway networks they are also used at the head of long carrier drains and where a filter drain discharges into a piped system, since filter drains shed fines steadily. Placing a catchpit at the low point of a construction phase surface water system is one of the simplest ways of keeping site silt out of a permanent asset.

Maintenance

A catchpit only works while there is capacity in the sump. Once full it passes sediment straight through, and the asset it was protecting begins to fill instead. Emptying is a vacuum tanker operation on a frequency set by observation in the first year or two of operation — six-monthly is a common starting assumption, but a car park catchment with an unbound surface may need far more. The maintenance schedule handed over at adoption should state the sump depth, the access arrangements and the initial frequency, and the asset should be recorded on the drainage schedule so it is not lost under landscaping.

Catchpit, silt trap and gully compared

The three overlap in everyday use. A gully is the road-surface inlet that takes run-off into the network, and most gully pots include a small sump of their own. A silt trap is the general term for any device whose primary function is to intercept sediment, which includes catchpits, gully pots and proprietary units. A catchpit specifically is an access chamber with a sump, on the pipe run rather than at the surface inlet. Where the treatment requirement is for hydrocarbons or fine sediment rather than grit, a catchpit is not sufficient and a bypass separator or hydrodynamic separator is needed.

Related ViaCon solutions

ViaCon supplies stormwater treatment and storage systems that depend on effective upstream silt capture. See our stormwater solutions range. Related glossary entries: silt trap, surface water drainage, road gully and attenuation tank.

Frequently asked questions about catchpit