Silt Trap is a chamber or structure that intercepts and retains silt, sand and other sediment carried by surface water run-off before it enters a drainage network, soakaway, attenuation tank or watercourse. By removing sediment at source, silt traps protect downstream components from blockage, loss of storage volume and pollution.
How a silt trap works
Run-off enters the trap and its velocity drops sharply in the enlarged chamber. Suspended particles settle out of the slower-moving water into a collection sump, while cleaner water discharges over an outlet weir or through a submerged outlet pipe. Some designs add baffles to lengthen the flow path or screens and filters to capture finer material and floating debris.
Types of silt trap
- Catchpit chambers — precast concrete or plastic chambers with a deep sump, installed at pipe junctions and upstream of outfalls.
- Proprietary silt and sediment interceptors — engineered chambers, including vortex-type units, sized for a defined treatment flow and sediment load.
- Filter and silt bags — temporary devices fitted in gullies or on pump discharges during construction.
- Sediment forebays and bunds — surface SuDS features that perform the same function upstream of ponds and basins.
Silt traps in SuDS and stormwater treatment
CIRIA C753 (The SuDS Manual) treats sediment management as the first stage of the treatment train. A silt trap or forebay upstream of infiltration and storage components — soakaways, permeable paving sub-bases and attenuation tanks — preserves their design volume and infiltration performance, and concentrates maintenance at a single accessible point. For sites with oil risk, silt traps are commonly combined with an oil separator or vortex separator.
Maintenance
Silt traps only work while the sump has capacity. Inspection is typically six-monthly and after major storms, with sediment removal by gully sucker when the sump is between half and two-thirds full. Maintenance access must be considered at design stage: chamber covers rated for the location, safe access positions and a route for tanker vehicles.
Related ViaCon solutions
ViaCon Treat Solutions include precipitators and separators that remove sediment and contaminants upstream of storage. See ViaCon Treat Solutions and our stormwater solutions range. Related glossary entries: vortex separator, attenuation tank and soakaway.
Frequently asked questions about silt trap
What is a silt trap?
A silt trap is a chamber that slows surface water run-off so that silt, sand and sediment settle into a sump before the water continues into a drainage system, soakaway, attenuation tank or watercourse. It protects downstream assets from blockage and pollution.
Where should a silt trap be installed?
At the upstream end of the drainage system, as close to the sediment source as practical — typically before soakaways, permeable paving sub-bases, attenuation tanks, ponds and outfalls, and at key pipe junctions via catchpit chambers.
How often should a silt trap be emptied?
Inspect every six months and after significant storm events, and empty the sump — normally by gully sucker — once it is between half and two-thirds full. Sites with high sediment loads, such as during construction, need much more frequent servicing.
Do I need a silt trap before a soakaway?
Yes. BRE Digest 365 and The SuDS Manual both recommend upstream sediment removal, because silt entering a soakaway progressively blinds the infiltration surface and permanently reduces its capacity. A silt trap or catchpit is the standard safeguard.