What is a Catchment Pit

A catchment pit, also known in some contexts as a silt trap, gully pot, or debris interceptor, is a chamber incorporated into a surface water drainage system designed to capture and retain silt, debris, and coarse particulate matter carried by stormwater. Its purpose is to prevent such materials from entering and obstructing downstream pipework, manholes, or treatment systems.

Installed at key points within a drainage network, particularly before water enters an attenuation system, soakaway, or public sewer, a catchment pit helps maintain hydraulic performance, prolong the life of drainage infrastructure, and reduce maintenance costs by trapping unwanted solids at an accessible location for removal.

In urban drainage, highways, construction sites, and landscaped areas, catchment pits are vital components of modern stormwater management strategies, especially in areas prone to high sediment loads or environmental pollution.

Purpose and Function of a Catchment Pit

The core function of a catchment pit is sediment interception. As stormwater flows across surfaces and into gullies or channels, it entrains a variety of materials—soil particles, leaf litter, gravel, plastic debris, and hydrocarbons. If allowed to travel unchecked into the underground drainage system, these materials can:

  • Reduce pipe capacity through sediment accumulation. 
  • Cause complete blockages and localised flooding. 
  • Damage sensitive SuDS components such as infiltration crates or permeable pavements. 
  • Increase wear in mechanical pumps or treatment plant equipment. 
  • Transport pollutants into receiving watercourses, breaching environmental regulations. 

A catchment pit acts as a gravity separator. Water flows into the chamber, slows down due to the increased volume and change in flow direction, and allows heavier solids to settle out of suspension. Cleaner water then continues through the outlet pipe, while sediment and debris accumulate at the bottom of the pit.

Catchment pits are commonly used in:

  • Car parks and paved areas. 
  • Access roads and driveways. 
  • Hard landscaped urban environments. 
  • Surface water drainage systems prior to soakaways or treatment units. 
  • Construction sites where heavy silt loads are expected during groundworks. 

Design and Construction

Catchment pits come in a variety of sizes, shapes, and materials, depending on the volume of water to be handled, the type of debris expected, and site-specific considerations. A standard catchment pit comprises the following components:

  • Inlet: Usually a pipe or open channel entering the pit near the top. Some designs include grated surface inlets that double as road gullies. 
  • Chamber: The main body of the pit, sized to allow adequate retention time for sedimentation. Depth is important to ensure sufficient silt capacity. 
  • Trap or sump: A lower section of the chamber where solids settle and accumulate. This must be periodically emptied to maintain effectiveness. 
  • Outlet: A pipe located above the sump level that conveys treated water to the next stage of the drainage system. 
  • Access cover or inspection lid: Allows maintenance teams to inspect, clean, and remove silt deposits. 

Materials used in catchment pit construction include:

  • Precast concrete: Durable and robust, widely used for highway and commercial applications. 
  • High-density polyethylene (HDPE) or polypropylene: Lightweight, chemically resistant, and easy to install, commonly used in domestic or lightweight commercial systems. 
  • Brick or block construction: Occasionally used in bespoke applications or older systems. 

The sizing of a catchment pit depends on:

  • Anticipated surface area draining to the pit. 
  • Local rainfall intensity and storm profiles. 
  • Expected silt loading (based on surface type and environment). 
  • Maintenance frequency and access. 

Where multiple catchment pits are used in series, the system may be designed with a primary interceptor and secondary treatment units (such as oil separators or filters).

Installation Considerations

Correct installation is essential for the effective function and durability of a catchment pit. The key aspects include:

Location

Catchment pits should be placed strategically at:

  • The lowest point in a catchment area, where surface water converges. 
  • Just upstream of sensitive SuDS components or proprietary treatment units. 
  • Entry points to soakaways or infiltration basins. 
  • Transition points between surface flow and piped systems. 

In highway and kerbed areas, catchment pits are often located under road gullies.

Invert Levels and Gradient

Proper alignment of inlet and outlet pipes is essential to ensure flow continuity and avoid backfall. Invert levels must be set to promote gravity flow into and out of the chamber, with sufficient drop from inlet to sump to allow effective sedimentation.

Bedding and Surround

Catchment pits, particularly those made of plastic, must be bedded on compacted granular material to prevent settlement or flotation. In areas subject to traffic loading, concrete backfill and load-rated access covers are required.

Access and Maintenance

A key design consideration is safe and easy access for maintenance. The pit cover must be removable without specialised tools, and the internal chamber must be large enough for manual or mechanical silt removal.

Maintenance and Operational Requirements

Catchment pits require regular inspection and desilting to maintain their performance. The frequency of maintenance depends on:

  • Catchment characteristics (e.g. tree coverage, unpaved areas). 
  • Pit size and sediment capacity. 
  • Local weather patterns. 
  • Upstream construction or disturbance. 

Typical maintenance tasks include:

  • Removing the access cover. 
  • Measuring silt depth to assess accumulation. 
  • Using suction equipment or buckets to remove solids. 
  • Checking inlet and outlet for blockages. 
  • Inspecting structural integrity and replacing damaged components if needed. 

Failure to maintain catchment pits can result in:

  • Overflow and flooding during rainfall events. 
  • Transfer of silt to downstream drainage features. 
  • Increased wear or clogging of treatment systems. 
  • Elevated pollutant loads in receiving water bodies. 

Some systems include alarms or remote monitoring for maintenance scheduling, particularly in sensitive or inaccessible locations.

Integration with Sustainable Drainage Systems (SuDS)

In SuDS schemes, catchment pits serve as pre-treatment components that protect the integrity and function of downstream features such as:

  • Swales. 
  • Infiltration trenches. 
  • Detention basins. 
  • Permeable paving. 
  • Green roofs and rain gardens. 

Including catchment pits in SuDS designs aligns with best practice guidance such as the CIRIA SuDS Manual (C753), which emphasises treatment trains and pollution prevention. In developments where pollutants such as hydrocarbons, metals, or organic waste are likely, catchment pits act as a first line of defence, capturing gross pollutants before they enter more sensitive features.

Regulatory and Planning Implications

In the UK, catchment pits are not typically subject to standalone regulation but are part of broader drainage and environmental management responsibilities under:

  • Building Regulations Part H (Drainage and Waste Disposal). 
  • Environment Agency guidance on surface water discharge and pollution prevention. 
  • Water company standards for adoptable drainage infrastructure. 
  • Planning permissions and SuDS approvals, particularly in flood-prone or sensitive locations. 

Designers must ensure that catchment pits do not discharge polluted water directly to ground or watercourses without adequate treatment. Where used in conjunction with soakaways, especially in Source Protection Zones or near water bodies, consultation with the Environment Agency may be required.

Common Challenges and Misapplications

Although straightforward in concept, catchment pits can become ineffective or problematic if poorly designed or maintained. Common issues include:

  • Undersized chambers that fill too quickly. 
  • Lack of maintenance access or awareness. 
  • Flat inverts that prevent sediment settlement. 
  • Incorrect placement resulting in bypassed flows. 
  • Use in inappropriate locations (e.g. high groundwater tables). 

To prevent these issues, collaboration between designers, installers, and maintenance teams is essential from the earliest planning stages.

Conclusion

A catchment pit is a simple yet vital component of effective surface water drainage systems. By intercepting silt and debris at key points, it protects downstream infrastructure, improves water quality, and ensures long-term system performance. Whether used in residential, commercial, highway, or industrial applications, catchment pits contribute significantly to the operational resilience of both conventional drainage and SuDS.

For drainage professionals, engineers, and developers, understanding the design, installation, and maintenance of catchment pits is essential in delivering efficient, sustainable, and regulation-compliant drainage solutions. As environmental standards rise and the importance of pollution prevention grows, these humble chambers continue to play an outsized role in shaping the quality and performance of modern drainage infrastructure.