Infiltration trench

An infiltration trench is an underground trench filled with a porous material such as gravel, designed to collect stormwater runoff. Stormwater infiltrates through the walls and base of the trench into the surrounding soil, while particulate and some dissolved pollutants are retained in the porous material.

How does an infiltration trench work?

In urban areas, more stormwater surface run-off is created by the surrounding impervious surfaces such as roads and buildings. These also form a barrier that prevents water soaking into the ground where it can evaporate and return to the atmosphere or soak deep down to water beneath the ground known as groundwater. An infiltration trench allows water to infiltrate into the soil and groundwater to restore the natural water balance.

Infiltration systems can be designed as trenches, wells or basins. Designs range from roof run-off flowing into a simple gravel-filled well to systems with a raingarden treating stormwater with underlying storages that minimise the frequency of surface run-off flows.

Stormwater infiltrates through the walls and base of the trench into the surrounding soil, while particulate and some dissolved pollutants are retained in the porous material. The infiltration trench's ability to remove pollutants is determined by the type of local soil and geochemistry, as well as the configuration of treatments.

Soils with low infiltration rates such as clays provide more effective treatment while sands can allow pollutants to pass through without effective treatment. However, infiltration trenches are smaller and more economical when the soil infiltration rates are higher.

Infiltration trenches protect waterways from damaging stormwater run-off flows by reducing the frequency, velocity and volume of stormwater run-off flows and flooding. They also increase the soil water levels and help to refill underground water storages called aquifers.

All infiltration treatment types require significant pre-treatment of stormwater to remove sediment before infiltration to avoid clogging the infiltration trench walls and base. Infiltration trenches are a secondary treatment or water disposal system because they need pre-treatment to remove other pollutants such as sediment, nutrients and heavy metals to protect groundwater from contamination.

Infiltration trenches are primarily intended to provide a temporary storage and pathway for stormwater to be discharged into the soil and groundwater after treatment. Infiltration trenches provide limited treatment through settling and trapping of particles.

Designing an infiltration trench

The main design considerations are:

the infiltration rate of the surrounding soil
the detention storage provided
the resulting hydrologic effectiveness, that is, the proportion of infiltrated stormwater

The storage can be sized using STORM or a continuous simulation model such as MUSIC. Calculations for a design event can be undertaken where the purpose of the system is to provide flood control. The inputs required are:

Underlying soil type (based on a soil test where available or Council maps)
Detention storage volume
Area and depth

The top surface of the fill in a trench can be covered with a layer of fibre fabric then covered with a shallow layer of topsoil and grass. The excavation can also be lined with a layer of open-weave geotextile fabric to prevent soil migration into the rock or gravel fill.

A washed gravel media is commonly used and can extend the life of an infiltration system by helping to trap any fine sediment that reaches the trench so it does not clog the walls and base of the system.

When to use an infiltration trench

In areas where you want to reduce nuisance flooding or the frequency of stormwater entering a waterway.

Infiltration trenches should generally not be constructed:

within close proximity to building foundations - see WSUD Engineering Procedures for set-back distances for different soil types
within 0.5m of the groundwater table
on slopes or areas with shallow soils and underlying rock
in areas with sodic or dispersive soils or reactive clays
in areas with groundwater salinity problems

Tips and advice

Only ‘cleansed’ stormwater free of sediment that is either roof run-off or treated stormwater should be allowed to enter an infiltration trench.

It is easier to keep sediment out of a trench than remove it once it’s inside. Ideally use a raingarden to prevent sediment or pollutants from entering. Otherwise a gravel media further traps sediment and extends the life of the system. Alternatively use a filter at the inlet that is easily and frequently cleaned.

The infiltration capacity is influenced by the surface area of the trench. In areas with low infiltration rates use a long narrow trench to achieve more surface area and allow the system to drain quicker.

Use a washed gravel to trap sediments or design the system so that it is easy to clean the filter surface.

Groundwater is complex and a geotechnical engineer or someone with knowledge of soils and soil water flow should be consulted during design of an infiltration trench.

For detailed information on how to design infiltration trenches, see the design guidance and examples provided in Chapter 11 "Infiltration measures" in the WSUD Engineering Procedures available for purchase from CSIRO Publishing.

WSUD Engineering Procedures