Infiltration trenches

An infiltration trench is an excavation filled with porous material, which collects stormwater run-off. Stormwater infiltrates into the surrounding soil, while particulate and some dissolved pollutants are retained in the porous media.

​Treatments and contaminants

Primary, secondary and tertiary treatment includes:

  • physical screening of sedimentation (coarse and fine)

  • infiltration of stormwater into soils

  • biological uptake and breakdown of pollutants and nutrients

  • retention of heavy metals and other toxicants

Typical retained contaminants include:

  • gross pollutants

  • coarse sediments

  • some nutrient removal

  • dissolved heavy metals

How infiltration trenches work

Infiltration systems can be configured in a variety of ways and range from simple gravel-filled trench systems to those topped with bio-retention systems for maximum pollutant removal prior to infiltration.

Infiltration trenches can increase the soil water levels and groundwater flow rates, as well as reduce stormwater flow velocities. Their ability to remove particulate and dissolved pollutants depends on local soil geochemistry, treatment measure configuration and grading.

To reduce their visual impact, they can be covered with a layer of fibre fabric and finished with a shallow layer of topsoil and grass. The trench can be lined with a layer of geotextile fabric, to prevent soil migrating into the rock or gravel fill.

Advantages and limitations

Advantages of infiltration trenches are that they:

  • replenish ground water and, therefore, dry weather flows to waterways

  • provide a relatively small surface footprint

  • offer water to plants (where vegetated) during dry periods

  • provide a scalable system that can be used at the lot, street and regional level

Limitations of infiltration trenches are that they:

  • become clogged with pollutants and sediment unless regularly maintained

  • result in groundwater contamination and low dissolved pollutant removal if soils are coarse

  • cannot be used if contaminated groundwater is present

  • are ineffective on steep slopes, loose or unstable areas

  • cannot be used near buildings

  • can cause bogging or damage to vegetation where car parking occurs

Tips and advice

Tips and advice for using infiltration trenches include:

  • use media to achieve better pollution removal and evapotranspiration

  • increase treatment storage and exfiltration potential with a scoria filled chamber below the system

  • do not intersect with the ground water table

  • drain the top 100mm of media in vegetated systems to support plants

  • ensure outlet rates are variable in vegetated systems to allow establishment

  • use geotechnical reports to provide critical information for design and modelling

For more information on the design of infiltration trenches, refer to Chapter 11 "Infiltration measures" in the WSUD Engineering Procedures available for purchase from CSIRO publishing.

WSUD Engineering Procedures - CSIRO Publishing


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