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Infiltration Systems

About Infiltration Systems

Infiltration systems are becoming a viable alternative or supplement to conventional drainage as it has many environmental and economic benefits.  Such benefits include reduced peak stormwater flows, reduced downstream flooding, reduced stormwater drainage capital costs, improved groundwater recharge, and improved stormwater quality.

Conventional stormwater drainage typically involves draining water from a residential property to a street drainage system.  This is great for removing stormwater from a site, but it also contributes to flooding risk, erosion, and sedimentation and water quality decline in downstream water catchments.  The modern infiltration systems are designed to minimise clogging by silt material and can be designed to overflow to landscaped areas or the street drainage system when their storage capacity is exceeded during heavy rainfall.  Infiltration systems contain a number of pollutant removal mechanisms including adsorption, straining, microbial decomposition in the gravel layer, and trapping of sediment in the pre-treatment areas.  If the system is correctly designed, the infiltration system can remove approximately 90 percent of sediment, 60 percent of phosphorus, and 60 percent of nitrogen from stormwater.

Using Infiltration Systems

Infiltration systems can be used on their own or with rainwater tanks, porous paving and landscape measures in order to manage stormwater.  The infiltration system collects the rain, stores it temporarily, and then releases it slowly onto the ground.  The three parts of the infiltration system are:

• Site drainage system – this is the roof gutters, downpipes, paths, and driveway, etc, that collects and delivers stormwater to the infiltration system.
  
• Pre-treatment system – this receives water runoff prior to it entering the infiltration system in order to remove gross pollutants, particular matter, and soluble pollutants.  This stage protects and maximises the life of the infiltration systems and improves the quality of the water entering the environment.
  
• The infiltration retention system – this stores the roof and stormwater until it can percolate into the surrounding soil.  The most commonly used systems for this are:
  
  • Leaky wells – consist of a vertical perforated pipe with a lid at the ground surface and an open bottom.  The stormwater enters via an inlet pipe at the top and an overflow pipe caters for excess stormwater.  The holes in the walls and the open bottom are covered with geotextile fabric to clean the stormwater as it percolates into the surrounding soil.  Before the water enters this, all stormwater should be filtered by a sediment trap.
    
  • Retention trenches – consist of a trench lined with geotextile fabric and filled with coarse gravel, and placed under a 300mm layer of sand or loam.  The stormwater is conveyed to the trench via an inflow pipe after passing through a sediment trap.  A perforated distribution pipe allows the water to percolate into the soil.  An overflow pipe directs excess water to the street drainage system.
    
  • Infiltration basins – collect and store stormwater runoff until it percolates into the surrounding soil and evaporates into the atmosphere.  By removing a portion of the stormwater runoff, the infiltration basin reduces the stormwater peak discharge and volume to downstream catchment and also improves the quality of stormwater discharged to the environment.  It is designed as a depression with good grass coverage over a layer of coarse gravel surrounded by geotextile fabric. A 300mm layer of topsoil is usually placed between the gravel and the surface.  The water that enters the basin is filtered to remove sediment, leaves, and debris by sediment traps, vegetated areas, or specially designed gutter systems.

What to Look for when Installing Infiltration Systems

When installing an infiltration system, you will need to check with your local council as to what their minimum clearance from buildings is.  Infiltration systems should not be installed in soils that are saline, sodic, or very shallow; wind blown or loose sands; clay soils that collapse in contact with water or have high shrink/swell characteristics; or soils that have a hydraulic conductivity of less that 0.36mm per hour.  Soil assessment and permeability testing will need to be done before any infiltration system is installed.  Infiltration systems should not be installed on steep slopes, that is on slopes greater than 5 percent.  Councils will generally require that the infiltration system is large enough to store the inflow for a one-in-three months average recurrence interval design storms with an emptying time of less than 24 hours.

Sewer Infiltration

Sewer infiltration is caused by surface water running freely into property sewers after heavy rain.  This is often because of a non-conforming disconnector gully grate and the solution to this is to install an anti infiltration/overflow relief device or AIORD.  The device has been developed to ensure that surface water cannot enter sewers at a level inlet fitted to a disconnector gully.  The AIORD fitting can be used instead of the standard open grating.  As well, the device allows the backflow of sewage to lift the cap clear of the sewer pipes and spill on the surrounding ground.  A special valve has also been incorporated to allow foot ventilation and the waste pipes to freely discharge through the disconnector gully.

The AIORD device sits in the top of a specially manufactured fitting installed at the top of a level inlet and is not easily removed once fitted.  It is non-captive and will pop clear in the event of a sewer surcharge.  After cleaning up, simply replace the device.  The AIORD may only be fitted to the top of an overflow relief gully and may be installed with a finished height of 25mm above the surrounding surface area.

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