Infiltration system types
Infiltration devices allow water to soak into the ground, provided that the subsoil is sufficiently permeable.
Infiltration systems consist of infiltration trenches, soakage wells or pits, and swales and basins, and are designed to retain a certain volume of stormwater runoff. The stored water permeates into surrounding soils, significantly reducing runoff volumes, having provided a pathway for treated runoff to recharge local groundwater aquifers.
Example | George Whittle Reserve: An 11m x 3m x 1m infiltration trench filled with 40mm aggregate was designed to ensure the 1-in-5 year ARI (0.18 AEP) design storm did not result in increased stormwater runoff from a 1500m2 of hard surfaces on the 3750m2 site (40% impervious) from entering City of Prospect’s stormwater network.
Protecting urban waterways from erosion (reduce total volume of stormwater runoff)Increase soil moisture to sustain tree health
Example | Dorset Avenue, Colonel Light Gardens, City of Mitcham: A 9000 x 750 x 750 mm infiltration trench delivers over 100,000 litres of water to the verge soils to passively irrigate street trees in an average year.
Source: Johnson T, Lawry D and Sapdhare H (2016) The Council verge as the next wetland: TREENET and the Cities of Mitcham and Salisbury investigate. In: Acta Horticularturae 1108: pp 63-70.
Dorset Avenue, Colonel Light Gardens – demonstration of canopy density and shade benefits of infiltration system: Left-hand side tree receives no infiltration, right-hand side tree with infiltration. Source: City of Mitcham.
Example | The Strand, Colonel Light Gardens, City of Mitcham: An infiltration system consisting of 66 m of 525mm slotted drainage pipe buried within a 20mm single sized aggregate filled trench (900mm wide x 965mm deep) diverts a minimum of 1megalitre per annum, in a dry year, from the stormwater network into the verge subsoil to passively irrigate trees.
Remove pollutants from stormwater runoff
Expected pollutant removal from infiltration systems
Replenish groundwater and therefore dry weather base flows to waterways
Example | Newenham ecological sponges: Small-scale wetlands manage (detain) frequent flow events up to the 1.5-year average ARI from a 6 hectare urban catchment, incorporating gravel filled infiltration wells at 2-3m depth that are integrated into the wetland liner to enable low flow, sub surface releases to the creek to encourage baseflows.
Field investigations must be undertaken to determine the soil type; hydraulic conductivity; presence of soil salinity, rock and other geological limitations; slope of the terrain; and groundwater level, depth and quality.
Infiltration should be avoided in the following soil types and contexts:
- saline, sodic or very shallow soils
- wind-blown or loose sands
- clay soils that collapse in contact with water or have high shrink/swell characteristics
- soils with a hydraulic conductivity of less than 0.36 mm/hr
- steep terrain (generally slopes >5% )
- if maximum seasonal ground water level is within 1m of the proposed base of the infiltration trench, pit or chamber
- fill material
- contaminated soils.
Schematic of a gravel trench
Source: City of Auckland (2017) Stormwater management devices in the Auckland Region: Guideline document 2017/001 Version 1. Pg 234, Fig. 47.
Tip 2 – Minimum setback distances from building footings (and property boundaries)
Infiltration characteristics of common soil types and set back distances from building footings and property boundaries
1 Melbourne Water (2005). WSUD Engineering Procedures: Stormwater. CSIRO Publishing.
2 Wong T (2006) Australian runoff quality: Guide to water sensitive urban design. Engineers Australia.
Pre-treatment methods to minimise sediments from entering the infiltration system include: a swale, grassed bugger strip, sumps (preferably with hung baffles), or filters.
Infiltration devices: Managing silt – bottomless SEP (1:39) | Tim Johnson
Infiltration devices: Managing silt – agricultural pipe in SEP (4:03) | Tim Johnson
Tip 4 – How to size your infiltration system
Size the device area to allow for complete infiltration within 72 hours.
Details for engineers for sizing an infiltration pit can be found in:
- Lot scale <1000m2 – Look up table Plan SA (2020) Ministerial Building Standard (MBS) 009, On-site stormwater retention
- All scales Argue J (2013) WSUD: Basic procedures for ‘source control’ of stormwater – a handbook for Australian practice, Student edition. University of South Australia
- All scales Wong T (2006) Australian runoff quality: Guide to water sensitive urban design. Engineers Australia.
Tip 5 – Construction – roughen sides of infiltration trench with a backhoe or excavator bucket with teeth
Tip 6 – Use of underdrains, overflows and observation wells
- If used for detention purposes, the storage capacity should be designed to accommodate for the required design storm.
- Underdrains are required where permeability of surrounding soils is too low to fully infiltrate the design storm.
- Infiltration systems should be fitted with an overflow system in case a storm event exceeds the infiltration and storage capacity.
- An observation well should be installed so that future inspections can determine whether the device is functioning as designed.
Tip 7 – Aggregate specifications
Typically, aggregate should consist of clean (fines free) drainage aggregate of 20-40mm diameter, with a defined void ratio of 0.3.
Tip 8 – Use of geotextile
Geotextile must be secured at edges and base, and all joins overlapped to prevent the movement of fine sediment between the infiltration device layer and base soils, and provide required tensile strength.