Treatment MeasuresInfiltration Trenches : Design Details

Treatment Measures

More Information

Infiltration trenches are highly susceptible to blockage problems. They are only suitable for use in areas where sediment yields are controlled, such as established urban areas. In addition a perforated pipe can be placed longitudinally within the gravel media to assist in the passage of stormwaters.

A grass buffer or filter is often located upstream of the trench to remove coarse particulate matter. An overflow berm may be located on the downstream side of the trench to encourage ponding of water over the trench to increase infiltration.

End view of the trench

Key design issues are the treatment flow rate, surrounding soil infiltration rates, porous media type and size and the potential for clogging. In addition, a trench should not be sited in a region of heavy sediment load as it will 'clog' the system, restricting flow.

Estimated Treatment Performance Summary

gross pollutants	L	Coarse sediment	M/H	medium sediments	M
fine sediments	L/M	Attached pollutants	L/M	dissolved	L
Installation costs	L	Maintenance costs	M/H	head requirements	L
N = Negligible, L= Low, M = Moderate, H = High, VH = Very high

Trapping Performance

More data is required to assess the pollutant retention capabilities of infiltration trenches.

Trench Site Selection

The bed should be at least 1.0 metre above the seasonal high water table, bedrock or a relatively impermeable layer;

The percolation rate should be a minimum of 0.8-1.3 mm per hour;

Soil Type	Hydraulic Conductivity	Distance to footings
Sand	>180 mm/hr	1m
Sandy Clay	180-36 mm/hr	2m
Medium Clay	36-3.6 mm/hr	4m
Heavy Clay	3.6-0.036 mm/hr	5m

The soil should be a loam, sandy loam or loamy sand. A clay content of <30%, with a clay and silt fractions at a maximum of 40%;
The trench should not be constructed upon fill material;
The maximum infiltration rate to groundwater should not exceed 60 mm per hour;
The slope should not exceed 5%; (1-3% are recommended)
Base flows should not enter the trench. This means that the presence of a high water table can limit the potential effectiveness of the trench, and saline groundwater regions should are not recommended for the installation of infiltration devices such as trenches.

Trench Sizing

There are no exact methods for sizing a trench based upon water quality objectives as yet. The following technique enables the size to be determined on maximum allowable drain time and flow rate (through porous media) parameters.

Estimate the base area of the infiltration trench using the following;
A = V/d

where:

A = area of infiltration surface (square metres)
V = effective volume of infiltration trench (cubic metres)
d = depth of trench (metres)

The effective volume of the trench (V) is the design storm run-off volume less the volume of rock within the trench-this commonly occupies 30 to 40 per cent of the trench volume.

The depth of the trench can be estimated from:

d = I.t/S

where:

I = infiltration rate (metres per hour)
t = infiltration time (hour)
S = factor of safety

Due to the difficulty in obtaining reliable percolation rate estimates, making several site measurements then adopting the lower value, in addition to adopting a factor of safety of two is a recommended method.

More accurate and comprehensive field measurements could result in a lowering of this factor of safety. Estimates of the infiltration rate can be obtained from soils texts, based on soil textural classes. If this approach were taken, a higher factor of safety would be appropriate.

The choice of an infiltration period is related to the inter-event period and the need to minimise the creation of anaerobic conditions in the underlying soil (these encourage the growth of algae during warmer periods, which may clog the soil). Reducing the infiltration time results in a smaller volume but higher surface area for a given soil type. Pollutant removal is enhanced by increasing the surface area of the bottom of the trench, which also reduces the risk of clogging.

Infiltration periods of 24 to 72 hours have been recommended by Camp Dresser & McKee (CDM) (1993) and Schueler (1987), with the lower periods applying when the inter-storm period in the wet season is relatively short. Auckland Regional Council (ARC) (1992) adopts an infiltration period for the mean storm of at least 50 per cent of the mean inter-storm period. As the majority of the infiltration occurs during the inter-event period, an approximate infiltration period could be determined from an analysis of the site's rainfall data history. Similar criteria to those of ARC (1992) could then be applied. These analyses have been carried out for major cities in Australia by Wong et al. (1998).
The second sizing technique is that described by Horner et al. (1994). Horner, R.R. and Skupien, J.J. (1994) Fundamentals of Urban Runoff Management: Technical and Institutional Issues, Terrene Institute, Washington DC.
This method calculates the surface area and infiltration volume based on Darcy's law, which describes flow through porous media. This is potentially a more accurate technique, but requires more information than the simple technique described above.

MUSIC Parameter definitions

(Music is a computer model used to determine the water quality improvement via the use of WSUD aspects. To find more information on this please go the references and links page of this site for a link)

The Surface Area (m2) would be the trench width by the length of the trench.

Bypass Flow (m3s-1, cummecs) is determined by the design flow of the trench with the bypass flow being the flow exceeding the hydraulic capacity of the inlet system.

Extended Detention Depth (m) is the holding depth of the overflow pits for a Bio-Retention systems before it flows over a weir into the conveyance pipes.

Trench Configuration

Pre-treatment of stormwaters must be carried out prior to these waters entering the infiltration trench to remove coarse particulate matter that can cause clogging. Methods currently used for this (e.g. Lynbrook Estate, Melbourne) are grass swales, filter strips or a sand filter, but most primary treatment measures are appropriate.

Clean, washed stone aggregate, typically 25-75mm in diameter should be used as fill. The size is dependent upon the hydraulic capacity of the trench, designed as to not wash away this media. The increased size of media increases the effective volume of the trench, however this will result in the reduction of particulate surface area and subsequently less area for bio-films to potentially develop.

The trench should be lined with geotextile fabric to prevent migration of soil into the fill media and this fabric can be extended to cover the top of the trench if (porous) topsoil is used. A sand layer at the base of the trench can assist in preventing upward piping of underlying soils.

References

Camp, Dresser and McKee (CDM), 1993, California Storm Water Best Management Practice Handbooks: Municipal, prepared for California Stormwater Quality Task Force.