GENERAL INFORMATION
1. Description
Infiltration trenches are long, narrow, rock-filled trenches with no outlet that receive
stormwater runoff. Runoff is stored in the void space between the stones and infiltrates
through the bottom and into the soil matrix. They are essentially linear soakaways.
Infiltration trenches perform well for removal of fine sediment and associated pollutants
because of their ability to maximize the infiltration surface area (Revitt, Ellis and Scholes,
2003).
Figure 1.1: Infiltration trench
2. Typology
There are two types of infiltration trenches (EPA, 1999):

Underground trenches: They receive runoff through pipes or channels.

Surface trenches: They receive sheet flow from the drainage area. The sheet
flow enters the trench through a layer of vegetated porous soil on the top of the
trench.
3. Suitable applications – advantages and disadvantages
3.1. Suitable applications:
Infiltration trenches take up little land and can be located on or close to residential sites.
However they are most suited to areas where runoff is relatively unpolluted and sediment
loads are low, in order to reduce the risk of clogging (Ciria C609, 2004). In general,
infiltration trenches are suitable for drainage areas up to 4 ha (SWRPC, 1991).
3.2. Advantages:
Apart from reducing the volume of runoff, infiltration trenches provide efficient pollutant
removal. The captured runoff infiltrates the surrounding soils and increases groundwater
recharge and base flow in nearby streams (EPA, 1999).
Infiltration trenches can easily be incorporated into the existing landscape, causing no
negative aesthetic impact.
3.3. Disadvantages:
Negative impacts include the potential for groundwater contamination and a high
likelihood of early failure if not properly maintained. It is difficult to restore functioning
of infiltration trenches once clogged, because replacement of clogged aggregate is
required (California Stormwater BMP Handbook, 2003).
Infiltration trenches are also not an appropriate BMP technique for industrial or
commercial sites where the release of large amounts or high concentrations of pollutants
is possible (Minnesota Urban Small Sites BMP Manual, 2001).
4. Performance
4.1. Pollutant removal:
Infiltration trenches remove well fine sediment and the pollutants associated with them,
mainly through adsorption and filtration (Scholes, Revitt and Ellis, 2004). Coarse
sediment may prevent the trench from operating properly and must be removed prior to
entering it.
They can be expected to remove up to 90% of sediments, metals, coliform bacteria and
organic matter, and up to 60% of phosphorus and nitrogen in the infiltrated runoff.
Biochemical oxygen demand (BOD) removal is estimated to be between 70 to 80%.
Lower removal rates for nitrate, chlorides and soluble metals should be expected,
especially in sandy soils. (Schueler, 1992)
Pollutant removal efficiencies may be improved by using washed aggregate and adding
organic matter and loam to the subsoil. The addition of organic material and loam to the
trench subsoil may enhance metals removal through adsorption. (California Stormwater
BMP Handbook, 2003)
The degree to which soluble pollutants are removed is dependent primarily on detention
time, bacterial activity and chemical bonding with the soil. The efficiency of an
infiltration trench to remove pollutants can be increased by increasing the surface area of
the trench bottom.
4.2. Hydraulic performance:
When the water enters an infiltration trench it flows downwards through the unsaturated
zone and after reaching the groundwater table, it moves sub-horizontally in the direction
of groundwater flow.
5. Design criteria
5.1. Soil and groundwater requirements:
Infiltration trenches are not suitable for areas with relatively impermeable soils
containing clay and silt (EPA, 1999). Acceptable soils are those with infiltration rates
greater than 3.5 × 10-6 m/s.
The seasonally high water table must be far enough below the bottom of the infiltration
trench (at least 1 m) to allow the structure to function hydraulically and to allow trapping
and treatment of pollutants by the soil. (Minnesota Urban Small Sites BMP Manual,
2001)
5.2. Hydraulic design:
The main features of an infiltration trench are shown in the following figure (Figure
5.2.1):
Figure 5.2.1: Typical infiltration trench design
[Source: Schueler, 1987]
Generally, the hydraulic design criteria of an infiltration trench are:

The design storm for an infiltration trench is typically a frequent, small storm
such as the one-year event. This usually provides treatment for the first flush of
stormwater runoff. (Minnesota Urban Small Sites BMP Manual, 2001)

Infiltration trenches should hold water for a minimum of six hours, in order to
provide efficient pollutant removal. However, they should drain within 72 hours
to maintain aerobic conditions, which favour bacteria that aid in pollutant
removal, and to ensure that the trench is ready to receive the next storm.

An infiltration trench is usually 0.9 to 3.7 m deep, backfilled with a stone
aggregate and lined with filter fabric (EPA, 1999). The bottom of the infiltration
trench can be covered with a 15 to 30 cm layer of clean sand in place of filter
fabric (Minnesota Urban Small Sites BMP Manual, 2001).

Pretreatment, such as filter strips or sediment forebays, should be established
adjacent to the infiltration trench to capture large sediment particles in the runoff
and protect the trench from clogging.

Infiltration trenches work best when the upgradient drainage area slope is less
than 5% (SWRPC, 1991). The downgradient slope should be no greater than 20%
to minimise slope failure and seepage (EPA, 1999).

The stone aggregate used in the trench is normally 2.5 to 7.6 cm in diameter,
which provides a void space of 40% (Schueler, 1987).

An observation well, consisting of a perforated vertical pipe within the trench, is
recommended to monitor water levels in the trench. (EPA, 1999)

A bypass system, such as overflow pipe, should be implemented for flows in
excess of design.
5.3. Erosion – Extreme events:
Infiltration trenches, just like soakaways, should be designed to take runoff from the
smallest practical catchment areas so that the volumes of water infiltrating are small and
consequently not erosive.
Extreme events above the capacity of the infiltration trench should be routed over the site
surface along acceptable flood routes. (Ciria C609, 2004)
6. Operation and maintenance
Inspection and maintenance is required for the proper operation of infiltration trenches.
The most critical maintenance factor for this BMP is the periodic removal of accumulated
sediment. If sediment is allowed to accumulate, the storage volume of the trench for
runoff will reduce as the space between rocks is filled with sediment. Eventually, the
infiltration trench should be rehabilitated, but with appropriate design and aggressive
preventive maintenance, this rehabilitation may not be necessary for a decade or more.
(Minnesota Urban Small Sites BMP Manual, 2001)
The recommended maintenance schedule corresponds to that of soakaways.
REFERENCES
California Stormwater Quality Association (2003). California Stormwater BMP
Handbook: New development and redevelopment. TC-10.
CIRIA (2004). Sustainable drainage systems: Hydraulic, structural and water quality
advice. Report C609, Construction Industry Research & Information Association,
London.
Minnesota Metropolitan Council (2001). Minnesota urban small sites BMP manual.
Prepared by Barr Engineering for Minnesota Metropolitan Council, Environmental
Services, St Paul.
Revitt D.M., Ellis J.B., and Scholes L. (2003). Report 5.1. Review of the use of
stormwater BMPs in Europe. DayWater Project. Middlesex University.
Scholes L., Revitt D.M., and Ellis J.B. (2004). Determination of numerical values for the
assessment of BMPs. DayWater Project. Middlesex University.
Schueler T.R. (1987). Controlling Urban Runoff: A practical manual for planning and
designing urban BMPs. Metropolitan Washington Council of Governments, Washington
DC.
Schueler T.R. (1992). A Current Assessment of Urban Best Management Practices.
Metropolitan Washington Council of Governments, Washington DC.
Southeastern Wisconsin Regional Planning Commission (SWRPC) (1991). Costs of
urban nonpoint source water pollution control measures. Technical Report No. 31.
United States Environmental Protection Agency (1999). Stormwater Technology
Factsheet: Infiltration trench. EPA 832-F-99-019, US EPA, Office of Water, Washington
DC.