Infiltration Trenches: Stormwater Management Facilities Ideal for Low Flows

Improving the resilience of municipal infrastructure to climate change comes back to the forefront after every major rainstorm. Source control stormwater management, on the property itself, is one of the measures that Quebec municipalities will increasingly promote.

Governments are already adopting by-laws that require stormwater retention when developments reduce pervious areas.

In this article, you will find basic information on infiltration trenches, also called percolation trenches: their definition, key design considerations, maintenance, and their advantages and disadvantages.

To learn more about other stormwater management facilities, read Nine Types of Stormwater Management Facilities Used in Quebec.

What is an infiltration trench?

An infiltration trench is a stormwater management facility that reduces runoff volumes and improves water quality. The trench is filled with a material with a high void ratio, which stores water before it percolates into the native soil.

Infiltration trenches are used for small areas such as single-family homes, small businesses and parking lots. Implementing these systems is part of an ecological development approach that aims to control stormwater at the source rather than conveying all of it to a single discharge point, the storm sewer.

Building stormwater management facilities such as infiltration trenches helps, among other things, to reduce peak flows in urban drainage systems during heavy rainfall events.

Designing infiltration trenches

The design of an infiltration trench must take into account soil conditions, land use and the flows or volumes of water to be treated.

In general, these facilities are 1 to 2.4 m deep. The trench is filled with clean (well-washed) stone wrapped in geotextile. The porosity of the fill material is generally between 35% and 40%. The bottom of the trench may be covered with a sand filter or a geotextile.

The facility should be equipped with a pretreatment device to control the entry of solids and debris into the trench, reducing the risk of clogging and extending its service life.

Pretreatment devices are chosen based on how water enters the trench. An oil and grit separator, a catch basin, a filter strip or a grassed swale can be used for this purpose.

Water can infiltrate directly from the ground surface or, in the case of a buried trench, through a perforated underground pipe.

The trench should be designed to drain within 24 hours. An overflow pipe must also be provided to discharge water when the trench reaches its maximum capacity.

Infiltration trenches for sustainable parking lots

When runoff comes from a parking area, special attention must be paid to managing debris and sediment, since the risk of clogging is higher.

Two pretreatment devices connected in series are sometimes needed. For example, a filter strip could be used together with an oil and grit separator to pretreat water coming from a parking area. Installing an observation well in the trench is recommended to monitor water levels.

Physical design constraints for infiltration trenches

The Guide de gestion des eaux pluviales (2014) identifies the main physical constraints associated with infiltration trench design.

Parameter	Value
Area used	2% to 3% of the impervious drainage area
Topography	Maximum slope of 15%
Soils	Loam with a hydraulic conductivity ≥ 15 mm/h
Bedrock or water table	Distance from the bottom of the trench > 1 m
Drainage area	Less than 2 ha

Physical design constraints for infiltration trenches, according to the Guide de gestion des eaux pluviales

Special attention must be paid to the water table criterion. Poor knowledge of the depth of the water table can lead to its contamination.

The soil’s infiltration capacity is also an important criterion, since water must be able to penetrate the soil for the trench to be effective. In addition to these constraints, the trench must be located at least 4 m from the foundation drains of any building.

Advantages of infiltration trenches

It is important to carefully assess a project’s needs to choose the optimal stormwater management system. Infiltration trenches can be used to store small volumes of water.

The main advantages of infiltration trenches are that they:

• Reduce runoff volume
• Help resolve downstream overloading in the drainage system
• Reduce the size of downstream facilities and their construction costs
• Contribute to groundwater recharge

Disadvantages of infiltration trenches

The main disadvantages of infiltration trenches are that they:

• Require optimal design and detailed knowledge of in-situ soil conditions, land use and the depth of the water table
• Require regular maintenance
• May contaminate the native soil or groundwater if contaminants are poorly managed
• Can only handle stormwater from an area of up to 2 ha. For larger areas, a retention pond or infiltration chambers could be considered

Designing an infiltration trench is complex and requires expertise to properly assess local conditions and carry out the work without compromising the future effectiveness of the infiltration system. Various regulations governing the installation of an infiltration system on a property must also be followed.

References

• https://fcm.ca/sites/default/files/documents/resources/guide/infraguide-controles-a-la-source-et-sur-le-terrain-des-reseaux-pgam.pdf
• https://www.environnement.gouv.qc.ca/eau/pluviales/guide.htm