Highway stormwater BMPs: Structural

Highway stormwater BMPs can be been divided into two main categories:

• structural
• non-structural

The list of structural BMPs given here is by no means comprehensive, but should provide the reader with a guide to some of the more common BMP types.

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Extended detention ponds

Depressed basins that temporarily store a portion of stormwater runoff following a storm event. May be berm-encased area, excavated pond or tank. Water is controlled by means of a hydraulic control structure to restrict outlet discharge. These ponds normally do not have a permanent water pool between storm events - water may be discharged by overflow, through pipes, by evapouration/transpiration, and/or by infiltration.

Wet ponds

Similar to the extended detention pond except that a permanent volume of water is incorporated into the design. Soluble pollutant removal is enhanced by biological processes. Wet ponds can have aesthetic and recreational value, as well as offering flood-control benefits.

Infiltration trenches

An excavated trench lined with a filter fabric and backfilled with stone to form an underground basin. Runoff is diverted to the trench and either exfiltrates into the soil, or enters a perforated pipe underdrain and is routed to an outflow facility. Smaller trenches are used for water quality, while larger trenches can offer stormwater quantity control. Three basic types of infiltration trenches are:

• Complete trench: all runoff is exfiltrated into the soil.
• Partial trench: includes a perforated underdrain.
• Water quality exfiltration system: designed to exfiltrate only the first flush volume.

Infiltration basins

Similar to infiltration trenches, except that they are generally used for larger drainage areas and water is stored in a visible pond. Normally designed to capture the first flush runoff volume for smaller catchments than extended detention ponds.

Sand filters

Designed to remove sediments and pollutants from the first flush volume of pavement and impervious area runoff. Filtration of nutrients, organics and bacteria is typically enhanced by a mat of bacterial slime. Major types include:

• Austin sand filter
• District of Columbia underground sand filter
• Alexandria dry vault sand filter
• Delaware sand filter
• peat-sand filter (normally includes a sorption layer and vegetative cover)

Water quality inlets

Commonly known as oil and grit separators, these are typically two- or three-chamber structures designed to remove sediment, oil, grease and large particulates. Usually implemented for smaller catchments. Outflow is routed to the storm drain system.

Grassed swales

Shallow vegetated channels used to convey stormwater. Pollutants are removed by settling, filtration through grass and infiltration into the soil. Runoff volume may also be reduced through infiltration. Require shallow slopes and soils that drain well. Often used as a pretreatment measure for downstream BMPs. May utilize check dams to increase storage, settling and infiltration.

Filter strips

Also known as vegetative buffer strips, these are vegetated sections of land similar to grassed swales except they are essentially flat with low slopes and are designed to promote sheet flow of stormwater runoff. Again, filter strips are often used as pretreatment for downstream BMPs.

Constructed wetlands

Provides physical, chemical, and biological water quality treatment of stormwater runoff where a continuous baseflow can be ensured. Wetlands support a variety of aquatic flora and fauna, and is characterised by a non-soil substrate which is saturated with water or covered by shallow water at some time during the growing season. Usually implemented in conjuction with other BMPs for large catchment areas.

Porous pavement

An asphalt or concrete based paving material that allows stormwater to quickly infiltrate the surface pavement layer to enter into a high-void aggregate sub-base layer. The captured runoff is stored in this reservoir until it either infiltrates into the underlying soil, or is routed through a perforated underdrain system to a conventional stormwater conveyance. Porous pavements operate in a similar fashion to infiltration trenches.

The information above has been compiled from Evaluation and management of highway runoff water quality, USDOT, pub. # FHWA-PD-96-032, June 1996.