CICEET News Release

The gravel wetland is a recent innovation in Low Impact Development (LID) designs that treat stormwater. Like other infiltration/filtration systems, it has a tremendous capacity to reduce peak flow and stormwater volume in general. It also has limited use as a replacement for septic systems.

This gravel wetland is designed as a series of horizontal, flow-through treatment cells, preceded by a sedimentation forebay. The device is designed to retain and filter the entire water quality volume (WQV)—10 percent in the forebay and 45 percent in each treatment cell.

For small, frequent storms, each treatment cell filters 100 percent of its WQV. Additionally, the wetland can detain a channel protection volume (CPV) of 4,600 cubic feet, and release it over 24 to 48 hours. WQV is filtered and drains offsite. Any storm volume exceeding WQV overflows into the adjacent swale. Since standing water of significant depth is not expected (except during heavy rains), swale side slopes are graded at 3:1 or flatter for maintenance.

Maintenance involves the periodic mowing and replacement of vegetation, as needed.

The fully vegetated gravel wetland [left top & bottom] exhibits excellent pollutant removal, provides subsurface anaerobic treatment, attenuates peak flow, and reduces flow volume. [Right] The gravel wetland’s forebay and retention cells just after installation.

Category Type
Stormwater Wetland

BMP Type
Low Impact Development Design

Design Source
Not Available

Basic Dimensions
Filter Beds:
15 ft L X 32 ft W

Forebay Top Width:
37 ft L X 56 ft W

Total Area:
5,450 sf

Specifications
Catchment Area: 1 acre
Peak Flow: 1 cfs
Treatment Volume: 3,264 cf

Treatment Function
Physical, Chemical, Biological

Cost Per Acre
$22,327

Maintenance Data
Maintenance Sensitivity: Low
Inspections: Low
Sediment Removal: High

Water Quality Treatment Process

This treatment removes pollutants, provides subsurface anaerobic treatment, attenuates peak flow, and reduces flow volume through evapotranspiration and infiltration. Biological treatment of water quality occurs through plant uptake and soil microorganism activity. Physical and chemical treatment happens in the soil through filtering and adsorption with organic matter and mineral complexes.

During lighter rains, each cell filters 100 percent of its water quality volume. The cells allow stormwater to pass horizontally through the microberich, gravel substrate and drain into a sump basin. The wetland is designed to continuously saturate at a depth that begins four inches beneath the treatment’s surface. This promotes water quality treatment and vegetation growth. To generate this condition, the system outlet pipe has an invert 4 inches below the wetland surface.