University of New Hampshire Stormwater Center
2007 Annual Report
Subsurface Gravel Wetland
About Subsurface Gravel Wetland
General Information Performance Data Water Quality Treatment Process Design
Subsurface gravel wetlands do an exceptional job of treating stormwater quality and managing water quantity. The design for the gravel wetland pictured above is helping New Hampshire’s Department of Transportation meet TMDL standards for a major highway widening project.
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About Subsurface Gravel Wetland
The subsurface gravel wetland is a recent innovation in Low Impact Development (LID) stormwater design. It approximates the look and function of a natural wetland, effectively removing sediments and other pollutants commonly found in runoff, while enhancing the visual appeal of the landscape. The subsurface wetland evaluated at UNHSC is a horizontal-flow filtration system that should not be confused with other stormwater wetlands that function more like ponds. Instead, it relies on a dense root mat, crushed stone, and a microbe rich environment to treat water quality. Like other filtration systems, it demonstrates a tremendous capacity to reduce peak flow and improve water quality.
Where to Use It
Subsurface gravel wetlands can be used in many regions, with the exception of those too arid to support a wetland system. Since they can be space intensive, they may not be appropriate for densely developed areas. However, they can be placed in existing dry ponds as a water quality retrofit. Large detention basins used for flood control can house a gravel wetland without affecting storage capacity—an innovation that would dramatically improve water quality treatment and peak flow control. Like any system that relies on infiltration or filtration, these wetlands should be lined and outfitted with subdrains that discharge to the surface if they are to be used in pollution hotspots. Dissolved oxygen levels may fluctuate in biologically active subsurface systems like the gravel wetland.
Constructed wetlands are widely used. While subsurface gravel wetlands are more costly and less common, they represent a dramatic performance improvement over surface wetland ponds. They have been used for some time in wastewater treatment. The installation cost of a subsurface gravel wetland large enough to treat runoff from one acre of impervious surface was $22,500. This does not include maintenance. Maintenance requirements for these systems are generally minimal. Their dense vegetation tends to experience fewer problems with invasive plants and insect infestations, and the use of 3/4 inch crushed stone for filtration and subsurface water storage further reduces the maintenance load. Learn more by contacting the UNHSC.
Table of Contents
About the Center
About the Field Site
How We Evaluate Performance
Stormwater Treatment Performance Comparison
How to Read this Report
Stormwater Treatment System Data
Resources for Land & Water Management
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This publication was produced in partnership with the UNH/NOAA Cooperative Institute for Coastal & Estuarine Environmental Technology.