Gravel Wetland

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Green Infrastructure

Gravel wetlands are similar to a regular stormwater wetland except it is filled with crushed rock or gravel and designed so that the rock or gravel is submerged. Wetland plants are rooted in the media along with thriving algae and microbes on the rocks. This structure relies on the pollutant-stripping ability of plants and soils to remove pollutants from runoff.

Maintaining Gravel Wetlands

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Routine maintenance should be performed on gravel wetlands. During the first year that the enhanced swale is built, maintenance may be required at a higher frequency to ensure proper establishment of vegetation in the practice. For more information on vegetation within a gravel wetlands, see [link to Appendix D: Planting and Soil Guidance].

In addition to routine maintenance, gravel wetlands require seasonal and intermittent maintenance. For example, in case of snow, ensure that snow does not impact infiltration capacity. During the winter, make sure the materials used to de-ice surrounding areas stay out of practice to avoid clogging and further pollution.

After a large rainstorm, keep drainage paths, both to and from the area, clean so that water can properly infiltrate into the gravel wetland. If the gravel wetland is not draining properly, check for clogging in the inflow and outflow structures.

Over a period of time, if the forebay or gravel wetland has received a significant amount of sediment, then the sediment at the bottom of the forebay- or gravel wetland may need to be removed. Sediment excavated from wetlands that are not stormwater hotspots are not toxic and can be disposed through land application or landfilling.

The embankment of the gravel wetland should be inspected regularly for trees and animal activity. Trees growing on the top or sides of the embankment should be removed. Animals can burrow holes weakening the structure of the embankment.

Fertilizers should only be used sparingly during the establishment of the practice. After the practice is established, fertilizers and pesticides should not be used.

Common Gravel Wetland Issues

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Here are some of the common problems to be aware of when maintaining a bioretention area. They include, but are not limited to, the following:

  • Sediment build-up
  • Clogging in the inlet and outlet structure
  • Establishing vegetation within the wetland area
  • Maintaining proper pH levels for plants
  • Pruning and weeding to maintain appearance
  • Mosquitoes breeding in the practice

Should the enhanced swale become a breeding ground for mosquitoes, the problem is likely with the soil media or the overflow structure.

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Gravel Wetland

Routine Maintenance Schedule
Schedule
Activity

Monthly
  • Ensure that inlets and outlets to each submerged gravel wetland cell are free from debris and not clogged.
  • Remove any accumulated sediment and debris from inlet and outlet structures.

Semi-annually
(Quarterly during first year)
  • Inspect wetland, side slopes and buffers for erosion. Replace vegetation in eroded areas.
  • Inspect wetland, side slopes and buffers for dead or dying vegetation.
  • Replace vegetation as needed.
  • Inspect wetland, side slopes and buffers for invasive vegetation and remove as needed.

Annually
  • Inspect for damage to the embankment and inlet/outlet structures. Repair as necessary.
  • Monitor for sediment accumulation in the facility.
  • Examine to ensure that inlet and outlet devices are free of sediment and debris and operational.
  • Inspect side slopes for erosion and undercutting and repair as needed.
  • Check for signs of eutrophic conditions (e.g., excessive algal growth).
  • Check for signs of hydrocarbon accumulation and remove appropriately.
  • Monitor sediment markers for sediment accumulation in forebays and permanent pools.
  • Check all control gates, valves and other mechanical devices.

As needed
  • Water side slopes and buffers to promote plant growth and survival.
  • Inspect wetland, side slopes, structures, and buffers following rainfall events. Plant replacement vegetation in any eroded areas.

5 years or after 50% of the total forebay storage capacity has been lost

  • Remove sediment, trash, and debris from inlets/forebay.

10 plus years or after 25% of the wetland storage volume has been lost

  • Monitor sediment accumulation in the wetland and remove sediment when the permanent pool volume has been reduced significantly, plants are “choked” with sediment, sediment buildup is preventing flow through the wetland, or the wetland becomes eutrophic. Replace with clean gravel and replant vegetation.