Dry detention ponds are basins with outlets that have been designed to store stormwater runoff for a specified period of time after a storm, usually 48 hours, to allow particles and pollutants to settle to the bottom. Unlike wet ponds, these facilities do not have a large permanent pool. The impounded water is discharged through an outlet that provides for prolonged release.
Dry ponds can be planted with wetland vegetation to provide additional nutrient and pollutant uptake. Dry ponds that incorporate a shallow marsh or wetland to improve pollutant removal are known as extended detention wetland basins or two stage detention ponds. It is important to determine whether standing water is by design or an indication that maintenance is required.
- Conduct routine mowing.
- Keep embankments and slopes stable.
- Remove sediment as necessary.
- Manage vegetation, seed and stabilize bare spots.
- Fill animal burrows.
- Keep trenches and basin free of debris and trash.
- Keep risers, spillways and outlets clear.
Wet retention ponds are constructed basins that hold a permanent pool of water throughout the year, or at least during periods of normal rainfall. Pollutant removal occurs through settling, as stormwater runoff is stored in the pond and released slowly at a controlled rate. the release is typically regulated by a combination of structures including risers, culverts, orifices and weirs.
The advantages of a wet pond over a dry pond are higher pollutant removal and less chance that pollutants will be re-suspended during a storm. Additional pollutant uptake of nutrients also occurs through biological activity. Wet ponds can also serve as an aesthetic or recreational amenity as well as habitat for some wildlife. Wet ponds are among the most cost-effective and widely used BMPs when space is available. However, wet ponds pose a higher safety liability than other practices.
- Conduct routine mowing.
- Keep embankments stable, and fill animal burrows.
- Remove sediment from pond when necessary.
- Manage vegetation, and seed and stabilize bare spots.
- Inspect and clean any mechanical components.
- Keep free of debris, and remove trash.
- Keep risers, spillways and outlets clear.
Biofilters, often referred to as rain gardens, are landscape features that store and treat stormwater runoff. Surface runoff is directed into shallow, vegetated depressions with underlying layers of soil, sand, and gravel. They may have an underdrain that sends excess runoff to the storm sewer. These areas are designed to mimic a natural ecosystem where pollutant removal occurs through soil infiltration and plant uptake. They are commonly located within, or adjacent to, parking lots or in residential open spaces.
- Maintain vegetation and replace mulch as needed.
- Remove trash and sediment.
- Watch for water standing more than 72 hours which can indicate clogging of subsurface layers and under-drain system.
Constructed wetlands, sometimes called stormwater wetlands, are shallow basins that receive stormwater for water quality treatment. The constructed wetland’s permanent pool is typically 6 to 18 inches deep (although it may have greater depths in some areas. The “microtopography” varies to promote a dense and diverse wetland cover. The wetland environment provides an ideal environment for settling of particles, biological uptake, and microbial activity to remove pollutants from stormwater runoff. Constructed wetlands can also help to reduce peak flows of stromwater, through detention storage above the permanent water level, however that storage does not provide any volume reduction (except through evaporation).
Maintenance of constructed wetlands should be driven by annual inspections to include the following:
- Control sediment accumulation levels in forebays and small pools within the wetland.
- Monitor the growth and survival of emergent wetlands and tree/shrub species, and replace or supplement as needed. Control growth of invasive plant species.
- Inspect stormwater inlets, upstream/downstream banks, internal/external side slopes, and outlet channel for erosion, undercutting, riprap displacement, slumping, animal burrows, boggy areas, and unwanted woody growth. Repair as needed.
- Inspect the condition of the principal spillway, riser, trash racks and other structural components for evidence of cracks, joint failure, leakage, clogging, corrosion, and debris accumulation. Repair as needed.
- Inspect maintenance access point(s) to ensure it is free of woody vegetation, and check to see whether valves, manholes and locks can be opened or operated. Repair as needed.
- Cleanups should be scheduled at least once a year to remove trash, debris and floatables.
Infiltration practices use temporary surface or underground storage to allow incoming stormwater runoff to exfiltrate into underlying soils. Runoff first passes through multiple pre-treatment mechanisms to trap sediment and organic matter before it reaches the practice. As the stormwater penetrates the underlying soil, pollutants are removed. If soil permeability exceeds ½ inch per hour, infiltration practices have the greatest runoff reduction capability of any stormwater practice. They are suitable for use in residential and other urban areas, but should not be used at sites with high pollution loads that could contaminate groundwater.
Annual site inspections are critical to the performance and longevity of infiltration practices:
- The drawdown rate should be measured at the observation well for three days following a storm event in excess of 1/2 inch in depth. If standing water is still observed in the well after three days, this is a clear sign that that clogging is a problem.
- Check inlets, pre-treatment cells, and any flow diversion structures for sediment buildup and structural damage. Note if any sediment needs to be removed.
- Inspect the condition of the observation well and make sure it is still capped.
- Check that no vegetation forms an overhead canopy that may drop leaf litter and other vegetative materials that could clog the infiltration device.
A sand filter is a stormwater practice that uses sand to filter particles and particle-bound constituents from runoff. There are two types of sand filters: infiltration sand filters and underdrained sand filters. Stormwater entering the sand filter is first conveyed through the pretreatment zone where trash, debris and coarse sediment are removed. It then passes through the treatment zone (sand) and out of the system through either an outlet pipe in an underdrained system, or through the subsoil via infiltration. Pollutants in runoff are treated in sand filters through the processes of settling, filtration and adsorption. Sand filters designed to infiltrate into the subsoil should never be used in areas where high pollutant or sediment loading is anticipated due to the potential for groundwater contamination and clogging of the filter.
- All structural components must be inspected, at least once annually, for cracking, subsidence, spalling, erosion and deterioration.
- Components expected to receive and/or trap debris and sediment must be inspected for clogging at least twice annually.
- Sediment removal should take place when all runoff has drained from the sand bed and the sand bed is dry.
Grassed swales are concave, earthen conveyance systems designed to simply transfer runoff safely. They can be designed as a water quality device, which is constructed to allow the stormwater to soak into the soil, and trap particles within the groundcover (usually turf grass). Some swales are constructed with berms across the width (small dams made of earth, rock, or wood) to slow the runoff by creating temporary ponds This can minimize erosion and help promote infiltration of stormwater into the soil.
- Maintain a healthy stand of grass and mow to a minimum height of 4 inches.
- Remove trash and sediment.
- Watch for water standing for more than 72 hours, which can indicate clogging of subsurface layers.
The Virginia Stormwater BMP Clearing House contains a list of manufactured BMPs, from a variety of companies, that have been approved by the Virginia Department of Environmental Quality (DEQ) for meeting certain nutrient reduction criteria. (DEQ does not endorse any of these devices and does not provide assistance with device selection.) Maintenance of these devices must be done according to manufacturer recommendations.
Permeable pavements are alternative hard surfaces that allow stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir, where it is temporarily stored and/or exfiltrated to the soil. A variety of permeable pavement surfaces are available, including pervious concrete, porous asphalt, permeable grid pavers, and interlocking concrete pavers. While the specific designs may vary, all permeable pavements have a similar structure, consisting of a permeable surface layer, an underlying stone aggregate reservoir layer, and a filter layer or fabric installed on the bottom.
While maintenance of all stormwater practices is always important, it is especially so for permeable pavement to ensure its long-term performance. The most frequently cited maintenance problem is surface clogging caused by organic matter and sediment, which can be reduced by the following measures:
- Periodic vacuum sweeping (usually at least once or twice a year, depending intensity of use and deposition rate)
- Protecting the bottom of the reservoir layer from intrusion by underlying soils
- Installing an observation well at the downstream end of permeable pavement systems
- Integrating landscaping within the permeable pavement system (This must be carefully planned to maximize runoff treatment and minimize the risk that sediment, mulch, grass clippings, leaves, nuts, and other organic material will clog the paving surface.)
“Filter strips” are vegetated areas that treat sheet flow delivered from adjacent impervious and managed turf areas by slowing runoff velocities and allowing sediment and attached pollutants to settle and/or be filtered by the vegetation. “Stream buffers” are filter strips along streams; the vegetation type for these buffers is typically forested. “Conserved Open Spaces” are large vegetated areas protected from development through legal deed restrictions or easements. In order to be effective, any of these vegetated areas must receive stormwater as sheet flow (not concentrated flow).
Vegetated Filter Strips that consist of grass/turf cover should be mowed at least twice a year to prevent woody growth. Additional maintenance needs are determined through annual inspections that should be conducted in the nongrowing season so that flow paths are visible. Inspections should ensure that:
- Stormwater flows continue to be received as sheet flow, and do not by-pass the overflow control section
- Debris and sediment has not built up in any areas
- Foot or vehicular traffic has not damaged any areas
- Scour and/or erosion have not occurred in any areas
- Sediment is cleaned out of Level Spreaders (if used as a component of the system)
- Vegetative density continues to exceed a 90% cover in boundary areas
Vegetated roofs (also known as green roofs, living roofs or ecoroofs) are alternative roof surfaces that typically consist of waterproofing, drainage materials, and an engineered growing media that is designed to support plant growth. Vegetated roofs capture and temporarily store stormwater runoff in the growing media before it is conveyed into the storm drain system. A portion of the captured stormwater evaporates or is taken up by plants, which helps reduce runoff volumes, peak runoff rates, and pollutant loads otherwise generated by rooftops. All vegetated rooftops should be designed and certified by an engineer to ensure that the structure can handle the weight of the maximum amount of water that may be stored on the roof.
Vegetated roofs in general are designed to have minimal maintenance requirements. Plant species are selected to need minimal irrigation or fertilization once vegetation is established. (It is sometimes recommended that slow release fertilizer be applied annually for the first five years.) It is best to utilize a roof system that is pre-vegetated prior to installation, as having established plants can reduce maintenance costs, weeds, and erosion risks. A vegetated roof should be inspected twice a year during the growing season:
- Hand-weed to remove invasive or volunteer plants; plants and/or media should be added to repair bare areas.
- The use of herbicides, insecticides, and fungicides should be avoided, since their presence could hasten degradation of the waterproof membrane.
- Power-washing and other exterior maintenance operations that could harm the plants should be avoided.
- Water as needed to promote plant growth and survival.
- Inspect the plants and replace any dead or dying vegetation.
- Inspect the waterproof membrane for leaking or cracks. If a roof leak is suspected, an “electric leak survey” (Electrical Field Vector Mapping) can pinpoint the exact location so that localized repairs can be made.
- Inspect roof drains and gutters immediately adjacent to the growing media to ensure they are not overgrown or have organic matter deposits. Remove any accumulated organic matter or debris.
- Mow or trim plantings in early April of each year as needed.
By disconnecting runoff from impervious areas, runoff is managed close to its source by intercepting, infiltrating, filtering, treating, and/or reusing it as it moves from the impervious surface to the drainage system. There are two kinds of disconnection: (1) simple disconnection for small areas where impervious surfaces are directed to pervious areas, and (2) disconnection leading to one or more alternate runoff reduction practice(s) adjacent to the impervious area
Inspect disconnection sites regularly to ensure the following, and repair as needed:
- Flows through the filter or flow path are not channelizing or short-cutting the system.
- Debris and sediment does not build up at the top of the flow path.
- Foot or vehicular traffic does not compromise any gravel diaphragms or energy dissipaters that may be components of the system.
- Scour and erosion do not occur within the flow path.
- Sediments and decomposed leaves or debris are cleaned out of any energy dissipaters.
- Vegetative density continues to exceed a 90% cover in the filter or flow path.