Chapter 5. Stormwater: Dry and Wet Ponds

Description of the Resource

According to the U.S. Environmental Protection Agency, stormwater is rain or snow melt that flows over land or impermeable surfaces and does not drain or infiltrate into the ground. Examples of impermeable surfaces are concrete surfaces, asphalt, and building rooftops. The amount of stormwater generated from an impervious surface depends on the amount of precipitation or snow melt present. Naturalized stormwater dry ponds and wet ponds, bioswales and rain gardens retain or detain surface water runoff. This prevents the water from causing erosion or other damage on adjacent properties, helps to prevent flooding and the discharge of sweage by overwhelmed combined storm sewer systems, and retains pollutants such as sediment that are often carried with the runoff, preventing them entering surface waters such as streams and wetlands.

Dry Ponds

Stormwater dry ponds are built to temporarily store excess stormwater and allow some pollutants to settle to the bottom of the basin. These ponds are not meant to store stormwater for long periods of time. The water from dry ponds will slowly drain back onto adjacent land features including wetlands and streams. The purpose of the dry pond is to allow sediment to settle out of the stormwater runoff and to discharge the water gradually, replicating the conditions of naturally vegetated areas. These types of ponds are normally dry and may have natural old field or even woody vegetation. More detailed information on dry ponds can be found here: http://www.stormwatercenter.net/Assorted%20Fact%20Sheets/Tool6_Stormwater_Practices/Pond/Dry%20ED%20Pond.htm

Wet Ponds

A wet pond is similar to a dry pond except that wet ponds are meant to store a specific volume of stormwater for a long period of time. They are also usually larger than dry ponds since they are meant to hold more water. Wet ponds look like small ponds or wetlands and may have a landscaped look around the edges in some communities. More information on wet ponds can be found here: http://www.stormwatercenter.net/Assorted%20Fact%20Sheets/Tool6_Stormwater_Practices/Pond/Wet%20Pond.htm

The size of both dry and wet ponds is dependent on the size of the drainage area as well as the amount of impervious surfaces. Both types of stormwater basins are typically designed for a specific storm event, e.g., a 2-, 5-, or 10-year storm. For more information on detention basins, see Appendices 5-1: Detention Ponds, 5-2: Detention Basin, and 5-3: Retention and Detention Basins.

Bioswales

Bioswales are referred to as any type of ditch, depression, or vegetative area that can convey stormwater and are similar to dry ponds. Typically, bioswales are linear and function similarly to small streams. The bottom, which should be wide and flat, is often vegetated and may consist of riprap stone. Bioswales are designed to maximize water retention time but also allow for water flow, similar to a stream channel. Bioswales are often constructed adjacent to parking lots to treat runoff from pavement. Bioswales are effective in removing sediment and some pollutants from runoff. For more information on bioswales, see Appendix 5-4: Bioswale Fact Sheet and http://www.crd.bc.ca/watersheds/lid/swales.htm

Rain Gardens

A rain garden is an area of land, large or small, that has been dug and planted with plants that will thrive in your climate and the ecosystem created by the garden. It is used to capture rain water from impervious surfaces allows some of the stormwater runoff water to absorb into the ground, rather than draining into storm sewer or onto adjacent properties. Not only do rain gardens capture water, but they can also capture and trap pollutants, attract birds and helpful insects, and enhance the aesthetics of a neighborhood. A rain garden is an excellent way to deal with stormwater runoff from rooftops and driveways. Gutters are easily directed into rain garden areas. The rain garden may be one of the most practical ways for the HOA and the residents to help control stormwater runoff in the community.

A variety of vegetation should be considered for use in rain gardens. Native plants are often suggested for use in rain gardens based on the belief that they are better adapted to local conditions than non-native plants. However, consider using all plants that do well in your area under the soil and site conditions found in your rain garden. Many plant species are tolerant of alternating dry and wet periods during the growing season. These plants are often ideal for rain gardens. There are many grasses, annuals, perennials, shrubs, and trees that can be used, that have showy foliage and/or flowers. Local nurseries, garden centers, and your Extension Service can help with selection of the appropriate plants for your area.

Rain gardens will require little maintenance, other than to prevent undesirable weedy plants from becoming established, and will be a great success if low maintenance plant materials adapted to the site and soils are used. Typically, rain gardens require the same or less care than a typical landscape bed. The rain garden should be maintained in a natural or at least semi-natural state. This has the added benefit of providing habitat for birds, bees, butterflies, and other wildlife.

For more information, see Appendix 5-5: Rain Gardens and http://www.dnr.state.wi.us/org/water/wm/dsfm/shore/documents/rgmanual.pdf.

Benefits

Benefits of naturalized stormwater basins include:

  • Reduced peak run-off rate of water during a strong storm
  • Reduced flooding of adjacent property and streams by retaining water on site
  • Effective trapping of pollutants, especially sediment
  • Reduced erosion and other damage to adjacent properties and stream channels from high run-off rates
  • Improved neighborhood aesthetics
  • Increased wildlife habitat

Technical Best Management Practices

Stormwater control structures, including dry and wet ponds as well as bioswales, are typically designed and constructed at the time of development. Federal, state, and local regulations specify how these structures must be designed and constructed. However, once constructed, maintenance of stormwater control structures may be left to the local HOA.

Dry Ponds and Bioswales

Dry stormwater ponds and bioswales should be planted with low-maintenance plant materials that have been shown to be well-adapted to your area and the site conditions they will be planted in. These are often native plants. The vegetation planted will depend on the frequency and duration of basin flooding. Appropriate plants should be selected for the anticipated “hydrological regime” (frequency and amount of water and saturation) of the basin. Stormwater basins are typically designed by an engineer or architect for a specific storm event over a defined area. Consult plans to determine how much water can be expected within the basin. Your local Soil and Water Conservation District, Stormwater District, or Extension Service can often provide assistance in using, designing, and constructing landscape stormwater management systems.

Wet Ponds

Wet ponds are often planted with vegetation depending on the water depth. Plants such as bulrushes and cattails are preferred for water levels up to two feet in depth. Deeper areas can be planted with floating-leaved plants such as water lilies.

There are numerous commercial sources for seeds and plants for varying degrees of wetness. For a stormwater basin or bioswale, plant species with a high biomass (total volume of living parts) are generally preferred for maximum uptake of nutrients and pollutants. Densely growing, vigorous plants also increase removal of sediment from the stormwater. Generally, herbaceous plants are preferred for stormwater basins because of their higher biomass.

Both dry and wet ponds can provide habitat for native plants and animals.

Naturalized stormwater control structures can be periodically mowed or otherwise maintained to prevent woody vegetation from becoming established. Woody vegetation (trees and large shrubs) can compromise the structural integrity of berms and water control structures as well as reduce the effectiveness of herbaceous vegetation for sediment and pollutant control. Inlet and outlet structures as well as berms should be maintained to keep them in working order and free from debris and obstructions. However, trees and shrubs can be allowed to grow around the perimeter which will provide further stormwater control benefits.

The amount of sediment carried by runoff, normally will drastically decrease after site construction is complete. However, it may be necessary to periodically reshape both dry and wet ponds to remove accumulated sediments. Many stormwater ponds are constructed with a small forebay that stormwater flows through before it enters the main pond. The purpose of the forebay is to allow some of the sediment to settle out before reaching the main pond; this reduces long-term maintenance costs. Sediment from the forebay can be removed much more easily than from the main pond. All stormwater ponds should maintain a volume close to what is specified in plans to make sure they function properly. After basin reshaping and sediment removal, some of the original soil material can be replaced so that appropriate vegetation can quickly re-establish across the basin.

Rain Gardens

Rain gardens require little maintenance after construction. The vegetation that you select for your rain garden should be well maintained and kept weed-free. Plant species that are attractive and fit well with home landscaping in your area can be used.

Owner/Management Best Management Practices

How to Hire a Consultant/Contractor

More likely than not, it will be the responsibility of the HOA to hire contractors to maintain existing stormwater control structures, create new ones, and manage the plants planted or found in these important public health and safety assets.

With the exception of rain gardens, which the individual homeowner can easily construct and maintain, most stormwater control structures require large equipment and professional design and installation. If a contractor is indeed needed, it is important to choose someone with experience in the stormwater field. Your county Natural Resources Conservation Service (NRCS) office will be able to provide references for experienced, reliable contractors. If your development was constructed in recent years, it is likely that a Stormwater Pollution Prevention Plan (SWPPP) was written for the project, and this document should be consulted to help plan for any additional stormwater structures or to repair or modify existing structures.

Obtaining Estimates

Ask for an estimate; often there is no charge for this service, and you should get more than one estimate. You can also consider creating a request for bid that details the work you want to accomplish and then provide this to several companies. This way, they are all bidding on the same work and it will be easier to compare their responses. You should not always select the lowest bid. When examining bids to determine which company will provide you the best service, take into account all information such as certifications, credentials, insurance coverage, work description, and cost. And most importantly, get the bid in writing.

Contract Specifications

Most companies provide a contract for services, or you can create your own. The contract should cover some general areas with specific details varying based on the type of project. Basic sections in most contracts are definitions, scope of work, liability and insurance requirements, and payment terms.

Qualifications and Industry Standards

Industry standards are based on federal, state, and local stormwater control regulations. Your contractor should have experience in the design, construction, and maintenance of stormwater control structures. If possible, it may be advisable to hire the contractor who originally constructed the stormwater control structures for the development.

Monitoring/Inspecting the work

It is important that any stormwater control structures be constructed in accordance with all federal, state, and local requirements. Depending on the scale of the project, a SWPPP may be required along with one or more permits. If not already required, request that someone from your county NRCS office come and inspect the work during and after construction.

Understanding Federal, State, and Local Laws and Ordinances

Most stormwater control structures are regulated under the Clean Water Act. A professional who is experienced in the stormwater control field should be involved in any project that involved construction of new stormwater control structures to ensure that all federal, state, and local regulations are followed. Maintenance of existing structures can likely be done without permits, but the specifications for the original structures should be consulted to ensure that maintenance work such as accumulated sediment removal is done correctly. Simple stormwater control projects such as rain gardens can be undertaken by individual homeowners and are not subject to any regulations.

Using Volunteers

Most stormwater control projects require large equipment and must be constructed to comply with federal, state, and local regulations. This work should be left to professionals. Simple stormwater control structures such as rain gardens are an excellent project for volunteers to create on private and HOA controlled areas. With some training and guidance, volunteers can also help with monitoring the amount, types, and conditions of vegetation in and around various stormwater basins, and then alerting HOA management when maintenance is needed. When converting a basin from simply turf to more natural plant materials, volunteers can assist with seeding, planting, weeding, and watering the newly converted areas.

Homeowner Education

Homeowners within subdivisions should have basic knowledge of stormwater control and how they can contribute to reducing stormwater runoff. Individual homeowners should be encouraged to construct rain gardens. The importance of limiting the amount of impervious surfaces should also be emphasized to homeowners. For example, gravel driveways should be promoted as an environmentally friendly alternative to concrete and asphalt. As a leader in your association, tell others about the importance of stormwater control.

Sources for Assistance

Your county NRCS office is an important resource. The NRCS staff will have extensive knowledge of stormwater regulations as well as the best stormwater management practices for your local area.

Conclusion

Naturalized stormwater dry and wet ponds along with bioswales and rain gardens are effective and attractive tools that reduce runoff and water-borne pollutants in the community and the larger ecosystem. When properly implemented and maintained, they can function to help reduce the impact of humans on the environment and can become assets to the HOA.

Appendix

5-1: BMPs Detention Basin Fact Sheet
5-2: NRCS Detention Basin
5-3: Detention and Retention Basins
5-4: Bioswale Fact Sheet
5-5: Rain Gardens

Helpful Websites

http://water.epa.gov/action/weatherchannel/stormwater.cfm
http://dsf.chesco.org/water/lib/water/pdf/brochures/retrofit.pdf
http://www.dauphincd.org/swm/BMPfactsheets/Detention%20Basin%20fact%20sheet.pdf
http://sudsnet.abertay.ac.uk/images/photos/Detention_basins/Jun24159.jpg (detention basin pic)
http://www.ncstormwater.org/pages/stormwater_faqspage.html#pollutedrunoff
http://www.dnr.state.wi.us/org/water/wm/dsfm/shore/documents/rgmanual.pdf
http://buildgreen.ufl.edu/Fact_sheet_Bioswales_Vegetated_Swales.pdf
http://www.pinehurstseattle.org/blogger/1.bp.blogspot.com/
_i7Z86WZgIVA/SKWX6K1t4nI/AAAAAAAAAjk/OQgF4S1v-NA/
s1600-h/IMG_2454.JPG
(bioswale pic)
Oregon Storm Water