Urbanization often leads to more expansive municipal infrastructure including roadways, residential housing, and commercial areas. Development is accompanied by increases in impervious surfaces and decreases in vegetative land cover. Less pervious area reduces stormwater infiltration while less vegetation means less evapotranspiration of water. This contributes to increased flooding and stream channel erosion. As the water flows over these surfaces, it picks up pollutants such as sediment, nutrients, metals, and bacteria and delivers them to waterways. These pollutants can be harmful to the environment and may contribute to eutrophication, algae blooms, low dissolved oxygen, and fish kills. To mitigate for these hydrologic changes and pollution inputs from stormwater flows, Stormwater Control Measures (SCMs), also called Best Management Practices (BMPs), have been developed. The goals of SCMs are to reduce runoff volume, mitigate peak flow rates, and improve water quality through a number of mechanisms including evapotranspiration, infiltration, detention, sedimentation, filtration, and biological/chemical activity. One of the most common SCMs is the wet retention pond (hereafter referred to as wet ponds).

Originally designed to control flooding during major storms, wet ponds were later adapted to improve stormwater quality. Wet ponds are basins that typically receive runoff from a mid-sized watershed (5 to 50 acres). They are designed to have a permanent presence (or pool) of water for removing sediment and sediment-borne pollutants. Additional capacity above the permanent pool stores and slowly releases detained runoff. Water surface elevations and water releases are controlled by an outlet structure, which includes normal and emergency spillways and a drawdown orifice. The drawdown orifice sets the permanent pool elevation (PPE) and controls the rate of release of the treated stormwater. Wet ponds are designed with a forebay that collects sediment for easy removal during routine maintenance (Figure 1, Figure 2a, and Figure 2b). Wet ponds constructed since the mid-2000s may also contain a vegetated shelf along the pond’s edge (Figure 1, Figure 2a, and Figure 2b).

To promote pollutant removal, wet ponds built in North Carolina since 2017 have been required to have a 6-foot wide aquatic shelf with a 6:1 side slope along the perimeter of the pond (Figure 2a and Figure 2b). The PPE can be located anywhere along the shelf, from its outer perimeter to that of its inner. This aquatic shelf is traditionally planted with vegetation that is adapted to being inundated with water. The vegetated shelf is shallow because it is, in part, a safety feature with water too shallow to become a drowning risk.

Vegetation plays important roles in wet ponds. Plant roots stabilize soil and decrease erosion by holding together the pond shelf. The leaves and canopy of the plants reduce erosive forces of intense rainfall. Vegetation provides a visual screen for Canada geese (Branta canadensis), making their habitation of the pond less likely. Plants also reduce nitrogen and phosphorous concentrations via root uptake and the facilitation of biological transformations. Fertilizer for plant establishment and long-term growth is typically not needed because of the nutrient-rich stormwater. Vegetation improves the aesthetics of a wet pond by adding additional color and life to the landscape. Moreover, since plants evapotranspire they may provide a minor contribution to stormwater volume reductions within the pond. Finally, plants promote biodiversity by providing pollen for pollinators and cover for song birds.

In the coastal plain and sandhill physiographic regions of North Carolina (Figure 3), the soils are often Hydrologic Soil Group (HSG) A, meaning they are highly permeable (often due to a high sand content). These soils thus allow for substantial amounts of infiltration. Because of infiltration, it is a challenge to maintain the PPE at the invert of the low-flow orifice when wet ponds are installed over HSG A soils. The result is that the water level drops below the intended PPE for extended periods of time, creating an infiltrating wet pond. When this occurs, the vegetation planted on the aquatic shelf is left high and dry (Figure 4). It is difficult for plants that are not drought tolerant to survive such dry conditions. Most design guidance recommends wetland species for aquatic shelves because this guidance is based on non-infiltrating wet ponds. If these plants die, there is a loss of the previously-described benefits.

Infiltrating Wet Ponds in Fayetteville, North Carolina

A North Carolina State University study assessed the hydrologic and water quality performance of infiltrating wet ponds in Fayetteville, North Carolina. Significant volume reductions were found when wet ponds were partially lined and installed over HSG A soils. At one pond, 54% of the inflow infiltrated, 6% evaporated, and 40% left as outflow. At another pond, 46% of the inflow infiltrated, 5% evaporated, and 49% left as outflow. The outflow water quality was not substantially different from standard (non-infiltrating) wet ponds. Throughout the study, it was observed that the water level was consistently below the intended PPE. The plants struggled to survive and bare soil was often exposed along the banks. The complete study is found in Baird (2015).

Figure 1. Plan view of basic wet pond design elements.

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Figure 1. Plan view of basic wet pond design elements.

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Figure 2a. Example of a wet pond with a forebay and small aquatic shelf in Brunswick County.

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Figure 2a. Example of a wet pond with a forebay and small aquatic shelf in Brunswick County.

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Figure 2b. A closeup of an aquatic shelf in Onslow County.

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Figure 2b. A closeup of an aquatic shelf in Onslow County.

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Figure 3. Physiographic regions of North Carolina.

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Figure 3. Physiographic regions of North Carolina.

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Figure 4. Plants struggling to survive along the vegetated shelf of an infiltrating wet pond.

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Figure 4. Plants struggling to survive along the vegetated shelf of an infiltrating wet pond.

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Select plant species for vegetated shelves around infiltrating wet ponds based on (1) expected frequency and duration of inundation and (2) expected duration of drought conditions. Because of fluctuating water levels, the palette of species for infiltrating wet ponds is more limited than that of standard wet ponds. Plants in an infiltrating wet pond experience rapid changes in (1) the soil moisture content and (2) the length of time they are inundated.

To prevent a monoculture, a variety of plants should be selected for wet pond aquatic shelves. Plant diversity may enhance nitrification/denitrification capability and improves both aquatic and terrestrial habitat as well as the aesthetics of wet ponds. Per North Carolina Department of Environmental Quality (NCDEQ) guidelines, a minimum of three different species need to be selected; however, faculty at NC State University recommend at least six species. Plants must be tolerant of dry conditions and inundation. Inundation will typically not last more than three days. Dry conditions, however, could persist for a month, depending on rainfall patterns. Thus, it is important to select plants that are drought tolerant. Two naturally occurring landscapes that mimic the hydrologic pattern of an infiltrating wet pond shelf are found in the North Carolina coastal plain: pocosins and riparian (floodplain) areas. These landforms are often quite sandy and have fluctuating water table conditions that cause moisture conditions to change rapidly. Vegetation becomes inundated when the river level rises or pocosins flood but then experiences low moisture conditions during droughts. Plants that naturally grow in these areas should be ideal candidates to survive and thrive on the vegetated shelf of an infiltrating wet pond.

Recommended plants are divided into two groups: shrubs/herbaceous species and grasses (Table 1). Many of these species produce colorful flowers that add to the aesthetics of infiltrating wet ponds.

Planting Specifications

Before planting, NCDEQ recommends that soils are sufficiently fertile to support plant growth. As such, NC State University recommends conducting a soil analysis before planting. NCDEQ recommends that shrubs be planted at a density of one shrub per 25 sq ft. This spacing allows the plants to grow without competing with one another. For herbaceous species and grasses, the most economical and readily-available size is small plugs. NC State faculty recommend a 2-foot spacing for plugs. Larger size plants can be established with greater spacing (quarts or 4-inch containers, 3-foot spacing; 1-gallon, 5- to 6-foot spacing).

Plant establishment is critical. Ideally, planting should be done in the spring or autumn, with spring (that is, late March through June) preferable due to wetter conditions and the potentially higher water level within the pond. If vegetation is planted in the autumn (September through November) or during a drought, irrigation may be required to aid in establishment. In any case, all plants will require thorough watering—whether from rain, pond water, or artificial irrigation—for three to four weeks following installation.

In addition, be careful not to over-top (completely submerge) herbaceous species and grasses before roots have established in the substrate. Even wetland species can “drown” if subjected to complete submerging during the initial two to three weeks following installation. However, total submergence is not an issue for established plants and during the dormant season (from late autumn to early spring).

Vegetation should also be planted on the upper bank, the area from the outer edge of the vegetated shelf to the existing ground (Figure 1). This includes the top of the dam and berms. This perimeter typically has steeper slopes and is subject to erosion, making stabilization critical. Plant turf grass to stabilize the embankment and create easier access for maintenance. For these locations, Bermuda grass (Cynodon dactylon) is recommended. Avoid planting trees and woody shrubs on the dam and berms because their roots jeopardize the integrity of the embankment.

We do not recommend planting cattails (Typha spp.) in any wet pond (or constructed wetland) as they tend to outcompete other vegetation and create a monoculture. Cattails provide a habitat that shelters mosquitoes from their predators. Remove cattails if they colonize at least 15% of a wet pond that is located near a school, neighborhood, or residential area (see Urban Waterways Bulletin AG-588-7, Maintenance of Stormwater Wetlands and Wet Ponds) and establish beneficial vegetation.

The deep areas of the wet pond other than the forebay can be planted with particular species appropriate for permanent pools. (Note: Forebays are sediment sinks and may be scooped out multiple times over the life of a wet pond. For this reason, we recommend against panting in them.) For deep pools, choose plants that are adapted to prolonged or permanent inundation. Examples are Nymphaea odorata (native water lily), Nuphar advena (spatterdock), and Nelumbo lutea (native lotus), as well as several of the bulrush species (Schoenoplectus/Scirpus spp.). See Urban Waterways Bulletin AG-588-12, Stormwater Wetland Design Update for further information about these plants. In addition, this area could sustain plants via floating treatment wetlands.

Despite lower water surfaces than standard wet ponds, infiltrating wet ponds should be designed with vegetated shelves. Shelves provide a key safety element to ponds and, when appropriately planted, are a deterrent for Canada geese. However, plants selected for standard wet ponds are often not capable of surviving the more variable conditions of an infiltrating wet pond’s shelf. Plant species selected for an infiltrating wet pond should be well adapted to dry conditions and be able to tolerate inundation. Once established, the species listed in this publication should be able to survive these conditions and help to improve the performance—and enhance the beauty—of infiltrating wet ponds.

Related Extension Bulletins of the Urban Waterways Series

• Mosquito Control for Stormwater Facilities. 2005. W.F. Hunt, C.S. Apperson, & W.G. Lord. NC State Extension, Raleigh, NC. AG-588-4
• Maintenance of Stormwater Wetlands and Wet Ponds. 2006. W.F. Hunt & W.G. Lord. NC State Extension, Raleigh, NC. AG-588-7
• Stormwater Wetland Design Update: Zones, Vegetation, Soil, and Outlet Guidance. 2007. W.F. Hunt. M.R. Burchell, J.D. Wright, & K.L. Bass. NC State Extension, Raleigh, NC. AG-588-12.

Other Resources

Baird, J. (2015). Evaluating the hydrologic and water quality performance of infiltrating wet retention ponds. (MS Thesis). Raleigh, NC: North Carolina State University.

Hunt, W. F., Apperson, C. S., Kennedy, S. G., Harrison, B. A., & Lord, W. G. (2006). "Occurrence and relative abundance of mosquitoes in stormwater retention facilities in Corth Carolina, USA." Water Science & Technology, 54(6), 315-321.

Leopold, L. B. (1968). Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use. Washington D.C.: United States Department of the Interior.

Mallin, M. A., Ensign, S. H., Wheeler, T. L., & Mayes, D. B. (2002). "Pollutant removal efficacy of three wet detention ponds." J.Environ.Qual., 31(2), 654-660.

NCDEQ. (2017). Stormwater design manual. Retrieved from https://deq.nc.gov/sw-bmp-manual

North Carolina Department of Environmental Quality—Division of Energy, Minerals, and Land Resources. (20007). Dam operation, maintenance, and inspection manual. Raleigh, NC.

Wu, J., Holman, R., & Dorney, J. (1996). "Systematic evaluation of pollutant removal by urban wet detention ponds." Journal of Environmental Engineering, 122(11), 983-988.

Acknowledgements

The authors would like to thank Dan Line of NC State University, Dr. Dan Hitchcock of Clemson University, and Dr. Eve Brantley of Auburn University for constructive input that improved the editorial quality. We also thank Charlotte Glen of NC State Extension for input on plant selection.