Maintenance of stormwater wetlands and wet ponds

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Maintenance of Stormwater Wetlands and Wet Ponds
Stormwater management practices must be kept in proper working order to maintain their intended functions and aesthetic appeal.
This publication presents maintenance guidelines for stormwater wetlands and wet ponds, two stormwater prac­tices that are being constructed across North Carolina.
OVERVIEW
As its name implies, a stormwater wetland is a wetland system designed to treat stormwater runoff. Wetlands typically have shallow water (ex­cept for intermittent deep pools) and dense vegetation. A well-functioning stormwater wetland will be a diverse ecosystem that includes many plant and animal species. It will also do an excellent job of removing pollution from stormwater runoff—its intended function. Stormwater wetlands are very efficient at nutrient removal. Recent studies conducted by North Carolina State University researchers indicate that a stormwater wetland re­moves 40 to 80 percent of all nitrogen and 50 to 70 percent of all phosphorus entering the wetland. Figure 1 depicts some wetlands located across North Carolina. (For more information on stormwater wetlands, see Designing Stormwater Wetlands for Small Water­sheds, AG-588-02, in the Urban Wa­terways fact sheet series.)
Wet ponds are typically much deeper than stormwater wetlands— their average depth ranges from 4 to 8 feet. They are designed so that most of the pond is open water. Wet ponds are the most common stormwater management practice in North Caro­lina and have been constructed since the 1970s in some parts of the state. More recent pond configurations in­corporate wetland features, such as an aquatic shelf (or wetland bench) and a forebay.
An aquatic shelf is a shallow-water zone of a pond, usually along the bank edges, planted with wetland veg­etation. These shelves flood during storms. A forebay is a pool where inflow first enters the pond, and heavier pollutants, such as sediment, initially settle there. Research con­ducted across the United States shows that wet ponds effectively remove sediment and the pollutants associ­ated with it from stormwater. Both wet ponds and stormwater wetlands can
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State Uni­versity and North Carolina A&T State University commit themselves to positive action to secure equal opportu­nity regardless of race, color, creed, national origin, reli­gion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State Univer­sity, North Carolina A&T State University, U.S. Department of Agriculture, and local gov­ernments cooperating.Figure 1
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be used for flood control as well. For more informa­tion on stormwater practices, see Urban Stormwater Structural Best Management Practices (BMPs) in the Urban Waterways series (AG-588-01).
MAINTENANCE GOALS
Maintenance of stormwater wetlands and wet ponds is performed to achieve four goals: efficient hydraulic flow and pollutant removal, aesthetic appeal, safety, and mosquito control. Most of the maintenance activi­ties associated with wetlands and wet ponds pertain to two or more of these goals. The following activities should be performed regularly to maintain stormwater wetland and wet pond efficiency:
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Remove sediment and gross solids from forebays.
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Keep the orifice (the drawdown hole) free-flowing.
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Clean away floating trash and debris.
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Remove vegetation along the dam face.
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Remove invasive plant species.
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Mow the perimeter of wet ponds.
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Control pests, such as muskrats and beavers.
REMOVE SEDIMENT AND GROSS SOLIDS FROM FOREBAYS
Forebays are located at the inlets to stormwater wet­lands and wet ponds. They are designed to slow in­coming water, dissipating the water’s energy, and to provide a location for sediment and other gross solids (such as leaves, other tree debris, cigarette butts, and trash) to settle and accumulate.
A forebay is typically 2 feet deep in a stormwater wetland and sometimes deeper in a wet pond. If the forebay fills with sediment and gross solids, these ma­terials will bypass the forebay and begin to accumulate in other portions of the wetland or wet pond that may be more ecologically sensitive.
To check sediment levels inside the forebay, record the depth of the forebay at the same time each year. Depending on the size of the forebay, a fish finder can be used from a small boat or someone can survey the depth along a grid of the forebay with a rod (Figure 2). If the forebay water is clear, the depth can often be determined visually.
Once the forebay is half full of sediment or the average sediment level is within 1 foot of the water
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surface, remove the sediment and gross solids.
This task is typically accomplished by a track hoe or backhoe (Figure 2). The water level inside the wet pond or stormwater wetland can be lowered, if needed, to aid excavation of the forebay. Depending upon the size of the forebay, cleaning it can require anywhere from a day to a week.
Once the excavated soil (or spoils) from the dredg­ing has begun to dry, either spread it in the watershed away from the banks of the wetland or wet pond and seed it, or take it to a landfill. Consider the location when disposing of the soil. Spoils from wet ponds downstream of industrial facilities may contain pol­lutants that need to be disposed of in a landfill, while those from a residential wetland or wet pond may not. If there is any concern as to proper disposal, samples of the excavated soil should be sent to a laboratory for chemical analysis. This can be costly.
A recent study by N.C. State researchers indi­cates that sediment and gross solids from forebays typically need to be removed (also known as dipped or dredged) once every 5 to 10 years. If wet ponds and stormwater wetlands are located in watersheds with active construction, however, spoils may need to be removed as often as once a year.
Like the forebay, the final deep pool of the pond or wetland near the outlet also must be inspected and maintained. The major difference between the two is that the final deep pool takes longer to fill with soil. The drawdown hole (located at the outlet and described in the next section) is where cap­tured stormwater slowly drains from the wetland or wet pond. It must be free of accumulated debris and sediment to work properly. Remove sediment and gross solids from the deep pool near the outlet whenever the material is within 1 vertical foot of the drawdown hole.
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KEEP
THE ORIFICE (DRAWDOWN HOLE) FREE FLOWING
Wetlands and wet ponds are designed to capture and detain stormwater from 2 to 5 days. On smaller ponds and wetlands, a relatively small hole or orifice is used to detain water for this period. The diameter of the orifice can be as small as 2 inches, which makes it susceptible to clogging (Figure 4). Because many ponds and all wetlands contain vegetation, dead plants can float to and clog the orifice. Moreover, floating trash and debris (see the next section) will potentially clog the orifice.
A clogged drawdown hole poses several problems, including the loss of storage to capture later storms and flooding of desirable plant species. When water levels remain too deep for the desirable plants to survive, stronger, usually invasive, plant species take
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The wetland or wet pond needs to store water between storms to perform its intended function. It cannot store water from the next storm effectively if the orifice is clogged and the wetland is continually full.
Unclogging the orifice is relatively simple. Clean the hole with a stick, a piece of wire, a pole, or your hand. Inspect it regularly—the drawdown hole can clog at any time. Visit the site once a month to make sure water is flowing freely through the orifice, and inspect the outlet after every rainfall event exceeding 2 inches.
CLEAN AWAY FLOATING DEBRIS AND TRASH
Stormwater wetlands and wet ponds are located in low elevations of the landscape. All water from sev­eral acres drains to wetlands and wet ponds. With this water comes trash and other debris, called floatage. It must be removed from wetlands and wet ponds for several reasons:
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It is unsightly, particularly when the wetland or wet pond is designed to be an attractive amenity.
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Floating trash, such as cups or plastic bags, often store small amounts of water in a sheltered environment. Studies have shown that mosquito larvae are more likely to be protected inside floating trash than in the exposed pond.
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Trash and other floating debris can clog the drawdown hole (the orifice), which is often used to slowly release captured runoff (Figure 5). Inspect wet ponds and wetlands for trash regularly
and frequently—typically once a month but occasion­ally once a week. On smaller wetlands and wet ponds, collect trash by simply wading along the edges. With
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larger facilities, a small boat or vac truck may be re­quired. Because most trash follows the movement of water, it tends to collect near the outlet of the wetland or wet pond. This makes trash easier to collect, but it increases the risk of clogging the drawdown orifice.
REMOVE VEGETATION ALONG THE DAM FACE
Dam inspection officials require earthen dams to be free of large shrubs and trees. Roots can conduct water through the dam from the open pond to the downstream side of the embankment. The movement of water along the roots is called piping, which can eventually lead to soil erosion and, if unchecked, dam failure. Piping tends to be a problem for large ponds and wetlands that have a large dam face. Some small wetlands and wet ponds and those with concrete dams do not have this problem. If a dam face is vegetated, it should be grassed exclusively.
Inspect the dam once a year, and remove all shrubs and trees from the dam top and both faces. If the wetland or wet pond has been regularly maintained and any shrubs and trees growing are juvenile, simply mowing the bank is sufficient. Otherwise, a weed wiper, which applies herbicides to plants more than 12 inches tall, can be used along the bank. The weed wiper will kill any plant it touches or scrapes.
If a bank is severely overgrown, trees and shrubs should be cut down and removed. A systemic herbi­cide can be applied to the freshly cut stumps, which will kill the root systems. This is a laborious process. If the dam face is heavily overgrown, a contrac­tor who specializes in removal should be consulted. Because dams of larger ponds and wetlands are re­sponsible for retaining large volumes of water, dam failure can be catastrophic if homes, businesses, or roads are downstream.
REMOVE INVASIVE PLANT SPECIES
Stormwater wetlands and wet ponds with aquatic shelves can become overgrown with invasive plants. The most common invasive plant is the cattail (Typha species, Figure 6). Cattails, while native to North Carolina, crowd out other, more desirable plants. Cat­tails tolerate a variety of conditions and do a good job of pollutant removal. From this functional standpoint, cattails can be considered good plants to have in a wetland. However, cattail monocultures fail to meet two very important design goals: aesthetics and mos­quito control.
A wetland or wet pond that is overgrown with cat­tails is not a diverse ecosystem. Ecosystem diversity is critical for mosquito control. Cattails provide a safe environment for mosquito larvae to mature to adulthood. When cattails go dormant in the fall, some of the fronds will form a protective thicket for mos­quitoes. For more information on mosquito control in wetlands and wet ponds, see Mosquito Control for Stormwater Practice Designers and Managers
(AGW-588-04) in the Urban Waterways series.
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Removing cattails can be challenging. It is almost
impossible to remove a mass of cattails by hand. Cattails grow from tubers that spread, and they also spread by seed. If a piece of cattail is left in the wetland or wet pond after removal, the stand will probably re-establish. Use a backhoe for mass cattail removal when a wet pond or wetland is completely overgrown by cattails.
If a wetland or wet pond has a variety of vegetation but cattails are beginning to colonize it, use an alter­native form of cattail removal, such as applying an aquatic formulation of the herbicide glyphosate (one trade name for this is Rodeo). Wear a chemical-resis-tant glove underneath a cloth glove. Soak the cloth glove in 2 percent glyphosate, and stroke the cattail leaves. Or brush the herbicide onto the leaves with a small weed wiper. Not every leaf needs to be touched by the herbicide because many of the cattails are con­nected by tubers. Within 10 to 12 days, the cattails fronds will wither and die (Figure 7).
The herbicide must be applied by hand rather than by broadcast spray because it will kill every herba­ceous plant it touches. Use only aquatic formulations of glyphosate because they do not harm fish and other aquatic species.
The frequency of cattail removal can vary. Several factors influence the need to apply herbicide to cat­tails: the density at which the wetland is planted with desirable species, the time of year the wetland is planted, and the maturity of the wetland. During the first year or two after wetland construction, remove cattails twice a year. As the wetland matures and desirable species begin to dominate, reduce the maintenance frequency to once a year. The amount of time needed to remove unwanted vegetation (via the glyphosate wipe) varies, but a well-maintained, mature wetland requires visits of about 2 hours per acre of wetland.
Other unwanted plant species include common reed (Phragmites species, Figure 6), various noxious floating aquatics (such as parrot feather, Myriophyl­lum aquaticum, and giant salvinia, Salvina spp.), and Asiatic dayflower (Murdannia keisak). Phragmites species can be removed in a manner similar to that described for cattails. Noxious floating aquatics may require careful chemical or physical removal. If you observe these exotic invasive species, contact your county Extension center.
MOW THE PERIMETER OF WET PONDS
Stormwater wetlands are not mowed to the water’s edge and tend to be surrounded by mature grasses. As a result, mowing the perimeter of wetlands is not a typical stormwater wetland maintenance activity. Many wet ponds, however, do have a grassed perime­ter that needs to be maintained. Mowing maintenance is almost purely aesthetic. The type of grass used, its growing season, and pond aesthetics dictate the height and frequency of mowing:
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Mow cool-season grasses to a recommended height of 4 inches and no lower than 2.5 inches. Cool-season grasses, such as fescue, tend to be used west of Interstate 95 in North Carolina.
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Mow warm-season grasses to a recommended
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height of 2.5 inches and no lower than 1.5 inches.
Warm season grasses include centipede, Bermuda,
and zoysia, and are principally found in eastern
North Carolina.
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Mow every one to three weeks during the growing season when the wet pond is part of an accessible landscape or treated as an amenity.
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Mow wet ponds that are located out-of-sight once or twice a year. The size and severity of slopes along the wet pond
determine the type of mower to use. For small ponds, a standard push mower is often adequate. Larger ponds or ponds with steep banks will probably require a specialized pond mower.
Grass clippings can be left adjacent to the pond to provide organic matter that encourages grass to grow. Do not discharge grass clippings into the water, as this will encourage the growth of algae and could poten­tially clog the drawdown hole.
CONTROL PESTS
Rodents such as muskrats and beavers are attracted to stormwater wetlands and wet ponds (Figure 8). Once
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Muskrats eat aquatic vegetation and burrow holes in the deeper pools. When muskrats actively burrow near the outlet of a wetland or wet pond, they will add sediment and increase turbidity to the outflow, in­creasing the release of pollutants from the wetland or wet pond. Moreover, muskrats will sometimes burrow holes around and through dams. These muskrat holes artificially lower the water level inside the pond or wetland, causing some plant species to die. At worst, the holes can lead to dam failure.
Beavers are attracted to the sound of running water. Once a beaver colonizes a wet pond or stormwater wetland, it will remove trees and shrubs surrounding the stormwater practice to build its lodge and dam. Beaver activity will clog or block the drawdown structure, thus raising the height of water inside the pond or wetland. This change in the depth of water inside wet ponds with aquatic shelves and stormwater wetlands will alter the types of vegetation that survive in the practice. Usually this change is undesirable.
Muskrat infestation is a difficult maintenance problem that usually must be addressed only when a pond or wetland has suffered from neglect. If the practice is infested, muskrats can be trapped under water, where they drown. Muskrats frequently escape traps, however, which makes live trapping difficult. Hire a licensed, experienced trapper who takes care to place traps where pets cannot be trapped by mistake. Once muskrats have been removed from the pond, their dwelling holes should be destroyed.
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If the stormwater practice has been regularly main­tained, muskrat populations can more easily be con­trolled. Encourage muskrats to move away from the wetland or wet pond by making it an uncomfortable place to live. If muskrat holes are observed around the perimeter of a wet pond or stormwater wetland, destroy them or fill them with soil (Figure 9). Identify and destroy muskrat holes during any regular main­tenance activity: whenever the wetland or pond is being inspected to verify that the drawdown is freely flowing and during mowing and trash removal.
Removing beavers is more difficult that removing muskrats. If a beaver is observed living in or around a stormwater wetland or wet pond, contact a profes­sional trapper who specializes in beaver removal.
SUMMARY
Well-designed stormwater wetlands and wet ponds remove pollutants and mitigate floods. To accomplish these goals and remain safe, aesthetically pleasing, and free of mosquitoes, they must be maintained properly to meet their design goals. Most stormwa­ter wetland and wet pond maintenance activities are simple and inexpensive. But without them, the effec­tiveness of these stormwater management practices will decline.
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TABLE 1. STORMWATER WETLAND AND WET POND MAINTENANCE TASKS AND FREQUENCIES
Task
Frequency
Notes
Remove sediment from
Varies. In stable watersheds, once every
In unstable watersheds (those with
forebay and deep pool
5 to10 years is typical.
active construction), the frequency
(dredging/ dipping).
increases to once a year, assuming the
forebay is correctly sized.
Monitor sediment depth in
Once a year.
In a large pond or wetland, a small boat
forebay and deep pools.
may be needed.
Maintaining free-flowing
Once per month and after every storm
Perform inspection regularly. Unclog-
orifice (drawdown hole).
exceeding 2 inches.
ging the hole when needed is simple.
Remove floating trash and
Depends on design aesthetics: once a
Remove trash whenever the drawdown
debris.
week to once a month.
hole is being inspected. Inspect for trash
more often if necessary, and remove as
needed.
Remove vegetation from
Once a year.
Dam top and faces should consist of
dam top and faces.
mowed grass, if vegetated.
Remove invasive species
In years 1 and 2, twice a year (spring
If spread of cattails is somewhat limited,
(particularly cattails).
and fall). From year 2 onward, once a
use the glyphosate-wipe method.
year (spring).
Mow the wet pond perim-
Depends on design aesthetics. Ranges
Wet ponds that are a design amenity will
eter.
from every 1 to 3 weeks to once a year.
require more frequent mowing (every 1
to 3 weeks).
Remove muskrats and
Muskrat hole inspection and destruc-
Contact a professional beaver trapper
beavers.
tion should occur every time the
to remove beavers. Use muskrat traps to
wetland or wet pond is visited (at least
remove muskrats, or contact a profes­once
a month).
sional trapper.
RESOURCES
Fact sheets in the Urban Waterways series, North Carolina Cooperative Extension, N.C. State Univer­sity:
Hunt, W. F. Urban Stormwater Structural Best Man­agement Practices (BMPs). AG-588-01. Online: http://www.bae.ncsu.edu/stormwater/ PublicationFiles/UrbanBMPs1999.pdf
Hunt, W. F., and B. A. Doll. Design of Stormwater Wetlands for Small Watersheds. AG-588-02. Online:http://www.bae.ncsu.edu/stormwater/ PublicationFiles/SWwetlands2000.pdf
Hunt, W. F., C. A. Apperson, and W G. Lord. Mosqui­to Control for Stormwater Facilities. AG-588-04. Online: http://www.bae.ncsu.edu/stormwater/ PublicationFiles/Mosquitoes2005.pdf
Hunt, W. F., and W.G. Lord. Bioretention Perfor­mance, Design, Construction, and Maintenance. AGW-588-05. Online: http://www.bae.ncsu.edu/stormwater/ PublicationFiles/Bioretention2006.pdf
Rodewald, A. D. Nuisance Canada Geese: How to Deal
with the Problem. Ohio State University Extension
publication no. W-3-2001.
Online: http://ohioline.osu.edu/w-fact/003.html
BAE Stormwater Group Web site: www.bae.ncsu.edu/stormwater
Obtain information on upcoming workshops (including BMP Inspection and Maintenance Certification Courses), publications, PowerPoint presentations, images to download, and design and construction specifications.
State of North Carolina Stormwater Web site: www.ncstormwater.org
ACKNOWLEDGEMENTS
All photographs were provided by the Department of Biological & Agricultural Engineering at N.C. State University except as noted for Figures 7 and 8.
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Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage reulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.
Prepared by
William F. Hunt, Ph.D., P.E. Assistant Professor and Extension Specialist Biological and Agricultural Engineering North Carolina State University and
Bill Lord
Area Environmental Agent
North Carolina Cooperative Extension
North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension North Carolina Cooperative Extension Published by North Carolina Cooperative Extension Service North Carolina Cooperative Extension Service North Carolina Cooperative Extension Service College of Agriculture and Life SciencesCollege of Agriculture and Life Sciences College of Agriculture and Life Sciences College of Agriculture and Life Sciences College of Agriculture and Life Sciences