Journal of the NACAA
Volume 5, Issue 1 - May, 2012
Improving Dry Detention Basins in New Jersey to Support Green Infrastructure Goals
- Haberland, M., Environmental and Resource Management Agent, Rutgers Cooperative Extension
Goodrow, S., Senior Project Manager, Rutgers Water Resources Program
Magee, C., Project Manager, Camden County Soil Conservation District
Mangiafico, S., Environmental and Resource Management Agent, Rutgers Cooperative Extension
Many existing stormwater basins in New Jersey were designed with the single purpose of providing flood control, but some may be improved to address the goals of green infrastructure, reduced maintenance costs, and wildlife habitat. A program was developed in Camden County, New Jersey to aid municipalities in implementing detention basin retrofits and assessing the results. Pilot projects suggested that basins maintained in turf grass cover and retrofitted to native vegetation showed the potential cost savings in reduced mowing schedules and resources, with some public works directors reporting savings of up to $4,000 for each basin. This pilot program further demonstrated the need to have the support of the local residents when changing the appearance of neatly mowed basins to alternative vegetation, addressing regulatory and engineering criteria, and preventing mosquito habitat.
Many existing stormwater basins in New Jersey were built prior to the mid 1980’s and were designed with the single purpose of providing flood control. These basins function by capturing stormwater from rain events and snowmelts, and then slowly releasing this water to a receiving stream or stormwater channel. This action effectively mitigates flooding by both decreasing peak flows downstream and delaying the timing of those peaks. Particularly in densely populated areas, it has been suggested that this style of basin could be improved to also contribute to green infrastructure goals such as infiltrating more water into the soil, protecting downstream water quality, and decreasing the volume of water managed by downstream stormwater infrastructure. Additional benefits of basin improvements may include improved wildlife habitat and reduced maintenance costs. The purpose of this paper is to describe the rationale and process of a program to retrofit stormwater basins in New Jersey.
Detention basins - sometimes called “dry ponds” or “dry detention basins” - are stormwater management structures that slow down stormwater runoff for a period of time to decrease downstream flooding. They differ from wet ponds - or, "retention basins" - because they are not designed to have large permanent standing pools of water. Instead, water will infiltrate into the soil or drain to a stream within a determined period of time. In New Jersey, detention basins have been used extensively in the last 30 years in suburban and commercial developments to provide flood control during storm events.
While detention basin design varies across regions of the country, those in New Jersey and other areas may suffer from limitations when considered in light of green infrastructure goals. These include:
A reduced capacity to infiltrate water into the soil. While the infiltration of precipitation and runoff water is an important green infrastructure goal, soils in these basins may suffer from reduced infiltration, due to soil compaction during construction or maintenance activities. Benefits of increased infiltration include the recharge of groundwater supplies, the decrease of total runoff handled by the stormwater infrastructure system, and removal of certain pollutants from water.
The inability of basins to treat small runoff events. In certain designs, detention basins may be designed with a concrete low flow channel, which channels the runoff from smaller storms directly through the basin without treatment. From a green infrastructure perspective, ignoring the cumulative effect of small storms may be a significant oversight. For example, it has been estimated that 90% of the storms in New Jersey are less than 1.25 inches of rainfall (Clayton and Schueler 1996). In cases of many small rainstorms, a significant volume of water passes through these basins without spreading into the vegetated portion of the basin and therefore without the opportunity to infiltrate into the basin soil.
The encouraging of nuisance species. Basins maintained in turfgrass may attract Canada geese in areas where the birds are nuisances. Where populations are high, feces from Canada geese may be a significant source of phosphorus and bacteria to rivers, lakes and coastal waters.
High maintenance costs. Depending on the design of the basins, the maintenance of vegetation can be significant in terms of both labor and resources for the municipal, county, or private owners. Turfgrass that requires frequent mowing during the growing season may be particularly burdensome.
One approach to overcoming these limitations involves both modifying the basin to allow water to spread beyond the concrete channel and planting native or other deep-rooted herbaceous vegetation. Such vegetation can improve the infiltration and pollutant removal of stormwater runoff (Winer 2000). Furthermore, such vegetation reduces maintenance, particularly mowing costs, for savings of time, effort, and money to the landowner. Finally, such vegetation has the potential to provide habitat for desirable wildlife species. Allowing the vegetation to grow and flower will enable birds and native pollinators to utilize this “urban refuge” for feeding, habitat, and for some species, reproduction (Figure 1.).
Figure 1. Flowering native vegetation can be an attractant for pollinators. (Photo credit: Mike Haberland, Rutgers Cooperative Extension).
A program was developed in Camden County, New Jersey to aid municipalities in implementing detention basin retrofits and assessing the results. Partners for the pilot program included Rutgers Cooperative Extension, Camden County Soil Conservation District, Cherry Hill Township, and the New Jersey Department of Environmental Protection. Rationale, methods, and initial results are presented here. Five detention basins in Cherry Hill, NJ, were selected for retrofit. The Public Works Department provided the equipment to modify the low flow channels, and volunteers were used to plant native herbaceous vegetation in the basin bottoms.
Basins identified as good candidates for retrofitting were those that had the following properties: 1) turfgrass vegetation maintained with regular mowing; 2) a low flow concrete channel (Figure 2.); 3) a discharge orifice greater than 3 inches; 4) sufficient soil drainage to prevent ponding of water or constantly wet conditions; and 5) no other designated uses when dry.
Figure 2. The concrete low flow channel in a detention basin (Photo credit: Sandra Goodrow, Rutgers Cooperative Extension Water Resources Program).
One of the main goals with retrofitting a basin is to get more of the stormwater runoff to spread out across the basin bottom to increase infiltration and filtration by the vegetation. This can be accomplished by one or more of the following methods: 1) removing the concrete low flow channel and replacing it with vegetation or stone; 2) placing a structure across the channel to deflect the stormwater out into the basin (Figure 3.); or 3) modifying the outlet structure (Emerson et. al. 2005). At this stage, assessing the infiltration capacity, compaction, organic matter, and chemical properties of the soil is helpful to determine if soil amendments or loosening of the soil will be beneficial (Mangiafico, 2011).
Figure 3. A barrier of gabion baskets spreads stormwater runoff in the basin (Photo Credit: Craig McGee, Camden County Soil Conservation District).
Any number of vegetation types can be appropriate replacements for high-maintenance turfs. These include native grasses, a wildflower mix, or other herbaceous vegetation planted in the basin bottom (Figure 4.). It is essential that local climate, weather, soils, and vegetation be considered when timing the planting of vegetation. Bare soil should be minimized, especially during rainy weather to prevent soil erosion and downstream sedimentation. However, planting should be timed to maximize the chances of establishment of the new vegetation. Appropriate erosion-control and sediment-capture methods should be used where needed. Likewise, alternative seed-planting techniques should be considered, including: 1) removing the sod layer and replanting with seed; 2) “drilling” seed through the existing grass; or 3) planting plugs of new herbaceous plants into the existing bottom. As a practical note, placing any shrubs as clumps or “islands” and not spread out through the basin is helpful to prevent them from being mowed down during annual maintenance.
Figure 4. Retrofitted basin showing growth after one year (Photo credit: Chasity Williams, Camden County Soil Conservation District).
Results and Discussion
Pilot projects suggested that basins maintained in turfgrass cover could be retrofitted to native vegetation. These projects showed the potential cost savings in reduced mowing schedules and reduced consumption of manpower resources for mowing. Some public works directors in these projects have reported savings of up to $4,000 per year in maintenance costs for each basin retrofitted, suggesting that in aggregate these savings can be substantial for local municipal governments. Such savings can be weighed against the costs of implementing green infrastructure projects and factored into willingness-to-pay assessments (Blaine and Smith 2006). Depending on vegetation and climate, properly retrofitted basins may need to be mowed only once per year, potentially when the ground is frozen to reduce compaction and rutting from equipment. Vegetation that is not mowed during the growing season can provide improved habitat for wildlife. An added benefit is that annual mowing allows for much taller vegetation in the basin. This acts as a natural deterrent to geese, which prefer low (mowed) vegetation.
Additional benefits include the continued functioning of the basins for stormwater control, runoff from small storms being forced out of the low flow channel and into basin vegetation, furthering of green infrastructure goals, and providing a refuge for native pollinators.
Costs to naturalize (retrofit) detention basins can vary a great deal depending on the size, how elaborate your design is, engineering costs (if needed), equipment, plant material, etc. Our goal was low cost and our projects ranged from $500 - $1,000 an acre.
Public Support and Education
This pilot program demonstrated the need to have support of the local residents, particularly when changing the appearance of neatly mowed basins to alternative vegetation. Educating residents about the benefits of the new planting to ensure its acceptance and allowing volunteers to aid in planting of the native vegetation with guidance and support from experienced staff was helpful. Local neighbors were invited to participate during the basin plantings and these basin naturalization projects are part of ongoing green infrastructure workshops.
Since detention basins are typically owned by a municipality, county, or managed by a residential development as community property, it is critical to work with public officials to ensure compliance with all public safety, engineering design, and stormwater management standards. There may be applicable stormwater regulations dictated by federal, state, or municipal authorities, and any changes to basin function may need approval by qualified engineers and applicable authorities. Often, basins will need to meet certain performance criteria if modified. Differences in climate, geology, topography, and vegetation will need to be considered in all cases. Sediment accumulation in the basin may be allowed, but should be evaluated on a site-by-site basis to ensure that the flood control capabilities of the basin remain intact.
Another important consideration demonstrated by the pilot program was the need to address any potential for creating a public health issue by supporting habitat for mosquito breeding. Since mosquitoes like to exploit stagnant, nutrient-rich water for breeding, an important basin design criterion is that the basin should not hold standing water long enough for mosquito larvae to mature. A 72-hour criterion may be used (Metzger 2004). Also, the outfall structures on these basins should incorporate a drainage orifice that is difficult to clog. Basin designs that incorporate ecological diversity using suitable habitat to enhance natural mosquito predators are highly encouraged (Megonigal 2009).
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