Regional Stormwater Management Ponds

(Conservation Currents, Northern Virginia Soil and Water Conservation District)

Not in my backyard. The NIMBY syndrome traditionally infects communities when it comes time to build landfills, chemical plants, and even group homes. NIMBY has a new victim in Fairfax County-stormwater management ponds.

In 1989, the Fairfax County Board of Supervisors adopted a plan for managing stormwater countywide. The plan identified 134 sites for building regional ponds that would control both the volume of runoff and the pollutants within it-quantity and quality, respectively. Thirty-two of those ponds would be wet and the rest would be dry.* Each regional pond would control a much larger area (at least 100 acres) than most on-site ponds.

dry regional pondRegional ponds elicit a strong reaction among their detractors and defenders. Neighboring homeowners claim that the ponds will devalue their property. Environmentalists argue that regional ponds don't do enough to protect streams. But County stormwater management staff asserts that regional ponds are an important tool in controlling flooding and protecting streams.

A public uproar over a proposed dry regional pond in the southwest section of the Difficult Run watershed led the County to reassess the regional stormwater management plan. Regional Pond D-40 was to be built on forested land that was dedicated by the developer to the County as a condition of building the development. Since 1989, the County has granted five detention waivers for 28 acres of upstream sites in anticipation of D-40 construction, according to County engineer Paul Shirey. "No water quality controls have been provided in the entire 317-acre drainage area to D-40 since the planned pond exempted those sites," said Shirey.

Mathew Taylor is a resident of the newest subdivision in the D-40 drainage area. He leads a group calling itself "Citizens for Responsible Stormwater Management." The group wants the community to play a role in deciding the best method to control stormwater. Taylor proposed that the County forgo this regional pond and in its place use innovative techniques such as rain gardens, wetlands, and swales. Taylor offered an easement on his property for the County to build a small rain garden to detain stormwater before it reaches the stream.

Even ardent advocates for rain gardens concede that unless at least half of the homeowners in the drainage area built and maintained a rain garden, Taylor's solution wouldn't eliminate the need for a pond. And while rain gardens remove pollutants from runoff, they do not provide flood control. Furthermore, bioretention as a replacement for conventional ponds should be integrated as soon as planning starts, not after construction of homes is nearing completion.

Is building a regional pond the best alternative to on-site stormwater detention? There are arguments on both sides.

  • PRO: Regional ponds require less clearing. One regional pond would perform the function of an average of 10-20 on-site ponds. Average clearing for a regional dry pond is approximately 0.4 acres compared to 7.5 acres to construct the equivalent in on-site ponds.
  • CON: This "tree save" argument is valid only if the land designated for on-site ponds is preserved, not used for building more houses. Furthermore, although limited clearing is done to make way for a regional pond and its dam, remaining trees are threatened by standing water in the pond. Often, lack of maintenance gradually turns a dry regional pond into a shallow wet pond or marshland. Plants not adapted to wet conditions die over a five to ten year period following construction.
  • PRO: Land developers recognize that economies of scale available at a single regional pond produce lower capital costs in comparison to the implementation of many on-site ponds and are able to pass savings on to home buyers.
  • CON: Construction of regional ponds makes it possible for developers to avoid considering more environmentally friendly and effective stormwater management and site design practices.
  • PRO: Properly planned and located, regional ponds are safer and more aesthetically pleasing than on-site pond areas that are often cleared and graded for construction.
  • CON: Wet ponds have the potential to look nice with sufficient landscaping. However, wet ponds can be a safety hazard, especially in areas where small children play. In Fairfax County, the majority of regional ponds are dry and ugly.
  • PRO: Regional ponds provide improved downstream channel protection. These ponds are designed to reduce outfall velocity and as a consequence control downstream erosion. Reducing erosion reduces sediment and pollutant transport, which also improves water quality.
  • CON: Although regional ponds protect the downstream channel, the portion of the stream between the pond and the upstream development is exposed to stormwater runoff without any quantity or quality control.
  • PRO: Since there are fewer facilities to maintain under a regional plan, the annual cost of maintenance is significantly lower. Regional facilities can be designed to ease maintenance activities.
  • CON: There is a significant lapse between the time that an on-site stormwater management waiver is issued and the regional pond is built. During this period the stream is exposed to uncontrolled stormwater runoff from the development unless temporary on-site stormwater management, a rarity, is provided.

If the County does not construct the regional ponds identified in its stormwater management plan, what are the alternatives? There is much information available on how other states and Canada manage their stormwater. While many areas still build regional ponds, it appears that the thinking has shifted toward finding ways of controlling stormwater closer to the site through alternatives such as low impact development. Traditional management solutions call for moving stormwater off site as quickly as possible. Low impact development adds landscape features that keep water on site, thus enhancing groundwater recharge and mimicking pre-development conditions.

If regional ponds are not built, many questions remain.

  • In 1992, the County created a funding mechanism for stormwater management. Developers would contribute to the cost of stormwater projects in the 30% of the County where upstream land was still developable. These stormwater projects would include, but not be limited to, regional ponds. In drainage areas where regional ponds were planned, the County allowed developers to contribute to a "pro rata share" program in exchange for not having to build on-site ponds. What should the County do with the pro rata shares it collected to pay for stormwater management in areas where there is none because the regional ponds were not built?
  • What should the County do about future development as it impacts stormwater?
  • How should the County pay the bill for extensive stream restoration and future stormwater management facility maintenance if regional ponds are assumed to have lower maintenance costs?
  • Should Fairfax County ask the voters to decide on a stormwater utility fee? Prince William County, Virginia, and Montgomery County, Maryland, have user fees based on the amount of impervious surface a property has. Prince George's County, Maryland, has a stormwater tax based on property value.

In the case of D-40, the Northern Virginia Soil and Water Conservation District has offered recommendations to the County staff. Urban Conservation Engineer Asad Rouhi is not a fan of regional ponds. "I recognize, however, that by not building this particular pond, stormwater from the developments in the drainage area will continue to discharge into the stream without any quality or quantity control," said Rouhi. "Therefore, at the end of the day I believe this pond will be built."

To decrease the negative impact of the pond on the environment and aesthetics of the neighborhood, Rouhi recommends exploring the possibility of decreasing the storage capacity of the pond and therefore the pond's footprint. This would require a complete reassessment of the design criteria.

Regional ponds are designed to hold the difference between pre-development and post-development runoff. However, certain assumptions are made in those calculations. For regional ponds, the pre-development runoff estimates are based on a drainage area that is 100% wooded and, therefore, has minimal runoff. Post-development runoff estimates are based on the runoff from the densest development in the drainage area. These numbers are the extremes and may not be applicable to the D-40 drainage area, according to Rouhi.

"If the difference between the pre- and post-development runoff is lower than previously estimated, the pond could be built with a smaller storage capacity," said Rouhi. "Decreasing the water detention time from a maximum of 48 hours to 36 or even 24 hours would also decrease the pond's storage capacity needs."

Don Demetrius, an engineer in the County's Stormwater Planning Division, is reviewing the specifications for D-40. "We are in agreement with Asad about reducing the size of the pond," Demetrius said, "but we differ on how to do it."

He explained that a regional pond must be designed to meet three objectives: water quality control, flood control, and protection against dam failure. Even if the pond were designed with a smaller footprint that could meet the first two objectives, the storage capacity required to protect the integrity of the dam in an extreme rainfall would have to be nearly as large as in the original design. Thus the footprint may not be noticeably smaller.

"We need to take a good hard look at how we do these regional ponds," said Demetrius. "Building wet ponds may be the answer because they do a much better job of controlling water quality and they look better." On the other hand, wet ponds require a larger dam and storage capacity and therefore more land clearing to build it. He also pointed out that wet ponds require more maintenance.

Even as the debate goes on and D-40 is on hold, construction of regional ponds continues. As more and more land is developed, Fairfax County planners must take a creative, aggressive approach to handling the increased runoff that is polluting and eroding our streams. To that end, the County is convening a multi-agency task force to develop a position on the role of regional ponds and other stormwater management tools in the County's stormwater management program. Stay tuned for more reports on this critical issue.

If you have any comments about this story or regional ponds in general, please e-mail the Conservation District.

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