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Digging Deep

Locating stormwater treatment facilities underground can free more land for development.

By John R. McAdams |

D evelopers should address stormwater management issues at projects' commencements, as storage and treatment facilities' sizes can alter the amount of available buildable land. Flood-zone regulations and stormwater treatment requirements affect the amount of land needed. If the land's opportunity cost is high, the only reasonable choice may be to locate the stormwater management facility underground. While this solution can cost four times or more of an equivalent aboveground facility, it frees the entire site for development. The following case study illustrates how underground stormwater treatment facilities can solve development problems.

Going Underground to Save Land

To comply with nitrogen removal requirements for the Neuse River Basin regulations, developer Wood Partners hired the John R. McAdams Co. to design a stormwater treatment facility for Station Nine, a midrise, 323-unit multifamily project in Durham, N.C.

McAdams encountered several challenges when determining the best stormwater treatment facility to design. The first challenge involved the small amount of available land -- approximately five acres -- on which the entire development was to be located. The amount of nitrogen that needed to be removed from the site runoff was another concern. The City of Durham required 40 percent reduction from pre-development nitrogen loading conditions. Typical stormwater treatment devices would not work, since all of the available land was required for the proposed development.

To contend with these challenges, McAdams determined that an underground sand filtration system was the best use of the land, although it would cost more than an aboveground facility.

The designed system comprised three 8-foot by 10-foot concrete vaults that were 110 feet long. The top of each vault was buried two feet to three feet below the ground's surface.

Each vault contained a sand chamber used to filter the nitrogen out of the runoff water. The stormwater ran through three side-by-side chambers to effectively solve the required 40 percent nitrogen removal.

The underground facility solution benefited the land, the surrounding community, and communities downstream. Since the project itself was relatively small, and the density required was high, the development would not have been feasible had any of the site been used for stormwater management purposes. Even though the estimated cost of the sand filters was more than $500,000, it made sense financially because it allowed full use of all of the developable area of the site. It fulfilled the Neuse River Basin regulations and dealt with the lack of available land. This approach allowed Wood Partners to use all the developable land without having to purchase additional land to provide stormwater treatment above ground.

John R. McAdams

John R. McAdams is president of the John R. McAdams Co., a land development design firm in Research Triangle Park, N.C. Contact him at (919) 361-5000 or mcadams@johnrmcadams.com. Managing Stormwater to Maximize Land When East West Partners Management Co. planned Meadowmont of Chapel Hill, its flagship mixed-use community, it sought to optimize the land\'s development yield. The developer followed a master stormwater management plan to comply with Chapel Hill, N.C.\'s regulatory requirements while maximizing the available land. Because the 435-acre project is located within the protected Jordan Lake watershed area, state and local regulations mandated different stormwater quality treatment for different aspects of the development. Thus, the developer divided the site into a high-density commercial area that required stormwater quality facilities and a low-density residential area that did not. The state permitted no more than 70 percent paved surface coverage in the high-density area and no more than 24 percent in the low-density area. Chapel Hill imposed stricter regulations, allowing only 50 percent impervious surface coverage in the high-density area to control pollution in the drinking water supply. To provide water features that met quality and quantity requirements while optimizing the available land, East West Partners approached the development as a whole rather than as individual parcels. Working with a land development design company, the developer located three ponds in high-visibility areas on various parcels that the Meadowmont Owner\'s Association would own and maintain. In contrast, separate stormwater plans for individual parcels might have resulted in 10 or more ponds, compromising available land and increasing both construction and facility maintenance costs. To track impervious surface limitations during construction, the land development design company created an accurate monitoring system that tracked each parcel and individual residential lot\'s paved area, including sidewalks, parking spaces, bike trails, landscape lots, and roads. When parcels were sold or developed, engineers inserted the impervious surface information into a spreadsheet and sent it to East West Partners, as well as Chapel Hill\'s planning department. Results and Benefits The benefits of starting with a master stormwater management plan are numerous, including no wasted land. By knowing the Meadowmont development\'s pond locations upfront, East West Partners could plan around them effectively. In addition, prior to purchase buyers know a parcel\'s precise impervious surface allocations, as well as where stormwater will be directed and the location of each parcel\'s drainage boundaries. At Meadowmont, the developer overlapped the individual basins\' drainage boundaries, allowing buyers the flexibility to build on high-density area parcels without worrying about getting the exact amount to draw to a specific basin. Also, once buyers purchased parcels, they could sell spare impervious surface allocations to other parcel owners located within the same sub-watershed (high- or low-density area), thus maximizing their investment. Ultimately, East West Partners\' potential buyers were better informed to make qualified decisions about development before closing their land purchases. East West Partners maximized Meadowmont\'s available land by implementing a stormwater management and treatement plan prior to development. Credit: John R. McAdams Co.

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