Development

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.