Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Detention Tanks and Vaults
Detention tanks and vaults are underground structures used to attenuate peak stormwater flows. They are usually constructed out of either concrete or corrugated metal pipe (CMP) and must consider the potential loading from vehicles on the vault or pipe (Figure 11). Pretreatment structures can be used at the inlet to treat stormwater runoff and remove trash and debris. In addition, flow splitters can be used to direct only a portion of the stormwater runoff to an underdrain detention.
Due to the costs associated with underground detention systems for construction and maintenance, these systems are primarily used when space is limited and there are no other practical alternatives. In the ultra-urban environment, costs for developable land may be high enough that these systems become a feasible alternative. Relatively expensive to construct, concrete vaults are used primarily to control small flows in areas where system replacement costs are high. Less expensive, CMP systems are typically used to control significant volumes of runoff in parking lots, adjacent to rights-of-way, and in medians, where they can be replaced or maintained if necessary.
In the ultra-urban environment, underground detention tanks have been used to decrease flows in combined sewer systems. The stormwater is stored in the tank and then can be released by a remotely controlled valve to the wastewater treatment plant after the peak flows have passed through the plant.
Underground detention structures are effective measures for stormwater runoff quantity control; however, these facilities do not provide significant water quality control or primary stormwater treatment, without extensive modifications. Consequently, they are more frequently used to attenuate and store peak flows. In addition to providing insignificant stormwater treatment without modifications, receiving waters can be very sensitive to releases of the stored volume from these underground detention systems.
Preliminary results of water quality monitoring of modified underground detention structures have demonstrated a total suspended solids (TSS) removal rate of between 60 to 80 percent; a total phosphorous (TP) reduction of between 20 and 40 percent; and a total lead reduction of between 40 and 70 percent. This facility, however, required weekly maintenance and cleaning out of the structure to maintain this efficiency (Northern Virginia District Planning Commission, 1992). In reality, few detention tanks and vaults receive weekly maintenance.
Siting and Design Considerations
The CMP systems used for large storage volumes are usually a series of pipes interconnected by a junction box or main pipe with an outfall structure. There should be a sufficient number of access holes and access points in the system to efficiently inspect and maintain both the outfall structure and the storage area. Whenever possible, the system should be located in an area where maintenance and potential repairs can be conducted with minimal disturbance to surrounding uses. Some design information on CMP systems is available in Design and Construction of Urban Stormwater Management Systems (ASCE, 1992).
Water quality controls, such as water quality inlets and sand filters, are often used to pretreat the stormwater before it enters the system. This is done to remove sediment and pollutants, which might clog the system. CMP systems can work in conjunction with infiltration to provide additional stormwater treatment.
When infiltration is used, perforations may be added to the pipe to allow the pipe to store the water until it can be exfiltrated into the soils below the pipe. In critical areas, such as under roads and parking lots, pipe joints may require gaskets and water-tight seals to protect the integrity of the pipe. Most systems have pipes or vaults inverts that are 1.8 to 3 m (6 to 10 ft) underground. Therefore, it may be difficult to obtain an adequate outfall for the system.
Another type of underground detention is the retrofitting of overcapacity storm drain pipes with baffles. The baffles cause the water to be stored in the pipes and to be released to the outfall at a slower rate (ASCE, 1992).
The cost and maintenance of these systems are major considerations. The systems must be designed so that they can have easy access for inspection and maintenance. Maintenance is usually conducted by periodically pumping out sediments and debris. In areas of high sediment flows, pretreatment is required to minimize the inflow of particulates so that the need to clean the system is reduced. An analysis of other management measures in the watershed is required to ensure that peak release rates are coordinated so that peak flows are reduced to predevelopment rates.
With the facilities located underground, inspection and maintenance are important issues because of the relatively high costs. In the ultra-urban environment, the facilities may require location under structures, such as buildings, parking lots, and roadways. Frequent maintenance is required to remove sediment and debris and to ensure that the outlet structure is functioning properly. Large-scale removal of accumulated sediment in the system may be difficult due to limited access. In addition, underground systems will be considered confined spaces that require additional safety requirements for inspection and maintenance.
Due to the high costs associated with concrete structure construction, the use of vaults is limited to small drainage areas. A preliminary cost estimate for the more expensive concrete vaults can be provided by the following equation (Wiegand et al., 1986):
C = 38.1 ( V / 0.02832 )0.6816
C = construction cost estimate (1995 dollars) and
V = volume of storage (cubic meters) for the maximum design event frequency.
Corrugated metal pipes have been used extensively in urban areas and are significantly less expensive than vaults for storing large amounts of water. Both concrete and CMP systems have long life cycles.
ASCE. 1992. Design and Construction of Urban Stormwater Management Systems. The Urban Water Resources Research Council of the American Society of Civil Engineers (ASCE) and the Water Environment Federation. American Society of Civil Engineers, New York, NY.
Northern Virginia District Planning Commission. 1992. Underground Detention Tanks (UDTs) As A Best Management Practice (BMP). Northern Virginia District Planning Commission, Annandale, VA.
Roberts, B.C. 1995. Water Quality Enhancement Using Subsurface Detention. National Corrugated Steel Pipe Association, Washington, DC.
Wiegand, C., T. Schueler, W. Chittenden, and D. Jellick. 1986. Cost of Urban Runoff Quality Controls. In Urban Runoff Quality - Impact and Quality Enhancement Technology, ed. B. Urbonas and L.A. Roesner, p.366-382. American Society of Civil Engineers, New York, NY.