In-line storage refers to a number of practices designed to use the storage within the storm drain system to detain flows. While these practices can reduce storm peak flows, they are unable to improve water quality and offer limited protection of downstream channels. Hence, EPA does not recommend using these practices in many circumstances. Storage is achieved by placing devices in the storm drain system to restrict the rate of flow. Devices can slow the rate of flow by backing up flow, as in the case of a dam or weir, or through the use of vortex valves, devices that reduce flow rates by creating a helical flow path in the structure. A description of various flow regulators is included in Urbonas and Stahre (1990).
In-line storage practices serve a similar purpose as traditional detention basins (see Dry Extended Detention Ponds fact sheet). These practices can act as surrogates for aboveground storage when little space is available for aboveground storage facilities.
In-line storage has significant limitations, including:
- In-line storage practices only control flow, and thus are not able to improve the water quality of stormwater runoff. As a result, other stormwater BMPs such as Green Roofs or Bioretention Rain Gardens should be considered and used if possible, particularly for new construction projects.
- If improperly designed, these practices may cause upstream flooding.
Siting and Design Considerations
Flow regulators cannot be applied to all storm drain systems. In older cities, the storm drain pipes may not be oversized, and detaining stormwater within them would cause upstream flooding. Another important issue in siting these practices is the slope of the pipes in the system. In areas with very flat slopes, restricting flow within the system is likely to cause upstream flooding because introducing a regulator into the system will cause flows to back up a long distance before the regulator. In steep pipes, on the other hand, a storage flow regulator cannot utilize much of the storage available in the storm drain system.
Flow regulators require very little maintenance, because they are designed to be "self cleaning," much like the storm drain system. In some cases, flow regulators may be modified based on downstream flows, new connections to the storm drain, or the application of other flow regulators within the system. For some designs, such as check dams, regulations will require only moderate construction in order to modify the structure's design.
The effectiveness of in-line storage practices is site-specific and depends on the storage available in the storm drain system. In one study, a single application was able to reduce peak flows by approximately 50 percent (VDCR, 1999).
Flow regulators are relatively low cost options, particularly since they require little maintenance and consume little surface area.
Urbonas, B., and T. Stahre. 1990. Stormwater Best Management Practices and Detention for Water Quality, Drainage and CSO Management. Prentice-Hall, Inc., Englewood Cliffs, NJ.
Virginia Department of Conservation and Recreation (VDCR). 1999. Watershed and Lake BMPs--Best Management Practices for Established Urban Communities. Virginia Department of Conservation and Recreation, Richmond, VA.