Extended Detention Design Specification

Dry Extended Detention Pond

Extended Detention Design Specification

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I must confess that dry extended detention (ED) ponds don’t exactly top my all-time list of favorite stormwater practices, but it is clear they occasionally have a role to play when full runoff reduction and channel protection cannot be achieved in upland areas. Dry ED ponds have been around for four decades and they remain extremely popular in some Bay communities, such as Baltimore County, MD and Fairfax County, VA.

Dry Extended Detention Pond
Dry extended detention ponds provide water quantity control.

Their persistent popularity is explained by two factors. The first is that dry ED ponds are the cheapest stormwater practice around on a cost per cubic foot treated basis, and the second is that they integrate water quality, channel protection and flood control in a single central facility.

Although these are certainly fine virtues, the reputation of dry ED ponds is tarnished by their mediocre nutrient removal and minimal runoff reduction capability. Indeed, it is doubtful whether dry ED ponds will be used as a stand-alone practice in the future, given the new environmental site design and runoff reduction requirements that are rolling out across the Bay.

The other thing about dry ED ponds is the seldom live up to their name — they frequently get quite soggy and hard to maintain and mow. Over time, they evolve into unintended semi-wetlands, willow thickets or cattail marshes. The frequent bounce in water elevations caused by extended detention favors invasive species that can withstand constant water fluctuation, and the repeated wet/dry cycles in the pond also promotes great mosquito breeding habitat of any stormwater practice (according to a half dozen public health studies).

With this in mind, the Network is inviting comment on a new draft ED pond design specification, the revised version of which can be accessed here. As always, there are a number of design issues worth arguing about:

  • For example, the new spec is clear that ED ponds are only the last element in the roof to stream design sequence, after all opportunities for upland runoff have been exhausted. This generally means that ED ponds will generally only be used to provide the last remaining channel protection storage at most sites (and not as the primary method to meet water quality volume requirements).
  • An unintended consequence of having smaller ED ponds has been raised by Lise Soukup of the City of Rockville . Quite simply, as the volume of ED is diminished by upstream runoff reduction practices, the diameter of the orifice needed to provide the required detention time becomes vanishingly small. Small orifices are especially prone to chronic clogging. So, if any designers have found an effective way to protect one to two inch pipe diameters, I love to hear about it.
  • Another area the spec needs some work is updating the “kerplunk” design method used to determine the volume needed for detention time (see Appendix A of the spec). Surely, there is a more elegant design technique out there? Even if there isn’t, we still need a defensible and consistent method to properly deduct the upstream runoff reduction volume from the ED calculations (and possibly increase time of concentration values). Anyone care to tackle this tricky modeling issue?
  • The new spec also limits the maximum allowable vertical height for extended detention to 5 feet for dry and wet ED design variations, and 2 feet for ED wetlands. The basic premise of this change is to create a more attractive and diverse native plant community in the pond over time.
  • Other spec changes include increased flow path and gentler side-slope design requirements (which may be further increased based on some recent research by Dave Hirschman and Laurel Woodworth in the James River ).
  • The spec also indicates that dry ED ponds should be discouraged in karst and coastal plain terrain, as other practices, such as linear constructed wetlands, are superior alternatives.
  • Lastly, I have dozens of photos of gnarly, scraggly and generally ugly dry ED ponds that have been in the ground for a decade or more, but I am still searching for a well-aged, blue ribbon ED pond that is really good looking. If you have one, please send it my way so I can use it as a cover shot for the spec.

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