Constructed Wetland Design Specification

constructed-wetland

Constructed Wetland Design Specification

More than 15 years ago, I published Design of Stormwater Wetlands, which became the foundation for most of the constructed wetland design specifications in the Bay states, and elsewhere in the nation, for that matter. The basic design of constructed wetlands has hardly changed since then, despite the fact that we have learned an enormous amount about what makes them work (or not work). We now have hundreds of good and bad wetland examples in the ground, and have a much better idea of the long-term trajectory of their wetland plant community.

So, after consulting with my colleagues at CWP and NCSU, CSN has taken a long, hard look at the ancient design, and has come up with a real wetland makeover. In addition to the Constructed Wetland Design Specification, some of the new wetland design concepts are illustrated in a slideshow.

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Constructed Wetland
Constructed wetlands should appear as naturally occurring ecosystems.

So, here is the ten minute summary on the major changes proposed for constructed wetland design:

  • The Demise of the Low Marsh Zone. Low marsh was defined as the zone where water depths were from six to 18 inches below the normal pool. Experience has shown that few wetland plants can colonize this “tweener” water depth zone. Plant coverage in the low marsh zone was always quite sparse due to the deep water and frequent water level fluctuations that thin out sensitive wetland species. The new design emphasizes deeper pools for nutrient removal, and vigorous wetland growth within the high marsh zone set at – 6 to + 6 inches, relative to the normal pool.
  • Constructed wetland on gold course
    Variety of plants and terrain features allow for a more successful wetland.

    Emergent vs. Mixed/Wooded Plant Community. The old paradigm was to create an emergent wetland plant community. The habitat quality created by these emergent wetlands, however, has been marginal. The new design approach emphasizes the creation of a more mixed wetland community that includes a combination of deep pools, emergent plants, and water-loving trees and shrubs. The basic idea is for the wetland to evolve over time into a forest with some open water areas.

  • The End of Extended Detention (ED)? Research has shown that the frequent bounces in water levels caused by temporary extended detention (ED) are the enemy of biological diversity in both natural and constructed wetlands. So, the old ED wetland design option has been dropped, and major limits on the vertical depth of ED are proposed.
  • The Pocket Wetland Sent Packing. This one-cell “micro” wetland was created to treat runoff from small drainage areas, but truth be told, it was not one of my better ideas. Most of the ones I have seen in the field are little McWetlands that are either overgrown “chia pets”, a public nuisance, or both. Submerged gravel or small upflow wetlands appear to show some promise, but need more testing.
  • Mosquito Bites. Back in 1992, West Nile Virus had not yet arrived on our radar screen, and residents and public health authorities were more accepting of the occasional mosquito. The Nile changed everything. Perceptions about mosquito breeding, often unfounded, sharply reduced the number of constructed wetlands built over the last decade. The new design has numerous design features to reduce the potential for mosquito breeding, and increase habitat for their predators.
  • Mandating micro topography. The original wetland design stressed the importance of a range of depth zones and internal microtopography, but it was a “should” and not a “shall”. Consequently, most stomwater wetlands built were flat as a billiard table, and to no one’s surprise, contained a mono-culture of invasive species. The new spec mandates that at least two forms of microtopography structure be included in every design. Low cost forms of micro-topography include snags, inverted rootwads, tree peninsulas, coir fiber island, internal pools and cobble sand weirs, to name a few.
  • Natural Geometry. The other thing you notice about the first generation of stormwater wetlands is that they all look like bathtubs – with steep grass side slopes descending into a shallow basin. The new approach is to design a much more linear system, with multiple cells, gentle side slopes and a dry weather flow path at least 2:1. Indeed, one of the best new designs to come on the scene is the Regenerative Stormwater Conveyance (RSC) system. The RSC is used in ravines and zero-order streams, and employs an alternating sequence of weirs and pools. Design specs and calculation methods for the RCS (also known as coastal plain outfalls) first developed by Underwood and Associates are included in the new spec, and can be accessed here.
  • Water Balance: The original wetland specs made a big deal about minimum drainage area, constant water elevations and minimum inflow rates to achieve a good water balance. These rules have been real confusing for designers, and in some cases, limited more widespread wetland application. The new spec argues that many of these arbitrary rules are no longer needed, and replaces them with an alternative water balance equation. The simple equation seeks to maintain some water in the deepest pools after a one month rain-free period. Wetlands are pretty tough; it’s OK to dry them out once in a while.
  • Seeing the Forest Through the Wetland: The new spec contains a more reliable and somewhat shorter list of emergent wetland, shrub and tree species that work well in most environments in the Bay watershed. Again, the big shift is to move toward a mixed forested wetland over time. Experience has shown that the initial emergent wetland community does not persist over time and is often invaded by less desirable plant species. Think about it for a minute: two thirds of natural wetlands in our region are of the forested wetland variety. It makes more sense to guide the wetland community to evolve into a natural forest.

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