Ecological Detention Basin Design
Detention basin design needs refreshing. Let’s move beyond the mown-turf design. We’re tired of looking at these…
Instead, lets design our detention basins to function as rich habitat for butterflies, amphibians, dragonflies, birds, and interesting plant species.
Below are some simple techniques that engineers and architects can employ to create real functioning habitat without increasing a project’s budget.
Create Different Hydrologic Zones
Flat-bottomed and heavily sloped detention basins that drain rapidly lack sufficient variability to create different hydrologic zones (wetter areas and drier areas) in patterns that mimic a diverse natural system. Different zones are easy to build including distinct ponding areas, connector streams, and bumpy bottoms. The relative proportion and extent of the zone variability depend on the type of soils, rainfall patterns, and regional native plant options. In the two pictures below, we implemented all three concepts. Note the large forebay pond, a smaller secondary forebay, a meandering connector stream, and variable elevations in the rest of the basin to create distinct wetter and drier areas. This approach allows us to interplant dry species, mesic species, marginal species, and submerged species, all in the same project.
Use Open Cell Revetments
The best thing about riprap is that its simple and cheap. The worst thing about riprap is that its simple and cheap. Instead, consider using one of many available brands of open celled revetment products. These provide the structural integrity required for outflow structures while also further creating a unique habitat element within in their cells, which can be easily mown. See the picture below.
Use Interesting and Appropriate Native Plants
The original specification for this case study project called for just one commercial riverbank rye-dominated seed mix, 50 plugs each of yellow flag iris and western scouring rush, and over 100 buffer trees including siberian spruce, kwanzan cherry, red maple, and arborvitae. The issues with this original specification should be obvious – boring, insufficient, inappropriate, and partly invasive. Specifying a functioning ecology in a detention basin design might require some outside assistance for many engineering and architecture firms, but this is becoming more readily available. For this project, the plant list and quantities were completely overhauled to include thousands of plugs, multiple custom seed mixes, and native tree species chosen to match the various microsite conditions, which ranged from dry gravelly soils on the core of the detention basin to wetland areas adjoining the basin. When laying out trees or shrubs in a design, trying using a randomized pattern instead of arranging them like soldiers; the varying densities create additional habitat diversity.
Use Open-Pollinated, Local-Ecotype (OPLE) Seeds and Plants
There was a time in our recent past when it was difficult to find native seed and plant vendors. Large super-regional suppliers emerged in response and they played a critical role in developing the native plant nursery industry. However, professionals are rapidly learning that these large suppliers sacrificed genetics for optimum production qualities, much like tomato and strawberry producers. Researchers and restoration experts are aghast that one of the major east coast native plant plug producers is shipping Asclepias tuberosa (butterfly milkweed) all over the country whose genetic origin is southern California (this is where they buy their seed). The plant geneticists know that each EPA EcoRegion essentially has its own unique gene pool for this species and that mixing genes results in reduced fitness in indigenous populations. Some policy makers are concerned that in our rush to use native plants, we could inadvertently be on a path to cause a net decline in native plant population due to unnatural genetic dispersal. The newest thinking is to use local seed and plant suppliers that have access to or are already growing stock from genetics collected within the EcoRegion that a project is being built. Click here for an EcoRegion map. For this project, the custom seed mixes were comprised of over 100 total species, all hand-collected from within EcoRegion 064a, and some were even collected on the project site as shown in the ironweed and aster pictures below. To adopt this practice on your project, find a local native plant nursery that can collect local seed and/or custom-grow plugs from local seed.
Tree Protection and Staking
Using 4″ corrugated drain pipe and 5′ wooden stakes to protect and stake a newly planted tree are commonplace, but there are issues with these practices. Corrugated pipe is often installed in its natural black color, which can overheat the bark of newly planted trees. The corrugated pipe can also hold too much moisture around the bark and root crown, causing problems with some species. Wooden stakes prevent new trees from flexing during wind and snow storms, which slows root development and makes them more susceptible to breakage as they grow older. Some specifications now instruct the landscaper to stake a tree loosely so that it can sway a little bit, but the benefit is only a little bit. To create a healthier tree for the long-term, instead of using plastic pipe, specify the use of a permanent deet repellant. The method shown in the picture below is nearly 100% effective against deer damage and avoids the issues related to corrugated pipe. To create a stronger tree for the long-term, use flexible stakes that allow trees to bend up to 60 degrees from vertical under wind and snow loads, but spring back into place later. This greatly strengthens tree roots and trucks. See the pictures below.
For more information about these techniques or for help in designing your next stormwater feature, call ArcheWild toll-free at 855-752-6862 to set a phone appointment.