Create a Water-holding Structure with Good Circulation
Bioretention ponds are similar to bogs. They can hold deep, standing water, so safety is a key issue. Common problems with retention basins is that they can be undersized, constructed incorrectly, or do not drain properly, resulting in over- flows. Slight over design is a small cost compared to basin failure. Once constructed, bioretention ponds can be planted with bog-type plants, ranging from those that tolerate constant standing water, to those that can handle dry soils.
Always make sure your soils percolate and, if needed, use an under-drain. Retention basins will have spillways with overflow drains and piping sized to handle the flow. You need to provide details for your basin, with existing and proposed contours. A lot of designers would like to do one big bio-retention area, but a treatment trains work a lot better. Try to work in grassy swales and level spreaders to slow water and to remove sediments before they reach the basin. Slow the flow of water and clean as you go.
You, of course, wouldn’t want trees around a basin, since the tree root systems can damage the structure as they grow, but stormwater retention basin buffers need to be planted with vegetation in order to stabilize the ground surface. Use a stabilizing structure on the slopes until the plants take root, though. A large-opening jute mesh blanket will allow the seeds contact with the soil.
For road projects with basins, you are faced with a very confined site and challenging soils. Bioretention basins are often very close to the road, and probably in the clear zone, so the basin will be in an area where trees also cannot be planted. So, the first rule for planting bioretention ponds is—no trees. This keeps the clear zone free of fixed objects and help avoid damage to the basin embankments.
Bioretention areas are typically more inundated or polluted than you might find in a stable natural system. The plants closer to the water’s edge will have wet feet at times. There should be no woody plant material on or near a bioretention basin embankment. Use only native shrub or grass species, with plants more adaptable to dryer conditions around the top half of the basin and plants more adapted to occasional wet feet around the bottom half of the basin—just a couple of species for each zone. Plant the entire basin vegetation area in a native, riparian seed mix to the water line. It is best to go with smaller plantings—plugs. Specify a fairly close spacing of less than three feet on center. Try to fill the system with vegetation, so that it will grow in rather quickly. Using overly-vigorous plant material can cause maintenance issues. Don’t try growing native flag iris and invasive cattails! They will clog the drainage flow.
Stormwater retention basins require maintenance. This is a surprise to some designers. Without hand-weeding, any area not mowed has the potential to grow trees and become a weedy mess. A basin located within a commercial apartment community, with professional horticulturalists on staff, can be planted quite differently from a neglected roadside basin. In an ornamental setting, wetland flowering plants and wildlife attractors can be used. For a basic, service basin with little to no maintenance, keep things simple and anticipate mowing once or twice a year. Retention basins are not self-sustaining ecosystems. Consider the maintenance issues involved as you design.
For initial ground coverage, stay away from ornamental mulches. Stick with clean straw, lightly spread to hold moisture and allow sunlight to reach a native/riparian seed mix for germination. Ornamental bedding mulch within the basin either floats or forms a decomposing, waterlogged mat that inhibits the water from infiltrating. Yuck! Use vigorous competition from native plants for weed suppression, rather than heavy organic mulch. Stone mulch will cause the same maintenance issues as rip rap—requiring chemical control of weeds instead of cyclical mowing. Bioretention basins need free flow of excess water through drains and overflows and ground surfaces that are easily maintained.