By: Jim Tolstrup
When developers build rooftops, parking lots, and other impermeable surfaces, rain, and snowmelt can no longer percolate into the ground. The resulting stormwater runoff must be managed to prevent flooding. It once was common practice to collect stormwater in concrete pits and dump it into nearby rivers and streams by piping it underground. In some cities, entire rivers have been channelized and sometimes buried. Many of these rivers have been restored in recent years after people realized the ecological harm that this practice caused, as well as the recreational, aesthetic, and educational benefits that may have been previously overlooked.
Over the last few decades, the standard for stormwater engineering has moved toward creating water-quality ponds that slow down the turbulence of the water and allow sediment to drop out. Ponds and conveyances that include vegetation can also remove nutrients the stormwater may have picked up in its flow. These nutrients come from natural deposition, landscape fertilizer, pet waste, soap used for washing cars, etc. Nutrients that reach ponds and lakes encourage the growth of algae, which can
reduce oxygen levels in the water, killing fish and other aquatic life.
Here at the High Plains Environmental Center, within the Lakes at Centerra neighborhood, we have far less algae than typical lakes within a residential development. The unmown native vegetation (shrubs, grasses, and wetland plants) in the setbacks
and stormwater conveyances around us function as a biological filter, helping to sequester nutrient run-off from adjacent developed sites. The proof of our strategy’s environmental success is measured and proven in periodic water quality testing.
Well-constructed, and well-managed, stormwater ponds can be aesthetically pleasing amenities that benefit wildlife while offering educational and recreational opportunities for residents and visitors
Stormwater ponds, though necessary, often are viewed by developers as a liability. They can be unsightly when not well-designed, fill up with trash carried along by storm flows, and take up space that would otherwise be buildable. Well-constructed, and well-managed, stormwater ponds, on the other hand, can be aesthetically pleasing amenities that benefit wildlife in the built environment while offering educational and recreational opportunities for residents and visitors. An artfully designed pond or natural area can allow residents access to nature while protecting wildlife from excessive disturbance. Although it can be hard to resist the temptation to put paths all around a pond, or throughout an open space, the benefits of limiting wildlife disturbance are well worth the effort. Providing a cover of vegetation around ponds can increase nesting habitat for songbirds
and allow wildlife to retreat to a place of peace and safety when necessary.
Wetland
One of the most difficult things about vegetating stormwater ponds is predicting the amount of moisture the pond will ultimately hold. Wetland plants tend to grow in very specific hydrological zones, and choosing the appropriate species can be challenging. We have addressed these issues by using diverse seed mixes that include various grasses, sedges, and rushes. Using this “shotgun” approach helps to ensure that there will be sufficient plant diversity. In the beginning, when the pond is first excavated, it may be drier. Over time, as the area around the pond gets built and runoff increases, the pond will become wetter, and plants will distribute themselves in accordance with the changing hydrological regime. In general, cattails, while native, should be discouraged in wetlands and stormwater channels. Although cattails provide filtration and cover for wildlife, they are aggressive and can reduce
habitat quality by creating a monoculture. Our goal should be to create a diverse wetland plant community.
When designing a pond, it is desirable to create an undulating shoreline and varying topography on the pond’s bottom. Shallow, flat-bottomed ponds should drain completely, or they can become a breeding ground for mosquitoes. Ponds that include plunge pools 5 feet deep or deeper can hold some water year-round and allow minnows to survive over winter. Holding back any amount of water in a stormwater pond may require water rights to offset the evaporative loss. People may be concerned about mosquitoes in deeper water, but mosquitoes can breed in an inch of water in concrete drains. Natural predators like minnows, tadpoles, and dragonfly larvae also will be present in the deeper areas of these ponds and can help control mosquito larvae.
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