Stormwater basins are structures built to hold stormwater runoff to prevent flooding and erosion downstream. The two most common types are detention basins, which temporarily store stormwater before releasing it slowly, and retention basins, which have a permanent pool of water to treat stormwater as it drains in. Native landscaping around basins can improve water quality, provide wildlife habitat, and enhance aesthetics. Proper siting and design of basins is important for effective stormwater management.

1. Often unnoticed, but very important to a development site's environmental integrity, stormwater
basins are impoundments built to hold stormwater and prevent high volumes from flooding or
eroding downstream areas. This is one of the most common conventional methods of managing
stormwater.
Native landscaping in and around the basin can help with nutrient reduction, water quality
improvement, wildlife habitat and esthetics.
Location
The location of a basin is very important, as it can make the difference between an effective or
an ineffective stormwater management tool. Ideally, a basin would be placed close to the source
of pollution so that it can slow down and filter the stormwater. This will help reduce downstream
flooding and sedimentation and provide a natural habitat for wildlife. Taking advantage of
existing topography, for example in low lying areas near to riverine flooding problems, can also
be an excellent place to site a basin.
There are a few different kinds of basins, though the two most commonly seen are detention
and retention. A detention basin (DB) is designed to store water during storm events and then
release it at a controlled rate into the surrounding stream, reducing downstream flood risk. DBs
typically remain dry between storm events, and are often located at the outfall point of a
development.
A wet detention basin or wet retention pond, on the other hand, has a permanent pool of water
in it and is usually designed to treat stormwater as it drains into the pond. Wet basins can be
used to reduce stream erosion, control sedimentation pollution and serve as artificial wetlands.
In general, wet basins are better equipped to remove nutrients such as nitrogen and
phosphorus from the water than dry basins. However, if the basin is not part of a treatment
system it will likely be ineffective at removing these pollutants.
Design
A common conventional method of managing stormwater runoff is through detention basins.
These basins are used to control the speed of water leaving a new development, preventing
downstream flooding and erosion. Although these basins are effective for flood control, they do
little to treat or infiltrate stormwater.
In recent years, landscape architects have begun to explore alternative detention basin designs
that improve water quality and reduce volume by naturalizing the basins through plantings.
These practices work in conjunction with, or in some cases replace the need for large storm
water structures such as detention basins.

2. Most traditional detention basins are drained through a structure called an outlet structure. The
outlet structure is usually a concrete low flow channel with one or more orifices along the edge
of the basin. These orifices are controlled by a grate, which is regulated to prevent clogging. An
emergency spillway is also located in the outlet structure to prevent the entire basin from
overflowing during a large storm.
The outlet structure is lined with an energy dissipating material such as riprap or a layer of
coarse gravel. The basin is then planted with a mix of native grasses and flowers. These
species are selected because they are drought tolerant, have shallow root systems, and provide
good ground cover. They also help to minimize mowing and chemical fertilizer use, which is
important since excess fertilizers and herbicides pollute the stormwater and local streams.
Planting
When stormwater basins are planted with native plants and maintained properly, they can
provide many environmental benefits. They can improve water quality through infiltration (water
soaks into the ground), reducing total runoff and pollutants, and can help restore and protect
soils that have been lost due to development. They can also provide habitat for wildlife, and they
can be aesthetically pleasing in residential settings.
Basins are most commonly designed with turf grass, but some cities and counties are beginning
to use smaller design practices like rain gardens, which can reduce the need for larger basins,
as well as a variety of other green infrastructure techniques. Many communities and developers
are installing wetlands and riparian buffers to protect their homes and neighborhoods from
flooding and provide water quality benefits.
Wet detention and retention basins can have the added benefit of attracting wildlife to the area
by providing habitat for birds, amphibians, insects and small animals. They may also be a
popular location for fishing, canoeing or kayaking.
Newly constructed wet basins are often ready to plant, but it is more difficult to re-vegetate
existing ones. If re-vegetating an existing basin, it is important to take into account the
surrounding natural landscape and to consider if the basin is located in a highly visible location
or a more secluded one. If the latter, esthetic consideration will play a bigger role in plant
selection.
Management
A stormwater basin is a man-made area that is designed to collect and manage stormwater
runoff after a rainfall event. Its purpose is to control and manage the water quality of stormwater,
the rate of flow and volume, as well as nutrient removal. It is commonly found in urban,
suburban and rural areas.

3. Unlike a natural pond, a stormwater basin only holds water temporarily, so the water level
quickly decreases after a rain event. During this period, the basin is improving the water quality
through natural processes such as sedimentation, decomposition and solar disinfection. This
process is called bioinfiltration.
The amount of treatment, or cleaning, that the water receives is limited. Dry detention basins,
which are commonly referred to as ponds, only control flood flows and provide some water
quality benefits by reducing sediments and attached pollutants. Wet detention basins, on the
other hand, are more effective at removing nutrients such as phosphorus and nitrogen.
Many communities are turning to smaller design practices such as rain gardens that work in
conjunction with, or in some cases can replace larger storm water basins. These small, simple
designs capture and infiltrate stormwater to reduce the need for oversized structures.
Maintaining the native vegetation in and around a stormwater basin is also important to its
function. By using locally adapted plants, the landscape manager can avoid high maintenance
costs such as mowing and lawn care chemicals. In addition, by replacing grass with wet
meadow and prairie plantings, the landscape professional can preserve, and potentially
improve, the basin’s ability to filter stormwater.