We draw on the following best practices, also called Integrated
Management Practices (IMPs), to design, implement, and evaluate
stormwater management efforts.
Bioretention cells
A bioretention cell or rain garden is a depressed area with porous
backfill (material used to refill an excavation) under a vegetated
surface. These areas often have an underdrain to encourage filtration
and infiltration, especially in clayey soils. Bioretention cells provide
groundwater recharge, pollutant removal, and runoff detention.
Bioretention cells are an effective solution in parking lots or urban
areas where green space is limited.
Curb and gutter elimination
Curbs and gutters transport flow as quickly as possible to a
stormwater drain without allowing for infiltration or pollutant removal.
Eliminating curbs and gutters can increase sheet flow and reduce
runoff volumes. Sheet flow, the form runoff takes when it is uniformly
dispersed across a surface, can be established and maintained in an
area that does not naturally concentrate flow, such as parking lots.
Maintaining sheet flow by eliminating curbs and gutters and directing
runoff into vegetated swales or bioretention basins helps to prevent
erosion and more closely replicate predevelopment hydraulic
conditions. A level spreader, which is an outlet designed to convert
concentrated runoff to sheet flow and disperse it uniformly across a
slope, may also be incorporated to prevent erosion.
Grassed swales
Grassed swales are shallow grass-covered hydraulic conveyance
channels that help to slow runoff and facilitate infiltration. The
suitability of grassed swales depends on land use, soil type, slope,
imperviousness of the contributing watershed, and dimensions and
slope of the grassed swale system. In general, grassed swales can be
used to manage runoff from drainage areas that are less than 4
hectares (10 acres) in size, with slopes no greater than 5 percent. Use
of natural, low-lying areas is encouraged and natural drainage courses
should be preserved and utilized.
Green parking design
parking refers to several techniques that, applied together,
reduce the contribution of parking lots to total impervious cover.
Green parking lot techniques include: setting maximums for the
number of parking lots created; minimizing the dimensions of parking
lot spaces; utilizing alternative pavers in overflow parking areas; using
bioretention areas to treat stormwater; encouraging shared parking;
and providing economic incentives for structured parking.
Infiltration trenches
Infiltration trenches are rock-filled ditches with no outlets. These
trenches collect runoff during a storm event and release it into the soil
by infiltration (the process through which stormwater runoff
penetrates into soil from the ground surface). Infiltration trenches may
be used in conjunction with another stormwater management device,
such as a grassed swale, to provide both water quality control and
peak flow attenuation. Runoff that contains high levels of sediments or
hydrocarbons (for example, oil and grease) that may clog the trench
are often pretreated with other techniques such as water quality inlets
(series of chambers that promote sedimentation of coarse materials
and separation of free oil from storm water), inlet protection devices,
grassed swales, and vegetated filter strips.
Inlet protection devices
Inlet protection devices, also known as hydrodynamic separators, are
flow-through structures with a settling or separation unit to remove
sediments, oil and grease, trash, and other stormwater pollutants. This
technology may be used as pre-treatment for other stormwater
management devices. Inlet protection devices are commonly used in
potential stormwater “hot spots”—areas where higher concentrations
of pollutants are more likely to occur, such as gas stations.
Permeable pavement
Permeable pavement is an alternative to asphalt or concrete surfaces
that allows stormwater to drain through the porous surface to a stone
reservoir underneath. The reservoir temporarily stores surface runoff
before infiltrating it into the subsoil. The appearance of the alternative
surface is often similar to asphalt or concrete, but it is manufactured
without fine materials and instead incorporates void spaces that allow
for storage and infiltration. Underdrains may also be used below the
stone reservoir if soil conditions are not conducive to complete
infiltration of runoff.
Permeable pavers
Permeable pavers promote groundwater recharge. Permeable
interlocking concrete pavements (PICP) are concrete block pavers that
create voids on the corners of the pavers (pictured to the right).
Concrete grid paver (CGP) systems are composed of concrete blocks
made porous by eliminating finer particles in the concrete which
creates voids inside the blocks; additionally, the blocks are arranged to
create voids between blocks. Plastic turf reinforcing grids (PTRG) are
plastic grids that add structural support to the topsoil and reduce
compaction to maintain permeability. Grass is encouraged to grow in
PTRG, so the roots will help improve permeability due to their root
channels.
Rain barrels and cisterns
Rain barrels and cisterns harvest rainwater for reuse. Rain barrels are
placed outside a building at roof downspouts to store rooftop runoff for
later reuse in lawn and garden watering. Cisterns store rainwater in
significantly larger volumes in manufactured tanks or underground
storage areas. Rainwater collected in cisterns may also be used in non-
potable water applications such as toilet flushing. Both cisterns and
rain barrels can be implemented without the use of pumping devices
by relying on gravity flow instead. Rain barrels and cisterns are low-
cost water conservation devices that reduce runoff volume and, for
very small storm events, delay and reduce the peak runoff flow rates.
Both rain barrels and cisterns can provide a source of chemically
untreated “soft water” for gardens and compost, free of most sediment
and dissolved salts.
Riparian buffers
A riparian, or forested, buffer is an area along a shoreline, wetland, or
stream where development is restricted or prohibited. The primary
function of aquatic buffers is to physically protect and separate a
stream, lake, or wetland from future disturbance or encroachment. If
properly designed, a buffer can provide stormwater management and
can act as a right-of-way during floods, sustaining the integrity of
stream ecosystems and habitats.
Sand and organic filters
Sand and organic filters direct stormwater runoff through a sand bed
to remove floatables, particulate metals, and pollutants. Sand and
organic filters provide water quality treatment, reducing sediment,
biochemical oxygen demand (BOD), and fecal coliform bacteria,
although dissolved metal and nutrient removal through sand filters is
often low. Sand and organic filters are typically used as a component
of a treatment train to remove pollution from stormwater before
discharge to receiving waters, to groundwater, or for collection and
reuse. Variations on the traditional surface sand filter (such as the
underground sand filter, perimeter sand filter, organic media filter, and
multi-chamber treatment train) can be made to fit sand filters into
more challenging design sites or to improve pollutant removal.
Soil amendments
Soil amendments increase the soil’s infiltration capacity and help
reduce runoff from the site. They have the added benefit of changing
physical, chemical, and biological characteristics so that the soils
become more effective at maintaining water quality. Soil amendments,
which include both soil conditioners and fertilizers, make the soil more
suitable for the growth of plants and increase water retention
capabilities. The use of soil amendments is conditional on their
compatibility with existing vegetation, particularly native plants.
Stormwater planters
Stormwater planters are small landscaped stormwater treatment
devices that can be placed above or below ground and can be
designed as infiltration or filtering practices. Stormwater planters use
soil infiltration and biogeochemical processes to decrease stormwater
quantity and improve water quality, similar to rain gardens and green
roofs but smaller in size—stormwater planters are typically a few
square feet of surface area compared to hundreds or thousands of
square feet for rain gardens and green roofs. Types of stormwater
planters include contained planters, infiltration planters, and flow-
through planters.
Tree box filters
Tree box filters are in-ground containers used to control runoff water
quality and provide some detention capacity. Often premanufactured,
tree box filters contain street trees, vegetation, and soil that help filter
runoff before it enters a catch basin or is released from the site. Tree
box filters can help meet a variety of stormwater management goals,
satisfy regulatory requirements for new development, protect and
restore streams, control combined sewer overflows (CSOs), retrofit
existing urban areas, and protect reservoir watersheds. The compact
size of tree box filters allows volume and water quality control to be
tailored to specific site characteristics. Tree box filters provide the
added value of aesthetics while making efficient use of available land
for stormwater management. Typical landscape plants (for example,
shrubs, ornamental grasses, trees and flowers) are an integral part of
the bioretention system. Ideally, plants should be selected that can
withstand alternating inundation and drought conditions and that do
not have invasive root systems, which may reduce the soil’s filtering
capacity.
Vegetated filter strips
Filter strips are bands of dense vegetation planted downstream of a
runoff source. The use of natural or engineered filter strips is limited to
gently sloping areas where vegetative cover can be established and
channelized flow is not likely to develop. Filter strips are well suited for
treating runoff from roads and highways, roof downspouts, very small
parking lots, and impervious surfaces. They are also ideal components
for the fringe of a stream buffer, or as pretreatment for a structural
practice.
Vegetated roofs
Green roofs consist of an impermeable roof membrane overlaid with a
lightweight planting mix with a high infiltration rate and vegetated with
plants tolerant of heat, drought, and periodic inundations. In addition
to reducing runoff volume and frequency and improving runoff water
quality, a green roof can reduce the effects of atmospheric pollution,
reduce energy costs, and create an attractive environment. They have
reduced replacement and maintenance costs and longer life cycles
compared to traditional roofs.
