Vegetated swales during
a wet weather event.
Source: LID Center
(click on thumbnail
for enlarged view)
Afforestation involves planting trees in an area where
they were absent for a significant period of time
(e.g. an old farm field or a riparian buffer).
Reforestation is the planting of trees in an area that
was forested in the recent past (e.g. an area that was
cleared for residential development). Plantings may be
seeds, seedlings, or semi-mature trees. Trees reduce
runoff volume through evapotranspiration (ET) and
interception and improve the infiltration capacity of
the soil, thereby reducing runoff potential.
In urban areas, downspouts are commonly directly
connected to the sewer system. The cumulative effect
of thousands of connected downspouts can greatly
increase the volume of stormwater entering the sewer
system. Downspout disconnection is the process of
separating roof downspouts from the sewer system and
redirecting roof runoff onto pervious surfaces, most
commonly a lawn. This simple act reduces the amount of
directly connected impervious area in a drainage area.
Filter strips are bands of densely vegetative slopes,
designed to increase water quality entering a specific
BMP. Filter strips increase water quality between a
runoff source (i.e., impervious area) and another BMP,
by filtration though the vegetated strip and
encouraging infiltration. Filter strips are important
components of a BMP treatment train.
Green roofs are designed to support plants and
mitigate the effects of urbanization on water quality
by filtering, absorbing, and detaining rainfall. There
are two basic types of green roofs: extensive and
intensive. Extensive roofs form a thin vegetated
sheath. Their low profile allows them to be added to
existing buildings. By contrast, intensive roofs are
integral to the roof structure, permitting the use of
trees and walkways. A greater depth of media and a
greater roof structural capacity may be required to
accommodate larger vegetation and surface features.
Infiltration practices include designs that enhance
water percolation through a media matrix that slows
and partially holds stormwater runoff. Infiltration
practices also promote groundwater recharge and
facilitate pollutant removal.
Pocket wetlands are constructed wetland systems
designed to control stormwater volume and facilitate
pollutant removal. Pocket wetlands generally have
less biodiversity than natural wetlands, but still
require a base flow through the wetland to support the
aquatic vegetation present. Pollutant removal in these
systems occurs through the settling of larger solids
and coarse organic material and also by uptake in the
Porous pavement includes pavers, asphalt, and concrete
that allow stormwater to pass through voids in the
surface and infiltrate into the subbase. The subbase
provides storage for stormwater. In unlined
systems, infiltration into the underlying soil may
also be possible.
Rain barrels are placed outside of a building at roof
downspouts to collect and store rooftop runoff, which
can later be reused for lawn and garden watering.
Rain gardens, also known as bioretention cells, are
vegetated depressions that store and infiltrate
runoff. Rain gardens are designed to encourage
vegetative uptake of stormwater to reduce runoff
volume and pollutant concentrations. A well design
rain garden has an engineered soil which maximizes
infiltration and pollutant removal while avoiding
stormwater ponding for longer than 24 hours.
Soil amendments make the soil more suitable for the
growth of plants and increase soil water retention
capabilities through the use of both soil conditioners
and fertilizers. Soil amendments also increase
pollutant removal and promote groundwater recharge.
Tree Box Filters.
Tree box filters are in-ground containers typically
filled with bioretention type soil media containing
street trees in urban areas. Runoff is directed to the
tree box, where it is filtered by vegetation and soil
before entering a catch basin.
Tree box filters enhance pollutant removal and are
ideal for ultra urban settings and spaces where rain
gardens are not practicable.
Natural and reintroduced vegetation provides
stormwater management and pollutant removal. Vegetated
areas intercept and infiltrate rainfall, decreasing stormwater volumes. Plants, trees, and other
vegetation remove pollutants from infiltrated
stormwater through root zone uptake. Incorporating
vegetation into the landscape is a stormwater
management technique that utilizes environmentally
beneficial mechanisms naturally occurring in the
Vegetated swales are broad, shallow channels designed
to convey and infiltrate stormwater runoff. The design
of swales seeks to reduce stormwater volume through
infiltration, improve water quality through
infiltration and vegetative filtering, and reduce
runoff velocity by increasing flow path lengths and