Low Impact Development (LID) is an innovative stormwater management approach
with a basic principle that is modeled after nature: manage rainfall at the
source using uniformly distributed decentralized micro-scale controls. LID's
goal is to mimic a site's predevelopment hydrology by using design techniques
that infiltrate, filter, store, evaporate, and detain runoff close to its
source. Techniques are based on the premise that stormwater management should
not be seen as stormwater disposal. Instead of conveying and managing / treating
stormwater in large, costly end-of-pipe facilities located at the bottom of
drainage areas, LID addresses stormwater through small, cost-effective landscape
features located at the lot level. These landscape features, known as Integrated
Management Practices (IMPs), are the building blocks of LID. Almost all
components of the urban environment have the potential to serve as an IMP. This
includes not only open space, but also rooftops, streetscapes, parking lots,
sidewalks, and medians. LID is a versatile approach that can be applied equally
well to new development, urban retrofits, and redevelopment / revitalization
projects.
Development of LID principles began with the introduction of bioretention
technology in Prince George's County, Maryland, in the mid-1980s. LID was
pioneered to help Prince George’s County address the growing economic and
environmental limitations of conventional stormwater management practices. LID
allows for greater development potential with less environmental impacts through
the use of smarter designs and advanced technologies that achieve a better
balance between conservation, growth, ecosystem protection, and public health /
quality of life. Today, bioretention is just one of the LID techniques available
to users. Other techniques, such as permeable pavers, tree box planters, and
disconnected downspouts, will all be presented here on the LID Design Site to
help users control pollutants, reduce runoff volume, manage runoff timing, and
address a number of other ecological concerns.
LID has numerous benefits and advantages over conventional stormwater management
approaches. In short, it is a more environmentally sound technology and a more
economically sustainable approach to addressing the adverse impacts of
urbanization. By managing runoff close to its source though intelligent site
design, LID can enhance the local environment, protect public health, and improve
community livability - all while saving developers and local governments money.
The need for such an approach has never been greater. Stormwater programs require
that a wide array of complex and challenging ecosystem and human health protection
goals be addressed. Many of these goals are not being met by conventional
stormwater management technology, and communities are struggling with the
economic reality of funding aging and ever-expanding stormwater infrastructure.
The challenge of how to restore stream quality in watersheds that have already
been densely developed is even more daunting. Simply relying on impervious
reduction and/or conventional detention ponds to address these issues is not
feasible, practical or sustainable. LID provides the key in its emphasis on
controlling or at least minimizing the changes to the local hydrologic cycle or
regime.
As a regulator, you can use LID to address a wide range of Wet Weather
Flow issues, including Combined Sewer Overflows (CSOs), National Pollutant
Discharge Elimination System (NPDES) Stormwater Phase II permits, Total Maximum
Daily Load (TMDL) permits, Nonpoint Source Program goals, and other Water Quality
Standards. Local permitting agencies can use LID as a model in revising
local zoning and subdivision regulations in favor of more cost-effective,
ecologically sound development practices. Developers can achieve greater
project success and cost savings through the intelligent use of LID, and
designers can apply these techniques for innovative, educational, and
more aesthetically pleasing sites.
A common concern is that LID-based projects will be more expensive because they
could require higher design and construction costs and a longer time to receive
project approval. This may or may not be true, depending on the experience of
the project consultants and contractors with these new techniques and the
receptiveness of local government officials to innovative practices. These
potential cost increases are not indictments of the concept of LID but of
inexperienced institutions, individuals, and bureaucracies that remain unaware of
the great necessity for and benefits of a new approach. This is changing!
For example, several years ago there were only a few permeable paver options
available. Today, the consumer can choose from a large number of these
innovative materials, and more widespread usage and acceptance of the technology
has led to lower costs.
Additional LID cost concerns include the potential for greater expenses due to
the increased use of on-site landscaping material. Despite these issues,
experience has shown that LID still saves money over conventional approaches
through reduced infrastructure and site preparation work. Case studies and pilot
programs show at least a 25 to 30% reduction in costs associated with site
development, stormwater fees, and maintenance for residential developments that
use LID techniques. This savings is achieved by reductions in clearing, grading,
pipes, ponds, inlets, curbs and paving. Far outweighing any of the cost increases
due to the use of LID, these infrastructure reduction savings enable builders to
add value-enhancing features to the property, to be more flexible and
competitive in pricing their products, or even to recover more developable space
since there is no need to waste land for a stormwater pond.
Costs are very site specific. Each project will be unique based on the site's
soil conditions, topography, existing vegetation, land availability etc. Here,
on the LID Design site, fairly exact cost estimates are given for some of the
individual techniques. Keep in mind, however, that the actual costs will vary
greatly based on the character of the individual site and the creativity of the
designer! Some commonly seen cost benefits of LID projects include:
Multifunctionality - In many projects, the LID practice was
originally designed as a landscaped feature before its functionality as a
stormwater control was introduced. In these situations, the landscaping and
construction costs for stormwater are essentially free. Additionally, the
cost of maintaining the landscaped areas was always expected for the project,
so one of the only major additional costs for stormwater maintenance is to
ensure that drainage areas are kept clear.
Lower lifetime costs - In any cost analysis, be sure to take into
account not just the initial capital costs but also those over the
structure's lifetime, which can include operation, repair, maintenance, and
decommissioning. Many LID techniques are self-perpetuating, easily
repairable, or can be left as natural areas at the end of their functional
lifetime, while conventional facilities may require high costs to take out
of commission and leave the area safe.
Additional environmental and social benefits - At the heart of LID are
the multiple benefits it provides, all of which are not readily measurable in
terms of cash. Not only do the techniques provide stormwater benefits, such
as groundwater recharge and cleaner streams, but they also increase the
urban forest, reduce the urban heat island, improve air quality, reduce
thermal stream pollution, enhance the appearance of a community, provide a
stronger sense of place, etc.
Reduced off-site costs - Since LID addresses stormwater at its source,
it is unlikely to incur major off-site costs in the form of sewers or
outfalls. Most conventional techniques will require an off-site sewer to
collect the stormwater from the on-site system, resulting in additional
project costs for the enhancement of downstream sewers as urban areas
expand.
Functional use of open space land - LID practices such as rain gardens
can usually be designed as part of the development's open space, without any
loss of developable area. Unlike large detention ponds, if these
multifunctional LID practices are distributed throughout set-aside open
space or previously designated landscaped land, they can contribute to a
more park-like and community-friendly setting without incurring costs for
land allocation to the drainage system.
Costs are relative - Cost considerations vary based on the user and
the project. For example, if a yard is retrofit to replace 1/2 of its area
with an LID infiltration practice such as a native vegetation rain garden,
does the homeowner perceive this as a loss of the use of the yard or a
benefit in the fact that there is now less lawn to maintain?
Opponents of the residential use of LID have tried to simplify the approach by
characterizing it as only relying on rain gardens and rain barrels that will not
be maintained by the property owner. LID is much more than this. It is a
comprehensive multi-systems approach that has built-in redundancy, which greatly
reduces the possibility of failure. Many LID techniques have nothing to do with
nor can they be significantly influenced by the behavior of the property owner.
These include basic subdivision and infrastructure design features such as
reducing the use of pipes, ponds, curbs and gutters; maintaining recharge areas,
buffer zones, and drainage courses; using infiltration swales, grading
strategies, and open drainage systems; reducing impervious surfaces and
disconnecting those that must be used; and conserving open space. LID’s
long-term success has much more to do with the knowledge, skills, and creativity
of the site designers than what the property owner does or doesn’t do.
Maintenance agreements can be used if a developer is uncomfortable about on-site
landscaping features that also serve as stormwater controls. However, the key
factor in the success of LID is to ensure that the landscape practices (such as
rain gardens) are attractive and perceived by the property owner as adding value
to the property. If these LID practices are viewed as assets, the primary
motivation for their long-term maintenance is that of property owners protecting
their vested economic interests. Additionally, experience has shown that
educational efforts can successfully promote active public engagement in
protecting our waters by the simple act of people maintaining their properties.
In actuality, LID site source controls reduce maintenance burdens for property
owners and local governments. The techniques are simple, need no special
equipment or high costs to maintain, and encourage property owners to be
responsible for the impacts associated with their land.
Traditionally, stormwater management systems have been designed to function well
under a single design condition, e.g. the 100 year flood, the 10 year storm, etc.
Designing control systems for a single extreme event does not mean that they will
perform adequately under other scenarios. For example, designing major floodways
for the 100 year event overdrains the system during more frequent storms,
degrades the natural stream system, and causes downstream water quality problems
by rapidly transporting pollutants through the urban area and into the receiving
waters.1 Flow control standards, which have their origin in ensuring
public safety and reducing property damage, have very little to do with ecosystem
protection. For preserving stream integrity, experience has demonstrated the
importance of a stormwater system that specifically addresses the frequent or
micro-storms that occur on a regular basis (weekly or monthly). By using
decentralized site-based source controls, LID uses the stormwater from these more
frequent events as a resource and is an effective ecosystem approach.
Additionally, if the full suite of LID controls and site design practices is
creatively used, LID is capable of automatically controlling the 10 and 100-year
storms through its primary strategy of restoring the built area's natural
rainfall-runoff relationship. The more techniques that are applied, the closer
to natural hydrologic function one gets. Where there are known flooding problems,
however, a hybrid approach may be needed to reduce liability and provide a sense
of safety. LID does not demand isolation from conventional technology. The LID
national design manual recommends hybrid systems if site constraints warrant it.
Although the term "low impact development" can be loosely defined (much
like sustainable development), the appropriate definition of LID is distinct and
should not be confused with other stormwater management and
development strategies. The key
distinction of LID from these other strategies is that it is an ecosystem based
approach. LID seeks to design the built environment to remain a functioning part of
an ecosystem rather than exist apart from it. The approach relies more heavily on
smarter and advanced technologies than it does on conservation and growth
management; it is not a land use control strategy. LID provides technological tools
to plan and engineer any type of urban site to maintain or restore a watershed’s
hydrologic and ecological functions. It does not sacrifice the environmental quality
of dense urban watersheds for greater protection of conservation areas. Growth
management strategies, such as Smart Growth, that emphasize the saving of green
space and the redevelopment of existing urban regions, can utilize this retrofit
capability of LID in order to promote ecologically-restorative infill and brownfields
development in impaired stream areas. In addition, the full LID process starts with
many of the same conservation and impact minimization principles inherent in other
strategies. The LID approach includes five basic tools:
encourage conservation measures
promote impact minimization techniques such as impervious surface reduction
provide for strategic runoff timing by slowing flow using the landscape
use an array of integrated management practices to reduce and cleanse runoff
advocate pollution prevention measures to reduce the introduction of pollutants
to the environment
The initial site planning stages of an LID design can incorporate components of
other popular strategies, such as Conservation Design and Better Site Design. LID's
distributed micro-scale practices can then be applied to create a hydrologically
functional landscape.
LID is simple and effective. Instead of large investments in complex and
costly centralized conveyance and treatment infrastructure, LID allows for the
integration of treatment and management measures into urban site features. This
involves strategic placement of distributed lot-level controls that can be
customized to more closely mimic a watershed’s hydrology and water quality regime.
The result is a hydrologically functional landscape that generates less surface
runoff, less pollution, less erosion, and less overall damage to lakes, streams,
and coastal waters.
LID is economical. It costs less than conventional stormwater management
systems to construct and maintain, in part, because of fewer pipes, fewer below-ground
infrastructure requirements, and less imperviousness. But the benefits do not stop
there. Space once dedicated to stormwater ponds can now be used for additional
development to increase lot yields or be left as is for conservation. The greater use
of on-lot multi-purpose landscaping / vegetation also offers human "quality of
life" opportunities by greening neighborhoods and contributing to livability,
value, sense of place, and aesthetics. Other benefits include enhanced property values
and re-development potential, greater marketability, improved wildlife habitat,
thermal pollution reduction, energy savings, smog reduction, enhanced wetlands
protection, and decreased flooding.
LID is flexible. It offers a wide variety of structural and nonstructural
techniques to provide for both runoff quality and quantity benefits. LID works in
highly urbanized constrained areas, as well as open regions and environmentally
sensitive sites. Opportunities to apply LID principles and practices are practically
infinite since any feature of the urban landscape can be modified to control runoff
and / or reduce the introduction of pollution. LID can be used to truly create a
"customized" watershed management design.
LID is a balanced approached. LID is an advanced, ecologically-based land
development technology that seeks to better integrate the built environment with
the natural environment. LID’s principles and practices allow the developed site
to maintain its predevelopment watershed and ecological functions.
1. The national LID Design Manual (Low Impact Development Design Strategies:
An Integrated Design Approach) and other LID information can be obtained from
one of the following sites:
Many of the ideas presented here came from an article written by Larry Coffman,
Associate Director of Programs and Planning Division for the Prince George's
County, Maryland, Department of Environmental Resources.
Coffman, L.S., 2001: Low Impact Development creating a storm of controversy.
Water Resources Impact, 3 (6), 7-9.
http://www.awra.org
1 Construction Industry Research and Information Association, 1995:
Infiltration drainage - appraisal of costs. Project Report 24. (London)