The "Lowdown" on Low-Impact Development (LID) Landscapes
Author: Chad Kennedy, Landscape Architect, ASLA
If you turn on your television this fall you most likely can't avoid fast-paced imagery of popping pads and crashing helmets. Professional and college football season is in full force and on nearly every channel. If you were to hear the term "low-impact development" (LID) mentioned amidst this, your thoughts may automatically trend toward new product developments in helmets pads or other protective gear. Though not related at all, in reality, the term is just as important to the health of society as protective sports gear might be to an athlete. LID in fact, addresses a much larger proactive method of protecting our cities and populations from the dangers related to storm water pollution, wildlife habitat loss, and diminished recreational opportunities along river systems. LID may have nothing to do with sports, but it has everything to do with our health and innate need for healthy living conditions.
What is LID?
The driving force behind low-impact development is to develop or re-develop land using management techniques for stormwater that closely imitate natural functions, keeping water as close to the original source as possible. This approach to water management uniquely celebrates stormwater as a resource rather than as a nuisance, which has traditionally been the case. In the past, the objective has always been to move water as efficiently as possible off site, away from structures, and into rivers, avoiding standing water which impacts the flow of pedestrian and vehicular circulation. Unfortunately, this has led to myriad issues downstream and has fundamentally changed the hydrology within areas we live and within downstream communities and habitats.
LID vs. Traditional Stormwater Management
As urban areas develop, the amount of impervious pavement increases through the addition of parking lots, walkways, plazas, rooftops, etc. This results in storm and nuisance water that cannot infiltrate the soil, but is transported across these surfaces, picking up sediments and harmful pollutants along the way. Eventually, the water is captured by traditional stormwater design elements including curbs, gutters, catch basins, positive drainage pipes, detention and retention ponds, manholes, pumps and outfalls. All of these features add up to very expensive high volume routing of polluted water to another location. Along the way, this water erodes, percolates, and discharges into local river, streams, lakes, and oceans, eventually depositing its sediment and pollutant loads.
In contrast, low-impact development aims to limit the transportation of storm and nuisance water by establishing innovative on-site solutions that utilize water for landscapes, facility re-use, cooling, or other useful needs prior to and often in lieu of discharge into a larger stormwater utility network. LID systems help watersheds, large and small, mitigate impervious surfaces by providing opportunities for soil infiltration, evapotranspiration, organic cleansing, groundwater recharge, and slow release into the larger watershed, if any. Some of the most commonly used LID systems are rain gardens, green roofs, above/below ground water storage, permeable pavements, bio-retention basins, vegetated swales, soil amendments, and tree box filters. Each of these has benefits and drawbacks and should be researched thoroughly before implementation.
Some of the general site specific and community-wide benefits typically associated with LID systems include the following:3
- Reduced pavement, curbs, and gutters
- Eliminated detention basins and pipe systems
- Increased property values
- Improved aesthetics
- Expanded recreational opportunities
- Increased marketing and faster sales
- Reduced stream channel damage and pollutants
- Reduced drinking water treatment
LID & Soils
Just as soils can differ greatly across even small projects, there are many different options and applications for LID soils. An incredibly important soil characteristic, texture, is in large part what determines infiltration rates and water storage capacity. In general, the more clay content in the soil, the longer it will take to drain and the less void space there is for storage. The more sand content is in the soil, the faster water moves through the profile and higher storage capacity. What often occurs when attempting to maximize LID principles is a combination of layers throughout the soil profile that cater to the specific purpose of that phase of water treatment. An example (as shown to the right) would be the construction of an upper horticultural soil surface of loamy texture to aid with plant growth, some water retention and pollutant capture. The next layer is often a sand layer that captures fine particles before water enters a third layer of high pore space gravels meant primarily for water storage and/or percolation into the fourth soil profile layer, native soil. Because soil makeup is so important to the success of an LID program, it is absolutely crucial that professionals be consulted to determine appropriate design for each component of the system ensuring that each component is designed to work collectively achieving the desired goal.
LID & Landscapes
Though LID does tend to be considered a sustainable and green approach to development, it does not come with the coveted "no maintenance" tag. It is important to understand that some systems will require soils and landscape features be replaced over time as they become over-saturated with pollutants and sediments. Similar to the description about soils, each project and region will have differing requirements that affect plant selections. In arid regions, low water use plants that don't mind periodic flooding are ideal, while in areas with regular rain, water loving plants may be a more appropriate selection. Each project and site will need to be reviewed and analyzed for sun exposure, temperatures, soil pH and organic content, level and duration of precipitation, and local evapotranspiration rates. Plant characteristics of particular importance are water use classification, drought and flood tolerance, ultimate growth size, longevity, invasiveness, maintenance needs and phytoremediation capabilities. Plants with phytoremediation capabilities are able to break down pollutants into smaller components that can be utilized by microorganisms, creating a scenario where plants and soils do not need to be harvested. The following is a list of organic pollutants (hydrocarbons, solvents, and pesticides) that can be cleansed with plants: Oil, Gasoline, Benzene, Toluene, PAHs, Methyl Teritiary Butyl Ether, trichloroethylene, PERC, Atrazine, Diazinon, Metolachlor, and Temik.
As there is a great deal of misinformation in the industry about what types of plants can be used for remediation, it is ideal to consult with a knowledgeable phytotechnology scientists during up-front planning exercises.
Just as the developers of sports equipment must find new innovative ways to protect athletes from injuries and long-term effects of impact sports, planners, engineers, and landscape architects are looking for long-term innovative ways to protect the environments we live in and that directly affect our health and quality of life. Through thoughtful LID landscapes many solutions are now being developed, with many more currently on their way. To learn more about related issues and other landscape architecture topics, please visit the publications section of our website by clicking here.
1. Barrier Buster Factsheet #1: How LID Can Protect Your Community's Resources (PDF)(2 pp, 1 MB, March 2012, EPA 841-N-12-003A)
2. Barrier Buster Fact Sheet #2: Terminology of Low Impact Development Distinguishing LID from other Techniques that Address Community Growth Issues (PDF)(2 pp, 601 K, March 2012, EPA 841-N-12-003B)
3. Barrier Buster #3: Costs of Low Impact Development: LID Saves Money and Protects Your Community's Resources (PDF)(2 pp, 2 MB, March 2012, EPA 841-N-12-003C)
4. Pollutant Purging Plants! September 15, 2015 in Plant Science, Restoration http://www.ecolandscaping.org/09/restoration/pollutant-purging-plants/