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Low-impact development (LID) is an approach to land development (or re-development) that works with nature to manage stormwater as close to its source as possible. LID employs principles such as preserving and recreating natural landscape features, minimizing effective imperviousness to create functional and appealing site drainage that treats stormwater as a resource rather than a waste product.
By implementing LID principles and practices, water can be managed in a way that reduces the impact of built areas and promotes the natural movement of water within an ecosystem or watershed. According to the Florida Department of Environmental Protection, to achieve the desired levels of nutrient load reductions in urban stormwater management systems, it is recognized that greater emphasis needs to be placed on nonstructural best management practices (BMPs) that can either reduce the generation of stormwater or decrease the amount of nutrients that get into the stormwater.
It is said, however, that implementing LID BMPs presents many challenges related to quantifying the pollutant load reduction benefits of such BMPs, consistent design and construction of these BMPs, and assuring their long-term operation and maintenance.
LID impacting design
The LID movement changes the fundamental stormwater management goal from draining a site efficiently and safely to retaining runoff such that the predevelopment hydrologic balance remains intact, said Vaikko Allen, CPSWQ, LEED-AP, regulatory manager, CONTECH Construction Products Inc. “Most structural LID BMPs including infiltration, rainwater harvest, evapotranspiration and filtration-based systems are not new,” he said.
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However, Allen said, following the “prevent then mitigate” philosophy, the requirement that all source-control and site-design BMP opportunities be explored prior to consideration of treatment or flow control BMPs, is taken much more seriously.
“When runoff prevention opportunities are exhausted, BMPs that retain runoff onsite are given first priority,” said Allen, adding that where site constraints limit retention, treatment controls are utilized.
Because LID shares the same basic principle as nature of managing rainfall at the source, new stormwater management and treatment devices tend to use design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source, said Gina Carolan, COO, CULTEC Inc. By keeping the runoff at its original location rather than conveying it offsite or to a river or stream, the predevelopment state is kept as closely intact as possible. That said, Carolan states designers of drainage systems prefer to store and infiltrate the water onsite whenever possible to recharge the aquifer and avoid flooding neighboring watercourses.
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Manufacturers are looking for new ways to implement their products for LID use; one way is water harvesting and reclamation. “Although this method of managing stormwater runoff is very popular in other parts of the world, the United States is slowly embracing the idea, and it is gaining popularity for green building solutions,” said Carolan. “The stormwater may be collected by various methods. One example is the use of an impervious liner under a subsurface plastic chamber bed that pumps the water to a storage holding tank.”
AquaShield also said it has seen incorporation of its stormwater treatment devices into sustainable use stormwater management systems in certain areas. For example, its units can be used in association with rainwater harvesting practices for a variety of onsite water uses. This approach allows for LID principles to be maintained while simultaneously allowing for efficient land use compared with development sites that use conventional approaches such as a detention pond, the company said. Other benefits of these designs include long-term water conservation and potential cost savings for the property owner.
Project sites lend themselves to LID practices in different ways, said Steve Kingsland, national sales representative for Oldcastle Precast Inc.’s marketing and engineering group. “There is a shift toward using BMPs that allow for groundwater recharge as the final step in the process,” he said. “In many instances, the treatment-train approach is used to remove pollutants from stormwater prior to infiltration into the ground.”
There is a national movement toward encouraging stormwater runoff-reduction design methods that mimic predevelopment site conditions, said Terry Siviter, general manager, Filterra Bioretention Systems. “The reality, however, is that these practices can require the use of more developable space than traditional BMPs require, and some LID practices like bioretention swales have a poor history of performance due to inconsistent design parameters (e.g., media specs), unsupervised construction practices, and lack of maintenance,” Siviter said. “In new construction areas where land may be less expensive, where the soils perk well, and well-engineered and constructed systems are installed, these techniques could be implemented.”
Stormwater management and treatment components are increasingly being designed and used as capture and re-use of gray-water mechanisms. According to Dustin Glist, media and information director, Invisible Structures Inc., the influence to reduce potable/municipal water use is coming from many directions — green initiatives, reduced aquifer supplies, tax incentives, and more.
A lot of cities and state agencies are making a push towards infiltration, re-use, and non-proprietary biotreatment systems. “This push is causing a decrease in demand of structural BMPs, such as catch-basin filters, hydrodynamic separators, and proprietary biotreatment systems,” said Zach J. Kent, stormwater engineer, Modular Wetland Systems Inc.
However, Kent said, there is a huge drawback to this trend toward LID design. “First, in many areas infiltration is not possible because of soils. Quality control and appropriate construction of non-proprietary BMPs is a major concern,” he said. “Also, these non-proprietary systems are being installed as standalone BMPs with no pretreatment. Therefore, clogging of these systems is a huge problem that is being overlooked.”
From a practical standpoint, Merideth Burkhart, water quality chemist, StormTrap, sees stormwater management changing in three ways — more focus on infiltration, more focus on rainwater harvesting, and more functional use of landscaped areas that were previously only for aesthetics.
“This seems to be the result of a new perspective where it is not enough to just detain and treat the stormwater; you have to reduce the amount that becomes runoff and therefore reduce the amount that needs to be treated,” said Burkhart. “LID is really a volume-reduction approach to stormwater. This can have important implications for water quality as well because it is typically more effective to capture the rainwater before it becomes polluted than trying to treat it afterwards.”
In most cases, LID practices are both fiscally and environmentally beneficial to communities. According to Joe Miskovich, president, Triton Stormwater Solutions, LID offers many other benefits as well such as improved aesthetics, expanded recreational opportunities, increased property values (because of the desirability of the lots and their proximity to open space), increased number of total units developed, value of increased marketing potential, and faster sales.
Answering the call
Companies are responding to LID trends with new products, upgrades, and research and development. In 2009, for example, CONTECH created the UrbanGreen brand to house products that are specifically designed to meet emerging LID requirements. So far, two products have been released under this brand, said Allen. The UrbanGreen BioFilter — a hybrid landscape planter and media filter system that can be integrated into streetscapes to provide superior water quality benefits; and the UrbanGreen Grass and Gravel Paver — a plastic grid paver that reinforces either gravel or grass surfaces so that they can withstand infrequent vehicular traffic without sacrificing permeability.
When infiltration is feasible, it is the preferred method of stormwater control for LID sites, said Allen. “Our ChamberMaxx system is an HDPE arch infiltration system that is installed below grade over a permeable sub-base,” he said. “This belowground system allows for infiltration to occur without using land that could otherwise be developed or preserved, which subsequently minimizes impact on the site.”
CULTEC is tracking product progress with its extensive range of chamber sizes to fit almost any site parameter. “Recently, we have developed and redesigned some of our large-capacity plastic chambers to have the capability of storing even larger volumes of water in smaller areas,” said Carolan. “By providing an ample subsurface storage void, clients are able to closely manage pre- and post-construction storm events without effecting adjacent properties.”
Modular Wetland Systems’ Linear product was the first system to provide a self-contained treatment train that includes screening, separation, media filtration and biological filtration processes, said Kent. The system can be used in conjunction with pre-detention to meet both water quality and quantity objectives.
AquaShield also recommends a combination of technologies that can work together for effective LID. It has partnered with other companies to offer LID-compatible systems such as permeable paving structures and underground water storage. For example, AquaShield’s hydrodynamic separator provides pre-treatment for a variety of LID applications such as bioswales, stormwater wetlands, and storage structures.
Burkhart said StormTrap has recognized the growing trends in LID and has adjusted its product line to better complement LID designs, specifically permeable pavers, infiltration, and rainwater harvesting.
Also in response to new product development, Triton launched two Stormwater Chamber lines — the C and M series — that provide users with many options. “This enables the utilization of our Chamber products in a number of combinations to solve any type of stormwater problem or harvest rainwater for reuse,” said Miskovich.
Kingsland said Oldcastle Precast developed and launched its Storm Capture line of products (patents pending) after hearing three major issues in the marketplace — the need for LID and sustainable solutions, the need for systems that can be easily accessed and maintained, and having systems that can function effortlessly in traffic-loading conditions. “The first two products include the Storm Capture Chamber and Storm Capture Module that are ideal systems for detention and groundwater recharge, and work well as part of a treatment train with other BMPs,” he said.
Filterra has invested heavily in nationally recognized third-party verification protocols, Siviter said. The company is working with Virginia Tech on testing various aspects of pollutant uptake within Filterra’s root and biomass system through a process called phytoremediation. “Findings are quite encouraging as to the role the plant plays in pollutant removal and the robust nature of each unit,” said Siviter.
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LID support
There are many design aids and resources available such as software, online calculators, and specification guidelines to help civil engineers identify and apply stormwater management and treatment devices appropriately for LID applications.
AquaShield has developed simple online tools on its website to assist specifying engineers in the design process. Sizing tool procedures allow for easier project designs and considerations in conjunction with LID-based applications, the company said.
Carolan said CULTEC recognized the need to have its Contactor and Recharger plastic stormwater chambers included in a modeling software program. “In 2003, we contacted and made an alliance with HydroCAD to be included in its program,” Carolan said.
The CULTEC StormGenie, released in May 2009, is an AutoCAD plug-in for designing CULTEC Stormwater Systems. Carolan said it is used in conjunction with CULTEC’s Stormwater Design Calculator to generate project-specific AutoCAD drawings for CULTEC’s Contactor and Recharger Retention and Detention Stormwater Systems. The StormGenie takes information from the Stormwater Design Calculator and creates rectangular beds based on the client’s project input.
Invisible Structures also has an online calculator to aid in designing and determining the number of components needed for its Underground Stormwater Detention System. “This device gives the user a clear number of units needed for storing ‘X’ amount of water, and then calculates the labor, excavation, backfill, base course, geogrid, geotextile fabric, and impermeable liner (if needed) for use with our Rainstore3 product,” said Glist.
Triton’s new website has been refreshed with a calculator that provides users with a complete breakdown of all required materials for a project. “It also allows the design of chamber layouts per specific requirements and creates a wire-frame drawing or shaded rendering to be used in the engineer’s drawing,” said Miskovich.
Mimicking the lines of design, Siviter said Filterra offers regional Design Assistance Kits (DAKit) for the proper planning and use of the company’s products.
According to Allen, CONTECH’s mission is to provide the industry’s best customer experience. “We realize that on any project, navigating local regulatory requirements, site constraints, and budgetary pressures can be a challenge,” he said. “That said, our sales engineers and supporting teams are specially trained to provide close assistance from initial site planning to post-construction operation and maintenance.”
Of course, for those “do-it-yourself” engineers, said Allen, the company offers many design tools such as “design-your-own” programs for all CONTECH detention systems.
Pamela Accetta Smith is a freelance writer based in Albuquerque, N.M.
