By Jennifer Steffens, E.I., LEED-AP, and Denise Pinto, P.E.
After reading this article you should understand:
- understand basic Low Impact Development (LID) principles;
- understand the design processes involved in creating LID sites;
- differentiate between engineered landscape and belowground techniques used in LID designs.
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The Low Impact Development (LID) approach to site development and stormwater management is rapidly becoming the required design approach in many areas of the United States. The basic principle is to use nature as a model and manage rainfall at the source. This is accomplished through sequenced implementation of runoff prevention strategies, runoff mitigation strategies, and finally, treatment controls to remove pollutants. Although Integrated Management Practices (IMPs) — decentralized, microscale controls that infiltrate, store, evaporate, and detain runoff close to the source — get most of the attention by engineers, it is crucial to understand that LID is more than just implementing a new list of practices and products. It is a strategic design process to create a sustainable site that mimics the undeveloped hydrologic properties of the site. It requires a prescriptive approach that is appropriate for the proposed land use.
Design using LID principles follows four simple steps. First, determine pre-developed conditions and identify the hydrologic goal (some jurisdictions suggest going to wooded conditions). Second, assess treatment goals, which depend on site use and local keystone pollutants. Third, identify a process that addresses the specific needs of the site. Fourth, implement a practice that utilizes the chosen process and that fits within the site's constraints.
The basic processes used to manage stormwater include pretreatment, filtration, infiltration, and storage and reuse.
Pre-treatment — Pre-treatment is recommended to remove pollutants such as trash, debris, and larger sediments. Incorporation of a pretreatment system, such as a hydrodynamic separator, can prolong the longevity of the entire system by preventing the primary treatment practice from becoming prematurely clogged.
Filtration — When stormwater is passed though a filter media, solids and other pollutants are removed. Most media remove solids by mechanical processes. The gradation of the media, irregularity of shape, porosity, and surface roughness characteristics all influence solids removal. Many other pollutants such as nutrients and metals can be removed through chemical and/or biological processes. Filtration is a key component to LID sites, especially when infiltration is not feasible. Filter systems can be designed to remove the primary pollutants of concern from runoff and can be configured in decentralized small-scale inlets. This allows for runoff to be treated close to its source without additional collection or conveyance infrastructure.
Infiltration — Infiltration reclaims stormwater runoff and allows for groundwater recharge. Runoff enters the soil and percolates through to the subsurface. The rate of infiltration is affected by soil compaction and storage capacity, and will decrease as the soil becomes saturated. The soil texture and structure, vegetation types and cover, water content of the soil, soil temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity. Infiltration plays a critical role in LID site design. Some of the benefits of infiltration include improved water quality (as water is filtered through the soil) and reduction in runoff. When distributed throughout a site, infiltration can significantly help maintain the site's natural hydrology.
Storage and reuse — Capturing and reusing stormwater as a resource helps maintain a site's predevelopment hydrology while creating an additional supply of water for irrigation or other purposes. Rainwater harvesting is an LID practice that facilitates the reuse of stormwater.
Figure 1: The continuous deflective separation (CDS) system is a
uniquely designed hydrodynamic separator that is able to remove
100 percent of neutrally buoyant material without blinding..
Pre-treatment — Belowground practice
Hydrodynamic separators (HDS) offer high-performance, proven stormwater treatment, and are widely accepted for effective solids removal. These systems are particularly effective at removing trash and debris and other floatable pollutants such as oils and greases (see Figure 1). HDS systems are a cost-effective way to remove a wide range of pollutants.
Basic design considerations include the following:
- Never place an HDS system downstream of detention.
- An HDS system can accommodate a variety of inlet configurations including inline, offline, grate inlet, and multiple inlet pipes.
- Design an HDS system to treat at least the water quality storm (as specified by local regulations). Bypass of peak flows is recommended to avoid re-suspension of previously captured pollutants.
There are five design steps:
- Determine the water quality storm per local requirement (the two-year, 24-hour storm is recommended if no requirements).
- Calculate peak flow rate.
- Choose an HDS unit that will treat the flow rate (refer to a product's verified treatment flow rate). If local verified flow rates are not given, see verified products at njstormwater.org.
- Calculate peak flow rate for the 10-year storm (or per local requirements).
- Confirm the HDS unit will pass peak flow without re-suspension of collected pollutants. If necessary, place the unit offline with a bypass structure.
Infiltration — Engineered landscape practices
Bioretention — Bioretention, perhaps the most recognized LID technique, incorporates landscaped features to treat stormwater aboveground in an aesthetically pleasing way. These systems most commonly replace the typical landscape islands found in parking lots with depressions designed to contain the water quality storm and promote filtration and infiltration. Bioretention systems in residential areas are more commonly referred to as rain gardens and can be tended by homeowners.
Bioretention systems are known for their versatility and come in just about any shape or size. The design of the system primarily depends on the drainage area and type of soil. If infiltration is not feasible, then a bioretention cell can be converted easily to a biofilter by adding underdrains to discharge the treated stormwater.
Basic design considerations include the following:
- The bioretention area should occupy 5 to 7 percent of the drainage area.
- Place in a space that can easily receive water.
- High-groundwater sites with frequently saturated soils are not preferred.
- Ponding depth should be 6 to 12 inches to allow for proper vegetative diversity.
- A typical bioretention cross-section consists of a 3 to 6 inch top layer of organic sandy loam and 3 to 4 feet of sandy loam or loamy sand fill soil.
- Biofilters include a perforated underdrain pipe wrapped in 2 to 4 inches of washed gravel.
There are five design steps:
- Determine the water quality runoff storage volume per local requirement (the two-year, 24-hour storm is recommended if no requirements).
- Calculate required surface area based on runoff volume and average ponding depth.
- Design the overflow. The water quality storm will typically account for 80 to 90 percent of the annual rainfall volume, but larger events will need to bypass the bioretention system. The use of an overflow pipe or box installed in the middle of the system with the top set at the ponding depth is commonly used.
- Determine type of soil used for media. Soil should have enough fines to support plant growth and permeable enough to pass water. A sandy loam is recommended.
- Design underdrain (if using a biofilter) with a flow capacity greater than that of the peak inflow. Peak inflow is determined using Darcy's Law.
Bioretention can become costly when land value is high. When land is not available to implement conventional bioretention, nature's ability to treat stormwater can be taken advantage of by using engineered soils with rapid infiltration rates. Media are available commercially with a hydraulic capacity 10 to 20 times that of conventional systems. This provides significant footprint reduction and potential cost savings. These biofiltration systems, also known as tree box filters, are described further below.
Permeable Pavement — Permeable Pavement is typically categorized into two general types: porous paving material and permeable pavers, which can be subcategorized into porous asphalt, pervious concrete, permeable interlocking concrete pavers, and grid pavers. The common goal of each of these systems is to produce less stormwater runoff by promoting infiltration on areas that would otherwise be impervious. The increased infiltration is enabled by the void space in the paver material itself or between the pavers. Treatment comes through natural filtration by the ground below the pavement section. The pavement can be placed over a stone bed that is designed to store the runoff volume temporarily while infiltration occurs. On sites that are not conducive to infiltration, underdrain pipes can be used to drain the stone bed. To utilize the storage space below the pavement effectively, structures such as lowprofile arch retention systems and perforated pipe can be used to provide more storage.
Basic design considerations include the following:
- In general, sites with good soil infiltration rates, low groundwater table, and low pollutant loading are preferable for permeable pavement.
- Typical applications include pedestrian areas, fire lanes, and low-volume roadways and parking lots.
- A storage bed, if used, should be designed to treat the total runoff generated by the design storm and, depending on local requirements, must drain in 24 to 72 hours.
- Consult a geotechnical engineer regarding the suitability of the site for permeable pavement. If the site has unsuitable soil, permeable pavement can still be used if an impermeable liner along with underdrains is in place. The liner prevents wetting of the underlying soils.
The design procedure includes five steps:
- Select the type of permeable pavement to be used based on site considerations Concrete block pavers have the highest load-bearing capacity and are recommended for use in parking lots, streets, driveways, and fire lanes. Porous asphalt and pervious concrete have lower load-bearing capacities and are not recommended in areas where large commercial vehicles are present. Plastic grid pavers are excellent for uneven terrain but do not provide the durability of concrete. They are recommended for lowvolume parking areas, residential driveways, emergency access roads, golf cart and bike paths, and sidewalks and other pedestrian areas.
- Determine the need for underdrains, especially in clay soils and sites with a high groundwater table.
- Determine effective site imperviousness and assess storage needs. Effective imperviousness depends on the pavement type. This calculation determines the runoff reduction achieved. If further runoff reduction is necessary, design the storage system below the pavement for the remaining volume of water.
- Installation of the permeable pavement is critical. Be sure that an experienced site contractor and geotechnical engineer are involved. Follow manufacturer's installation procedures.
- Understand maintenance requirements. Routine vacuuming will prevent clogging.
Infiltration — Belowground practices
When infiltration is feasible, it is the preferred method of stormwater control for LID sites. Since aboveground landscape practices are not always an economical solution, consider belowground practices such as vaults, chambers, and pipes with open bottoms or perforations. Belowground practices allow for infiltration to occur without using land that could otherwise be developed or preserved, which minimizes impact on the site.
Belowground infiltration provides an efficient underground groundwater recharge system, often meets detention requirements on a site, and provides a cost-effective method for reduced peak surface water discharges.
Basic design considerations include the following:
- Use the maximum depth available for storage. Deeper storage depth reduces system footprint and minimizes land disturbance.
- High groundwater, bedrock, or karst soils require lowprofile arch systems and small-diameter pipe.
- Use void space in gravel fill to maximize storage capacity and minimize footprint.
- Belowground systems come in many shapes, sizes, and materials. Work with the product manufacturer to determine the system that is best suited for each specific site.
The design process includes five steps:
- Calculate the volume of water to be infiltrated.
- Determine the depth available for underground storage. If groundwater is present, it is typically recommended to keep 1 foot above the peak groundwater level.
- Speak with manufacturers to determine the best material for the site. Corrugated metal pipe (CMP) can be sized and shaped to meet most site-specific needs. Variable sizing, material economy, faster installation, and durability combine to make CMP systems an economical method for controlling stormwater runoff. Open-bottom precast concrete vaults and concrete arch systems offer a wide range of span and rise combinations for design flexibility. System modularity allows for quick and easy installation while off-site fabrication ensures tight adherence to specifications and quality control. Arch shapes can support high live loads efficiently, which lowers material costs and ultimately capital costs. Open-bottom plastic arch systems are designed to economically collect, detain, retain, and infiltrate stormwater runoff. These chambers offer maximum storage capacity when low storage depth is available (see Figure 2).
- Choose system size based on the manufacturer's recommendation.
- Install per the manufacturer's recommendation.
Figure 2: Plastic arches, such as the ChamberMaxx system shown here,
are acceptable belowground practices for infiltration.
Filtration — Engineered landscape practices
Tree box filters — Tree box filters (see Figure 3) are enhanced biofiltration systems that utilize biological and engineered media. They are specifically designed to treat small catchment areas and can be easily combined with underground infiltration. This decentralized approach to managing stormwater runoff is a core principle of LID. These highly adaptable filters can be used where bioretention is desired but site conditions make it impossible, such as clay soils, high groundwater, steep terrain, contaminated soils, karst topography, and sites with high pollutant loads. Tree box filters can be integrated into the landscape, making them both functional and aesthetically pleasing. Basic design considerations include the following:
- Place in an area where both aesthetic and hydraulic needs are met.
- Typical dimensions range from 6 feet by 4 feet to 6 feet by 10 feet.
- Treatment capacity depends on the engineered media and system size.
- Engineered biofiltration media operate between 50 and 100 inches per hour.
- Select native vegetation with non-aggressive root structure.
- High flow rates should bypass the biofilter. Choose a tree box filter that treats the initial runoff with high pollutant concentrations via the biofilter and treats higher flows by media cartridges.
- Some tree box filters bypass peak flow rates externally, while others have integrated high-flow bypass.
The design process has four steps:
- Choose curb inlet location based on site topography and hydraulics.
- Calculate expected peak flow rate to the filter. Filters with internal bypass typically have a bypass capacity of 2 to 3 cubic feet per second. If peak flow rate is expected to exceed this, then use an external bypass structure.
- Speak with manufacturers to determine treatment capacity of the system and system sizing recommendations.
- Installation, activation, and planting should be done per the manufacturer's recommendation.
Figure 3: The UrbanGreen BioFilter shown here is an example of a tree box filter.
Filtration — Belowground practices
Sand filters and cartridge-based filter systems — Underground filtration systems are common stormwater treatment systems. Some local regulations dictate sizing guidelines and should be referenced when available. Underground filtration products meet the most stringent regulatory requirements for stormwater treatment. Using sustainable media, filtration solutions remove the most challenging pollutants including solids, soluble heavy metals, oil and grease, and total nutrients. Basic design considerations include the following:
- Choose a system with extensive field verifications to prove performance.
- Consider which structure configuration (drywell, curb inlet, catchbasin, precast) best fits site hydraulics.
- Analyze available hydraulic drop. Low-drop options can operate with as little as 1.8 feet of headloss.
- Sand filters are designed to capture and treat a water quality volume while cartridge-based filters typically operate to treat a flow rate. Consult with a manufacturer to determine the most economical solution for each site.
- Choose a system with low maintenance costs and proven longevity in the field.
Maintenance is the most discussed concern when it comes to underground filtration systems. Like any effective stormwater treatment system, captured pollutants must be removed periodically to restore the system to its full efficiency and effectiveness. Maintenance requirements and frequency are dependant on the pollutant load characteristics of each site. In general, cartridge-based filtration systems are easier and more economical to maintain than sand filters. Maintenance compliance programs offered by manufacturers can ensure the longevity and effective operation of the system.
LID is not simply substituting green practices for manufactured practices, it includes both engineered landscape and belowground. Clearly identifying the hydrologic and water quality goals upfront and following the guidelines discussed here will ensure that the appropriate process is used and the best available practices are implemented. Each site is unique, and a one-size-fits-all solution will not meet sustainability goals. The most environmentally sustainable designs are best fostered by clear performance standards and flexibility to use a combination of innovative approaches — aboveground or belowground — to meet each site's specific needs.
Jennifer Steffens, E.I., LEED-AP, Mid-Atlantic regulatory manager for CONTECH Construction Products, Inc., works with regulatory and non-profit groups to evaluate pollutant removal and operational performance of proprietary BMPs. Steffens also serves on several technical committees for stormwater design manuals.
Denise Pinto, P.E., area engineer – stormwater products for CONTECH, designs stormwater BMPs and works with engineers to help solve stormwater challenges. Previously, she worked in engineering consulting with a focus in water and wastewater treatment.
- DeProspo Philo, Lisa; Joubert, Lorraine; and McNally, Catherine, March 2007, Permeable Pavement: What's It Doing on My Street? The University of Rhode Island Cooperative Extension, in partnership with Rhode Island Department of Health, Source Water Protection Program.
- Forgotten Rain L.L.C., www.forgottenrain.com.
- Hunt, William, June 2001. Designing Rain Gardens (Bio-retention Areas), Raleigh, N.C., Urban Waterways and N.C. State Univ., A&T State Univ. Cooperative Extension.
- Jurries, Dennis, Jan. 2003, BioFilters (Bioswales, Vegetative Buffers, & Construction Wetlands) For Storm Water Discharge Pollution Removal, Oregon Department of Environmental Quality.
- LID Center, Urban Design Tools, www.lid-stormwater.net.
- NRCS, 2005, Bioswales Fact Sheet.
- N.J. Department of Environmental Protection, Feb. 2004, N.J. Stormwater Best Management Practices Manual, Chapter 9.7: Standard for Pervious Paving Systems.
- Stormwater Manager's Resource Center, www.stormwatercenter.net, Stormwater Management Fact Sheet: Bioretention, Tool 6 — Stormwater Practices.
- Urban Drainage and Flood Control District, Oct. 2005, Drainage Criteria Manual V.3, Structural Management Practices, 3.0 Porous Pavement.