Stormwater Treatment and Onsite Infiltration — LID at the Core

By Douglas E. Krause, P.E., and Vaikko P. Allen II, CPSWQ

Learning Objectives

After reading this article you should understand:

  • Understand the basic regulatory and design setting of low impact development (LID);
  • Differentiate design parameters that meet LID objectives;
  • Understand the potential for increased scope of work for civil engineers

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Overview

Green solutions. Sustainable design. Low-impact development (LID). On one hand, these new design principles represent revolutions in site design, and on the other, they represent the continuum of an innovative society attempting to use its resources wisely. For civil engineers, the principles offer an opportunity to practice the craft with more tools and a wider scope. Integrated greenscapes with hydraulic functions, water reuse systems connecting outdoor drainage to indoor plumbing, and more complicated flow-control designs are a few of the many applications.

In the neighboring field of indoor water recycling and reuse, a recent GE white paper on best practices highlighted the following barriers to designers and manufacturers in bringing recycled water solutions to the marketplace:

 

  • Regulations that require that all water meet potable standards,
  • Plumbing codes that do not allow dual piping, and
  • Stringent permitting and inspection requirements for recycled water.

In comparison, regulators widely accept stormwater LID practices. At the core of the various definitions of LID is the central unifying theme of mimicking predevelopment hydrology to minimize the impact of development on downstream water bodies and biota. Natural landscape features, often referred to as integrated management practices (IMPs), are being utilized to detain, infiltrate, and treat runoff. This approach is widely endorsed by agencies at all levels as they renew their national pollutant discharge elimination system (NPDES) permits and best management practice (BMP) manuals.

LID is promoted as a distinct alternative to underground, offsite conveyance. For onsite stormwater management, evaluation of reasonable design options that meet low-impact objectives is fairly straightforward, and should occur at the feasibility phase. This includes manufactured, underground, or landscaped options. This article originated via policy presentations at StormCon 2007, and provides background and case studies for civil engineers on LID objectives and design, highlighting the effectiveness of manufactured BMPs in combination with LID principles. The citations and case studies derive from the Western United States, but are universal. LID practice started via proactive resource management in Prince George's County, Md.

Although this paper focuses on treatment and onsite infiltration of stormwater, the key to LID design is hydrologic site assessment at the conceptual stage. More rigorous evaluations of design options, such as more intensive geotechnical and groundwater investigations, will increase the engineer's routine scope, but can yield significant capital and maintenance cost savings. The designs may often employ innovative pervious pavement products or geotechnical improvements such as geogrids or soil amendments.

Stormwater design framework

The genesis of site stormwater design is a jurisdiction's NPDES permit within its identification of pollutants of concern and downstream impairments, especially any Clean Water Act, Section 303(d) listings. Once pollutants are identified, unit processes — the chemical, physical, biological, and hydrologic processes inherent in various BMPs — are identified that affect these pollutants for a given land use and setting (Figure 1). Then, specific treatment components or unit operations utilizing those processes are designed and assembled into a stormwater treatment system adequately sized to mitigate the targeted pollutants. Finally, once systems are installed, as-built characteristics are verified against plans and an inspection and maintenance program is initiated to ensure optimal performance in the long term.

Figure 1: BMP Selection Matrix indicates the appropriate BMP to address pollutants of concern.
BMP Selection Matrix

However, recent permits in California include new LID requirements. Some require minimum LID practices on all sites, which are logical for practices such as conserving natural areas, minimizing street widths, and using pervious materials in areas without heavy pollutant loading. Other requirements do not fit, where separation of pollutants and natural features is desired. For example, requirements to route flow from impervious areas through pervious features before discharge can endanger the functionality and safety of those features as pollutants such as trash, sediment, and oil and grease accumulate.

Although other management approaches are widely used, LID in the form of vegetated, land-based BMPs is the stated preference of many agencies. It would make more sense from a water-quality-protection perspective to eliminate a formbased hierarchy and replace it with a simple requirement that the most effective treatment controls be considered first on all sites regardless of whether they are green, grey, natural, or manufactured. If engineers are accurately assessing the pollutants of concern and weighing other land use priorities, they will necessarily be designing integrated, multi-benefit strategies. They should be allowed to use innovative devices as long as performance expectations are justified.

Deconstructing LID practices

From a unit process-based approach, it is clear that small-scale, multipurpose site design features are valuable options to reduce the impact of a site. When properly designed and maintained, they may infiltrate, filter, absorb, transform, or detain water and pollutants, thereby reducing or even eliminating the need for conventional treatment controls.

A common concern with the use of IMPs on residential developments is that they are treated more as typical landscape features instead of stormwater control practices. That is, they are far more likely to be fertilized, irrigated, mowed, and filled in over time than maintained to consistently provide the infiltration and storage capacity that they are designed for. Proper inspection of infiltrating practices requires an assessment of percolation rates. Proper inspection of detention practices requires assessment of remaining storage volume and outlet control status. Rather than retaining an engineer or hydrologist to do these inspections at many locations throughout a site, a developer may choose to install a centralized treatment and infiltration BMP that is easier to inspect and maintain.

On private developments, it is common for engineers to locate stormwater features in or close to the public right of way where a public agency can ultimately assume inspection and maintenance responsibility. This again may lead a developer to a centralized treatment strategy. Assuming that the same level of pollutant and runoff volume mitigation is provided, these non-IMP strategies may be equally low impact and ultimately more suitable.

Stormwater retention and maintenance optimization

An example from the arid Western United States is Heritage Court in Indio, Calif., a 2.9-acre mixed commercial development completed in 2006 and anchored by a Walgreens store. With slightly more than 3.1 inches of average annual precipitation in the area, irrigation demand is high for any vegetated landscaping and typical stormwater management practices such as swales and vegetated filter strips are impractical. On this site, stormwater is routed through three manufactured screening units that remove all trash and debris greater than 2.4 mm in diameter, coarse sediment, and oil and grease (Figure 2). The units are designed with minimal standing water to minimize the potential for putrification of captured materials in the long, dry periods between storms.

Figure 2: The proprietary screening system will remove all trash and debris greater than 2.4 mm in diameter from runoff prior to discharge into the infiltration system.
The proprietary screening system

Runoff passing through the screening units enters an infiltration system comprised of 112 underground plastic chambers. The open-bottom system is designed to hold a maximum volume of 13,600 cubic feet of water within the chambers and the 12-inch gravel bedding. Water percolates through a filter fabric layer surrounding the chambers into the surrounding stone, and ultimately into the ground. The entire system is designed to infiltrate the water quality volume within 72 hours, entirely eliminating runoff from the site for most storms.

Maintenance of the screening units is expected every three to five years and will remain the responsibility of the site owner. The infiltration system was sized assuming percolation rates four times slower than measured on site. Given the minimal annual runoff volume and screening pretreatment, the 5,000-square-foot infiltration system is expected to be maintenance free for the foreseeable future.

A municipal perspective

From a municipal perspective, the goal is typically to reduce discharge to the maximum extent practicable (MEP). Municipalities that are faced with meeting total maximum daily load requirements (TMDLs) are often substantially built out. In such cases, regional treatment strategies located close to the discharge location may be most suitable and cost effective.

Research by the Pew Oceans Commission has shown that water quality impairments virtually always occur when the percentage of impervious area in a watershed increases over 10 percent. At first glance, an obvious remedy would be to limit impervious area in a development to no more than 10 percent. The draft Ventura County NPDES permit takes it a step further and states a maximum of 5 percent impervious area. Closer consideration reveals that it is not the imperviousness itself that is the problem. It is the lack of proper management of the resulting runoff and pollutants. From this perspective, the call to limit the percentage of effective impervious area on new developments and significant redevelopments is critically misdirected. On many redevelopments it will be unachievable and encourages sprawl.

A more reasonable approach would be simply to require that both predevelopment peak runoff rates and total runoff volumes are not exceeded post development for the range of erosion-causing events. This would require engineers to use increasingly innovative management approaches as the percentage of impervious surfaces increases in their designs. The ultimate balance of imperviousness and onsite mitigation chosen would be governed by developed land values, engineering feasibility, and BMP costs.

Figure 3: Using permeable pavement can provide a traffic-bearing surface
that resists compaction and contributes to significant runoff reduction.
permeable pavement

IMPs can help maintain an acceptable level of density by replacing conventional design elements to provide inexpensive and attractive treatment. For example, simply decreasing street widths, using pervious paving options (Figure 3) for areas without heavy vehicular use, and designing concave medians can significantly reduce runoff volumes with little additional cost (California Stormwater Quality Association Manual). However, pressure to maximize land values leads to many new developments that are still far greater than 10 percent impervious. Urban redevelopment projects are especially difficult to bring under the 10-percent imperviousness threshold without major renovation to include features such as green roofs. In such cases, space-efficient storage and infiltration BMPs, particularly those that can be installed underground, can be used to eliminate post-development runoff volume and magnitude impacts.

Large, publicly funded, urban retrofit projects are becoming necessary to meet new TMDL targets where the developed land base lacks adequate BMPs. Implementation of IMPs can be challenging at this scale. Access to potential sites is only the first hurdle. Even if private land owners agree to modifications to their land, they are not likely to accept the additional inspection and maintenance burden that comes with any stormwater management practice. Municipal stormwater BMP maintenance is typically performed by public works personnel with sweepers, vacuum trucks, and other heavy equipment. Maintaining vegetated BMPs for optimal hydrologic performance, especially those needing hand maintenance, is beyond their expertise.

Disinfection and nutrient removal are top priorities in coastal areas that rely on their waters for contact recreation. Unfortunately, these are among the most difficult conventional pollutants to address. Even if disinfection is nearly 100-percent effective at a particular point, concentrations may rebound just a few hundred yards downstream. Inputs from wild animals, especially birds, exacerbate the difficulty. Vegetated BMPs may have a net uptake of nutrients if the vegetation is regularly harvested and removed. Unfortunately, grass swales and other terrestrial vegetation are commonly irrigated, fertilized, and dosed with pesticides and herbicides that end up contributing a toxic and enriched leachate. If not actively managed, non-infiltrating IMPs may actually increase these pollutant loads.

High-capacity treatment train and groundwater recharge

Sun Valley Park is an urban retrofit project in Los Angeles designed to alleviate flooding issues in the Sun Valley area and recharging the regional groundwater table. The park site was chosen for its location in the watershed and because it was already publicly owned. The site is designed to accept as much as 30 cubic feet per second (cfs) of offsite runoff, which represents the 50-year storm peak flow rate. The water quality flow rate on the site is based on treating the first 0.75 inches of runoff. Pollutants of concern include sediment, oil and grease, heavy metals, and trash.

Figure 4: Media-filled passive filtration systems are able to remove pollutants and keep
them separate from the environment until they are removed during maintenance.
media-filled passive filtration systems

Stormwater entering the site is treated by two swirl concentrators with a combined capacity exceeding the 50-year flow rate. These separators are designed to remove fine sand, trash, and oil and grease. As much as 3.5 cfs of effluent from the separators is conveyed to a manufactured filter unit designed to remove finer particles and metals (Figure 4). Filtered water enters one of two underground modular basins that are designed to infiltrate the entire 50-year storm. Flows in excess of the 3.5-cfs filter unit capacity are routed directly from the separators to the infiltration basins.

Maintenance of all structures will be the responsibility of the Los Angeles County Department of Public Works. The county has contracted with the provider of the filter unit for maintenance of that system. In this example, surface water impacts are all but eliminated through the exclusive use of underground manufactured treatment and infiltration systems. The park continues to serve the public without contamination from off-site urban runoff as had previously been the case.

Pollutant-recovery approach

A sensible alternative to the conventional IMP-based approach is to design systems with the goal of providing maximum pollutant recovery at the lowest cost. Following this approach, BMPs are selected for ease of maintenance and for their ability to remove and contain pollutants away from the natural environment so that they are prevented from cycling through the food chain and migrating through the water cycle. It is an approach that recognizes that pollutants are inherently dangerous and should be managed similarly in the urban environment to the way they would be managed in an industrial setting. That means closing the loop by first eliminating pollutant sources wherever possible and then tracing the pathways of distribution, transformation, and storage, and designing in steps along those pathways to collect and remove them.

A pollutant-recovery model might take an opposite view of multiuse and directly connected impervious areas from conventional LID design. Instead, polluting land uses, especially those associated with vehicles, could be linked together so that they are effectively flushed to a treatment system that would also serve as a discrete reservoir. Gardens, landscaping, and natural areas would remain relatively unfouled by urban pollution, thereby protecting their value for multiple uses. Uses such as recreation, habitat and aesthetic improvements, and pollutant storage may be incompatible.

Reduction of development: Urban design

Efficient use of land space, which may be enabled by the use of innovative non-LID techniques, can also shrink the footprint of development and encourage pedestrian-friendly, dense, multi-use development typical of new urbanist designs. A good example is Orenco Station in Hillsboro, Ore. This 190-acre site was redeveloped starting in 1997 with a focus on providing light rail and pedestrian commuting options for its 2,600 residents. It includes 1,850 residential units and more than 230,000 square feet of retail and office space. The site incorporates many features familiar to LID practitioners such as shared driveways, sidewalks with adjacent planter strips, and narrow streets. It also relies on three underground filter treatment systems to treat a combined 4.7 cfs of runoff to meet the treatment goals set by the city of Hillsboro.

Figure 5: An underground BMP, such as this filtration system installed in Orenco Station,
means stormwater treatment is invisible to the general public.
An underground BMP

Exclusive use of land-based treatment systems was not feasible on this site because of high land values and the desire to integrate commercial, residential, and transportation elements in a density that encourages carless travel. Use of the manufactured filters also provides a reservoir for pollutants where they are invisible to the general public and out of contact with natural site elements (Figure 5). An inspection and maintenance contract between the site manager and the BMP provider has been in place since installation in 1999. Since that time, the systems have been cleaned every one to two years and approximately 30 cubic yards of material have been recovered, not including the spent filter media. The benefit of containment and disposal of pollutants and the reduced risk on development are often overlooked.

Summary

The most suitable stormwater system designs are often a combination of approaches that take into account local economic, political, and environmental concerns. These designs are best fostered by clear performance standards and flexibility to use innovative approaches to meet them. Civil engineers will increasingly be asked to undertake more rigorous evaluations at the site layout and feasibility phases. New design parameters are around the corner. The LID movement has helped pave the way for cost savings and green solutions from roadways to rooftops. LID principles will foster the continuing innovation and creativity of the nations' civil engineers.


Douglas E. Krause, P.E., is product engineering manager, his experience in civil engineering includes site design, dredging and remediation of contaminated tidelands, and surveying.

Vaikko P. Allen II, CPSWQ, is Southwest regulatory manager, for CONTECH Stormwater Solutions. Allen previously served as technical manager of Vortechnics, Inc., and has worked with regulatory groups to evaluate pollutant removal and operational performance of proprietary BMPs.


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