Designing BMPs for Drought
A study of the Southeast
Friday, December 31, 2010
By Jerry Regenbogen, Richard S Keagy
While the Southeast United States is no stranger to dry conditions in the summertime, for most of 2007 and 2008 the region suffered its worst drought in decades. Around the same time, many developers and municipalities, spurred on by the water-quality requirements of the National Pollutant Discharge Elimination System (NPDES) Phase II permit, had begun to focus on better stormwater management and sustainable site design practices. As a result, bioretention structures, those with a natural pollutant filtering capacity, were being more aggressively integrated into their projects. When the drought conditions took hold, however, these features not only became blights to the landscape as plantings died, but also posed potential problems for flooding, runoff, and water-quality issues when the rain returned.
Throughout this time, engineers and landscape architects at Stantec began implementing and experimenting with different stormwater management and low-impact-development measures to learn which ones, and which combinations of techniques, would be most sustainable during variable weather conditions, including drought. The project teams studied tailored combinations of best management practices (BMPs) to help determine how varied treatment trains can best accommodate bioretention capabilities, while limiting the risk of failure during periods of drought. They further identified basic parameters of bioretention design that allow this BMP to function more effectively in both wet and dry conditions.
Drought Wreaks Havoc on Southeast
By the fall of 2007, the southeastern US was reeling from a months-long, record-shattering drought. Between 80 and 90% of the region was in some stage of drought condition as monitored by the National Oceanic and Atmospheric Administration (NOAA) Drought Monitor service, and, for first time in more than 100 years, many areas reached the most severe category of drought. As reservoir levels dropped precipitously, cities across the region were forced to restrict water. While the lack of rain impacted agriculture, recreation, and wildlife, it also revealed drawbacks in the design and operation of structures that capture, store, and treat stormwater.
The drought coincided with a time when many municipalities were implementing NPDES Phase II requirements related to stormwater management, particularly those designed to improve water quality through nutrient removal. To meet the requirements of NPDES Phase II, many developers had begun to take a closer look at best management practices that added a water treatment, or filtering, component to the traditional treatment regime. In most cases this took the form of introducing bioretention structures such as rain gardens, bioswales, and wet ponds, all of which provide a mechanism to store runoff, slow the rate of discharge, and, most importantly, filter pollutants via a carefully selected palette of native plantings and soil bed.
During the Southeast drought, a number of these bioretention BMPs failed and required costly, possibly avoidable, reconstruction. Understanding the role of bioretention structures in treatment systems, and designing them for utmost flexibility, were among some of the lessons learned during this period.
Adapting BMP Methods for the Future
Although bioretention is an effective means to remove pollutants such as phosphorus and nitrogen from stormwater, it cannot replace a properly balanced and integrated stormwater management system. Applying research from the EPA and the Water Quality Control Design Center to local municipal bids for stormwater management infrastructure, Stantec reviewed a host of BMPs to better understand how combinations of methods and their sequence—the treatment train—would enable developers to meet local permit requirements in the most cost-effective manner. At the same time, the exercise provided a framework within which bioretention can be applied in a judicious manner, reducing overreliance on a practice that may not withstand long-term drought conditions.
A recent review of BMP standards against a series of criteria such as total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) removal efficiency and quality control revealed there are typical removal rates in most locations in the Southeast. The values are indicated in Table 1.
Too often, engineers recommend the smoothest route to permit approval or the most common best management practice preferred by a local
jurisdiction. Frequently, local jurisdictions drive the selection of best management practices for specific projects, and very often rain gardens are a preferred approach. A treatment train analysis such as depicted in Table 1 provides an opportunity for engineers to help jurisdiction staff explore a toolbox of different methods that can adapt to the specific project site while also considering future drought conditions. This flexibility can result in a solution that reduces the footprint of the more vulnerable bioretention structures without sacrificing their vital function in the stormwater management and treatment process.
As part of the BMP study, engineers and landscape architects reviewed specific types of structural and nonstructural BMPs and how to use them in a treatment train. The standards were subjected to a comparative cost analysis, looking at the cost of nine BMPs per square foot and the cost of a typical 5-acre drainage area (DA) application (Table 2).
The drainage basin for these areas was key, as many jurisdictions in the Southeast limit the drainage area to 5 acres for some BMPs. This acreage limit is based on infiltration rates for those specific treatment types. The costs per surface square foot, shown in Table 2, are based on numerous cost evaluations including USEPA estimates; local Charlotte, NC, and Atlanta, GA, construction bids; Mecklenburg County, NC, cost estimations; and several other resources. The costs are the median value from all the sources and also were very close to the construction cost estimates found in the two prototypical cities analyzed.
From this information, it is possible to develop a treatment train comparative cost analysis, responding to local treatment requirements, for any combination of BMPs (Table 3). Many engineers and jurisdictions are familiar with treatment trains. This is a standard way to combine BMPs to provide the required TSS, TN, and TP removal as well as other requirements, while adapting to project site
conditions.
The comparative cost analysis in Table 3 provides an approximate idea of the value of the potential treatment trains. The analysis provides design engineers a gauge by which to analyze how to keep BMP costs at budgeted levels and still allow some flexibility in the type of BMPs applied to a project. The owner or jurisdiction must also consider the maintenance cost for the BMP in their evaluations.
Upon analysis of Table 3, and factoring in the desire to reduce the amount of plantings (and therefore negative exposure to drought conditions), it is apparent that a grassed swale with an optimal bioretention area, or a grassed swale with a wet pond and a treatment wetland, may be the best option in situations where water treatment requirements necessitate some bioretention capability. As indicated in this study, there are in fact several treatment train options that could be considered. However, when trying to reduce liabilities during drought conditions, the best option would be to use the most cost-effective solution with the least amount of drought-vulnerable plantings.
Click on image to enlarge
Designing Flexible Bioretention Structures
Using an effective treatment train can mitigate overreliance on any one solution, but the fact is that bioretention may still be required to reach nutrient removal, total suspended solids removal, or detention requirements. So, what are the components of an effective bioretention BMP, one that can better weather long periods of below-average precipitation? Armed with
lessons learned from the prolonged Southeast drought, engineers and landscape architects developed a series of recommendations for municipalities and
developers.
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Photo: Black Creek Watershed Association
Rain gardens, such as this one at West Cary Middle School, provide an aesthetic component while serving an important stormwater management function. |
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Photo: Charlotte-Mecklenburg Stormwater Services
Linear rain garden captures runoff from adjacent impervious surface and utilizes flowering annuals and perennials for seasonal color. |
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Photo: Stantec
Roadside rain garden captures runoff directly adjacent to asphalt. Note the variety of textures attained through multipleplant types. |
Successful bioretention structures integrate sound principles of both landscape architecture and civil engineering, combining specific stormwater management calculations and soil profiles with aesthetically pleasing design features and compatible vegetation. Encompassing a variety of different low-impact-development systems such as vegetated swales, rain gardens, sunken medians, retention ponds, and wetlands, bioretention BMPs serve a dual purpose of reducing peak runoff rates and volumes while efficiently removing suspended solids, heavy metals, adsorbed pollutants, nitrogen, phosphorus, pathogens, and temperature.
Rain gardens in particular are gaining popularity outside their traditional role in residential landscapes. They are particularly useful on densely developed urban sites and are a solid
option for retrofitting failing stormwater management structures. Rain gardens can be small, formal, homeowner style gardens; large complex bioretention gardens; or multiple distributed units providing treatment in a large drainage area.
When rain gardens are added to the treatment train, they require only a few crucial, basic elements for success. The planting medium should contain the right mixture of topsoil, sand, and
compost. The plantings should generally be native perennial plant species that will not require much care after establishment. And they should be covered in shredded hardwood mulch to keep the soil moist and ready to soak up rain.
Other general recommendations include:
- An underdrain system that drains filter media within 48 hours
- Pretreatment and energy dispersion via other BMPs
- Sheet flow conditions into the facility
- Maximum contributing drainage area of 10 acres
- Maximum contributing drainage of 5 acres per inflow point
- Maximum ponding depth of 12 inches for water quality
Importance of Soil Conditions in Plant Selection. Across the Southeast, a variety of conditions exists, from the red clay of the Piedmont to the sandy soils of the coastal plains. A geotechnical investigation is the best way to understand the nature of local soil conditions. Soil conditions will dictate plant selection. The filter bed of a rain garden (controlled by its mixture of soil) allows water to infiltrate the garden’s soil. If the filter bed is predominantly clay soils, it will have excellent water-retention properties. However, it will lack the ability to allow surface runoff to move quickly into the soil. Consequently, large-particle-sized amendments will need to be mixed into the filter bed. Plant materials in clay soils therefore must have strong roots systems in order to penetrate these compact conditions.
Conversely, soils of a sandy nature do not hold water well, requiring frequent watering. Infiltration rates, however, are often four to five times faster than clay. These soils require amendments with small particle size to improve retention. Plant materials in sandy soils therefore must have fibrous roots that will hold the granular elements together, holding the moisture for longer periods.
The Role of Non-Native Species. Most bioretention structures are populated with native plants best adapted to local growing conditions. However, the requirement for nutrient removal adds another layer of complexity to the plant selection. There may be times when non-native plants are introduced because of their ability to better treat runoff or succeed in drought conditions. Often, rain gardens are constructed in hot, urban environs near impervious surfaces. In most instances, the native soil has been modified, greatly affecting the water, oxygen, and nutrient makeup. Native plants will prosper in their preferred native habitat. However, in these harsh environs, non-native species may have a greater chance of survival due to a higher immunity to pests and diseases.
In general, bioretention schemes modeled on a grassland community rather than a wetland environment are better able to survive varying dry and wet conditions.
Maintenance and Soil Replacement
Unlike natural wetlands, bioretention BMPs require ongoing maintenance and soil replacement. Basic rules of thumb include:
- Inspect and repair erosion issues monthly
- Add or freshen mulch every six months
- Replace mulch/sand mixture every two to three years
- Keep plants alive and remove/replace dead plants
- Enlist a licensed engineer or landscape architect to inspect BMPs annually
Owners would also be wise to require an operation and maintenance agreement and plans for all BMPs. In addition to documenting best practices, an agreement can serve as a legal document to ensure maintenance and to clearly establish a maintenance schedule and responsible parties.
Another recommended practice is to invest in BMP maintenance and warranty bonds. These bonds ensure that funds are secured to maintain BMPs if the owner should fail to do so. In such cases, the controlling authority can cash the bond to fund necessary maintenance. Bonds are posted for a minimum of two years from the final approval of the BMP.
A Sample Plant Palette for the Piedmont
The Southeast drought of 2007–2008 primarily followed the I-85 Corridor through the Southeast, from Montgomery, AL, through Atlanta, GA; Charlotte, NC; and Raleigh, NC, also known as the Piedmont area. The sample palette for a rain garden BMP is this area is provided in Table 4. Materials are selected based on their proven ability to weather both drought and rainy season conditions, given proper maintenance.
Drought-Tolerant Plant Characteristics
Drought-tolerant plants have been used as an integral element in gardens and landscaping for hundreds of years. Some of the earliest examples could be found in fifth- and sixth-century Persian gardens, as well as Moorish gardens from the thirteenth century, where water played a primary role in their design and function. Since water was in short supply in these dry, hot climates, it was managed with great care. Plants were chosen that functioned best under low-water conditions while at the same time helped to create an oasis-like environment.
Drought-tolerant plants must be able to withstand low-water and high-heat conditions while still maintaining their aesthetic and functional qualities. Plants that thrive in naturally dry conditions sometimes have small or divided leaves that are waxy or hairy. These characteristics help plants hold in moisture, reducing water loss through transpiration.
Sometimes the plants have leaves that are spiny or lack leaves entirely. Some plants have leaf, stem, and root structures that promote evapotranspiration, which is the movement of water through a plant. Water enters a plant through its roots, then moves through the stalk, stems, or trunk and eventually evaporates into the atmosphere through the leaves and flowers. One of the primary goals of drought-tolerant landscaping is to use plants with low rates of evapotranspiration.
Some drought-tolerant plants survive periods of reduced summer water by going dormant and then resuming growth during the winter and spring. For example, think of beautiful spring bulbs, like tulips, daffodils, and irises. They grow, flower, and die before the dry season ever arrives. Many low-water-use plants actually avoid drought by producing wide-spreading or long roots to reach stored groundwater.
A plant’s natural growing conditions contribute to its appearance and characteristics. A drought-tolerant plant that thrives in dappled shade may have large, leathery leaves, whereas a plant that prefers full sun may have smaller, firmer foliage. Plants in high-heat areas may be drought-deciduous and lose their foliage altogether during times of low water. Leaf color generally depends on light exposure and tends to be lighter, silvery, or grayish for plants that live in full sun; plants from more shaded areas generally are darker.
Conclusion
Periods of drought are a natural part of the climate cycle, but drought doesn’t have to spell doom for bioretention BMPs. These structures stand a better chance of survival when they are constructed within an overall treatment train and when plant materials are carefully selected for versatility. When the bioretention footprint is reduced and strengthened, the overall performance and cost effectiveness of the stormwater management and treatment process can be improved.
By examining a series of BMP methods against nutrient removal standards and costs per surface square foot data, it is possible to develop a treatment train comparative cost analysis, responding to local treatment requirements for any combination of BMPs. This analytical tool can assist public and private owners, and their engineers, in arriving at a responsive and cost-effective solution.
Author's Bio: Richard S. Keagy, P.E., is a vice president for urban land engineering for Stantec Consulting Services Inc. in CHarlotte, NC. |
Author's Bio: Jerry Regenbogen, RLA, ASLA, is with Stantec in Charleston, SC. |
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