6. National Demonstration of Advanced Drainage Concepts using Green Solutions for CSO Control; US EPA and TetraTech, 2008 – 2013
The Kansas City demonstration project on the use of “green infrastructure” to minimize combined sewer overflows (funded by the US EPA and supported by a wide range of national and local agencies) uses a variety of integrated practices and modeling approaches. “Green infrastructure” includes a wide variety of stormwater runoff volume and pollutant reduction tools that can be applied in existing urban areas. Those being examined during this project include beneficial use of runoff, rain gardens, and biofilters. This extensive project is collecting data before, during, and after implementation of a variety of control practices in a 100 acre test watershed, and in a parallel control site. The reduction of discharges to the drainage system during wet weather will be calculated using models and verified through field monitoring. The continuous models determine the decreased amount of stormwater discharged for each event as the storage and infiltration facilities dynamically fill and drain over an extended period of time. Both developed stormwater and combined sewersheds can benefit from the added storage from areas retrofitted with bioretention cells or rain gardens and other management practices, e.g., inlet retrofits or curb-cuts with tree plantings.
Kansas City curb cut biofilter with monitoring station. / Kansas City porous concrete sidewalk in study area.The overall key project objectives are to:
· Demonstrate the integration of green solutions with traditional gray infrastructure in an urban-core neighborhood having a combined sewer system
· Develop a methodology for implementation of Green Solutions
· Measure the changes in the peak flow, total volume and pollutant mass of storm events in the receiving system or the reduction of combined wastewater volumes, pollutant loads and overflows
· Develop a model for predicting the quality and quantity benefits of implementing Green Solutions
· Compare economic costs and benefits of integrated green and gray solutions
Percentage reduction in annual runoff from directly connected roofs with the use of rain gardens. / Reduction of annual runoff from directly connected roofs with the use of runoff storage and irrigation.The watershed model (WinSLAMM) and the sewerage model (SWMM) are being calibrated for this area using the pre-construction flow and water quality data. Both dry and wet weather flow data are being recorded. The calibrated models were used early in the project to predict the benefits of the upland controls, and these predictions are being verified as the controls are installed. After the models are calibrated and verified for the demonstration area, they will be used to predict the benefits of wider application of the upland controls across the city. Specifically, the models will predict the decreased runoff volumes and peak runoff rates associated with upland stormwater controls to alleviate problems in the combined sewer system. Water quality benefits associated with stormwater pollutant discharge reductions of wet-weather flow particulates (including particle size distributions), nutrients, bacteria, and heavy metals are being quantified. WinSLAMM is used to calculate the stormwater contributions to the combined sewerage system during wet-weather by providing a time series of flows and water quality conditions, for various types of upland controls, while SWMM, with its detailed hydraulic modeling capabilities, focuses on the interaction of these time series data with the sewerage flows and detailed hydraulic conditions in the drainage system. Both models will be used interactively emphasizing their respective strengths.
The land survey found that about 65% of the area is landscaped, with most being in turf grass in poor to good condition. This information was used in conjunction with regional evapotranspiration data to calculate the amount of supplemental irrigation needed to meet the ET requirements of typical turf grass, considering the long-term rainfall patterns. Most of the supplemental irrigation would be needed during the months of July and August, while excess rainfall occurs in October through December (compared to ET requirements during these relatively dormant months). A single 35 gallon rain barrel per home is expected to reduce the total annual runoff by about 24% from the directly connected roofs, if the water use could be closely regulated to match the irrigation requirements. If four rain barrels per home were used (such as one on each corner of a house receiving runoff from separate roof downspouts), the total annual volume reductions could be as high as about 40%. Larger storage quantities result in increased beneficial usage, but likely require larger water tanks. Water use from a single water tank is also easier to control through soil moisture sensors and can be integrated with landscaping irrigation systems for almost automatic operation. A small tank about 5 ft in diameter and 6 ft in height is expected to result in about 75% total annual runoff reductions, while a larger 10ft diameter tank 6 ft tall could approach complete roof runoff control.
The use of rain barrels and rain gardens together at a home is more robust than using either method alone: the rain barrels would overflow into the rain gardens, so their irrigation use is not quite as critical. In order to obtain reductions of about 90% in the total annual runoff, it is necessary to have at least one rain garden per house, unless the number of rain barrels exceeds about 25 (or 1 small water tank) per house. In that case, the rain gardens can be reduced to about 80 ft2 per house.
Reduction in annual runoff from directly connected roofs with the use of rain gardens and roof runoff storage and irrigation.
The “best” combination of control options is not necessarily obvious. The CSO program must meet their permit requirements that specify certain amounts of upland storage in the watershed. Other elements, including costs, aesthetics, improvements to street-side infrastructure, and other benefits, also need to be considered in a decision analysis framework.
6.1 Stormwater Controls to Satisfy Developing Regulations
Newly proposed stormwater regulations being promulgated by state and federal regulatory agencies are stressing significant reductions in runoff volumes for new development, even areas of poor soils. Many of the above research projects, along with our past research results, have been incorporated into stormwater management models and demonstration projects that can show how these regulations can be addressed. However, it is important that various precautions are considered in challenging conditions.
The Energy Independence and Security Act of 2007” was signed into Law on Dec. 19, 2007. Title IV (“Energy Savings in Building and Industry”), Subtitle C (“High Performance Federal Buildings”) Sec. 438 (“Storm Water Runoff Requirements for Federal Development Projects”) requires that: “The sponsor of any development or redevelopment project involving a Federal facility with a footprint that exceeds 5,000 square feet shall use site planning, design, construction, and maintenance strategies for the property to maintain or restore, to the maximum extent technically feasible, the predevelopment hydrology of the property with regard to the temperature, rate, volume, and duration of flow.” This new provision requires much more attention to controlling runoff volume, in addition to other hydrologic features. Current proposed state regulations require runoff volume restrictions so that post-development runoff volumes meet pre-development runoff volumes for the 2-year rainfall (about 4 inches for the central Alabama area).
Our past research has examined regional soils and how they may affect infiltration capacity:
Loss of infiltration capacity due to soil disturbance and compaction during construction.
We have also researched groundwater contamination potential for stormwater infiltration. Potential groundwater problems are affected by a stormwater pollutant’s abundance in the stormwater, its mobility through the unsaturated zone above the groundwater, and the treatment received before infiltration. Basically, with surface infiltration with minimal pretreatment (grass swales or roof disconnections), mobility and abundance are most critical. With surface infiltration with sedimentation pretreatment (treatment train: sedimentation then media filtration), mobility, abundance, and treatability are all important. With subsurface injection with minimal pretreatment (porous pavement in parking lot or dry well), only abundance affects groundwater contamination potential. We have found that infiltration devices should not be used in most industrial areas without adequate pretreatment. Runoff from critical source areas (mostly in commercial areas) needs to receive adequate pretreatment prior to infiltration. However, runoff from residential areas (the largest component of urban runoff in most cities) is generally the least polluted and should be considered for infiltration.
Considerations for the use of porous pavement in Central Alabama:
• Soils having at least 0.1 in/hr infiltration rates can totally remove the runoff from porous pavement areas, assuming about 1 ft coarse rock storage layer. Porous pavement areas can effectively contribute zero runoff, if well maintained.
• However, slow infiltrating soils can result in slow drainage times of several days. Soils having infiltration rates of at least 0.5 in/hr can drain the pavement structure and storage area within a day, a generally accepted goal.
• These porous pavements can totally reduce the runoff during the intense 2-year rains.
• Good design and construction practice is necessary to prolong the life of the porous pavements, including restricting runon, prohibiting dirt and debris tracking, and suitable intensive cleaning.
Considerations for the use of green roofs in Central Alabama:
• Green roofs can contribute to energy savings in operation of a building, can prolong the life of the roof structure, and can reduce the amount of roof runoff.
• They can be costly. However, they may be one of the few options for stormwater volume control in ultra-urban areas where ground–level options are not available.
• Irrigation of the plants is likely necessary to prevent wilting and death during dry periods.
Annual roof runoff reductions for local green roofs.
• Vegetated green roofs can reduce up to about 70% of the annual roof runoff during typical conditions, if the complete roof is planted.
• The plants would likely wilt and die as the evapotranspiration (ET) drives the substrate to the plants’ wilting point during the late summer, early fall period, requiring substantial irrigation.
Considerations for the use of rain gardens for controlling roof and paved area runoff in Central Alabama:
• Simple rain gardens with extensive excavations or underdrains can be used near buildings for the control of roof runoff, or can be placed in or around the edges of parking areas for the control of runoff from parking areas.
• Rain gardens provide greater groundwater contamination protection compared to porous pavements as the engineered soil fill material should contain significant organic material that hinders migration of many stormwater pollutants. This material also provides much better control of fine sediment found in the stormwater.
• Rain gardens can be sized to control large fractions of the runoff, but maintenance to prevent clogging and to remove contaminated soils is also necessary.
Annual runoff reductions from paved areas or roofs for different sized rain gardens and soils.
• Local rain gardens should be located in areas having soil infiltration rates of at least 0.3 in/hr. Lower rates result in very large and much less effective rain gardens, and the likely clay content of the soil likely will result in premature clogging.
• Rain gardens should be from 5 to 10 percent of the drainage area to provide significant runoff reductions (75+%).
• Rain gardens of this size will result in about 40 to 60% reductions in runoff volume from the large 4 inch rain. Rain gardens would need to be about 20% of the drainage area in order to approach complete control of these large rains.
• Roof runoff contains relatively little particulate matter and rain gardens at least 1% of the roof area are not likely to clog (estimated 20 to 50 years).
• Paved area runoff contains a much greater amount of particulate matter and would need to be at least 10% of the paved area to have an extended life (>10 years).
Newly published federal construction site and stormwater regulations will require much more careful site planning. Runoff volume controls during large events will require extensive use of infiltration practices. The sizes of practices for the same land use is not very sensitive to soil conditions (less runoff increases compared to pre-development conditions with poorer soils and therefore lower volume reduction goals). However, use of infiltration controls in poor soils is not a very robust/sustainable practice, and needs to be done with caution and over-sizing.