Rain gardens should look and perform like gardens. Because the process is relatively new, there has been some controversy over the soils used in bio-retention systems. Unfortunately, the complexity of the soils and influx of silt into the system from the runoff can result in the clogging and failureof the system. Most systems using natural soil parameters may result in higher maintenance or total replacement in five years. If the nutrient content is too high, the opportunities for the nutrients to be discharged into a creek or other body of water during the establishment period are greater. The other concern is the aesthetics of the plants. It is difficult to find plant material that can survive saturated soil for a period of time and then survive during the dry periods for an undefined period of time. A bed of weedy suffering plants does little to promote the use of rain gardens.
Using expanded shale, clay or slate (ESCS) fines to provide additional air space deep in the system profile showed increased oxygen for the plant roots. Plant roots provide the perfect environment for the microbes to do their job while increasing phosphorous removal. Do not underestimate the value that plants have in this process; not only for cleansing the runoff but also by providing an attractive alternative to stagnant holding ponds. Extensive and vital research projects with UNC Charlotte involving laboratory column testing of the expanded slate fines and field trials at NCSU were completed. The research demonstrated that the expanded slate fines used in bio-retention media functioned in a manner acceptable to the North Carolina DENR Division of Water Quality. The field trial research also demonstrated that plants establish quicker and survive during dry periods. The aeration capabilities of the media allow roots to grow longer and deeper within the system. ESCS fines enhance the natural physical and biological processes in vegetated bioretention by promoting healthy root systems that contribute to the remediation of pollutants found in stormwater runoff.
The angular shape of the particles includes micropores providing an additional porous surface area containing 54% available pore space on the particle surface allowing water to be retained and released slowly within the system. This angular shape also can trap fines from runoff. This reduces the chance of the migration of fines to the base of the system, thereby helping to prevent future clogging. Internal storage will contain oil and grease contamination thereby preventing pollution from overflow releases resulting from subsequent rain events. Additional treatment continues by way of adsorption and retention. When compacted, the porosity and angular shape allows the expanded slate fines material to meet the required infiltration rates. The system will be more sustainable because silt contamination from site erosion and sludge from pavement is trapped on the media surface and can easily be skimmed and removed.
Case studies show how the selection of growing media can impact success or failure. Established North Carolina projects in Raleigh, Charlotte and Asheville will be showcased such as, Northlake Mall in Huntersville, NC and an award winning project at NCSU. Alternative media allows planners and developers a broader palette of BMP designs to meet the required nutrient load reductions, aesthetics and maintenance costs. Engineers and planners dedicate hours to designing stormwater control systems. Therefore, the soils and filter media must be thoughtfully selected in order to fulfill the concept from the drawing board to the field.
See the related article below on the Best Urban Best Management Practice in the Bay Award contest (BUBBA) recognizing the best urban BMPs.
by Chuck Friedrich, RLA, ASLA, Carolina Stalite Company
New to the BUBBAs this year, this award category of urban best management practice presented by the Chesapeake Stormwater Network looks for nontraditional solutions for the management or mitigation of stormwater at a site that can be applied locally to benefit the Chesapeake Bay watershed.
One honoree, The Artists’ Backyard, is a multiphase stormwater retrofit project on the NC State University campus. Implemented from 2011-2014, the project was designed and built by the NC State Department of Landscape Architecture Design+Build Studio in partnership with NC State University Housing. The project addresses numerous stormwater issues using bioinfiltration (rain garden media using 80%expanded slatefines blended with 20% pine bark fines)and conveyance devices, permeable paving, structural and amended soils, a 500-gallon cistern and hydro-zoned plantings.