Design Manual: Biological Filtration Canal (Bioswale) Dayna Yocum, Bren School of Environmental Science and Management, University of California, Santa Barbara Figure 1. Bioswale along roadside. Photo courtesy of CalTrans Description: A biological filtration canal is a shallow depression created in the earth to accept and convey stormwater runoff. A biological filtration canal uses natural means, including herbaceous vegetation and soil, to treat stormwater by filtering out contaminants being conveyed in the water. Canals require shallow slopes that drain well, and function best under light to moderate runoff conditions (Clar et al. 2004). Purpose: storm water treatment, pollutant removal (suspended solids, nitrogen, phosphorus) by vegetation uptake, vegetation slows flow down and encourages sedimentation, cleans water by biota consumption, encourages infiltration into the subsurface zone, which reduces flow volume. Optimum design of channel dimensions, longitudinal slope, type of vegetation, and use of check dams will improve pollutant removal rates. Figure 2. Diagram of typical bioswale (adapted from UDFCD 1999) 1
Table 1. Optimal Design Measurements (City of Salem 2005) Optimal Design Measurements Minimum Maximum Optimum Depth (m) 1 4 2.5 Bottom Width (m) 1 4 Channel Width (m) 7 12 Length (m) 30 Slope (%) 1% 6% 3% Sideslope (%) 20% 25% 20% Drainage Area (hectares) 1 4 Vegetation Height (mm) 100 150 150 Height above Groundwater (m) 3 3 Figure 3. Check dam (USDA - Natural Resources Conservation Service) (left), diagram of check dam positioning (adapted from King County 2005) (center), check dam (North Carolina Department of Environment and Natural Resources) (top right). 2
Materials Organic Compost (m 2 ) Ideal Location: Down-gradient of impermeable surfaces or contaminated areas. Contributing area should be less than 4 hectares. Soil (m 2 ) Tractor Seeds for vegetation and top soil (m 2 ) Gravel (m 2 ) Small Stones (m 2 ) Shovels Large stones for check dams Cost: $200 4,000 for a 200m 2 bioswale (7m x 30 m), depending on availability of free labor and materials such as compost and soil (NCGBTD 2003). Figure 4. A bioswale (without check dams) during a storm. Steps for Construction 1. Identify site according to specifications a. Contributing drainage area should be less than 4 hectares b. Canal should be downstream of impervious surfaces c. Canal must be able to dry between storms the bottom of the canal should be at least 3 feet above the groundwater to prevent bottom from remaining too wet d. Canal should be at least 30 meters in length and 7 meters in width e. Residence time of water in the canal should be from 5 10 minutes. 2. Decide on date of construction canal construction should be finished 3 months before rainy season so that vegetation can fully establish itself 3. Calculate flow and estimate 2-year storm height The bioswale should be built so that it can accommodate the flow from the size of a storm that occurs every two years. This can calculate by knowing the drainage area, and rainfall intensity for a 2-year storm. Consult an engineer to choose the depth, length, and width necessary for that flow level. 3
4. Design canal and decide where to put check dams check dams create shallow ponds to increase infiltration and settling a. Check dams should be located such that the upstream limit of ponding from one check dam is just below the downstream edge of the adjacent check dam. b. Include orifice at bottom of dam to permit water passage during low flows c. To prevent mosquito development, check dams should be designed to pond water to a depth that will infiltrate within 24 hours of the end of the storm. 5. Design inlet to allow peak flows to bypass Figure 5. Bioswale with check dams - this prevents high velocity flows of large storms from eroding the bioswale 6. Gather materials use table below to do make a list of material quantities a. To calculate quantity of compost and natural soil add the area of compost and soil needed for the bottom plus area needed for the slopes: Quantity of compost (m 2 ) = length x width of bottom x 0.150m + length x width of lateral section x 0.075m 2 Quantity of soil (m 2 ) = length x width of bottom x 0.150m + length x width of lateral section x 0.075m 2 b. To calculate quantity of gravel: Quantity of gravel (m 2 ) = length x width of bottom x 0.150m c. Seeds for vegetation have enough seeds to cover the area of the bottom: Area of bottom = length x width of bottom d. Vegetation must: i. Provide a dense cover and strong root structure ii. Stand upright in strong water flows iii. Tolerate periodic flooding and drought iv. Cannot be dormant during rainy season v. Should receive sun most of the day 7. Dig canal to specifications determined by diagram (page 1), and calculations of 2-year storm flow 8. Mix soil and compost together this adds infiltration capacity and nutrients for vegetation growth. Bioswales with compost added to bed material have been shown to experience faster growth, thicker coverage, and higher removal efficiency than in bioswales with regular soil (The Clean Washington Center 1997) 9. Lay gravel to a thickness of 150mm 10. On top of gravel, lay compost / natural dirt mixture to a thickness of 150mm 4
11. Plant and irrigate seed as normal a. Vegetation should provide a dense cover and a strong root or rhizome structure that resists erosion (Jurries 2003) b. Be sure that seed is not inundated for too long during germination this will hinder growth c. Seeds should be planted in enough time for the vegetation to be very strong when the first storm occurs 12. Construct a flow diversion in the case of early storms flow should be diverted until vegetation becomes established 13. Provide maintenance as needed a. Remove and replant vegetation when it dies this removes metals and pollutants that have been filtered by plants b. Remove invasive species c. Remove sediment when it accumulates d. Irrigate when needed to maintain vegetation (City of Salem 2005) Additional Information 1. Note: Soils with more organic matter remove a greater amount of metals due to the high cation exchange capacity (Jurries 2003) 2. Common Problems with Construction: a. Workers are unfamiliar with the concept behind bioswales b. Timing of the construction of the bioswale is delayed and vegetation is unable to establish itself before storm occurs (Mazer) c. Excessive compaction of the bioswale subsoil and subsequent failure to correct the problem d. Poor timing of bioswale construction, such that the bioswale is inundated with sedimentladen runoff, seeding is washed off by the first rains, or erosion occurs in the bioswale and turbidity laden waters are discharged e. Cost cutting occurs in the selection and planting of the vegetation f. Poor preparation of the swale channel to accept flows, for example check dams are not installed 5
6 This bioswale is located at the low point of a grassy area. Goleta, CA
References CalTrans. "Design of Biofiltration Swales and Strips." Ed. California Department of Transportation. City of Salem. "What Is a 'Bioswale'?" City of Salem, Oregon Public Works Website, 2005. Clar, Michael L, Billy J. Barfield, and Thomas P. O'Connor. "Stormwater Best Management Practice Design Guide, Vegetative Biofilters." 2 (2004). Jurries, Dennis. "Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal." Ed. State of Oregon Department of Environmental Quality, 2003. King County. "Surface Water Design Manual." Ed. King County Water and Land Resources Division, Washington, 2005. Mazer, Greg. Environmental Limitation to Vegetation Establishment and Growth in Vegetated Stormwater Biofilters: Center for Urban Water Resources Management. North Carolina Department of Environment and Natural Resources. "Rock Check Dam." Ed. rockcheckdam.jpg: North Carolina Department of Environment and Natural Resources. North Carolina Green Building Technology Database (NCGBTD). "Bioswale Case Studies." North Carolina Green Building Technology Database, 2003. The Clean Washington Center and E & A Environmental Consultants, Inc. "Study of Compost Use in Bioswales for Compost Market Expansion." (1997). Urban Drainage and Flood Control District (UDFCD). "Conceptual Schematic of Trapezoidal Grass-Lined Swale Section." 1999. USDA-Natural Resources Conservation Service - Illinois. "Rock Check Dam." Ed. urbst905.jpg: Natural Resource Conservation Service. Yu, Shaw L, et al. "Field Test of Grassed-Swale Performance in Removing Runoff Pollution." Journal of Water Resources Planning and Management.May/June 2001 (2001). 7