Rain Gardens Water Quality the Way Mother Nature Intended Maintenance Discussion Design Discussion Susan Morgan, Ph.D, PE Southern Illinois University Edwardsville smorgan@siue.edu (618) 650 5014 Michael Rosborg, PE, CFM Hoelscher Engineering mike.rosborg@hoelscherengr.com (618) 624 8610
Rain Garden / Bioretention Background
Bioretention Bio Swales Bioretention Basins Engineered Wetlands Rain Gardens are Bioretention Basins
On its website, Prince George s County, Maryland, claims possession of bioretention early development: Bioretention is one of the main components of the Low Impact Development (LID) approach for managing stormwater runoff. Bioretention was developed by Prince George's County around 1990 and has become the "Integrated Management Practice" (or IMP) of choice around the world. The County continues to refine the bioretention IMP design and its effectiveness has been documented by studies that show significant pollutant loading reductions. (http://www.princegeorgescountymd.gov/der/esg/bioretention/bioretention.asp)
Bioretention Regulation With Prince George s County paving the way, the regulatory design authority for bioretention has de facto become the Maryland Department of the Environment and their publication of the Maryland Stormwater Design Manual. U.S. Green Building Council acknowledges the manual for LEED Reference Guides St. Louis Metropolitan Sewer District has adopted the manual in implementing its water quality regulations in 2006.
Bioretention Usage Relatively new to St. Louis Since 2006, approximately 200 Rain Gardens permitted in St. Louis County (Jay Hoskins MSD) Invented in early 1990 s Engineering standing water? Scary proposition to an owner.
Bioretention Usage Environmental Site Design The use of structural water quality treatment will now be discouraged strongly, and subsequently bioretention will be encouraged. The Maryland Stormwater Design Manual has a revised Chapter Five that encourages Environmental Site Design. Other monikers for ESD include low impact development, green building, smart building, etc. (3)
Bioretention Pre Design Best Management Practices Reduce Impervious Area Provide only minimum required parking spaces / Minimum Widths (De educate) Infiltration type pavement Pervious Concrete Pervious Asphalt Permeable Pavers
Bioretention Pre Design
Bioretention Design Capture and treat the runoff from 90% of the average annual rainfall. (2) For St. Louis, P = 1.14 inches in 24 hour Duration Determine Drainage Area to Rain Garden Maryland Manual 5 acres maximum Prince George s County 2 acres maximum What if drainage areas are larger?
Bioretention Design Determine Soils Types USDA Web Soil Survey http://websoilsurvey.nrcs.usda.gov/app/ Measure Impervious Area Pavement Roofs Gravel Measure Pervious Area Grass Mulch
Bioretention Design Determine Water Quality Volume Requirement WQ v = (P) (R v ) (A) / 12 P = 90% Rainfall 1.14 inches by MSD standard 1 inch for humid climates by LEED standard R v = Volumetric Runoff Coefficient = (0.05) + (0.009) (I) I = Percent impervious cover on site A = Onsite area in acres Bypass offsite area around BMP
Bioretention Design *Prince George s County Bioretention Manual:
Bioretention Design Filter Area is sized based on principles of Darcy s Law A f = (WQ v ) (d f ) / [ (k) (h f + d f ) (t f )] A f = Surface Area of filter bed (ft 2 ) WQ v = Water Quality Volume (ft 3 ) d f = Filter bed depth (Can be 2.5 to 4.0 feet at the discretion of designer) k = Coefficient of permeability of filter media (0.5 ft/day for bioretention soil)* h f = Average height of water above filter bed (0.25 feet, residential to 0.5 feet, commercial) t f = Design filter bed drain time (2 days for bioretention)
Bioretention Design Pretreatment Rain Gardens are unique; pretreatment is provided when: 20 foot Grass Strip (where there s room) Sand Strip (where there s not room) Gravel Diaphragm Mulch Layer
Bioretention Design Entire system shall store 75% prior to filtration V temp = 0.75 * (WQ v ) Check actual storage provided For bioretention, V temp = V treatment V treatment = (h ponding * Af) + [ (df) (Af) (φ) ] A f = Surface Area of filter bed (ft 2 ) WQ v = Water Quality Volume (ft 3 ) d f = Filter bed depth (2.5 to 4.0 feet) φ = Porosity of Filter Media (0.4) h ponding = Max. height of water above filter bed (1.0 ft, commercial)
Bioretention Design What else? Provide specifications for planting soil, mulch, pea gravel, geotextile, gravel diaphragm, sand Pick plantings Provide installation specifications Prepare an Inspection & Maintenance plan
Bioretention Design Example:
Bioretention Design Example: School District New Campus Water Quality Volume Required (Area to Bioretention) Water Quality Volume (WQv) WQv = [ (P) (Rv) (A) ] / 12 90% Rainfall Event (P) P = 1.14 in. Total Area & Percent Impervious Area (A & I) Type Area (Ac.) Impervious 0.912 Pervious 2.296 A = 3.209 Ac. I = 28.4 % Volumetric Runoff Coefficient (Rv) Rv = 0.05 + 0.009 (I) Rv = 0.306 WQv = 0.093 Ac. Ft. WQv = 4061 Cu. Ft.
Bioretention Design Example: School District New Campus Filter Area Required Filter Area (Af) Af = (WQv) (df) / [ (k) (hf + df) (tf) ] Water Quality Volume (WQv) WQv = 4061 cu. ft. Filter Bed Depth (df) df = 2.50 ft. Coefficient of Permeability of Filter Soil (k) k = 0.50 ft/day Average Height of Water Above Filter (hf) (Max. Ponding 1-foot) hf = 0.50 ft Design Filter Bed Drain Time (tf) tf = 2.00 days Af = 3384 Sq. Ft.
Bioretention Design Example:
Bioretention Design Example:
References: 1. Green Building Design and Construction, LEED Reference Guide for Green Building Design and Construction; (U.S. Green Building Council, 2009 Edition) 2. Maryland Stormwater Design Manual (Maryland Department of the Environment, October 2000 & 2009 Update) 3. Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices (U.S. Environmental Protection Agency, December 2007) 4. Bioretention Manual (The Prince George s County ESD, December 2007) 5. Metropolitan St. Louis Sewer District, Development Review Process Plan Review Presentations http://www.stlmsd.com/msd/pgmsprojs/planreview/search