Retrofit of the Gwinnett County DWR Facility Using Low Impact Development Practices SESWA 2012 Annual Conference October 19, 2012 Jeff Herr, P.E., D.WRE National Stormwater Leader jherr@brwncald.com
Acknowledgements Steve Leo, Director, Stormwater Management Division, Gwinnett County Department of Water Resources Pete Wright, Stormwater Management Division, Gwinnett County Department of Water Resources Chuck Graham, Stormwater Management Division, Gwinnett County Department of Water Resources Jill Stachura, Project Manager, Brown and Caldwell
Federal LID Requirements are Expected New EPA stormwater treatment requirements will likely include volume reduction (use of LID practices). Requires newly developed and redeveloped federal facilities (> 5000 sq ft) to provide MEP volume reduction (focus on LID practices) for the 95 th percentile storm, typically 1.5 2.0 inches.
Project Objectives Determine if it is possible to infiltrate and store/reuse on-site the runoff from the 95 th percentile storm event Determine effectiveness of different LID practices Determine construction and operation and maintenance requirements for infiltration and storage/reuse facilities Determine capital, O&M and life cycle costs for different types of LID practices Use constructed project to educate developers, engineers, and the public on the use of LID practices Promote the use of viable LID practices in Gwinnett County
Project Site Aerial Photograph ~31 acres total, 18.5 acres impervious, 60% impervious. Decided only the immediate site would be used for treatment; no off-site treatment. Wanted to demonstrate LID can be used for any level of imperviousness allowed by County code; residential, commercial, or industrial.
Completed Project Site Drainage Sub-Basin Delineation
Developed Historical Annual Rainfall Probability Distribution for Atlanta 1.73 inch event = 95 th % Total average annual rainfall = 49.6 inches.
Performed Subsurface Testing and Evaluated Site Soil Types
Completed In-Situ Infiltration Testing at Proposed Bottom of Engineered Soils Measured infiltration rate = 1 to 6 inches per hour at 6-8 ft below existing grade; proposed bottom elevation of infiltration practices.
Goal to Include at Least One of Each of These LID Practices in the Design
Initially Evaluated Three Different LID BMPs per Sub- Basin Evaluated effectiveness for runoff volume reduction and cost.
Developed Three Alternative Overall Site Retrofit Plans- Alternative A
Unit Capital, O&M, and Life Cycle Costs Units Capital Cost ($) Annual O&M Cost ($) LID Practice Bioretention area $/sq.ft. 5 40 0.2 2.00 Rain garden $/sq.ft. 5 40 0.28 2.00 Green roof $/sq.ft. 15 25 0.54 1.00 Vegetated swale $/sq.ft. 2 7 0.05 0.50 Filter strip $/LF 40 400 10 40 Planter boxes $/box 2,500 10,000 250 1,000 Cisterns HDPE $/gal 0.70 1.40 0.05 0.20 Cisterns Concrete $/gal 1.50 5.00 0.05 0.40 Permeable asphalt $/sq.ft. 5 15 0.05 0.50 Porous concrete $/sq.ft. 6 20 0.05 0.50 Grass pavers $/sq.ft. 5 10 0.05 0.50 Concrete paver blocks $/sq.ft. 8 15 0.05 0.50 Planting Trees/Urban forestry $/tree 200 1,000 10 30 Hydrodynamic devices $/device 10,000 100,000 2,000 8,000
Preliminary Plan LID Practices Utilized
Estimated Runoff Volumes and Load Reductions for Sub-Basins and Project Site
Final Preliminary Plan Met with all DWR Department Managers and incorporated comments.
Moving Forward Estimated Construction Cost = $1,000,000 Estimated Annual O&M cost = $90,000 Submitted 319 (h) grant application to GA EPD; Requested $600,000 with $400,000 local match Post construction monitoring proposed for each type of LID practice Construction of Phase I (bioretention area and roof rainwater harvesting system) completed in July 2011 using another 319 (h) grant
Rain Barrel and Garden
Phase I Construction Completed in one month, July 2011
Pre-Construction Site Area Drainage area = 1 ac Impervious area = 0.7 ac (parking area) Parking drains to one curb inlet. 0.3 acre raised grass area available. Curb inlet for parking area
Excavation and Underdrain Installation Excavated area for engineered soils (3-4 ft) and loosened soil to a depth 2 ft. below proposed bottom of engineered soils. Installed 6-inch HDPE underdrain and gravel trench with 15-inch risers.
Engineered Soils and Gravel Backfill Installed engineer soils in 1 ft lifts and watered to consolidate. Installed gravel trenches across area to aid in runoff distribution.
Backfill Completion and Final Grading Installed remaining engineered soils in 1 ft lifts and watered to consolidate. Final surface grading 3-5H:1V slopes.
Surface Treatment Cobble, mulch, plants and sod
Completed Project Frequent watering was initially required due to sandy soils. Installed a 5,000 gallon tank next to building and roof rainwater harvesting system in September for watering bioretention. (1-inch rain = 1,000 gallons of water)
Completed Project Rainwater Harvesting 5,000 Gallon Storage and Irrigation Reuse
Completed Project 3-inch tall curb around inlet; runoff overflows into curb inlet when pond is full. Underdrain and risers have closed valve; Installed to be used only if pond does not infiltrate naturally. Construction cost = $100,000 Primary cost was engineered soil purchase. New curb Existing Inlet New curb cuts
May 2012
Monitoring WERF LID Study
Plan Refinement and 319 Grant Application Submitted new 319 grant application for retrofit of remaining front portion of site $666,000 total/$400,000 grant Announcement March 2013 Contract awarded October 2013
Questions Jeff Herr, P.E., D.WRE jherr@brwncald.com 770-673-3673