West Virginia Stormwater Manual Training Workshop A Chesapeake Bay Stormwater Training Partnership Workshop December 5, 2012 9:00 AM 4:30 PM Robert C. Byrd Health Sciences Center Eastern 2500 Foundation Way Martinsburg, WV 25401 Objective: To provide practitioners and reviewers with information and design guidelines related to innovations in urban stormwater management. The Chesapeake Stormwater Partnership: A training program for stormwater management professionals created by the Chesapeake Stormwater Network and the Center for Watershed Protection. It is sponsored by the National Fish and Wildlife Foundation s Chesapeake Bay Stewardship Fund. WV DEP is providing logistical support for the workshop. Instructors: Tom Schueler, Chesapeake Stormwater Network Cecilia Lane, Chesapeake Stormwater Network Dave Hirschman, Center for Watershed Protection Alana Hartman, West Virginia Department of Environmental Protection Sherry Wilkins, West Virginia Department of Environmental Protection Matt Pennington, Eastern Panhandle Regional Planning & Development Council ( Region 9 ) 9:00 9:15 Welcome/Introductions 9:15 9:45 Stormwater Management in West Virginia 9:45 10:15 West Virginia Stormwater Manual: Methods CWP 10:15 10:30 Break 10:30 12:00 West Virginia Stormwater Manual: Stormwater BMPs 12:00 1:00 Lunch Alana Hartman, WVDEP Sherry Wilkins, WVDEP CWP 1:00 2:00 Site Design Exercise Guided CSN 2:00 3:45 Site Design Exercise Break-out Sessions CSN 3:45 4:15 Site Design Exercise Report-out CWP 4:15 4:30 Wrap-up, Discussion and Evaluations Matthew Pennington, Region 9
The Chesapeake Stormwater Network (CSN) is an organization whose mission is to improve on the ground implementation of more sustainable stormwater management and environmental site design practices in each of 1300 communities and seven states in the Chesapeake Bay Watershed. The Network is coordinated by Tom Schueler and Cecilia Lane is located in Ellicott City, MD. Tom has more than 30 years experience in practical aspects of stormwater practices to protect and restore urban watersheds. As executive director of CSN, Tom works on a number of projects in the Bay including the Chesapeake Bay Stormwater Training Partnership, which provides webcasts, workshops and on-line training modules to train engineers on new practices. Tom also serves as the stormwater technical coordinator for the EPA Chesapeake Bay Program. Cecilia joined the Network in January coming from the Center for Watershed Protection where she served as a Watershed Technician for 3 years working on stormwater management issues and specifically retrofitting. At the Network, Cecilia is heading up several tasks on the Chesapeake Bay Stormwater Training Partnership including developing training content on LID Maintenance and Retrofitting. Since 1992, the Center for Watershed Protection (CWP) has been working in numerous communities to provide the solutions for clean water and healthy natural resources. Their work is based on sound scientific research and guided by a passion for advancing the state-of-the art, ensuring practitioners have the right tools, and promoting the widespread implementation of the most effective watershed management techniques. Dave Hirschman is a program director at the Center for Watershed Protection. Dave joined the Center staff in 2005 as a Senior Water Resources Specialist. He has worked for 20 years in the public, private, and non-profit sectors on a variety of water resources and environmental health issues. In his life prior to the Center, Dave managed a water resources program for a local government, managed a non-profit organization, ran a small environmental consulting office, and taught water resources courses at the university level. His areas of concentration include innovative stormwater design and implementation, stormwater program development, land use planning, and training. Dave works for the Center from the satellite office in Charlottesville, Virginia. He has a B.A. in Biology from Duke University and a Masters of Urban & Regional Planning from Virginia Tech. Alana Hartman is one of four Basin Coordinators at WVDEP who are responsible for coordinating local efforts to implement water quality improvement projects. Basin Coordinators foster and support volunteer watershed associations and groups, educate citizens on nonpoint source pollution issues, help identify local stakeholders, partners and funding sources, and facilitate Project Teams to get improvements on the ground. Alana also works on the Chesapeake Bay cleanup effort in West Virginia, helping to reduce nitrogen, phosphorus, and sediment entering local streams and reaching the Bay.
Sherry Wilkins has worked at the West Virginia Department of Environmental Protection for 19 years. She is currently the MS4 program coordinator for the WV DEP. She has more than 29 years experience in water protection and regulatory programs. Her past experiences include working with a stormwater contractor in Columbus, Ohio, private consulting on stormwater issues for projects in West Virginia and 11 years in the field as an environmental inspector with WVDEP s office of Environmental Enforcement. Sherry holds a Bachelor of Science degree in Biology from Florida Atlantic University. One of her favorite pastimes is traveling and enjoying the water resources she endeavors to protect. Matthew Pennington earned his degree in Urban and Regional Planning. He has worked for public planning commissions reviewing development plans, as well as private engineering firms designing development projects for review and approval from zoning and planning boards. Matthew is currently working for a regional planning council as their Chesapeake Bay Coordinator who assists local government and nongovernment organizations improve water quality in their area.
West Virginia Stormwater Management Manual: Methods Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff. www.cbstp.org www.chesapeakestormwater.net
Chesapeake Bay Stormwater Training Partnership 2
Things That Go Together Well Stormwater Manual Design Compliance Spreadsheet Cistern Design, Checklists, other Tools Chesapeake Bay Stormwater Training Partnership 3
Performance Standards for Site and Neighborhood Design 5.a.ii.A.1.:... require management measures that keep and manage on site the first one-inch of rainfall... Means: require stormwater practices that are designed and sized to capture and treat the runoff from a 1-inch rain event. Chesapeake Bay Stormwater Training Partnership 4
Performance Standards for Site and Neighborhood Design 5.a.ii.A.1. (continued): Runoff volume reduction can be achieved by canopy interception, soil amendments, evaporation, rainfall harvesting, engineered infiltration, extended filtration and/or evapotranspiration and any combination of the aforementioned practices. Means: The permit establishes that a wide array of runoff reduction pathways are considered acceptable Chesapeake Bay Stormwater Training Partnership 5
Manual Outline Chapter 1: Introduction Chapter 2: WV Stormwater Regulations Chapter 3: BMP Selection & Design Methodology Chapter 4: BMP Specifications Chapter 5: Stormwater Hotspots Chapter 6: Design Examples Chesapeake Bay Stormwater Training Partnership 6
Manual Outline Appendix A: Checklists Appendix B: Infiltration Testing Appendix C: Stormwater Design in Karst Appendix D: Soil Amendments Appendix E: Treatment Volume Peak Flow Rate Appendix F: BMP Landscaping & Plant Lists Appendix G: Resources for Dry & Wet Ponds Appendix H: Considerations for Impaired Waters Chesapeake Bay Stormwater Training Partnership 7
Chapter 4: BMP Specifications 4.1. Better Site Design 4.2.1. Sheet Flow to Filter Strip or Conservation Area 4.2.2. Impervious Disconnection 4.2.3. Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden 4.2.4. Permeable Pavement 4.2.5. Grass Swale Chesapeake Bay Stormwater Training Partnership 8
Chapter 4: BMP Specifications 4.2.6. Infiltration 4.2.7. Regenerative Stormwater Conveyance 4.2.8. Rainwater Harvesting 4.2.9. Vegetated Roofs 4.2.10. Filtration Practices 4.2.11. Stormwater Wetlands Chesapeake Bay Stormwater Training Partnership 9
Methods 1. Runoff Reduction 2. Treatment Volume 3. BMPs in Series 4. Design Volume 5. Level 1 & Level 2 Design Chesapeake Bay Stormwater Training Partnership 10
1. Runoff Reduction Runoff Reduction processes & capabilities vary by practice Infiltration Canopy Interception Evaporation Transpiration Rainwater Harvesting Extended Filtration Chesapeake Center Bay for Stormwater Watershed Training Protection Partnership 11
Typical Extended Filtration Chesapeake Bay Stormwater Training Partnership 12
Chesapeake Bay Stormwater Training Partnership 13
2. Treatment Volume: Runoff Coefficient for Land Cover Types Chesapeake Bay Stormwater Training Partnership 14
Treatment Volume Calculation Chesapeake Bay Stormwater Training Partnership 15
Treatment Volume Achieve Treatment Volume reduction at site scale Plan BMPs for each drainage area on the site 15 Acres, 25, 0.6 acre lots Impervious = 3 ac. Managed Turf = 11 ac. Forest/Open Space = 1 ac. Treatment Volume (Tv) = 19,130 cubic feet Chesapeake Bay Stormwater Training Partnership 16
3. Practices in Series Chesapeake Bay Stormwater Training Partnership 17
Treatment Volume = 18,100 cubic feet BMP 3 BMP 2 BMP 1 Chesapeake Bay Stormwater Training Partnership 18
4. Design Volume (Dv) For Treatment Volume = 18,100 cubic feet Dv = 5,100 cf Dv = 5,500 cf Dv = 7,500 cf Chesapeake Bay Stormwater Training Partnership 19
Drainage Area (DA) BMP 1 BMP 2 Treatment Volume (Tv) = DA size x runoff coefficient x 1.0 inch BMP 3 Tv = Design Volume BMP 1 + Design Volume BMP 2 + Design Volume BMP 3 Chesapeake Bay Stormwater Training Partnership 20
5. Level 1 and Level 2 Design Specifications Level 1 -- Standard Design Features (included in all designs) needed to maintain proper BMP: Function Safety Appearance Safe conveyance Performance longevity Maintenance Level 2 -- Enhanced Design Features include specific design features to improve the BMP s Total Performance Chesapeake Bay Stormwater Training Partnership 21
Chesapeake Bay Stormwater Training Partnership 22
Chesapeake Bay Stormwater Training Partnership 23
Methods 1. Runoff Reduction 2. Treatment Volume 3. BMPs in Series 4. Design Volume 5. Level 1 & Level 2 Design Chesapeake Bay Stormwater Training Partnership 24
Chesapeake Bay Stormwater Training Partnership 25
West Virginia Stormwater Management Manual: Practices Created to deliver targeted training on new tools and practices to improve the quality of stormwater runoff. www.cbstp.org www.chesapeakestormwater.net
Chapter 4: BMP Specifications 4.1. Better Site Design 4.2.1. Sheet Flow to Filter Strip or Conservation Area 4.2.2. Impervious Disconnection 4.2.3. Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden 4.2.4. Permeable Pavement 4.2.5. Grass Swale 4.2.6. Infiltration 4.2.7. Regenerative Stormwater Conveyance 4.2.8. Rainwater Harvesting 4.2.9. Vegetated Roofs 4.2.10. Filtration Practices 4.2.11. Stormwater Wetlands Chesapeake Bay Stormwater Training Partnership 2
Chapter 4: BMP Specifications: Site Design 4.1. Better Site Design 4.2.1. Sheet Flow to Filter Strip or Conservation Area 4.2.2. Impervious Disconnection Chesapeake Bay Stormwater Training Partnership 3
Chapter 4: BMP Specifications: Runoff Reduction 4.2.3. Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden 4.2.4. Permeable Pavement 4.2.5. Grass Swale 4.2.6. Infiltration 4.2.7. Regenerative Stormwater Conveyance 4.2.8. Rainwater Harvesting 4.2.9. Vegetated Roofs Chesapeake Bay Stormwater Training Partnership 4
Chapter 4: BMP Specifications: Pollutant Removal 4.2.10. Filtration Practices 4.2.11. Stormwater Wetlands Chesapeake Bay Stormwater Training Partnership 5
Chapter 4: BMP Specifications: Site Design 4.1. Better Site Design 4.2.1. Sheet Flow to Filter Strip or Conservation Area 4.2.2. Impervious Disconnection Chesapeake Bay Stormwater Training Partnership 6
4.1. Self-Crediting Better Site Design Chesapeake Bay Stormwater Training Partnership 7
Site Design is First Slice of Salami Reduce Impervious Cover Reduce Site Distrubance Reduce Managed Turf Chesapeake Bay Stormwater Training Partnership 8
Reduce Impacts By Design Chesapeake Bay Stormwater Training Partnership 9
Not So Good Good Chesapeake Bay Stormwater Training Partnership 10
Watershed Protection Elements (Part 2C5.i.A.) 1. Minimize Impervious surfaces 2. Preserve, protect, create and restore ecologically sensitive areas 3. Prevent or reduce thermal impacts to streams 4. Avoid or prevent hydromodification of streams and other waterbodies 5. Protect trees and other vegetation 6. Protect native soils Chesapeake Bay Stormwater Training Partnership 11
Checklist Format Chesapeake Bay Stormwater Training Partnership 12
4.2.1. Sheet Flow to Vegetated Filter Strip or Conservation Area Chesapeake Bay Stormwater Training Partnership 13
4.2.1. Sheetflow to Conservation Area or Vegetated Filter Strip Re-Engineering the Filter Strip: Two Filter Designs Vegetated Filter Strip Sheet Flow to Conserved Open Space Chesapeake Bay Stormwater Training Partnership 14
Chesapeake Bay Stormwater Training Partnership 15
Design Considerations Sheet Flow Boundary Spreaders Level Spreaders for concentrated flow Maximum slopes Chesapeake Bay Stormwater Training Partnership 16
Stream Buffers Designated during initial lot recordation/subdivision; Protected with legal instrument to prevent alteration of the land or vegetation; Width varies (35. ft to 150 ft.); Must establish sheet flow into buffer Chesapeake Bay Stormwater Training Partnership 17
Level Spreader Applicable to filter strips, buffers, and conserved open space with concentrated inflow. Henrico County, VA Level Spreader 30 to 40 ft in length; 50 ft width at level lip Chesapeake Bay Stormwater Training Partnership 18
How Big A Slice? 4 9 cf/each 100 square foot of conservation area Chesapeake Bay Stormwater Training Partnership 19
4.2.2. Impervious Surface Disconnection Chesapeake Bay Stormwater Training Partnership 20
4.2.2. Impervious Surface Disconnection Rooftop/Small Impervious Area Disconnection Simple disconnection Soil amended filter path Disconnection to small-scale runoff reduction practice: rain garden, infiltration, rainwater harvesting COMPENSATORY PRACTICE Chesapeake Bay Stormwater Training Partnership 21
Simple Disconnection Filter corridors from the downspout to the street Level spreader to distribute runoff over 15 25 width Criteria for: Contributing IC area Length Width Slope of receiving area Chesapeake Bay Stormwater Training Partnership 22
Appendix D: Soil Amendments Chesapeake Bay Stormwater Training Partnership 23
Soil Amended Filter Path Enhanced runoff reduction in C/D soils Can Be Use for Multiple Practices Chesapeake Bay Stormwater Training Partnership 24
Soil Amendments for Other Practices Chesapeake Bay Stormwater Training Partnership 25
How Big A Slice? 2 4 cf/each 100 square foot of pervious receiving area* Runoff reduction for compensatory practices varies based on the practice: see bioretention, infiltration, rainwater harvesting specs Chesapeake Bay Stormwater Training Partnership 26
Design Example Chesapeake Bay Stormwater Training Partnership 27
Treatment Volume -- Typical Layout 15 Acres, 25, 0.6 acre lots Impervious = 3 ac. Managed Turf = 11 ac. Forest/Open Space = 1 ac. Treatment Volume (Tv) = 19,130 cubic feet Tv applies to each drainage area + site Chesapeake Bay Stormwater Training Partnership 28
Treatment Volume Site Design Measures 15 Acres, 25, 1/4 acre lots Impervious = 2 ac. Managed Turf = 5 ac. Forest/Open Space = 8 ac. Treatment Volume (Tv) = 10,890 cubic feet 19,130 10,890 = 8,240 cubic feet Chesapeake Bay Stormwater Training Partnership 29
Treatment Volume Additional RR Practices 13 lots = rain gardens = 0.65 ac. 12 lots = sheetflow to conservation (½ roof + ½ yard) = 0.30 ac. IC + 1.13 ac. turf Remaining Treatment Volume (Tv) = 7,608 cubic feet Total Reduction from Site Design = 11,522 Chesapeake Bay Stormwater Training Partnership 30
Chapter 4: BMP Specifications: Runoff Reduction 4.2.3. Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden 4.2.4. Permeable Pavement 4.2.5. Grass Swale 4.2.6. Infiltration 4.2.7. Regenerative Stormwater Conveyance 4.2.8. Rainwater Harvesting 4.2.9. Vegetated Roofs Chesapeake Bay Stormwater Training Partnership 31
4.2.3. Bioretention Chesapeake Bay Stormwater Training Partnership 32
4.2.3. Bioretention Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden Chesapeake Bay Stormwater Training Partnership 33
Chesapeake Bay Stormwater Training Partnership 34
Level 1 Chesapeake Bay Stormwater Training Partnership 35
Level 2 -- Infiltration Chesapeake Bay Stormwater Training Partnership 36
Level 2 Extended Filtration Chesapeake Bay Stormwater Training Partnership 37
Level 2 -- Infiltration Sump Chesapeake Bay Stormwater Training Partnership 38
Level 2 Upturned Elbow (Internal Water Storage Zone) Chesapeake Bay Stormwater Training Partnership 39
How Big A Slice? 60 -- 100% (55% for Level 1 WQS) Chesapeake Bay Stormwater Training Partnership 40
4.2.4. Permeable Pavement Chesapeake Bay Stormwater Training Partnership 41
Chesapeake Bay Stormwater Training Partnership 42
Level 1 Chesapeake Bay Stormwater Training Partnership 43
Level 2 -- Infiltration Chesapeake Bay Stormwater Training Partnership 44
Level 2 Infiltration Sump 45% RR 100% RR Chesapeake Bay Stormwater Training Partnership 45
How Big A Slice? 45 -- 100% Level 2: 45% above UD; 100% for sump Chesapeake Bay Stormwater Training Partnership 46
4.2.5. Grass Swale Chesapeake Bay Stormwater Training Partnership 47
Chesapeake Bay Stormwater Training Partnership 48
One Design Level: Depends on Soil Types, Soil Amendments Chesapeake Bay Stormwater Training Partnership 49
Check Dams Chesapeake Bay Stormwater Training Partnership 50
How Big A Slice? 0.1 0.2 inches per CDA Chesapeake Bay Stormwater Training Partnership 51
4.2.6. Infiltration Chesapeake Bay Stormwater Training Partnership 52
Chesapeake Bay Stormwater Training Partnership 53
One Design Level Chesapeake Bay Stormwater Training Partnership 54
Additional Pipe/Arch Storage Chesapeake Bay Stormwater Training Partnership 55
How Big A Slice? 100% for Design Volume Chesapeake Bay Stormwater Training Partnership 56
4.2.7. Regenerative Conveyance Chesapeake Bay Stormwater Training Partnership 57
Chesapeake Bay Stormwater Training Partnership 58
Riffles, Pools, Sand/Woodchip Bed Chesapeake Bay Stormwater Training Partnership 59
How Big A Slice? 100% for Design Volume Chesapeake Bay Stormwater Training Partnership 60
4.2.8. Rainwater Harvesting Chesapeake Bay Stormwater Training Partnership 61
Chesapeake Bay Stormwater Training Partnership 62
Year-Round Use OR Chesapeake Bay Stormwater Training Partnership 63
Secondary On-site Runoff Reduction Practices Grass Filter Strip Grass Channel Bioretention (micro, small) Dry Swale Infiltration Rooftop Disconnection Storage & Release in Foundation Planter BMP Clearinghouse http://www.vwrrc.vt.edu/swc/nonproprietarybmps.html Chesapeake Bay Stormwater Training Partnership 64
Cistern Design Spreadsheet - INPUT 1.) Drop Down Menu to select 1 of 4 regions 2.) Enter Area of Rooftop to be Captured in SF 3.) Enter Area to be Irrigated (if any). Also Enter inches/week (1 /SF assumed) and start and end day of year of application 4.) Enter Internal demand usage for flushing of toilets and urinals (if any) based on per Person or designer estimate. Also enter day of week to begin & end Chesapeake Bay Stormwater Training Partnership 66
Enter Runoff Reduction from Cistern Spreadsheet in Design Compliance Spreadsheet Runoff Reduction Practice 1. Vegetated Roof (4.2.9) Vegetated Roof 2. Impervious Surface Disconnection/Rooftop Practices Simple Disconnection to A/B Soils or Soil-Amended Filter Path in C/D Soils (4.2.2, Appendix D) Simple Disconnection to C/D Soils (4.2.2) To Residential Rain Garden (4.2.3.C) To Rainwater Harvesting (4.2.8) Impervious Cover in Contributing Drainage Area (acres) Managed Turf in Contributing Drainage Area (acres) Volume Received by Practice (cubic feet) N/A 0 N/A 0 N/A 0 N/A 0 N/A 0 Description of Credit 100% volume reduction for the Design Volume of the practice Reduce volume conveyed to conservation area by 4 cu. ft per each 100 sq. ft. of pervious receiving area. Reduce volume conveyed to conservation area by 2 cu. ft per each 100 sq. ft. of pervious receiving area. 60% volume reduction for the Design Volume of the practice Subtract a variable % of the provided Design Volume based on annual beneficial use (see Cistern Design Spreadsheet). % Credit 100% N/A N/A 60% To Urban Bioretention (4.2.3.B) N/A 0 60% volume reduction for the Design Volume of the practice 60% Chesapeake Bay Stormwater Training Partnership 67
How Big A Slice?? -- 100% Chesapeake Bay Stormwater Training Partnership 68
4.2.9. Vegetated Roof Chesapeake Bay Stormwater Training Partnership 69
Chesapeake Bay Stormwater Training Partnership 70
Extensive Green Roof Chesapeake Bay Stormwater Training Partnership 71
Intensive Green Roof Chesapeake Bay Stormwater Training Partnership 72
How Big A Slice? 100% for Design Volume Chesapeake Bay Stormwater Training Partnership 73
Chapter 4: BMP Specifications: Pollutant Removal 4.2.10. Filtration Practices 4.2.11. Stormwater Wetlands Chesapeake Bay Stormwater Training Partnership 74
4.2.10. Filtration Practice Chesapeake Bay Stormwater Training Partnership 75
Surface Sand Filter Underground/Vault Sand Filter Perimeter Sand Filter Chesapeake Bay Stormwater Training Partnership 76
How Big A Slice? 0% Chesapeake Bay Stormwater Training Partnership 77
4.2.11. Stormwater Wetland Chesapeake Bay Stormwater Training Partnership 78
Wetland Basin Multi-Cell or Pond/Wetland Subsurface Gravel Wetland Chesapeake Bay Stormwater Training Partnership 79
How Big A Slice? 0% Chesapeake Bay Stormwater Training Partnership 80
Chapter 4: BMP Specifications 4.1. Better Site Design 4.2.1. Sheet Flow to Filter Strip or Conservation Area 4.2.2. Impervious Disconnection 4.2.3. Bioretention Water Quality Swale Urban Bioretention Residential Rain Garden 4.2.4. Permeable Pavement 4.2.5. Grass Swale 4.2.6. Infiltration 4.2.7. Regenerative Stormwater Conveyance 4.2.8. Rainwater Harvesting 4.2.9. Vegetated Roofs 4.2.10. Filtration Practices 4.2.11. Stormwater Wetlands Chesapeake Bay Stormwater Training Partnership 81
Design Compliance Spreadsheet West Virginia Design Compliance Spreadsheet Drainage Area A Drainage Area A Post-Development Land Cover (acres) Land Cover Type HSG A soils HSG B Soils HSG C Soils HSG D Soils Totals Forest/Preserved Open Space 0.00 0.00 0.00 0.00 0.00 Managed Turf 0.00 0.00 0.00 0.00 0.00 Impervious Cover 0.00 0.00 0.00 0.00 0.00 Total 0.00 Rv(turf) 0.00 Rv(impervious) 0.95 Apply Runoff Reduction Practices Runoff Reduction Practice 1. Vegetated Roof (4.2.9) Vegetated Roof 2. Impervious Surface Disconnection/Rooftop Practices Simple Disconnection to A/B Soils or Soil-Amended Filter Path in C/D Soils (4.2.2, Appendix D) Simple Disconnection to C/D Soils (4.2.2) To Residential Rain Garden (4.2.3.C) To Rainwater Harvesting (4.2.8) To Urban Bioretention (4.2.3.B) Impervious Cover in Contributing Drainage Area (acres) Managed Turf in Contributing Drainage Area (acres) Volume Received by Practice (cubic feet) N/A 0 N/A 0 N/A 0 N/A 0 N/A 0 N/A 0 Description of Credit 100% volume reduction for the Design Volume of the practice Reduce volume conveyed to conservation area by 4 cu. ft per each 100 sq. ft. of pervious receiving area. Reduce volume conveyed to conservation area by 2 cu. ft per each 100 sq. ft. of pervious receiving area. 60% volume reduction for the Design Volume of the practice Subtract a variable % of the provided Design Volume based on annual beneficial use (see Cistern Design Spreadsheet). 60% volume reduction for the Design Volume of the practice Chesapeake Bay Stormwater Training Partnership 82