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Wake County Stormwater Workshop

Wake County Stormwater Workshop. Guidance on the New Stormwater Ordinance and Design Manual August 29, 2006. Introduction. Wake County met with a group of stakeholders and formed a committee to create a new stormwater ordinance

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Wake County Stormwater Workshop

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  1. Wake County Stormwater Workshop Guidance on the New Stormwater Ordinance and Design Manual August 29, 2006

  2. Introduction • Wake County met with a group of stakeholders and formed a committee to create a new stormwater ordinance • After a year long process, of education, discussion, and debate - the committee came to an agreement. Wake County Stormwater Presentation August 2006

  3. Introduction • Decisions of the Committee: • Runoff volume management is the goal. (not peak discharge management) • Runoff volume management will be achieved by managing the curve number. • Target curve numbers are set and proposed developments should match the target or manage the excess runoff for a 3-inch precipitation depth. • Excess runoff will be infiltrated or stored and released over a period of 2 to 5 days. Wake County Stormwater Presentation August 2006

  4. Introduction • Target Curve Numbers Wake County Stormwater Presentation August 2006

  5. Introduction • Goals of Ordinance: • Protect streams and other resources. • Encourage low impact design approaches. • Allow flexibility for designer. • Meet the Neuse Rules. Wake County Stormwater Presentation August 2006

  6. Design Manual • Intent: • Provide procedure to meet the ordinance. • Curve Number Method (TR-55) • SCS Runoff Equation (TR-55) • Storage/Infiltration Devices • Discuss design approaches. • Conventional design • Low impact design • Hybrid design Wake County Stormwater Presentation August 2006

  7. Curve Numbers • Integers ranging from 30 to 98. • Based on Land-use and Hydrologic Soil Group. • Hydrologic Soil Group is determined by soil series. • Curve numbers represent the amount of runoff expected from a specific area. A high curve number represents more runoff than a low curve number. Wake County Stormwater Presentation August 2006

  8. Wake County Stormwater Presentation August 2006

  9. Curve Numbers Wake County Stormwater Presentation August 2006

  10. Procedure • Curve Number Method • Calculate Target Curve Number for Proposed Site based on Zoning and Soil Composition • Calculate Proposed Site Curve Number based on Soil Composition and proposed land use data Wake County Stormwater Presentation August 2006

  11. Example • Calculate Target Curve Number • Proposed Site zoned as R-40W • 75% B Soils, 25% C Soils (0.75*62) + (0.25*75) = 65.25 • Target Curve Number for site is 65. Wake County Stormwater Presentation August 2006

  12. Example • Calculate Proposed Curve Number • 10-acre Site • Proposed Woods, HSG B (CN=55) = 4 acres • Proposed Open Space, HSG B (CN= 61) = 1.5 acres • Proposed Open Space, HSG C (CN=74) = 2 acres • Proposed Impervious (CN=98) = 1.5 acres • Proposed Impervious in R/W (CN=98) = 0.5 acres • Proposed Open Space in R/W (CN=80) = 0.5 acres (55*4)+(61*2)+(74*2)+(98*2)+(80*0.5) = 695.5 695.5/10 = 69.55 • Proposed Curve Number is 70. Wake County Stormwater Presentation August 2006

  13. Procedure • Calculate Runoff Volume to be stored or infiltrated using SCS Runoff Equation. • SCS Runoff Equation (TR-55): Q* = (P – 0.2S)2 (P + 0.8S) Where: Q* = runoff in inches P = Precipitation Depth in inches (3 inches) S = Potential maximum retention in inches Wake County Stormwater Presentation August 2006

  14. Procedure • In order to calculate Q* - first calculate S. S = (1000/CN) – 10 • Q* must be calculated for the Target and Proposed curve numbers and then multiplied by the site acreage to determine the total runoff volume for each CN • The difference between the two volumes must be handled by stormwater devices Wake County Stormwater Presentation August 2006

  15. Example • Calculate Runoff Volume. • Site acreage = 10 acres • Target CN = 65 • Proposed CN = 70 Starget = (1000/65)-10 = 5.38 inches Q*target = (3+ (0.2*5.38))2/(3+(0.8*5.38)) = 0.51 inches Sproposed = (1000/69)-10 = 4.49 inches Q*proposed = (3+ (0.2*4.29))2/(3+(0.8*4.29)) = 0.71 inches Q*proposed - Q*target = 0.71 – 0.51 = 0.20 inches 0.20 inches * 10 acres = 2.0 acre-inches = 7,405 ft3 Wake County Stormwater Presentation August 2006

  16. Procedure • Neuse Rules dictate that any development of 15% impervious or greater must not cause an increase in the post-development 1-year, 24-hour storm from the pre-development conditions. • Wake County has adopted the Neuse Rules for the Cape Fear River Basin as well. • Therefore, sites of 15% impervious or more must meet both the Wake County Stormwater Ordinance and the Neuse Rules Wake County Stormwater Presentation August 2006

  17. Procedure • For these sites – the peak flow must be calculated using the SCS method. • Time of concentrations for pre- and post-development must be calculated using methodology in TR-55. It is also included in the design manual. • Type II SCS curve with a 3-inch total depth should be used. Wake County Stormwater Presentation August 2006

  18. Design Approaches • Three approaches can be used for volume management and peak flow management, if required: • Conventional approach • This is the typical approach to stormwater on a subdivision and usually incorporates a wet or dry pond. • Low impact design approach • This includes trying to mimic the natural hydrologic conditions by allowing for as much infiltration and transpiration as possible. • Hybrid of the two approaches Wake County Stormwater Presentation August 2006

  19. Low Impact Development • Minimize runoff by: • Maximizing wooded areas by preserving woods on lots and in common open space. • Minimizing impervious areas by using narrow roads, reducing on lot impervious, and/or replacing with pervious pavers. • Maximizing opportunity for infiltration and transpiration by using open ditch section roads instead of curb and gutter and disconnecting impervious areas. Wake County Stormwater Presentation August 2006

  20. Low Impact Development Wake County Stormwater Presentation August 2006

  21. Land Use Practices • Preservation of Wooded Areas and Reforestation • Important tool used to lower volume of runoff by lowering the proposed curve number. • Due to the locations of houses, driveways, and septic fields being largely unknown at the construction drawing stage – it may not be practical to depict a boundary on lots of preserved wooded areas at the preliminary plan or construction drawing level. • Therefore – the County has allowed a set assumption on lots to be made without designation of a boundary. • Preserved wooded areas and reforested areas can be set aside in common open space as well and does not have a set assumption as lots do. Wake County Stormwater Presentation August 2006

  22. Land Use Practices • Preservation of Wooded Areas and Reforestation • Use maximum assumption of woods on lots. • If exceeding assumption – boundary for all preserved woods on lots must be provided. Wake County Stormwater Presentation August 2006

  23. Land Use Practices • Disconnected Impervious Areas • TR-55 describes as impervious areas in which the runoff is spread over a pervious area as sheet flow. • TR-55 provides an equation in order to reduce the composite curve number to reflect the reduction in runoff for impervious surfaces that has the opportunity to infiltrate. • The opposite of this would be impervious surfaces that are connected by pipe systems or do not have adequate opportunity to infiltrate. Wake County Stormwater Presentation August 2006

  24. Land Use Practices • Disconnected Impervious • TR-55 Equation: CNc = CNp + [(Pimp/100)*(98-CNp)*(1-(0.5R))] Where: CNc = Composite Curve Number CNp = Pervious Runoff Curve Number Pimp = Percent Impervious R = Ratio of Disconnected Impervious Area to Total Impervious Area Wake County Stormwater Presentation August 2006

  25. Land Use Practices • Disconnected Impervious Surface • How is “opportunity to infiltrate” quantified? • Wake County: • Rooftop • Minimum of 50 ft in direction of flow across pervious surface before channelization • Maximum of 500 sq. ft. of rooftop to downspout • Splashblock or equivalent area of gravel dissipater • Roadway • Minimum flow length across pervious surface before channelization is equal to flow path across impervious surface • Maximum flow path across impervious surface is 75 ft • Maximum slope of pervious surface receiving flow is 5% • Pervious pavers and pavement is automatically disconnected. Wake County Stormwater Presentation August 2006

  26. Land Use Practices • Disconnected Impervious Surface • As with preserved wooded areas, a maximum assumption can be used for the amount of impervious surface that is disconnected. • This is due to the fact that house and driveway locations, areas to each downspout, and grades are largely unknown on the lots at the preliminary plan and construction drawing level. Wake County Stormwater Presentation August 2006

  27. Land Use Practices • Disconnected Impervious Surface • With this assumption, an R-80 subdivision would apply an R=0.5 to the TR-55 equation • If exceeding assumption, disconnection must be shown on plans. Wake County Stormwater Presentation August 2006

  28. Integrated Management Practices (IMPs) • Low impact development’s coined phrase for microscale and distributed management techniques. • Typically used to manage stormwater at the source. • Therefore – are most commonly used on lots for residential uses. Wake County Stormwater Presentation August 2006

  29. Integrated Management Practices (IMPs) NCSU-BAE Wake County Stormwater Presentation August 2006

  30. Best Management Practices (BMPs) NCSU-BAE Wake County Stormwater Presentation August 2006

  31. Example Site • R-40W • 53.25 acres • 100% B Soils • Cluster S/D • 44 Lots Wake County Stormwater Presentation August 2006

  32. Conclusionwww.wakegov.com/environment/water/stormwater.htmQuestions?Conclusionwww.wakegov.com/environment/water/stormwater.htmQuestions? Wake County Stormwater Presentation August 2006

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