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Low Impact Development Overview

Low Impact Development Overview. Alternative to end of pipe approach to SWM Maintain hydrologic function of local ecosystem Treat stormwater close to the source of runoff Decentralized small scale devices Maintain runoff rates and connection with groundwater History

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Low Impact Development Overview

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  1. Low Impact Development Overview • Alternative to end of pipe approach to SWM • Maintain hydrologic function of local ecosystem • Treat stormwater close to the source of runoff • Decentralized small scale devices • Maintain runoff rates and connection with groundwater • History • Prince Georges County Maryland, 1980’s • Means to address economical, environmental and physical shortcomings of traditional stormwater designs • Key Elements • Uses common stormwater BMPs • Combination of devices results in more efficient land use

  2. LID-EZ • Development • Similar programs in use in Wake County and Manteo. • Local and NC Coastal Federation Funding • Cooperation with NC DWQ • Wilmington Version • Written to comply with proposed Coastal Rules • Quantitative approach to LID developments • Based on local ordinances and NC DWQ BMP manual

  3. LID Calculations • SCS Method • Described in TR-55 • Per NC DWQ, allowable method for LID Projects only • Accounts for soil conditions on site • NC DWQ Involvement • No changes required for new Coastal Rules • Permitting guidelines in development by DWQ • Clarification of policies • Disconnected Impervious Area • Pervious Pavement • First Flush Calculations

  4. Connected / Disconnected Impervious Area • Connected Impervious Area • Directly connected to drainage conveyance • Minimal opportunity for volume reduction before reaching analysis point • Disconnected Impervious Area • Runoff has contact with pervious surfaces before reaching analysis point • Recommended 50’ sheet flow or sheet flow length equal to width of impervious surface • Benefit is dependant on soil type • Net result is a reduction of CN

  5. Calculating Runoff Depth, Q [in] – TR-55 Chapter 2 • Q [in] = (P – Ia)2 / (P + 0.8S), • when (P – Ia) > 0; • otherwise Q[in] = 0 in • P = Precipitation depth in inches • Ia = Initial hydrologic abstraction = 0.2S • S = Potential maximum retention after runoff begins in inches • S = 1000/CN – 10

  6. Example Site • 5 acres • Area = 5.00-ac • Single-Family Residential • Curb & Gutter • 1.6 ac Total Impervious • 0.85 ac disconnected

  7. Calculating Q1-YR [in] TR-55 Composite CN Method: (with disconnected impervious area) • Calculate CNcomp: CNcomp = CNp + (Pimp / 100)*(98 - CNp)*(1 - 0.5R) CNcomp = Composite Curve Number CNp = Pervious Curve Number Pimp = Percent Total Impervious R = Aimp(disconn) ÷ Aimp(total) • Calculate Q1-YR for CNcomp

  8. Calculating Q1-YR [in] - Continued • CNp = 61 (in this example) • P [in] = 3.41 in (in this example) • Pimp = Aimp(tot) ÷ ATot = (0.75 ac + 0.85 ac) ÷ 5 ac = 32 % • R = Aimp(disconn) ÷ Aimp(total) = 0.85 ac ÷ 1.60 ac = 0.53

  9. Calculating Q1-YR [in] - Continued • CNcomp = CNp + (Pimp / 100)*(98 - CNp)*(1 - 0.5R) = 61 + (32 / 100)*(98 - 61)*(1 - 0.5*0.53) = 70 *Note – Without Disconnection CN = 73 • S = 1000/CNcomp - 10 = 1000 / 70 - 10 = 4.29 • Q1-YR = (P – Ia)2 / (P + 0.8S) = (3.41 – 0.2*4.29)2 / (3.41 + 0.8*4.29) = 0.95 in

  10. First Flush Calculations • 0.75-ac Connected Impervious • AImp (conn) = 0.75-ac • 0.85-ac Disconnected Impervious • AImp (disconn) = 0.85-ac • 3.40-ac Open-Space • Apervious = 3.40-ac

  11. First Flush Calculations • Two Separate Calculations: 1) Qimp(conn) • 2) Qremain

  12. Calculating QFF [in] – First Flush (1.5”) Discrete CN Method: • Obtain CN for Connected Impervious Area • CNimp(conn) = 98 • Calculate CN for Remaining Area • CNremaining = CNp + (Pimp / 100)*(98 - CNp)*(1 - 0.5R) *R = 1 always because connected impervious area has already been accounted for • Calculate QFF. for each CN • Obtain the Area-Weighted Average QFF.

  13. Calculating QFF [in] – First Flush (continued) • CNp = 61 (in this example) • P [in] = 1.5 in (in this example) • CNimp(conn) = 98 • Pimp = Aimp(disconn) ÷ (ATot - Aimp(conn)) = 0.85 ac ÷ (5 ac - 0.75 ac) = 20 % • R = 1

  14. Calculating QFF [in] – First Flush (continued) • CNremain = CNp + (Pimp / 100)*(98 - CNp)*(1 - 0.5R) = 61 + (20 / 100)*(98 - 61)*(1 - 0.5*1) = 65 • Sremain = 1000/CNremain - 10 = 1000 / 65 - 10 = 5.38 • Qremain = (P – Ia)2 / (P + 0.8S) = (1.5 – 0.2*5.38)2 / (1.5 + 0.8*5.38) = 0.03 in

  15. Calculating QFF [in] – First Flush (continued) • Simp(conn) = 1000/CNimp(conn) - 10 = 1000 / 98 - 10 = 0.20 • Qimp(conn) = (P – Ia)2 / (P + 0.8S) = (1.5 – 0.2*0.20)2 / (1.5 + 0.8*0.20) = 1.28 in • QF.F. = [(QA)remain + (QA)imp(conn)] / ATot = [(0.03 in * 4.25 ac) + (1.28 in * 0.75 ac)] / 5 ac = 0.22 in

  16. LID-EZ Features • Storage devices increase effective soil storage capacity, reducing CN • “Effective Volume” varies based on storm event • Effective Volume used in Peak Flow calculations • Disconnected Impervious • Pervious Pavement • Land Use or Storage Area • Lakes and Wetlands • Coastal Wetlands • Pollutant Removal • BMPs in series

  17. LID-EZ – Residential Development - Lakeside Example Site: Lakeside • 42.62-ac Parcel • “B” Soils • Predevelopment – 100 % Pervious, Natural Area • 35% Open Space, 64% Woods • Post-Development • 24 % Impervious (Lots and Roadways) • 14 % Managed Open-Space Stormwater Management: • 8 Bioretention Cells, 4 Vegetated Swales • Total Storage Volume = 167,729 ft3 • Total Effective WQV = 33,197 ft3

  18. LID-EZ – Condominium Development Example Site: • 9.38-ac Parcel • “A” Soils • Predevelopment – 100 % Pervious, Natural Area • Post-Development • 62 % Impervious (Connected) • 38 % Managed Open-Space Stormwater Management: • 1 Wet Pond, 4 Sand Filters, 6 Infiltration Basins, 1 Bioretention Cell • Total Storage Volume = 29,390 ft3

  19. LID-EZ – Quick Calculator – Retrofit Site Example Site: House Addition • 0.22-ac Lot • “A” Soils • Pre-Construction – 21 % Impervious (CN = 52) • Post-Construction – 25 % Impervious (CN = 54)

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