Low Impact Development Overview

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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

10. 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

11. 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

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

13. 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.

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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)