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

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

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

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

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

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

Calculating Q 1-YR [in] TR-55 Composite CN Method: (with disconnected impervious area) 1.Calculate CN comp : CN comp = CN p + (P imp / 100)*(98 - CN p )*( R) CN comp = Composite Curve Number CN p = Pervious Curve Number P imp = Percent Total Impervious R = A imp(disconn) ÷ A imp(total) 2.Calculate Q 1-YR for CN comp

 CN p = 61 (in this example)  P [in] = 3.41 in (in this example)  P imp = A imp(tot) ÷ A Tot = (0.75 ac ac) ÷ 5 ac = 32 %  R = A imp(disconn) ÷ A imp(total) = 0.85 ac ÷ 1.60 ac = 0.53 Calculating Q 1-YR [in] - Continued

 CN comp = CN p + (P imp / 100)*(98 - CN p )*( R) = 61 + (32 / 100)*( )*( *0.53) = 70 * Note – Without Disconnection CN = 73  S = 1000/CN comp - 10 = 1000 / = 4.29  Q 1-YR = (P – I a ) 2 / (P + 0.8S) = (3.41 – 0.2*4.29) 2 / ( *4.29) = 0.95 in

First Flush Calculations  0.75-ac Connected Impervious -A Imp (conn) = 0.75-ac  0.85-ac Disconnected Impervious -A Imp (disconn) = 0.85-ac  3.40-ac Open-Space -A pervious = 3.40-ac

First Flush Calculations  Two Separate Calculations: 1) Q imp(conn) 2) Q remain

Discrete CN Method: 1.Obtain CN for Connected Impervious Area  CN imp(conn) = 98 2.Calculate CN for Remaining Area  CN remaining = CN p + (P imp / 100)*(98 - CN p )*( R) *R = 1 always because connected impervious area has already been accounted for 3.Calculate Q FF. for each CN 4.Obtain the Area-Weighted Average Q FF. Calculating Q FF [in] – First Flush (1.5”)

 CN p = 61 (in this example)  P [in] = 1.5 in (in this example)  CN imp(conn) = 98  P imp = A imp(disconn) ÷ (A Tot - A imp(conn) ) = 0.85 ac ÷ (5 ac ac) = 20 %  R = 1 Calculating Q FF [in] – First Flush (continued)

 CN remain = CN p + (P imp / 100)*(98 - CN p )*( R) = 61 + (20 / 100)*( )*( *1) = 65  S remain = 1000/CN remain - 10 = 1000 / = 5.38  Q remain = (P – I a ) 2 / (P + 0.8S) = (1.5 – 0.2*5.38) 2 / ( *5.38) = 0.03 in

 S imp(conn) = 1000/CN imp(conn) - 10 = 1000 / = 0.20  Q imp(conn) = (P – I a ) 2 / (P + 0.8S) = (1.5 – 0.2*0.20) 2 / ( *0.20) = 1.28 in  Q F.F. = [(QA) remain + (QA) imp(conn) ] / A Tot = [(0.03 in * 4.25 ac) + (1.28 in * 0.75 ac)] / 5 ac = 0.22 in Calculating Q FF [in] – First Flush (continued)

 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 LID-EZ Features

LID-EZ – Residential Development - Lakeside Example Site: Lakeside  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 ft 3  Total Effective WQV = 33,197 ft 3

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 ft 3

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