Permeable Pavement: A New Chapter

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1 Permeable Pavement: A New Chapter Annette Lucas, PE (919) annette.lucas@ncdenr.gov NC Division of Water Quality Wetlands & Stormwater Branch

2 Final Chapter Released: October 16, 2012 We Bring Engineering to Life

3 WHY did we revise the chapter? WHAT does the new chapter say?

4 WHY did we revise the Permeable Pavement Chapter? Traditional BMPS are NOT protecting the receiving stream Permeable Pavement can RESTORE natural hydrology Permeable Pavement can WORK across the state Other benefits besides water quality

5 Hydrology 101

6 Land Use Planning Commercial 60-70% of built-upon area on a site is parking, streets and sidewalks. % of Built-Upon Area Roofs Parking Streets Sidewalks 0

7 Most common stormwater BMP Stormwater in Stormwater out 7

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9 Here s the Hydrograph!

10 Lower low flows

11 Urban stormwater is the #1 cause of stream degradation.

12 WHAT if we went to the source of the problem?

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14 Stores STORMWATER in an aggregate layer to enable infiltration. If water cannot infiltrate, then it can be detained & released.

15 Standard permeable pavement cross section Pavement Course Aggregate Base Underdrain Subgrade We Bring Engineering to Life

16 Permeable Pavement has different forms

17 Dry streets Dry shoes Safety Other Benefits besides Water Quality!

18 Permeable Pavement costs more per sq ft than conventional pavement, but it allows developers to avoid other costs.

19 Protecting streams is also an economic issue!

20 New Permeable Pavement Chapter: DWQ Goal Design standards that insure long term excellent performance of permeable pavement for both water quality and structural goals.

21 Here are the TOP FIVE changes to the new BMP Manual Chapter...

22 #1: Statewide Use and Credit Before: Credit awarded No credit awarded in/hr soil infiltration After: Credit awarded state-wide but design allowances must be made for differing soil permeabilities.

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26 Approximate Soil Infiltration Rates (Ferguson 2006) Texture Infiltration Rate (inch/hour) Sand 8.27 Loamy sand 2.41 Sandy loam 1.02 Loam 0.52 Silt loam 0.27 Sandy clay loam 0.17 Clay loam 0.09 Silty clay loam 0.06 Sandy clay 0.05 Silty clay 0.04 Clay 0.02 Note: 0.52 in/hr = 13 in/day, more than one quarter of the annual rainfall expected in Raleigh (47 in).

27 But there still are some siting constraints: Seasonal high water table Site slope Buffers and setbacks Stormwater hotspots Redevelopment sites Water supply wells

28 #2: More BUA Credit Awarded Before: Permeable pavement received a BUA credit as 60% or 40% pervious depending on the type of pavement and the depth of the aggregate. Why? This is a more accurate way to estimate the effectiveness of permeable pavement. After: Permeable pavement receives BUA credit based on the soils, not the location of the pavement.

29 BEFORE: Coastal Plain 40% 60% 60% 40% Permeable pavement Rest of state 60% 40% No credit

30 AFTER: Permeable pavement A & B soils C & D soils 25% 75% 60% 40% 50% 50% Why? These numbers account for permeable pavement s performance while also accounting for potential maintenance issues.

31 How BUA Credit Can Affect a Site Need this size lot to be low density for Phase 2. Low density for Phase 2 C and D soils. Building Conventional Asphalt Building Permeable Pavement No piped conveyances.

32 How BUA Credit Can Affect a Site Need this size lot to be low density for Phase 2. Low density for Phase 2 A and B soils. Building Building Conventional Asphalt Permeable Pavement No piped conveyances.

33 #3: Credit for Pollutant Removal Before: Permeable pavement did not receive credit for removing pollutants. After: Significant pollutant removal credit is awarded to permeable pavement based on the design of the system.

34 Pollutant Removal Credits Infiltrating systems: 85% TSS 30% Total Nitrogen 60% Total Phosphorus Why? The research data supports pollutant removal credit. Detention systems (explained in a minute): 70% TSS with an impermeable liner 85% TSS with no liner 10% Total Nitrogen 10% Total Phosphorus

35 Permeable Pavement is Joining a New Club 85% TSS Removal: Wet detention pond Stormwater wetland Bioretention cell Sand Filter Infiltration basin/trench Permeable pavement * <85% TSS Removal: Dry detention pond Grassed swale Vegetated filter strip BUA Credit Only: Permeable pavement * This is the only device in the 85% club that also gets BUA credit.

36 #4: Adjacent BUA can be Treated by Permeable Pavement Before: Permeable pavement may not have any adjacent areas on the site discharging to it. After: Runoff from site BUA can be directed to the pavement to receive pollutant removal credit.

37 The area of additional BUA draining to the pavement shall not exceed the area of the pavement itself. Why? This has worked in other states and if designed correctly, can work in NC too.

38 Downspouts shall not drain more than 1,000 sf of rooftop area to a single point onto the permeable pavement.

39 #5: New Detention Design Before: Permeable pavement was designed exclusively for infiltration. After: Permeable pavement can be designed to detain and release stormwater. Why? Some soils are not appropriate for for permeable pavement.

40 Detention Design Required if: In-situ soils become unstable when saturated. Design storm cannot drain in 5 days. Site is a stormwater hotspot. Site has contaminated groundwater or soils.

41 How does permeable pavement designed for detention work? Pavement course Aggregate base Subgrade

42 Top Five Changes: In Summary #1 Statewide use & credit #2 More BUA credit awarded #3 Credit for pollutant removal #4 Adjacent BUA can be treated #5 New detention design Questions?