Permeable Pavement Treatment Capacity

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1 Permeable Pavement Treatment Capacity April This investigation will analyze the pollutant removal capacity of various types of permeable paving techniques. Daniel Sullivan Joseph Fleury

2 Contents Objectives... 3 Principles of Permeable Pavement... 3 Design Parameters... 3 Design Procedure... 4 Design Example... 7 Bibliography... 7 Figure 1 - Reservoir Depth (No Underdrain)... 4 Figure 2 - Depth of Reservoir (W/ Underdrain)... 5 Figure 3 - Sample Pollutant Removal Rates... 6 Figure 4 - Treatment Capacity of Level 1 and 2 Pavements... 7

.. 7 Figure 1 - Reservoir Depth (No Underdrain).

3 Objectives The objective of this analysis is to determine the potential treatment capacity of various types of permeable pavement techniques. The sediment and solids removal, nutrient removal, and runoff reduction will be analyzed as a function of the pavement design specifications. Pre and post treatment nutrient loadings will be the main parameter analyzed in order to determine the treatment ability for generic permeable-paved surfaces. Source treatment of stormwater runoff allows for less expensive collection treatment at water treatment facilities or downstream of the point of generation, making such technologies as permeable pavement a promising option for stormwater management. Principles of Permeable Pavement There are three main types of permeable paving: pervious concrete, porous asphalt, and permeable interlocking concrete pavers. The design specifications vary for each type, yet the structure of the pavements are similar in nature, with a permeable surface layer, underlying aggregate reserviour layer, and base filter fabric layer. The reserviour layer serves to retain stormwater while supporting the traffic load transferred from the surface of the roadway or path. Any captured runoff that does not infiltrate into the soil is drained through a perforated underdrain pipe in the aggegate layer. Sediment control is necessary for avoiding clogging of the porous paving; proper maintenance keeps the structure capable of providing high runoff volume reduction, nutrient removal, and suspended solids removal. There are two levels of design for calculating the effects of runoff and pollutant reduction provided by permeable paving. The main goal of permeable pavement is to maximize nutrient removal and runoff reduction. Design Parameters Space constraints for permeable paving are equal to the space requirements for a normal paving project; additional depth may be designed for higher volume of storage if desired. Soil conditions do not constrain the use of permeable paving, but will determine if an underdrain is necessary (generally the case for type C and D soils). Fill soils must usually be lined with a impermeable fabric and underdrain assembly. If no underdrain is designed, then the soil must have an infiltration rate of at least.5 inches per hour, as confirmed by on-site infiltration tests; native soils must contain less than 40% silt/clay and less than 20% clay content. If there is an external drainage area contributing runoff to the permeable paving area, it should not exceed twice the surface area of the permeable-paved area, and should be mainly impervious area (in order to avoid sediment runoff). Upgradient drainage areas leading into permeable surfaces can lead to sediment within runoff and increased clogging of permeable voids. The surface slope should be moderate (less than 3% ), and base slope of pavement a maximum of 1% slope in the longitudinal direction and 0% lateral in order to retain even distribution of infiltration. The pressure head required for transmitting flow through the structure is negligible, however some drains may require 2-4 feet head (at a slope of at least.5%). Permeable paving should only be used for areas with a water table depth of greater than 2 from the natural ground surface. Proper setbacks from buildings and structures must be used in order to avoid seepage. Areas with high loading of sediment should not use permeable paving.

Pre and post treatment nutrient loadings will be the main parameter analyzed in order to determine the treatment ability for generic permeable-paved surfaces.

4 Design Procedure The designing engineer must determine the level of design which the pavement meets in order to determine treatment capacity; Level 1 design is the baseline parameters, while Level 2 represents the enhanced design which maximizes nutrient and runoff removal. The depth of the reservoir layer (aggregate) can be calculated as a function of the depth of runoff, infiltration rate, time to fill, void ratio, and ratio of total area to pervious-paved area. For a design with no underdrain unit (HSG A or B): Figure 1 - Reservoir Depth (No Underdrain) The maximum depth of the reservoir is constrained by drain time, which is calculated as (Infiltration Rate/2) *(Max Drain Time) / Void Ratio. If an underdrain is required, the outflow from the drain unit in feet per day (assumed as one 6 diameter outlet) can be calculated as the hydraulic conductivity in ft/day (k, assume 100 ft/day) * pipe slope (ft/ft). The depth of the reservoir layer can then be calculated as a function of the design storm, with the following equation

The depth of the reservoir layer (aggregate) can be calculated as a function of the depth of runoff, infiltration rate, time to fill, void ratio, and ratio of total area to pervious-paved area.

5 Figure 2 - Depth of Reservoir (W/ Underdrain) As with the no-underdrain assembly, the depth of reservoir is bounded by the time of draining as: The volume of the detention space may be calculated as inflow minus outflow divided by the porosity of the trench area. The flow capacity of the outdrain must be able to handle a flow equal to blockage (typically.5) * orifice discharge coefficient (.6) * Area of orifice * Total area orifices * sqrt (2*gravity*max height above pipe) (Policies)

porosity of the trench area. The flow capacity of the outdrain must be able to handle a flow equal to blockage (typically.

6 The emptying time for the detention volume is typically between 12 and 84 hours, with target rates between 24 and 48 hours. It is computed as the ratio of volume of water (storage * porosity) to the filtration rate through filter layer (hydraulic conductivity * porosity) and outlet pipe(above) (Policies). Overflow pipes must be placed at the height equal to the head loss in pipe and head loss in the entry and exit of the system (Policies). It can be difficult to determine the specific treatment capacity for each site with permeable paving due to varying site conditions. The initial loadings of suspended solids, phosphorus, nitrogen, and other nutrients and metals vary greatly depending on the area and type and level of runoff generated. In general, permeable paving can remove over 80% of TSS, greater than 50% of Total Phosphorus and Nitrogen, and upwards of 50% of all metals. Figure 3 - Sample Pollutant Removal Rates Application Location TSS Metals Nutrients Permeable Concrete Parking lot Tampa, FL 91% 75-92% -- Permeable Interlocking Concrete Pavers Driveways Jordan Cove, CT 67% Cu: 67% Pb: 67% Zn:71% TP: 34% NO 3 -N: 67% NH 3 -N: 72% Parking lot Goldsboro, NC 71% Zn: 88% TP: 65% TN: 35%/td> Parking lot Renton, WA --- Parking lot King College, ON 81% Porous Asphalt Cu: 79% Zn: 83% Cu: 13% Zn: 72% -- TP: 53% TKN: 53% Highway (friction course only) Austin, TX 94% 76-93% 43% Parking lot Durham, NH 99% Zn: 97% TP: 42% (EPA)

Overflow pipes must be placed at the height equal to the head loss in pipe and head loss in the entry and exit of the system (Policies).

7 Figure 4 - Treatment Capacity of Level 1 and 2 Pavements Design Example Pervious paved road in Boston, MA (HSG C), for minor residential access roadway and sidewalk area of 1 acre, total drainage area equal to two areas, all impervious (homes). Bibliography DCR, Virginia. "Virginia DCR Stormwater Design Specification No. 7: Permeable Pavement." EPA, US. Pervious Concrete Pavement. 10 September March 2011 < p=137&minmeasure=5>. Policies, Gold Coast Planning Scheme. "13.11 Porous and Permeable Paving." 2007.

"Virginia DCR Stormwater Design Specification No. 7: Permeable Pavement." 2010. EPA, US. Pervious Concrete Pavement. 10 September 2009.

Pervious Pavers. By: Rich Lahren. Hebron Brick & Block Supply

Pervious Pavers. By: Rich Lahren. Hebron Brick & Block Supply Pervious Pavers By: Rich Lahren Hebron Brick & Block Supply Stormwater Management and Control Issues Past emphasis was on flood control Today s emphasis is also on pollution More impermeable areas are