Storm Water Quality Modelling Several jurisdictions place restrictions on the amount of Total Suspended Solids (TSS) that can be discharged from a site to the receiving storm water systems. There are two generally recognized methods of TSS management with permeable pavements. The first is filtration/straining as surface water infiltrates down through the jointing aggregate between the permeable pavers. Research at Florida Gulf Coast University determined that removal efficiency is a function of the size distribution of the particulate and the grain size of the jointing aggregate. Assuming ASTM #8 stone in the joints, the projected removal efficiency of an NJCAT gradation material is between 61 and 74%, while the removal efficiency of an MTO winter sand approaches 100%. The second method of preventing TSS from being discharged to the storm water system is, like with other infiltration practices, related to the infiltration capacity of the subgrade soils. To quantify the TSS removal resulting from infiltration, a water balance needs to be conducted to define what percentage of water that enters the base/subbase infiltrates into the subgrade (versus overflows/discharges through the drain). Depending on the native soil type and system design, the percentage of infiltration can range from 0 to 100%, with the resulting reduction in the remaining TSS being proportional.
Storm Water Quantity Modelling
Storm water quantity modelling is performed to calculate and compare the following conditions: pre-development, post-development (uncontrolled), and post-development with BMP practices in place. Since there are no default values for PICP using the Soil Conservation Service (SCS) Curve Numbers (CN) method, it is up to you to determine them. Start by calculating the expected runoff from the surface of the pavers based on the typical CN for impervious surfaces (CN=98) using the traditional SCS equations below. Remember that a typical 85%-95% solid PICP surface experiences losses similar to traditional pavements due to the cooling/wetting of the paver surface. Where: Q = Total runoff depth (in.) P = Total precipitation depth (in.)
Q = (P - Ia)2 / (P - Ia + S) S = 1000/CN – 10
Ia = Initial abstraction of losses before runoff begins (in.) S = Potential maximum retention after runoff begins (in.)
With traditional pavements, excess water collects and sheet flows off the pavement surface. With PICP, excess water infiltrates through the joints between the pavers and into the base/sub-base. Surface overflow occurs only after the infiltration capacity of the sub-grade and/or the storage depth of the reservoir is exceeded. The equations used to calculate adjusted flows (Qadj) and adjusted CN (CNadj) are as follows: Where:
Qadj = Q – Ts - Ti
CNadj = Adjusted curve number Qadj = Adjusted runoff depth (in.) TS = Depth of water storage within aggregate reservoir (in.) Ti = Depth of water infiltrating into the subgrade over the duration of the design storm (in.)
1000 10 + 5P + 10Q adj – 10(Q adj 2 + 1.25Q adj P) 1/2
CN adj =
Examples: 100 yr 24 Hr duration precipitation depth (P) = 8 in; for an asphalt pavement with CN = 98, Q = 7.76 in
Over silty clay soil using a Parial Exfilration System
Over silt using a Full Exfilration System
Over clay soil using a No Exfilration System
Ts = 4.8 in (using a 12” thick base) Ti = 1.44 in/day (see page 16) Q adj = 7.76 - 4.8 - 1.44 = 1.52 CN adj = 43
Ts = 4 in (using a 10” thick base) Ti = 6.48 in/day (see page 16) Q adj = 7.76 - 4 - 6.48 < 0 CN adj = 0 (No underdrain used, balance of stored water would infiltrate)
Ts = 4.8 in (using a 12” thick base) Ti = 0 (system is lined) Q adj = 7.76 - 4.8 - 0 = 2.96 CN adj = 57 (Underdrain discharge would be controlled using an orifice plate or similar)
(Underdrain raised to minimize discharge, balance of stored water would infiltrate)
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APPLICATIONS AND SOLUTIONS
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