The cornerstone of sustainability and green infrastructure is multifunctionality. In conventional infrastructure systems, every component has one job to perform. For example, asphalt provides the driving surface, inlets collect the runoff from the pavement, pipes carry the water away, detention ponds store water to reduce the peak flow, and water quality BMPs cleanse the water before it is released into the river. Permeable Interlocking Concrete Pavements (PICP) provide these five functions in one system:
- 1) Pavers provide a heavy duty driving surface
- 2) The entire paver area captures stormwater
- 3) Aggregate filled joints filter out sediment
- 4) Open-graded aggregates below the pavers convey water downstream
- 5) Voids in the aggregates (approx. 40%) provide storage
By utilizing the multi-functionality of PICP systems, expensive infrastructure components, including inlets, pipes, water quality devices, and detention ponds, can be eliminated creating additional space for development and lowering overall stormwater management costs. Permeable pavers also last twice as long as conventional pavements and cost less to maintain.
Hydrologic Design of PICP
The hydrologic design will typically govern the final configuration of the system. The most common design methodologies used are event-based hydrograph estimations or continuous simulation modeling. The most common event-based estimation method is the Watershed Hydrology Program (WinTR-20). The most common continuous simulation models are the US EPA Stormwater Management Model (SWMM) and the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS).
These methodologies rely on developing a Curve Number (CN), which is an empirical parameter related to the runoff response of a watershed. Because PICP can improve the hydrologic function of a developed area, a reduced CN is used. ASCE 68-18 provides a means for calculating the reduced CN accounting for the storage in the system and infiltration into the soil. The reduced CN is typically slightly greater than or equal to the pre developed CN. The Rational Method is not generally used for designing PICP systems, but in some cases regulatory agencies require it. When using the Rational Method, a C-value, which represents the percentage of rainfall that becomes runoff, can be calculated for a PICP system by dividing the total calculated outflow by the total calculated inflow. Typical calculated C-values range between 0.25 to 0.40 depending on the design of the system.
Slope affects the design of the system and once the surface slope exceeds 0.5% flow dams should be considered. The detail below shows how flow dams can be used to increase the efficiency of the system to store and manage stormwater.
The flow dams have orifices cut into them and the top of the flow dam is a weir allowing each storage cell created to be modelled as detention basins in series. The lowest storage cell can be connected to an outlet control structure used to attenuate peak flows and meet the allowable release rate of the regulatory agency.
Credit for Pervious Surface
Correctly designed, installed, and maintained, PICP systems have surface infiltration rates higher than that of almost any natural soil, and several times greater than the maximum possible rainfall intensity. This is why a PICP surface should be given complete credit for “100% perviousness,” as would a meadow or forest.