The goal of PICP is to mimic, if not improve upon, a site's predevelopment hydrology by retaining as much stormwater as possible in the base/subbase, so that it can infiltrate back into the ground.
Permeable Interlocking Concrete Pavement (PICP) is considered a Low Impact Development (LID) Best Management Practice (BMP) for stormwater management.
As a distributive infiltration practice, PICP conserves space by providing five functions in one Stormwater Control Measure:
1) Pavement wearing course
2) Stormwater capture
3) Stormwater conveyance
4) Water quality
5) Detention/retention
Stormwater Control Measure in one system. Various methods are used to model the site hydrology and calculate runoff flow rates and volumes. Depending on the hydrology model used, a curve number or a runoff coefficient is needed to represent the PICP site condition.
Belgard permeable pavers have a variety of shapes and surface openings. Based on the type of aggregate used in the surface openings (No. 8 or No. 9 stone), new installations can expect minimum infiltration rates of between 500 to 750 inches/ hours when tested in accordance with ASTM C1781.
Curve Number for PICP System
The Natural Resources Conservation Service (NRCS) method characterizes site runoff based on hydrologic soil type, land cover and amount of rainfall using a parameter known as a curve number (CN). Curve Numbers predict direct runoff from rainfall excess, and can range between 30 to 100, with lower numbers indicating lower runoff potential.
ASCE 68-18 - Permeable Interlocking Concrete Pavement provides a methodology to calculate CNs for permeable paver systems. Caution should be applied when using CNs for permeable pavement. Results can underestimate runoff in small watersheds (under 5 acres) for small storm events (below the 2-year storm), so for these cases, calculations should be verified by another method. Sample CNs per USDA Technical Release-55 are listed on this page. However, traditional hydrologic modeling requires CN modifications to properly model permeable pavement to account for reservoir storage.
CNs for PICP systems can range anywhere between 45 for A soils to between 70-80 for D soils.
Surface Runoff Coefficient for
PICP Systems
A common question that is asked is “What is the runoff coefficient (C) of the PICP system? C represent the percentage of rainfall that becomes runoff based on the surface type and is used in the Rational Method to determine peak flow rates.
It is overly simplistic and does not account for rainfall intensity, duration, or reservoir drainage. However, a C value can be calculated for the system by determining the flow into and out of the PICP system, and then dividing the outflow by the inflow the result being the C value for the system.
A C value of between 0.25 and 0.40 depending on subgrade permeability is appropriate for PICP systems when using the Rational Method.
Credit for Pervious Service
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.
Water Volume Control
PICPs can detain or retain water quality volume through storage in the aggregate base and subbase. Most design storm requirements are easily controlled in the underground reservoir until the subgrade soils infiltrate the water or until underdrains release the volume at a controlled rate.