Permeable Pavement Hydraulic Conductivity Indices for Rainfall-Runoff and Particulate Matter Loadings

Abstract

Permeable pavement (PP) can be a primary source-area component of a sustainable drainage system. PP facilitates runoff reduction and particulate matter (PM) separation. PM separated by the PP surface reduces hydraulic conductivity (K) and influences PP functionality and maintenance. Pragmatic and reproducible methods are necessary to identify K reduction and maintenance. Because K is the proportionality constant between specific discharge and hydraulic gradient (Darcy's law), disparate test methods and conditions produce disparate K results. Consequently, evaluations based solely on K can lack guidance when the goal is runoff reduction. Herein, PP response is examined based on K, outflow time (OT) through the PP across a fixed driving head range, and volumetric runoff coefficient (C). The PP test matrix parameters are (1) total porosity from 0.15 to 0.25, (2) rainfall loading from 0 to 150 mm/h, (3) PM aerial loads from 0.5 to 2.0 kg/m2 as a silty-sand schmutzdecke, (4) slopes from 2.5% to 7.0%, and (5) PP maintenance. Results demonstrate (1) higher PM aerial loads increase OT (K decreases), (2) higher total porosity decreases OT (K increases), and (3) higher rain rates inconsistently produce lower OT compared with the no-rainfall control at higher PM aerial loads. For all tests (N=285) with C measurements, lumping all other parameters, the mean C is 0.07, the standard deviation is 0.11, and the maximum is 0.67. These C results are significantly lower than impervious pavements. As the PP slopes and PM aerial loads increased, C increased and K decreased but with no clear trend between K and C. For tests where K<0.01 mm/s and OT<300 s, C was <0.35. An OT limit of 300 s provides guidance for PP conditions that provide runoff reduction, with C<0.20 for 2.0% slopes. PP conditions in this limit provide discernable runoff reduction. OT and C can be pragmatic adjuvants with disparate K methods.