Sensoring Permeable Pavement to Document Performance and Crediting

The design and implementation of stormwater best management practices (BMPs) is based on regulations and guidance that tries to anticipate and model the impacts of each system. The deployment of in-situ sensors along with stormwater BMPs has allowed communities to more accurately understand BMP performance before, during, and after rain events. This has led to a more accurate accounting of BMP performance and reduced costs by municipalities in complying with their MS4 permits and TMDL compliance goals. Our research has specifically focused on documenting the performance of permeable pavement systems, which are often governed in their implementation by the soil on which they are installed. The permeable pavement system chosen for this work is PaveDrain, a system comprised of permeable articulating concrete blocks (P-ACBs). P-ACB systems differ from traditional permeable pavers because they have 1) open joints and 2) are able to handle heavy traffic loads. The open joints means this system handles more rain water, and is easier to maintain (cleaning process is required less frequently and is easier to perform). The suitability to handle heavy traffic loads means that PaveDrain can be used in additional applications like roadways, traffic aisles, and truck stops. For this work, the following sites were studied: Table 1. Studied Sites The sensor system monitors water level in the stone base, with a nearby weather station monitoring rainfall, temperature, barometric pressure and relative humidity. As can be seen in the figures below, water enters the base quickly during rainfall events, and over time exfiltrates from the stone base into the soil [where present, the height of internal drainage structures is notated]. Figure 1 Measured rainfall and water level with corresponding subgrade infiltration rates. In both sites, most runoff draining into the BMP has exfiltrated into the soil, reentering the natural water cycle in a decentralized fashion within 24 hours of the end of a rainfall event. Instantaneous rates of exfiltration with high hydrostatic head pressure reached close to 1.00 in/hr. Average rates of exfiltration over the first 48 hours are 0.40 in/hr0.50 in/hr. For most jurisdictions stormwater designs would expect clay soil to infiltrate at 0.04 in/hr, a full order of magnitude less than the rates being measured in the field. The discrepancy between observed and expected values is attributable to the heterogenous nature of soil. While clay soil is present, so are more permeable soils which form 3-dimensional networks and paths for water to exit the BMP system leading to significantly elevated performance. Furthermore, based upon these values, modeling was done to understand the value of TSS and TP removal beyond what is typically credited to a permeable pavement system. Source loading and management modeling software (WinSLAMM) was used to reevaluate the installed system in Cudahy, utilizing site-specific parameters including measured subgrade infiltration rate and product-specific performance properties representative of the P-ACB system (PaveDrain) [Figure 2]. It was determined that pollutant removal costs required to close compliance gaps can be reduced by a factor of 12 when modeling implements measured data and parameter definitions that reflect system capabilities. [Table 2] Figure 2. Subgrade sediment accumulation impact on pollutant removal. Table 2. Economic impact of SLAMM enhancement and in-situ sensor system. In communities that have high percentages of impermeable cover (e.g., urban cores), the use of low-maintenance, high-performing BMPs like P-ACBs, would allow all communities to more effectively address stormwater challenges, especially in the face of more extreme rainfall events.