Comparative Evaluation of Different Types of Permeable Pavement for Stormwater Reduction—St. Louis Green Alley Pilot Study

Abstract

The City of St. Louis is served by a combined sewer system and faces the problem of combined sewer overflows (CSO). Green infrastructure such as permeable pavement provides on-site stormwater management for volume reduction and pollutants removal from storm runoff, can offer a cost-effective means to address the CSO problem. A pilot study was conducted in St. Louis at three alleys in residential neighborhood, where the existing impermeable pavement at each site was replaced by one of the three different types of permeable pavement (permeable concrete, asphalt, and paver, respectively). The objective of the pilot study was to evaluate the effectiveness of permeable pavement on volume reduction in combined sewers. The flows in the combined sewer at each site were measured for approximately one-year during each time period of before and after the change in pavement. To determine the stormwater volume in the measured flows, the base flows (i.e. sewage) were separated from the combined flows. For a given rainfall, the measured flows from the dry days that preceded or followed the rain days were taken as the base flows. The measured flows from the rain days were taken as the combined flows. The difference in flows between the rain and dry days were used to calculate the stormwater volume. To adjust for the difference in rainfalls between the pre- and post-installation, the separated stormwater volumes were normalized by the amount of rainfalls before the volume reduction was evaluated. The results showed that, permeable pavement resulted in stormwater volume reduction in the combined sewers at each of the three sites. The volume reduction ranged from 46% by permeable paver, 36% by permeable concrete, to 13% by permeable asphalt. The analysis on flow hydrographs from the combined sewers revealed that the application of permeable pavement reduced peak flows as well. The pilot study indicated that the installation of permeable pavement can result in stormwater volume reduction in combined sewers; the effectiveness of reduction differed among different types of pavements. The study recognizes that many factors can affect the flows in sewers, and recommends extended flow monitoring over a longer period, so that the long term performance of permeable pavements can be evaluated.