Permeable pavements are considered as an effective technique for reducing stormwater runoffs and pollutants. Studies showed that permeable pavements can reduce stormwater runoffs up to 100%. There are many factors affecting the efficiency of permeable pavements to reduce stormwater runoff volume, which may include, but are not limited to the following: type of the permeable pavements, design, installation, site slope and rainfalls. Being studied in this research were the effects of rainfalls on the stormwater volume reduction of three types of permeable pavements: permeable concrete, permeable asphalt and permeable interlocking concrete blocks.

The City of St. Louis started a pilot study in 2008 to evaluate the effectiveness of permeable pavements on runoff volume reduction and quality improvement. For the purpose of the study, three sites were selected in the St. Louis metropolitan area, and their surface covers were replaced with permeable pavements. Then, data were collected for these sites under the conditions existing in 2008 and also after the installation of the permeable pavements in 2011. The collected data, which included rainfalls and flow rates in the combined sewers during the period of the pilot study, were used in this research.

After detailed calculations and analysis of the collected data and comparison of the results, it was found that rainfall intensity affected the volume of stormwater runoff generated from the three sites. When rainfall intensity increased, the volume of the runoff generated per each inch of rainfall increased as well. The results showed that the reduction rates in the volume of the runoff generated due to the use of permeable pavements decreased with the increase in rainfall intensity. All storms that occurred during the study period were divided into four groups based on their rainfall intensities. For group 1, which included all storms whose rainfall intensities were between 0.01–0.25 in/hr, the reductions were 60%, 36% and 69% for permeable concrete, permeable asphalt and permeable interlocking concrete blocks, respectively. For group 2, which included all storms whose rainfall intensities were between 0.25–0.50 in/hr, the reductions were decreased to 28%, 24% and 53%, respectively. For group 3, which included all storms whose rainfall intensities were between 0.50–0.75 in/hr, the reductions were decreased to 21%, 15% and 27%, respectively. For group 4, which included all storms whose rainfall intensities were between 0.75–1.00 in/hr, the reductions were decreased to 10%, 9% and 22%, respectively.