Laserfiche WebLink
G:\Group Data\Municipal\Clients - Cities - Counties\St. Anthony\City Engineer\Letters_Memos\MEMO-3417 Skycroft Retaining Wall-052120.docx7 0 1 X E N I A A V E N U E S | S U I T E 3 0 0 | M I N N E A P O L I S , M N | 5 5 4 1 6 | 7 6 3 . 5 4 1 . 4 8 0 0 | W S B E N G . C O M Memorandum <br />To:Mr. Charlie Yunker - City Manager, St. Anthony Village <br />From:Justin Messner, PE – City Engineer <br />Date:05/29/2024 <br />Re:Permeable Pavement and Pavers as Pervious Surface <br />This memo has been prepared in response to a recent request to accept permeable pavements <br />and pavers as pervious surface. Currently when reviewing a site plan for impervious surface <br />calculation, St. Anthony Village considers permeable surface materials (asphalt, concrete, pavers), <br />artificial turf, and playground surfacing to be impervious. <br />Regulating impervious surface areas is an environmental effort for the following reasons: <br />To minimize the volume of stormwater runoff (infiltration) <br />To improve water quality (filtration) <br />To reduce urban heat islands <br />To reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes, <br />and streams <br />Impervious surface areas displace living vegetation and reduce ecological productivity, and <br />interrupt atmospheric carbon cycling <br />Per the US EPA National Pollutant Discharge Elimination System (NPDES) guidelines, permeable <br />pavements and pavers can be effective at reducing stormwater discharges and pollutant <br />concentrations, though their effectiveness can be variable and depends more on the design <br />of underlying layers and surrounding environmental conditions than surface type. <br />Throughout the majority of St. Anthony Village, the underlying soils are predominantly tight, clay <br />soils with low hydraulic conductivity, thus not allowing infiltration and therefore offering minimal <br />reduction in stormwater runoff. Although reduction in stormwater volume is generally a function of <br />subsoil infiltration rate and base storage capacity, permeable pavements and pavers can reduce <br />pollutant concentrations through several processes. The media layers filter stormwater and <br />promote pollutant removal through physical filtration and biological processes. The subgrade soils <br />are also a major factor in treatment. <br />A research study conducted at the Water Resources Research Institute of the University of North <br />Carolina on the performance of permeable pavement over a tight, clay soil concluded the overall <br />volume reduction via exfiltration and evaporation was moderate (22%) which was expected given <br />the limitations of the underlying soil. Volume reduction was understandably lower than studies of <br />permeable pavements built over infiltrative soils but was achievable to moderate levels by the <br />inclusion of an internal water storage zone. Furthermore, pollutant loads for nutrients, sediment, <br />and metals were typically reduced by 70%. <br />Recommendations: <br />1.Permeable pavements (asphalt, concrete, pavers), artificial turf, and playground surfacing that <br />is designed to infiltrate stormwater when maintained is considered impervious surface, but may <br />be used as a volume-reduction practice in accordance with the table provided below: