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Highway Stormwater Control: Porous Vegetated Road Shoulders

Written by William G. Handlos, P.E.

Highway Stormwater Control: Porous Vegetated Drivable Road Shoulders

Gravel road shoulders are not porous. Rainwater sheet flowing off highway pavement passes right across the aggregate shoulder carrying with it sediment and other pollutants that ultimately end up in freshwater creeks and rivers. Aside from pollutant loading, re-grading shoulders is a continuous activity for State and County crews to repair eroded sections after several rain events.

Some counties have taken a smart solution approach to this problem and are using geocell cellular confinement systems to reduce maintenance costs and to improve water quality.  Shown below are the cross sections detailing a GEOWEB® geocellular drivable vegetated shoulder solution.

Typical Cross-Section Tower Drive Stabilized Shoulder and Swale.jpg

The infill to the geocell is made up of 2/3rd clear stone aggregate (void ratio 30 – 40%) and 1/3rd topsoil. The topsoil remains uncompressed within the interstitial spaces between the stone, and the stone is the structural component that carries the load. The topsoil allows for the vegetation roots to grow and feed upon the nutrients and water in the topsoil.  This system percolates runoff from the highway while offering proper load support for vehicles that pull off the side of the road. The GEOWEB® cells confine the infill, offering an apparent cohesion to the aggregate/topsoil mix.  The cellular structure eliminates movement of the infill and rutting that can occur with unconfined materials.

This approach has been available for years and was first used in Wisconsin, where the solution functions well over ten years later and is used on the other side of the world for swales and shoulders.

William Handlos, P.E.