ALDOT’s Road to Roadside Green Infrastructure
A state Department of Transportation (DOT) experiences challenges in implementing a post-construction stormwater management program that are different than those the typical MS4 (e.g., municipality) faces. Such challenges include (i) having limited space available on roadways for installing post-construction stormwater management devices, (ii) placing post-construction stormwater management devices at discrete locations along continuous, linear roadways, (iii) accounting for motorist safety in the design of post-construction stormwater management devices placed in close proximity to roadway travel lanes, (iv) creating permeable ground surface able to support large volumes of heavy, high-velocity vehicles, and (v) maintaining a large and widespread network of roadways. Many widely-used post-construction stormwater management practices, such as retention basins, permeable pavers, green roofs, rainwater harvesting, and complex bioretention areas, are generally not suitable for most roadway facilities operated by a state DOT. Thus, preferable post-construction stormwater management practices for a DOT are ones that can approximate natural stormwater runoff attenuation processes (e.g., infiltration) while occupying a small amount of roadside area, requiring minimal maintenance, and preserving motorist safety. Employing green infrastructure practices strategically can allow for better navigation of the DOT-specific challenges. In this presentation, the path that the Alabama DOT (ALDOT) has taken to implement green infrastructure on its roadways feasibly and effectively will be described.
The presentation will begin with a brief orientation that will (i) highlight the possible negative environmental consequences of converting a natural, pervious surface into a significantly more impervious, developed surface, (ii) explain the intended function and potential advantages of green infrastructure in offsetting hydrology changes expected from development, and (iii) situate post-construction stormwater management and, by extension, green infrastructure within the standard MS4 program framework. Then, the unique set of post-construction stormwater management considerations of a state DOT will be examined in relation to green infrastructure features and options. Discussion of specific ALDOT policies set, procedures developed, and design decisions made over time to implement green infrastructure on ALDOT roadways will follow.
The “infiltration swale,” an ALDOT green infrastructure practice, will be emphasized because of the promise it has shown for broader implementation. Other green infrastructure practices employed by ALDOT, such as step pools, vegetated filter strips, grassed channels, and riparian buffers, will also be discussed, but those practices are used for “supplemental” attenuation; the infiltration swale performs the primary attenuation of increased runoff volume. Details concerning the infiltration swale that will be provided include key implementation site variables, current design specifications, overall performance assessment, factors adversely impacting performance, and ongoing efforts to improve design specifications.