The average motorist passing by the interchange of I-40/85 and Maple Avenue may not notice NCDOT’s latest efforts to protect Little Alamance Creek because most of the action happens below ground! To the casual observer the grassy area within the interchange appears much the same as it always has, but with one important exception, the interchange now serves as a treatment system to filter and cleanse stormwater runoff from portions of the interstate and Maple Avenue before it reaches the nearby creek. This summer NCDOT completed construction of the ‘ecological interchange’ by regrading the area within the interstate on-ramp and installing a underground french drain system overlaid with two feet of engineered filter media. Prior to this retrofit project stormwater runoff from the highway was conveyed through a network of pipes and open channels to the creek with little or no treatment. Now the highway runoff is directed into the new basin within the interchange where it infiltrates through the grass and down into the subsurface engineered media where pollutants are filtered out and trapped. Clear, clean water is released through the french drains into the existing drainage network before ultimately reaching Little Alamance Creek.
The photograph above shows installation of the sandy filter media over top the french drain system within the basin. This media traps pollutants washed off the highway after each rainfall.
Photograph of the filtering basin taken soon after construction was completed in the summer of 2016. Two feet of engineered filter media and a french drain system lies below the grass surface. The floor of the basin is equipped with several access ports (white square shaped objects) to allow maintenance engineers to clean the under drain system should it become clogged.