In order to keep roadways safe for driving, they are designed and constructed to allow storm water runoff to drain quickly and efficiently to the road shoulder. Storm water runoff from the State of Hawaii Department of Transportation, Highways Division’s (DOT-HWYS) roadways typically drains through a network of pipes (the Municipal Separate Storm Sewer System, or “MS4”) directly to outfall locations at natural stream channels or to the ocean. Unfortunately, this increased storm water flow often results in stream channel erosion and increased pollution into our natural waterways. As part of the Action Plan for Retrofitting Structural Best Management Practices (BMPs), the DOT-HWYS is reconstructing portions of their existing storm drainage system in order to restore more natural flow patterns and remove pollutants from roadway storm water runoff.
For the Ala Wai Watershed Storm Water BMPs on Oahu Project, the DOT-HWYS is using the median areas between the H-1 freeway off ramps at University Avenue (referred to as the “cloverleaves” due to their shape) to construct bioswales that will provide an opportunity for storm water infiltration, evaporation and treatment to reduce pollutants. The bioswales consist of an under drain system, which includes a perforated drain pipe and engineered soils that provide filtering. On the surface, native shrubs and grasses are planted to slow and filter storm water as it first enters the bioswales. Additionally, the design includes an outfall structure that forces storm water to pond in the median allowing more time for infiltration into the under drain or evaporation. This project is just one of many ways that the DOT-HWYS is working to improve the quality of storm water runoff from its roadways and protect our natural resources.