The Kaneohe Watershed Storm Water Best Management Practices (BMP) on Oahu project is part of the HDOT Highways, Oahu District’s ongoing effort to reduce pollution from storm water runoff. Storm water from roadways typically drains through a network of pipes (the Municipal Separate Storm Sewer System, or “MS4”) directly to outfall locations into stream channels, or directly to the ocean. Unfortunately, roadway runoff often contains trash, oils and greases and unnaturally high nutrients that can have negative impacts on streams and the ocean.

As part of the Action Plan for Retrofitting Structural BMPs and the Trash Reduction Plan, DOT-HWYS is designing and constructing storm water treatment systems throughout their MS4. A Nutrient Separating Baffle Box (NSBB) was one of several treatment systems that were constructed as part of the Kaneohe Watershed Storm Water BMP project. The NSBB is a large concrete vault that consists of three chambers that trap sediment; a box screen that collects trash, leaves, and other floating debris; and an absorbent boom that removes oils and other hydrocarbons from storm water. Construction required the excavation of a large hole around an existing storm drain pipe. A section of the pipe was then removed to allow for construction of the NSBB. All the storm water that flows through this pipe is now treated in the NSBB before reaching its discharge point just a few feet downstream. This project was managed by DOT-HWYS Kaneohe Field Office and was successfully completed in 2016. Next time you’re sitting at the light on the town-bound H-3 off-ramp to Kamehameha Highway, look for the three large manholes in the landscaped median that indicate the location of this NSBB!

Excavation and removal of a section of the storm drain pipe at the new NSBB location.
Excavation and removal of a section of the storm drain pipe at the new NSBB location.
Construction of the NSBB in progress.
Construction of the NSBB in progress.
 
The completed NSBB is located under these three manholes at the town-bound H-3 off ramp to Kamehameha Highway.
The completed NSBB is located under these three manholes at the town-bound H-3 off ramp to Kamehameha Highway.