Storm water from roadways typically drains through a network of pipes (the Municipal Separate Storm Sewer System, or “MS4”) to outfall locations into stream channels, or directly to the ocean. Over time, the pipes and outfall structures at many of these locations have degraded, resulting in soil erosion and negative water quality impacts. As part of the Action Plan to Address Erosional Outfalls, DOT-HWYS is permanently repairing degraded outfall structures and stabilizing eroded areas at these sites.

One of these repairs was recently completed at an outfall in Kailua that drains to the Kawainui watershed. Prior to repair, the end of the pipe at this outfall was severely corroded and broken, resulting in significant soil erosion. The design for this repair required a solution to fix the corroded pipe, stabilize the eroded slopes around the outfall, and provide flow dissipaters to reduce the potential for erosion in the future. A new lining was installed inside of the existing storm drain pipe to restore its functionality and prevent further corrosion. The eroded slopes around the outfall were stabilized using concrete cloth, which is a pliable fabric that is embedded with un-cured concrete. Once the slope was prepared, the concrete cloth was applied and anchored in place over the previously eroded areas. Then, water was applied to the fabric causing the concrete to activate and harden, creating an extremely durable and stable surface. In order to reduce storm water flow velocity and prevent future erosion “A-Jacks” concrete blocks were installed at the base of the outfall.

The repair of this outfall is just one of many accomplishments that DOT-HWYS has completed in their ongoing efforts to improve the quality of storm water runoff from our highways.

This site was part of the Central Oahu Best Management Practices, Phase 2 project managed by DOT-HWYS Salt Lake Field Office.

Damaged and eroded storm water outfall in prior to repair.
Damaged and eroded storm water outfall in prior to repair.
Repaired and stabilized storm water outfall after project completion.
Repaired and stabilized storm water outfall after project completion.