Wailele Stream Flood Risk Management

The Wailele Stream Flood Risk Management (FRM) study is currently in the feasibility phase.  The integrated Feasibility Report and Environmental Assessment (IFR/EA) presents analysis and evaluation of alternative plans to manage flood risk along Wailele Stream in Laie, Oahu, Hawaii.  The study is being conducted under Section 205 of the Flood Control Act of 1948, as amended.  Section 205 allows for the study, design, and construction of small flood risk management projects in partnership with non-federal government agencies.  These smaller scale FRM studies are capped at a $10 million federal per-project cost share limit. The non-federal sponsor is the City and County of Honolulu (CCH).  The feasibility study was initiated in January 1999 upon signing of a Feasibility Cost Sharing Agreement (FCSA) between the U.S. Army Corps of Engineers (USACE) Honolulu District and CCH Department of Design and Construction. A Draft IFR/EA was released in June 2019. The Final IFR/EA is anticipated to be released in Fall 2023.

Wailele Stream is susceptible to periodic overtopping, resulting in damage to structures and infrastructure, conveyance of pollutants from the urbanized area into the ocean, and increased risk to community safety of residents and visitors within the floodplain.  Alternative plans evaluated include full detention, full conveyance, or a combination of the two to reduce flood risk in the Wailele watershed.  Alternative 2B, an optimized version of the full conveyance plan, was selected as the Recommended Plan at a cost of $16.6 million (FY22 price level). The Recommended Plan includes improvements to the existing Wailele Stream channel geometry, construction of a weir and overflow/diversion channel, two culverts traversing beneath Kamehameha Highway, and appurtenant structures required for energy dissipation at the outlet of the overflow channel.