Oceanit was contracted by the Maui County Department of Public Works to bring Kahana Nui Reservoir up to present-day dam safety standards. Currently, the Kahana Nui Dam falls under high hazard classification.
Historically, dams and reservoirs in Hawaii were predominately developed by the agriculture industry in the early 1900s. Today, dams and reservoirs continue to be used by the agriculture industry, in addition to providing flood control and serving other purposes.
At Kahana Nui, a 50-feet (15 m) high earthen dam – with emergency spillway – was built upstream from Honoapiʻilani Highway to manage water from high flow storm events. Flow is held in the basin until it reaches the height of the principle spillway.
Spillway structures must be able to safely pass the peak Probable Maximum Precipitation (PMP) event inflows while maintaining a minimum of 2-ft freeboard at the dam crest. In hydrologic and hydraulic modeling run by Oceanit, the Kahana Nui dam was shown to overtop during a PMP storm event.
One main project objective is to provide adequate spillway capacity to pass inflow from a Probable Maximum Precipitation (PMP) Event in the watershed.
Oceanit investigated two dam improvement alternatives to meet dam safety requirements and prevent dam overtopping: 1) Elevating the dam crest by adding a “parapet” wall on top of the dam; and 2) Lowering the crest elevation of the secondary spillway.
Both alternatives prevented dam overtopping and maintained a minimum of 2-ft freeboard at the dam crest. Oceanit examined the downstream effects of both alternatives, comparing downstream flooding under each alternative to the existing condition for a PMP storm event.
Oceanit is investigating possible solutions to resolving dam deficiencies that have less impact on maintenance staff including possibly installing a parapet wall on a concrete cap placed atop the existing dam wall.
Work with Maui County continues, including work to improve low flow discharge intake and controls at the primary spillway and construction of an earthen berm spanning the middle of the reservoir basin. With the evolving challenges of climate change, flood control and dam engineering will be a critical need in the future as extraordinary storm events become more frequent.
