Hawaii Water Management – Waiāhole Reservoir System Reservoirs 155 and 225 Improvements Project

The Waiāhole Ditch Irrigation System was constructed between 1912 and 1916 to irrigate sugarcane fields on the western side of Oʻahu and consists of a 26-mile-long transmission system of ditches, tunnels, siphons, and reservoirs that provides a source of irrigation water to local farmers from the windward side of the island of Oʻahu. The State of Hawaiʻi Agribusiness Development Corporation (ADC), an attached agency to the Hawaiʻi Department of Business, Economic Development, and Tourism (DBEDT), operates and maintains the Waiāhole Ditch Irrigation System, including two reservoirs within the system: Reservoirs 155 and 225, both of which are unlined, earthen storage basins used to store irrigation water for adjacent farmers and are fed directly by the Waiāhole Ditch. These reservoirs have lost holding capacity due to years of sediment accumulation.

The United States Army Corps of Engineers (USACE) is proposing to improve the safety and operation of Reservoir 155 and Reservoir 225 (Proposed Action, or project). In accordance with the Hawaiʻi Dam Safety Act of 2007, the State of Hawaiʻi Department of Land and Natural Resources (DLNR) has jurisdiction over the enlargement, repair, and alteration of jurisdictional dams, to protect the health, safety, and welfare of the citizens of the State of Hawaiʻi by reducing the risk of failure of the dams and reservoirs. The USACE is working with the DLNR to ensure that all proposed modifications are consistent with state law.

The Proposed Action is authorized under Section 1(a)(4) of the Consolidated Appropriations Act of 2001 (Public Law 106-554, Appendix D, Chapter 5 (114 STAT 2763A-190)), which authorized and directed use of $2 million of appropriated Construction General Funds to initiate design and construction of the Hawaiʻi Water Management Project, which includes the Waiāhole Ditch Irrigation System. The 905(b) Report (USACE 2002) was approved by Headquarters USACE on February 12, 2003, allowing implementation of design and construction of repairs and rehabilitation of publicly owned irrigation systems to the extent of the funds appropriated.

Reservoir 155 is a regulated dam located within agricultural fields approximately 1.0 mile west of Kunia Road (State Route 750). Reservoir 155 is classified as a high hazard, small dam due to the following factors: a total height of 25 ft (ft), a holding capacity greater than 50 acre-feet (ac-ft), and potential downstream impacts in the event of a failure. Per the Hawaiʻi Administrative Rules (HAR) Chapter 13-190.1, a "high hazard" dam classification is defined as that in which the failure of the dam or reservoir will result in probable loss of human life.

Reservoir 225 is located east of Kunia Road, adjacent to the roadway and is not currently listed as a regulated dam. However, recent calculations show that Reservoir 225 exceeds a capacity of 50 ac-ft, which would result in the reservoir being listed as a regulatory dam and require DLNR to take jurisdiction over the reservoir.

Several historic studies, reports and inspection forms were reviewed to determine the existing conditions of the two reservoirs for the purposes of project design. In general, the identified deficiencies include over-steepened slopes, uneven and marginal crown width, excessive vegetation, inadequate outlet and spillway works, and compaction/stability of the earthen embankment. The main purpose of the Proposed Action is to ensure that each reservoir meets dam safety criteria. To meet this purpose the water storage capacities of Reservoir 155 and Reservoir 225 would both be reduced. In addition, the reservoirs would be lined to reduce water losses and leakage in the system.