Puʻukapu Reservoir Irrigation System Reservoir Safety Improvements

Hawaii State Irrigation System Reservoir and Dam Safety Improvements Project

The Puʻukapu Reservoir is an irrigation water storage reservoir located near the town of Waimea, Hawaiʻi County, on the island of Hawaiʻi, upstream from rural developments. The project is one of many dam safety projects that Oceanit has worked on in recent years, like Kahana Nui dam on Maui, to bring the State’s dams and reservoirs up to present-day safety standards.

The Puʻukapu Reservoir features a circular-shaped dam that was completed in 1957, and receives the majority of its water from the upstream Upper Hāmākua Ditch System fed by Kawainui, Alakahi, Koiawe and Waimea intake sources. Puʻukapu is capable of holding approximately 60 million gallons of water and is operated as part of the Waimea Irrigation System (WIS) by the Agriculture Resource Management Division of the Hawaiʻi State Department of Agriculture.

WIS provides irrigation water to local farms in the nearby area. Water from Puʻukapu can also be fed to the downstream Puʻu Pulehu reservoir (which holds approximately 100 million gallons) during drought conditions.

Puʻukapu Reservoir is classified as an intermediate-sized dam based on the storage capacity and its maximum structural height, which is approximately 50 feet along one perimeter. It is also classified as a high hazard dam, due to existing development downstream of the reservoir.

Oceanit was contracted as a Civil Engineering subconsultant to develop storm flow hydrographs, using HEC-HMS models and Probable Maximum Precipitation (PMP) data issued by the Hawaiʻi Department of Land and Natural Resources (DLNR). Oceanit analyzed the adequacy of the Puʻukapu Reservoir and its emergency spillway, and the results of the hydrologic and hydraulic analyses informed the design of improvements to meet minimum requirements for regulated dams and reservoirs per Hawaiʻi Revised Statues (HRS) §179D-6.

The scope of the Puʻukapu Reservoir safety improvements project included lining the reservoir basin with an HDPE liner and geocomposite sub-drain, along with construction of a curb wall along the crest to improve outlet flow control. In addition, plans called for construction of a new intake box, gate valve and stem, and new monitoring wells and embankment monuments for future monitoring reference.

Construction of these safety improvements along with the design of improvements to the spillway structure and piping downstream of the spillway are currently being undertaken as of this writing.