PDP was engaged by Bay of Plenty Regional Council (BoPRC) to provide advice on upgrading the Lake Ōkāreka outlet following a period of consistently high lake levels. PDP assessed options to increase the flow out of Lake Ōkāreka to manage the water level and flooding risk to property and infrastructure. The options assessed included extending an outlet pipeline to Lake Tarawera and assessing potential for construction of a hydroelectricity generation scheme.
PDP carried out detailed design and construction phase support for the preferred option to upgrade the Lake Ōkāreka outlet pipeline to Waitangi Stream, which flows to Lake Tarawera and has significant ecological values. Erosion protection and ecological enhancement structures were installed at the head of Waitangi Stream to accommodate the increased flows. The project involved carefully coordinating hydraulic, ecological, and geotechnical design to overcome several design and construction challenges. The challenges included marginally stable slopes, complex pipeline hydraulics, deep excavation within a narrow corridor, significant energy dissipation, and maintaining flow during construction.
The design of the new works was also cognisant of cultural matters and local public expectations during the design and construction phase. In particular, the outlet design was via an up-flow structure that not only dissipated energy but also emulated a flow from a natural spring as requested by the local iwi and was located to avoid damaging a natural spring that was important to local landowners for water supply.