The Eastern Bay of Plenty experienced two large rainfall events during April, the first due to Ex-Cyclone Debbie on 4 and 5 April and the second due to Ex-Cyclone Cook on 12 and 13 April. Because of the rainfalls associated with ex-cyclone Debbie the Rangitaiki River experienced its largest flood on record.
Matahina’s normal consented operating range is between RL73.15m and RL76.2m. In preparation for Ex-Cyclone Debbie Trustpower (TPL) started lowering the reservoir to its minimum normal operating limit on 31 March. On 4 March, following authorisation of the Bay of Plenty Regional Council (BOPRC) Trustpower started lowering the reservoir below its minimum consented operating limit in accordance with the flood management plan. By 21:30 on 5 April the reservoir level was at RL71.6m which is the minimum flood control level defined in the flood management plan and authorised only during emergency situations. As anticipated prior to reaching this level, at RL71.75m the ability to generate ceased and the scheme was entirely reliant on its spillway for discharge.
With inflows significantly increasing overnight, the lake level rose steadily and generation resumed at a lake level of RL73.6m at 07:30 on 6 April. The maximum flood inflow into Matahina reservoir of calculated 915 m3/s had happened two hours prior at 05:30 on 6 April. Trustpower capped the total scheme outflow to 780 m3/s reducing the peak flows for downstream stakeholders significantly. The peak spillway discharge was calculated to be approximately 730 m3/s, about 32% of its maximum spillway capacity of 2,300 m3/s. The “normal” flood management range of the reservoir is between RL76.2m and RL76.8m with higher levels only during extreme inflow events. The maximum reservoir level of RL76.48m was recorded at 18:30 on 7 April.
The dam spent approximately 26 hours in its flood range, returning to maximum normal operating level at 04:00 on 8 April.