Fact Sheet: Flood Management at Matahina Dam

Press Release – TrustPower

SEQUENCE OF EVENTS TO 4:00PM, FRIDAY 7 APRIL, 2017 Due to flood inflows during the week beginning March 27th, Lake Matahina was full at the start of Friday 31st March. At Trustpowers initiative, the process of lowering the lake level began on Friday …Flood Management at Matahina Dam

SEQUENCE OF EVENTS TO 4:00PM, FRIDAY 7 APRIL, 2017

Due to flood inflows during the week beginning March 27th, Lake Matahina was full at the start of Friday 31st March.
At Trustpower’s initiative, the process of lowering the lake level began on Friday 31st March in anticipation of forecast rain linked to Cyclone Debbie.
The formal flood management process, in conjunction with Regional Council, commenced on Tuesday 4 April when the lake was at 73.4m (11.30am), and then further lowered to 71.7m by Wednesday 5th April – being the minimum allowable limit in the resource consent.
At the commencement of inflows arriving at the dam, Regional Council instructed Trustpower to release water to match inflows to retain storage in the lake in anticipation it would be required to attenuate the peak of the flood event.
When flows further increased, Regional Council asked that Trustpower continue to release water within the limits of the downstream stop-bank capacity.

Since 10.30pm on 5th April to current time (5pm Fri 7th), inflows into the lake have exceeded outflows, i.e. lake storage has been utilised.

This resulted in approximately twenty percent less water flowing down the river, during the peak of the flood than would have occurred if there were no dam in place.
Overnight of 6th/7th April, discharge from the dam was progressively reduced – during this period inflows had also begun to reduce. Again inflows remained higher than outflows.
At 9.30am on 7th April, following discussions with Regional Council, outflows were reduced to 350 m3/s to allow the Regional Council to safely access areas downstream for assessing flood damage, etc. This reduced flow was held to 5pm on Friday 7th April.
Planning is now underway for lake management over the weekend in collaboration with Regional Council.
The flood management collaboration with Regional Council considers the vulnerability of the downstream population during night time hours. In this regard, Trustpower will continue to cooperate with Regional Council to manage releases to enable downstream inspection and works to be conducted safely.
Our priority is always to keep the dam and the people below the dam safe.

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