Safety measures have been in place to mitigate any risk of a sudden failure for years, as required under the Building Act 2004.
“To mitigate the ongoing risk, we aim to keep the reservoir level of the larger dam between one and two metres under the spillway, which gives us capacity to handle heavy rain events and overflow from the smaller dam,” Benadie said.
The new classification may require at-risk properties, already noted as being in a flood hazard zone, to now include the classification in its Land Information Memorandum.
The council is currently working to determine if this will be necessary and plan to notify residents accordingly.
Flooding channels shown in recent inundation mapping of the unlikely event of a catastrophic failure of the whole reservoir are close to identical to what council already knew from previous flooding events.
Council and Stantec have worked to create an Emergency Action Plan in the event of either dam structure failing, which is currently going through a review process before being finalised.
“A failure of the B Dam structure itself without a trigger, event, or warning is highly unlikely. However, we have a plan in place should the unlikely eventuate,” Benadie said.
A new water supply for Marton is being assessed through a trial and testing period to ensure bore water and innovative nanofiltration system standards.
Options are being discussed by the council for what will happen to the dams once the new Marton water supply is put in place.
Potential options may include reducing water levels to below risk level, or full decommissioning of the dams.
Horizon Regional Council have been notified of the “dangerous dam” classification change as regulators of the Building (Dam Safety) Regulations 2022.
It is required under the Building Act 2004 to have a policy in the case of dangerous dams, earthquake-prone dams and flood-prone dams in alignment with Dam Safety Regulations.
