New dam safety regulations lead to new classification of Marton B Dam

Marton B Dam is the smaller upper dam located at Tūtaenui Reservoir, with the larger capacity Marton C Dam downstream.

Whilst the classification is new under the Building (Dam Safety) Regulations 2022, the risk that the Marton B Dam poses has not changed and is well known to Council. Mitigations have been in place to manage the risk for many years as required under the Building Act 2004. A recent independent engineering assessment has also identified the risk of a sudden failure as highly unlikely.

“The greatest risk to the Marton B Dam is a significant rain event causing overtopping into the Marton C Dam,” says Council’s General Manager - Assets and Infrastructure, Arno Benadie.

“To mitigate the ongoing risk, we aim to keep the reservoir level of the larger dam between one and two meters under the spillway, which gives us capacity to handle heavy rain events and overflow from the smaller dam.

“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,” adds Arno.

Inundation mapping of a catastrophic failure of the whole reservoir shows that the flood channels are almost identical to what is already known from previous flooding events.

Council will now be working to determine if the dangerous dam classification will have to be noted on LIMs of properties that are already noted as being in a flood hazard zone. If this is the case, residents will be notified.

Options for what will happen to the dams once the new Marton Water Supply is commissioned are also being assessed. These options could include reducing the reservoir water levels to below the risk level, or decommissioning the dams entirely.

Marton’s new water supply is currently going through a robust trial and testing period to ensure the bore water and innovative nanofiltration system will be able to deliver what Marton residents expect from their drinking water.

In an abundance of caution, Council and Stantec have drafted an Emergency Action Plan in the unlikely event of either dam structure failing. This Action Plan is now going through a robust review before being finalised to ensure all possible eventualities are considered.

As the regulator of the Building (Dam Safety) Regulations 2022, Horizons Regional Council were also notified of the "dangerous dam" classification of Marton B Dam. Under the Building Act 2004, Horizons Regional Council must have a policy on dangerous dams, earthquake-prone dams and flood-prone dams for the region that aligns with the Dam Safety Regulations. This policy outlines their approach and priorities as the regulator.

The Marton B Dam located at Tūtaenui Reservoir is the upper and smaller dam. Marton C Dam is the lower and bigger.