Farm advice: New regulations on dam safety

By: Stephen McNally, IrrigationNZ principal technical advisor

The new regulations on dam safety announced on 13 May by the MBIE may impact a variety of dam owners

The new regulations on dam safety were announced on Friday, 13 May 2022 by the Ministry of Business, Innovation and Employment (MBIE).

The regulations may impact a variety of dam owners, including owners of large agricultural dams used for irrigation water storage and some irrigation distribution canals. The regulations require dam owners to determine whether their structure exceeds the height and volume thresholds for the reservoir to be defined as a classifiable dam.

The depth for classifying the dam is only concerned with water held above the external ground level

Many water storage structures and canals in the New Zealand farming and food production sector have been well-engineered. The proposed regulations take a risk-based approach to safety management and place obligations on owners of dams proportionate to the risk their dam is likely to pose.

Irrigation New Zealand has worked with MBIE over the last year participating in a widely experienced technical advisory group to help shape some pragmatic amendments to the regulations. There have also been some useful guidance documents developed for the farming sector. These resources should help alleviate concerns that all rural dams will become a regulatory burden; they won’t.

The key focus is that only water held above natural ground level that could escape in a catastrophic failure event such as an earthquake or flood overtopping is used to calculate the volume, not the total storage capacity. Most farm dams are simply too small and due to their remote proximity are of such low risk to be of concern.

The regulations define dams as classifiable if they exceed height and volume thresholds; any dams that are greater than: four metres or higher with a volume of 20,000 cubic metres (approximately 2.1-hectare surface area), or one metre or higher with a volume of 40,000 cubic metres (approximately 4.2 hectares surface area).

The size of these structures is quite significant as seen from the surface area equivalent in each category. A dam sizing resource will be made available to help farmers determine if their dam falls below the classifiable threshold. As dam owners can calculate the classifiable status themselves, this helps in becoming self-determining in this regard.

If the dam is large and is determined to be a classifiable dam, the owner then needs to assess, if in the event of failure, it could harm downstream populations, and damage property, the environment (including flora and fauna), and culturally significant sites.

While a dam owner can conduct an initial potential impact classification assessment, they will need that to be signed by a suitably qualified engineer.

Where dams have been built on farms, these generally have detailed engineering design reports prepared for consents that should allow the necessary threshold calculations and risk assessment to be determined. For dams that exceed the thresholds but are determined as having a low potential impact category, these will not require significant future actions on behalf of the owner.

If a dam is determined to have a medium to high potential impact classification, in that case, it’s sensible that these dams address any safety concerns, and any design features that are creating unacceptable risks are addressed through a well-thought-out safety assurance plan.

The classification will need to be certified by a recognised engineer and then submitted to the regional authority for approval. This must be done no later than three months after regulations come into force or no later than three months after the dam is commissioned.

There will be a two-year lead-in period before the regulations come into play, starting when they are made. MBIE will provide a range of resources during 2022 to help dam owners better assess and understand their dams and how the regulations might apply to them.

One concern is where dams have been built by filling across a gully to flood the valley behind it. There are examples of dams built well in excess of the threshold values (four metres and 20,000 cubic metres) and examples where the design and construction process has not been well controlled or resourced. It’s concerning to hear anecdotal cases of dam walls and spillways only a few years old already suffering from wave action erosion and spillways not coping with peak rainfall events. While a leaking wall is not necessarily a sign of failure, as some seepage can be part of the design process, owners need to be confident in their structure so should seek good advice from engineers specifically experienced in dam design if they have concerns.

IrrigationNZ is concerned that there may be a rush and bottleneck in accessing a limited capacity in suitably qualified dam engineers to sign off the Potential Impact Classifications. However, looking at the sizes of classifiable dams in the final version of the regulations, there may not be as many structures as originally estimated; most small structures will be eliminated at the stage of calculating volumes that the owner can do
for themselves. 

IrrigationNZ is also concerned the regional councils aren’t ready for their role in implementing these regulations and probably don’t have a suitable database system (connected nationally back to MBIE) or staff with suitable qualifications to handle incoming risk registrations and information they’ll have to assess. Irrigation NZ understands there are only three regional councils approved to handle dam-building consents and they share this resource across all other regional councils. They may have to do something similar with a new dam safety and risk database.

IrrigationNZ will continue to look at developing specific guidance on the risk assessment process and work with some of our universities on tools that could quickly size up dams using remote sensing to help eliminate them from the regulatory pathway as flagged in the original policy statement to cabinet.

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