Balmoral Water Storage Facility

These guidelines require a dam break flood hazard and consequence assessment to be completed. This assesses the consequences of an uncontrolled release of the pond’s contents via a dam failure and establishes the criteria for which dam design, construction and operational safety assurance are based.

This work was undertaken by Riley Consultants and is a standard tool used across the industry. This classified the BWSF as a High Potential Impact Classification (PIC). Earlier modelling of smaller dam options were also classed High PIC and this classification is considered appropriate for preliminary design. A High PIC classification also requires the investigation, design and construction to be peer reviewed, and we have involved a peer reviewer since the early stages of this project.

Identifying potential scenarios is an important step in the design process as it allows for improved understanding of the processes leading up to a dam safety incident or failure and therefore what mitigation or defensive measures should be incorporated into the design to prevent or reduce their likelihood or severity.

As the BWSF has no contributing catchment and is not at direct flooding risk from the Hurunui River, potential failures occurring during wet weather conditions are not expected and the modelling was based on sunny day conditions when the BWSF is most likely to be at capacity.

The assessment looked at the worst-case scenarios in terms of population at risk and combined damage levels, which was a piping type failure of the eastern embankment. At this location, the embankment is near its highest, and any dam break flood wave would be directed over land for a considerable distance before entering the Hurunui River.

Using a computer model developed from a hydraulic modelling tool, a large portion of water would be directed east and north and not directly south-east into the Hurunui River. The resulting water depths were estimated across the plains and the Hurunui River using the hydraulic model so that the damage and population at risk could be estimated. Riley also advise that vehicles travelling SH7 are at risk should a breach occur.

Modelling of alternative breach locations along the northern, western and southern embankments indicate a broader envelope of hazard, but with similar or slightly less consequences. A breach on the southern or western embankments would result in a larger proportion of water flow into the Hurunui River, and less on the floodplain. Damage or destruction to downstream properties may still occur, however the velocity of water inundating SH7 would be lower, and the flow within the Hurunui River of up to 1300m3/s comparable to a 1 in 500-year annual exceedance probability (‘AEP’) flood event.

If a breach were to occur on the northern embankment, Riley advises that the maximum breach height would be reduced by at least 5m, with a lower volume of water released from the breach. The depth and velocity of water inundating SH7 would be similar to the modelled critical breach location, and the inundated stretch of road would be the same, if not slightly shorter, when compared to the selected breach.

The most likely Potential Failure Modes (PFMs) associated with this type of structure are:

1. Liner failure

2. Extreme seismic ground shaking causing deformation

3. Separation at the interface with the main conduit or other concrete structures

4. High winds generating large wave runups on the upstream slope causing overtopping of the embankment

5. Erosion damage of the spillway chute, main conduit, or energy dissipators

The design process will address all relevant failure modes in greater detail and implement measures as necessary to mitigate the associated risks.

In response to the BWSF’s high PIC rating, plans and systems must mitigate the identified risks associated with the BWSF and ongoing operations will need to comply with a dam safety management system to ensure its continued safe operation.

The following measures are proposed for safety management:

A Dam Safety Management System (‘DSMS’): This system provides AIC with a timetable to complete dam safety management activities. It also provides a risk-based decision-making framework for addressing dam safety issues should they arise. To implement the DSMS, a network of instruments, deformation survey marks and visual inspection points will be installed during construction to allow for performance monitoring once operational. Design of the network will be undertaken during the detailed design phase to allow for modification during construction, as necessary. The system will be robust and be able to be monitored by non-specialist personnel within AIC.

An Emergency Action Plan (‘EAP’): The EAP’s purpose is to minimise the potential for negative effects by implementing both preventative and emergency actions should a potential safety emergency arise. If failure cannot be prevented, the purpose of the EAP is to limit the effects of the failure on people, property, and the environment downstream of the BWSF.

Safety Inspection Schedule: In addition to the regular safety management procedures undertaken by AIC, a dam safety inspection by a suitably qualified and experienced dam engineer will be undertaken annually. A comprehensive dam safety review by an independent and suitably qualified expect will be undertaken every five years. The proposed safety inspection and review are consistent with recommendations in the New Zealand Society on Large Dams (NZSOLD) guidelines.