When we engaged Opus International Consultants Ltd to assess the flood risks around the margins of Lake Taupo, they also looked at the potential risk from wave activity (known as wave run-up). Their wave run up report details where the flood waters from a 1% annual exceedance probability event would go. In addition they also considered the risk from extreme wave activity. Although the flood risk and the risk from extreme wave activity both occur around the lake margins, they are different natural hazards, with different causes and management responses.
While there are historical records for levels of the lake, there is none for wave activity. To judge wave activity, Opus needed to combine the actual lake level record with the results from a computer model.
The computer model (lakewave) used a variety of measures like beach profile, grain size of the sand on the beach and wind strength. There are a limited number of wind recording stations in the Taupo District. The official wind recording station is at the Taupo airport and this record was used to inform the computer modelling.
Given the size of the lake, it was necessary to group similar areas together and 10 different areas were identified.
Opus identified these results were more indicative of possible rather than actual wave activity, but that they still indicate some areas will be more affected by wave activity than others.
Although the modelling work enabled Opus to identify properties potentially affected by wave activity, the modelling could not take into account features like fences, vegetation or buildings. Such features would impact on how far a wave might move in from the shoreline.
While Opus have clarified that this information is useful as a guide, the constraints of the data availability (eg. limited number of wind recording stations) and the modelling mean that analysis for individual properties is not possible. Comments from Opus on the level of confidence in the information can be viewed here.
We have been working with Opus and NIWA to establish what work is required to increase the level of confidence in the modelling results. They have indicated that we need to collect data on actual wave activity around the lake to enable the computer model to be calibrated. There would also need to be mapping of the different shoreline features around the Lake. Council will need to balance further investigation of the extreme wave activity hazard with other natural hazards which may be of greater consequence such as earthquakes.
We have visited the parts of the lakeshore that are potentially at risk from extreme wave activity and discussed the likely level of risk with officers from Waikato Regional Council. We believe that the level of risk is relatively small and can be managed through the existing rules in the Taupo District Plan. In particular, the District Plan identifies the foreshore protection area, a 20 metre buffer area right around the margins of the lake. Any building work proposed in that buffer area requires resource consent allowing Council to consider the risks from extreme wave activity. The District Plan also requires resource consent for urban scale subdivision in rural areas. Lastly, section 106 of the Resource Management Act enables Council to consider issues related to natural hazards as part of any subdivision application.
We are currently identifying a potential risk from extreme wave activity on all Land Information Memoranda (LIMs) issued in accordance with our legal obligations under the Local Government Official Information and Meetings Act 1987. This affects all LIMs issued throughout the district, because the information is not certain enough to say exactly which properties might be affected by extreme wave activity.