Lake Taupo flood study identifies flood zones around foreshore

A new Lake Taupo flood study identifies two flood zones around the foreshore based on a 100 year flood event.

The flood study by Opus Consultants has been jointly funded by Environment Waikato and Taupo District Council to identify the extent of the flood risk around the Lake Taupo foreshore. The study says the extent and depth of potential flooding results from a combination of physical processes (rainfall, run off, lake levels, waves, tectonic deformation, future climate change, seiching*) and human and economic factors (lake level management for hydro power generation, land use change.) *seiching is the slopping back and forth of water in an enclosed basin.

The study identifies that the various factors affecting the lake combine in an infinite number

of different ways that vary over time to produce different flood events. The terrain around the lake also determines the extent and depth of flooding from site to site e.g. low lying areas are more susceptible.

Following analysis of all the factors the study has identified two potential hazard zones. An initial “inundation hazard zone” based on those areas which are likely to be covered with water and a second “wave hazard zone” which identifies areas above the inundation zone where wind causes waves to push water up onto the land. In both zones there is a 1% chance that this flood level could be exceeded in any one year (1 in 100 year flood event).

The two zones have been separated because future development in either zone may need to have different planning and building controls applied to reflect the different risks. Both hazard zones include adjustments for the estimated effects from climate change and tectonic movement over the 100 year timeframe.

Tonkin & Taylor have peer reviewed the methodology used to identify the flood hazard zones and consider that it is suitable given the complexities of the factors influencing the lake when it floods.

Maps showing the hazard areas are being prepared for each of the settlements areas around the Lake Taupo foreshore. In line with legislative requirements, once the Council is satisfied that the mapping accurately reflects the reality on the ground, the new flood information will be included on Land Information Memoranda (LIM) and Project Information Memoranda (PIM) for affected properties. Letters will also be sent out to property owners in the affected areas informing them of the study findings and how their property might be affected. Further information on specific properties around the entire lake foreshore will be available once maps are completed.

Mayor Rick Cooper says the flood study findings will be used by Taupo District Council in consultation with communities over coming months to confirm appropriate planning measures.

“In the meantime we are legally obliged to include the new information on LIMS and PIMS,” he said.

Mayor Cooper and Environment Waikato’s Taupo representative Laurie Burdett say the new flood hazard information will allow both Councils to be proactive when it comes to protecting people and property from the risks of future flooding.

Mr Cooper said that changes may ultimately be needed to the Taupo District Plan to ensure that development in areas affected by flooding could be appropriately managed. While work has only just commenced on what those potential changes could be, Mr Cooper suggested that they might include things like controlling the floor height or location of buildings.

Council staff will be on hand to talk about the findings in the Lake Taupo Flood Hazard Study and the recently completed Lake Taupo Erosion Study at open days

Tues 8 April, 12 midday to 7 pm at the Great Lake Centre, Taupo
Wed 9 April, 12 midday to 5 pm at the Turangi Service Centre
Thurs 10 April, 12 midday to 5 pm at the Mangakino Council Offices
Sat 12 April, 10 am to 3 pm at the Acacia Bay Hall, Taupo

Final recommendations and actions will be included in a Lake Taupo Erosion and Flood Strategy that is due for completion towards the end of the year. Formal public consultation will be sought before the Strategy is adopted for implementation in the 2009/2010 financial year.

Key Facts Lake Taupo Flood Study


Lake Taupo Erosion and Flood Strategy

Consultants have been employed by Taupo District Council and Environment Waikato to prepare technical studies on the erosion and flood risk around the Lake Taupo Foreshore.
The technical findings from both studies will be used as a basis for development of the Lake Taupo Erosion and Flood Strategy
The Strategy is being developed during 2008 in consultation with key stakeholders, affected property owners and the public.
The Strategy will clarify roles, responsibilities and funding and guide future management decisions.



Lake Taupo Flood Study

While the District Plan currently shows the flooding hazard associated with the Tongariro and Tauranga Taupo Rivers, it does not identify flood hazards around the rest of the lake foreshore.
Further studies will be undertaken to review the flood risk from the major rivers entering the lake such as the Tauranga Taupo and Tongariro Rivers.
Maps are currently being finalized based on the flood study findings to determine which areas and how many properties are at risk of flooding. (see flood risk section)
The potential flood hazard areas and different options for managing the risk will be summarized in an issues and options paper and taken to iwi and the community for comment and discussion.


Technical background

The extent and depth of flooding around the foreshore is a combination of physical processes ( rainfall, runoff, lake level, wave regime, seiching, tectonic deformation, future climate change) and human and economic factors (lake level management for hydro power generation, land use change) *seich effect is the slopping back and forth of water in an enclosed basin
These various factors combine in an infinite number of ways that might cause flooding. The importance of each factor varies at different areas around the lake with some factors being more important in particular locations than in others. The importance of some of the factors also changes over time, such as the accumulated tectonic subsidence and climate change.
A building block approach was used by the consultants to analyse the variability and importance of each factor. The effect of each factor could then be added together to produce the final flood level. These levels were then overlaid on a high resolution digital terrain model (DTM) to identify which areas would be flooded.
Flood risks

Following analysis of all the factors the study has identified two potential hazard zones based on a one hundred year event, which is accepted as best practice for hazard planning
An initial “inundation hazard zone” is based on those areas which are likely to be covered with water during a 1 in 100 year flood event (a 1% chance that the flood level could be exceeded in any one year).
The second “wave hazard zone” identifies areas above the inundation zone where the wind causes waves to push water up onto the land
The two zones have been separated because future development in either zone may need different controls to reflect different risks
The area within the inundation and wave zones varies around the lake because of differences in tectonic deformation, terrain and wind
Tonkin & Taylor have peer reviewed the methodology used to identify the flood hazard zones and consider it suitable given the complexities of the factors influencing the lake when it floods
TDC and EW are still refining the mapping of the different settlements around the Lake shore to remove anomalies and ensure that all maps are accurate.
Once the maps have been refined to remove any anomalies the new information will be added to LIMs and PIMs for potentially affected properties to reflect the Flood Study findings
Property owners in affected areas will be sent letters letting them know that that their property may fall within a flood hazard zone and that the information will be included on LIMs and PIMs
Once the maps are completed and information is on LIMs and PIMs, people wanting to find out more about their property and its flooding risk can contact Steve Richards, GIS Manager Ph: 07 376 0899
Maps showing the settlement areas will also be made available at the front counter of the Council’s main office and service centers


Where to next

TDC and EW are working on some options for managing the flood hazard in those areas identified in this study. These may include possible controls of floor height or location of buildings within hazard zones
TDC will be consulting with communities during April and May to discuss the study and possible options for managing the flood risk
Management options will be developed further as part of a Lake Taupo Erosion and Flood Strategy. A draft is due for completion towards the end of the year.
Formal public consultation will be sought before the Strategy is adopted for implementation in the 2009/2010 financial year.