Tsunami Modelling Study 4

Executive Summary

Northland Regional Council contracted NIWA to undertake an initial study on the risk of tsunami inundation facing communities in the Northland Region. The following credible sources were identified:

• Remote source: South American origin. Return period 50-100 years. This represents the most probable tsunami risk in the next 100 years. 
• Local/regional source: Tonga Kermadec. Two events were modelled, Moment magnitude scale (M*) 8.5 and M* 9.0. The return period of these events is much longer (500-2000 years) but these represent a worst-case scenario for a tsunami striking the Northland coast.

The current study investigated tsunami propagation into the Whāngārei Harbour, including inundation at six communities, for each of the above scenarios using computer simulation. Inundation modelling was performed assuming that the tsunami arrives at Mean High Water Spring (MHWS) at current sea levels and for a mean sea level elevated by 50 cm, representing the 100 year projection of sea level rise by the IPCC Fourth Assessment Report.

The simulation results presented here cover the likely range of tsunami that might be expected in Whāngārei Harbour, from the relatively common, generally smaller South American tsunami to the largest likely event emanating from the Tonga-Kermadec subduction zone (TKSZ). This report provides information on the likely range of impacts on which to base local contingency planning.

The South American tsunami source caused the greatest inundation, in particular to Marsden Cove and Whāngārei. Other areas inundated include Otaika, Oakleigh, Takahiwai and Whāngārei Head. These six areas were also inundated, to a lesser extent, by the TKSZ M*9.0 scenario, and less again by the TKSZ M*8.5 scenario. When the effect of sea level rise is included, the extents of inundation increased for each of the scenarios. With sea level rise, the South American source again produced the greatest level of inundation followed by TKSZ M*9.0, and TKSZ M* 8.5 scenarios. The TKSZ M* 8.5 source with sea level rise included (MHWS + 0.5 m) inundates more than the South American scenario at current MHWS.  The Bercich Drain was not inundated in this study. A dynamic analysis of flow within the culvert may be required to determine if water entering the culvert at the seaward end will spill into the refinery. Finer resolution of the Bercich Drain grid area in future studies would permit more accurate modelling of this aspect.

In this study, results are summarised and recommendations for improving the modelling and reducing the uncertainty in the results are made. In previous modelling, the TKSZ sources produced the greatest inundation in most areas. This study found that, within Whāngārei Harbour, the South American scenario had the greatest effect. The modelling study is dependent on, and therefore limited by, the initialisation of the tsunami for each earthquake source, the quality of the LiDAR topographic data and also the quality of bathymetric data in inshore waters. A lack of knowledge of the effects of buildings and land features on wave drag also adds uncertainty to the simulations. Despite these limitations, we believe that the current modelling exercise provides the best possible estimate of inundation in Northland from remote and regionally sourced tsunamis available to date.

Download the report

File size: Please note that this report is very large due to the number of maps and graphs. It has been separated into sections to make it easier to download. The Table of Contents .pdf outlines what information is in each section. 

Find out what a tsunami is and what we are doing to prepare for managing one if it does happen: www.nrc.govt.nz/tsunami