Past tsunami record for the Birdlings Flat area

The purpose of this project is to better understand the magnitude and frequency of past tsunamis in and around the Birdlings Flat area. There is a growing public awareness of the tsunami hazard for the region in general, but it is difficult to develop effective coastal management strategies in the absence of a reliable longer term record of past events.As a result of the work undertaken for this report we were able to identify two palaeotsunamis that inundated the area within the past 1000 years, one around 1300AD and another sometime between 1500 and 1700AD.No older events were found which suggests that Kaitorete Spit acts as an effective barrier to inundation. However, in areas where the spit has little influence on inundation we were still unable to find evidence for older events. One problem in attempting to trace palaeotsunamis around Banks Peninsula is the almost ubiquitous signal of European colonisation. This is manifested as thick layers of reworked loess in coastal environments resulting from the erosion of land cleared of forest vegetation. It is possible that some earlier events are buried at depth beneath this covering.We infer that the tsunami hazard is low. Historical evidence tends to suggest that distant sourced tsunamis from South American fault ruptures have little or no affect in the area. It is a concern though that what appear most likely to be locally sourced events have so few potential known sources. There are no known faults of sufficient size to generate the runup heights (up to 29.0 metres above sea level) and inundation distances (510 m) reported. While submarine slope failures cannot be excluded, gas hydrate releases seem to be the most reasonable known sources at this time. Whether they are capable of being sufficiently tsunamigenic is unclear.To clarify the nature of potential tsunamigenic sources we suggest undertaking:a) Numerical modelling of gas hydrate releases;b) More detailed bathymetric surveys to identify possible submarine slope failures;c) Numerical modelling of the 1575AD and 1604AD tsunamis from South America.If additional geological work is undertaken, we recommend that a study be carried out around the ancient bays (now infilled) along the southwest side of Banks Peninsula adjacent to SH75 between Motukarara and Okoura valley. These would have been exposed to tsunamis prior to the formation ofiiKaitorete Spit and at other times over the past 8000 years. It is possible that inundation by the ~1300AD event at Okoura removed evidence of earlier events.Finally, detailed discussions with tangata whenua were beyond the scope of this study, but are recommended in order to extend, add value and richness to the findings detailed in this report