DISCLAIMER BIBLIOGRAPHIC REFERENCE

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2 DISCLAIMER This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Environment Southland. Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of, or reliance on any contents of this Report by any person other than Environment Southland and shall not be liable to any person other than Environment Southland, on any grounds, for any loss, damage or expense arising from such use or reliance. The data presented in this Report are available to GNS Science for other use from November BIBLIOGRAPHIC REFERENCE Prasetya, G.; Wang, X.; Palmer; N.; Grant, G Tsunami inundation modelling for Riverton and New River Estuary Southland, GNS Science Consultancy Report CR 2011/ p. Project Number: 410W1371

3 CONTENTS EXECUTIVE SUMMARY INTRODUCTION BACKGROUND OBJECTIVE METHOD Numerical model Model grids Roughness Coefficient DATA GEOGRAPHIC SETTING POTENTIAL TSUNAMIGENIC EARTHQUAKE SOURCES Local Sources Fiordland Subduction Zone Puysegur Subduction Zone Downes et al. (2005) scenario Hayes and Furlong (2010) scenario Distant sources RESULTS Tsunami modelling for Local Sources Fiordland Subduction Zone Water Level at Mean Sea Level (MSL) Sea Level at Mean High Water Spring (MHWS) (1.105 m above MSL) Puysegur Subduction Zone Downes et al., (2005) (M w 8.6) Scenario Water level at MSL Sea level at MHWS (1.05 m above MSL) Hayes and Furlong (2010) (M w 8.3) Scenario Water level at MSL Sea level at MHWS (1.105 m above MSL) Hayes and Furlong (2010) (M w 8.4) Scenario Sea at MSL Sea level at MHWS (1.105 m above MSL) Hayes and Furlong (2010) (M w 8.6) Scenario Sea at MSL Sea level at MHWS (1.105 m above MSL) Hayes and Furlong (2010) (M w 8.7) Scenario Sea at MSL Sea level at MHWS (1.105 m above MSL) Tsunami modelling for Distant Sources Peru M w 9.1 Scenario Sea at MSL Sea level at MHWS (1.105 m above MSL) Peru M w 9.4 Scenario Sea Level at MHWS (1.105 m above MSL) SUMMARY OF SIGINIFICANT TSUNAMI RISK AND INUNDATION AREAS Fiordland Scenario Mw Puysegur Downes et al Mw Puysegur Hayes and Furlong (2010) Mw MITIGATION MEASURES Land threat Marine threat Limitations of the current work ACKNOWLEDGEMENTS GNS Science Consultancy Report Riverton Southland Modelling 1

4 12.0 REFERENCES EARTHQUAKE AND TSUNAMI WARNING NOTIFICATION SOURCES (WEB RESOURCES) GLOSSARY FIGURES Figure Schematic diagram for runup height and inundation for different beach slopes (simplified from Prasetya et al. 2011). For the same incoming tsunami height at the coast, those areas with a gentle slope inland will experience lower runup heights but longer inundation distances compared to the areas that have steeper slopes Figure The nested configuration with four resolution levels was implemented for the numerical simulations for local sources Figure The second-level grids (i.e., layer 02) range from to in longitude and from to in latitude, covering the south end of the South Island including Stewart Island Figure The fourth-level grids (layer 04) for detailed inundation modelling. The topography data within and around the New River Estuary were based on SRTM Figure The fourth-level grids (layer 04) for detailed inundation modelling. The topography data within and around Riverton were based on an RTK field survey Figure RTK survey lines around Riverton to provide detailed topographic height for the site and correction to the SRTM 90 m data in combination with LINZ Topography spot height data Figure Location map. Red boxes show the areas where detailed inundation modelling was carried out (Riverton and New River Estuary) Figure The local source regions based on Downes et al. (2005) (red rectangles) and Hayes and Furlong (2010) (solid green line) are located along the Alpine Fault and Puysegur Trench. The present-day seismo-tectonics of the Australia-Pacific (AUS:PAC) plate boundary south of New Zealand (Hayes and Furlong, 2010) show the earthquake distribution since 1973 and their slip vectors, the Benioff zone and relic fractures zone from the past seafloor spreading (inset). Contours of the Benioff zone are in red dashed lines between 10 and 90 km, and blue dashed lines between 110 and 130 km. Circles represent earthquake locations for events between 1973 and the present (2010), scaled by magnitude as well as the Centroid Moment Tensor Solutions (CMTs). The 1979 (M w 7.3) was used by Hayes and Furlong to develop the tsunami scenario Figure Initial sea surface deformation of the Fiordland fault rupture scenario (M w 7.9). Scale bar unit is in metres Figure Initial sea surface deformation of the Puysegur fault rupture scenario (M w 8.6). Scale bar unit is in metres Figure Initial sea surface deformation of the Puysegur Hayes and Furlong (2010) fault rupture scenario (M w 8.3). Scale bar unit is in metres Figure Initial sea surface deformation of the Puysegur Hayes and Furlong (2010) fault rupture scenario (M w 8.4). Scale bar unit is in metres Figure Initial sea surface deformation of the Puysegur Hayes and Furlong (2010) fault rupture scenario (M w 8.6). Scale bar unit is in metres Figure Initial sea surface deformation of the Puysegur Hayes and Furlong (2010) fault rupture scenario (M w 8.7). Scale bar unit is in metres Figure Maximum tsunami amplitude distribution for the Peru (M w 9.1) fault rupture scenario, showing the tsunami directivity towards New Zealand. Scale bar unit is in metres Figure Distribution of maximum tsunami elevation shows that the Fiordland region is most affected by the tsunami from this source scenario, while to the east along the Southland coast, the effects of tsunami is minimal. Scale bar unit is in metres Figure Distribution of maximum tsunami elevation shows the Fiordland regions are highly impacted by tsunami from this source while to the east the impact is minimal. Scale bar unit is in metres Figure Distribution of maximum tsunami elevation along the New River Estuary shows the impacts of tsunami from this scenario at MSL is minimal and no inundation. Scale bar unit is in metres Figure Distribution of maximum tsunami elevation at Riverton from this source scenario at MSL shows the tsunami mostly affected the beach front (up to the highest water mark). No significant inundation occurs. Scale bar unit is in metres GNS Science Consultancy Report Riverton Southland Modelling 2

5 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Distribution of maximum tsunami elevation shows the impact of a tsunami for Fiordland regions from this source scenario at MHWS are increased as well as other places along the southland coast. Scale bar unit is in metres Distribution of maximum tsunami elevation at Riverton from this source scenario at MHWS is increased along the coast and inside the estuary. Small inundation occurs near to the mouth of estuary. Scale bar unit is in metres Distribution of maximum tsunami elevation at New River Estuary from this source scenario at MHWS is increased as shows along the coast and inside the estuary. Small inundation occurs near to Waihopai areas. Scale bar unit is in metres Distribution of maximum tsunami elevation from this source scenario shows that the tsunami the tsunami energy is directed into east-west direction and passed through the relatively shallow Campbell Plateau through shoaling, refracting and diffracting processes that diverted the tsunami propagation toward the eastern part of Southland region. Scale bar unit is in metres Distribution of maximum tsunami elevation along the Southland coast shows that tsunami from this scenario affected the Southland coast from Fiordland to Toetoes Bay, and the west coast of Stewart Island. The impact to the east coast of Stewart Island as well as some areas between Riverton and Tiwai Peninsula are minimal as it is located on the shadow zone, even though the refraction and diffraction processes provide a high tsunami elevation further east along Tiwai Peninsula. Scale bar unit is in metres Distribution of maximum tsunami elevation at Riverton from this source scenario at MSL shows the tsunami inundated the low-lying areas at Riverton. Scale bar unit is in metres Distribution of maximum tsunami flow speed at Riverton shows a high flow speed occurs up to 8.0 m/s. Scale bar unit is in m/s Distribution of maximum tsunami elevation along the New River Estuary shows the tsunami from this scenario at MSL causing small inundation at the north-end of the Estuary to the areas around the Waihopai Rivers and at the low-lying areas at the western edge where tsunami overtopped the Stead Street. Scale bar unit is in metres Distribution of maximum tsunami flow speed inside the New River Estuary, showing high flow speeds at some places around the entrance and tidal flat areas. Scale bar unit is in m/s Distribution of maximum tsunami elevation shows the impact of tsunami for Fiordland regions from this source scenario at MHWS are increased as well as other places along the southland coast. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation at Riverton from this scenario at MHWS shows the township along Taramea and Mitchells Bays at Riverton are inundated further inland. Scale bar unit is in metres Distribution of maximum tsunami flow speed from this source scenario at Riverton during MHWS shows strong currents up to 8.0 m/s (~16 knot) occur along the beach front of both side of the estuary entrance. Inside the estuary, the current velocity ranges up to 3.0 m/s and occurs near to the entrance. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation areas at New River Estuary from this source scenario at MHWS are also extending further inland as tsunami overtopped the Stead Street, and penetrate further inland at the west end of this northern end of estuary and Waihopai River to the east. Tsunami inundates the reclamation areas, the airport and river banks. Scale bar unit is in metres Distribution of maximum tsunami flow speed inside the New River Estuary, showing high flow speeds at some places around the entrance and tidal flat areas. Scale bar unit is in m/s Distribution of maximum tsunami elevation along the Southland coast from this source scenario shows the concentration of tsunami waves along the Southland coast from Fiordland to Toetoes Bay, and the west coast of Stewart Island. The impact to the east coast of Stewart Island as well as some areas between Riverton and Tiwai Peninsula are minimal as it is located on the shadow zone, even though the refraction and diffraction processes provide a high tsunami elevation further east along Tiwai Peninsula. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation at Riverton from this source scenario shows a relatively small inundation occurs up to the highest water mark. Scale bar unit is in metres Distribution of maximum tsunami flow speed around the shallow entrance at the Riverton estuary and inside the estuary at tidal flat areas. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation at New River Estuary shows a localised inundation occurs at around the Waihopai Rivers. Scale bar unit is in metres GNS Science Consultancy Report Riverton Southland Modelling 3

6 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Distribution of maximum tsunami elevation shows the impact of tsunami for Fiordland regions from this source scenario at MHWS are increased as well as other places along the southland coast. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation at Riverton shows the inundation at most of low-lying areas along the coast of Taramea and Mitchell Bays during this source scenario at MHWS is increased further inland. Scale bar unit is in metres Distribution of maximum tsunami flow speed at Riverton show a high tsunami flow speed occurs at the Estuary entrance. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation at New River Estuary show a larger inundation area at the northern end of estuary occurs as tsunami overtopped the Stead Street, and penetrate further inland, and through the Waihopai River. Tsunami inundates the reclamation areas, the airport and river banks. Scale bar unit is in metres Distribution of maximum tsunami elevation for this source scenario shows the directivity of tsunami energy mainly towards Southland coast from Fiordland to Riverton and west coast of Stewart Island. Beyond these regions the tsunami impact are minimal. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation along the Riverton coast show the extent of inundation occurs at the low-lying areas of Tamarea Mitchell Bays and the Golf course. Scale bar unit is in metres Distribution of maximum tsunami flow speed shows the strong currents occurs along the Riverton nearshore region, estuarine entrance and tidal flat inside the Estuary. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation from this source scenario show a localised inundation occurs at the northern-end of the estuary through the Waihopai River. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation along the Southland coasts show an increasing of tsunami elevation for this source scenario during the sea level at MHWS compare to the sea level at MSL. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation along the Riverton coast show the inundation extents are increased for this source scenario at MHWS. Scale bar unit is in metres Distribution of maximum tsunami flow speed at Riverton for this source scenario at MHWS. Strong inundation flow speed u to 8.0m/s occurs along the inundated areas, and estuary entrance. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation at New River Estuary show an increase of tsunami elevation and larger inundation areas for this source scenario at MHWS. Scale bar unit is in metres Distribution of maximum tsunami elevation along the Southland coast form this scenario show a significant tsunami impact to the coastal areas from Fiordland to New River Estuary, and along the west coast of Stewart Island, and less towards Tiwai Peninsula, Toetoes Bay and further east. Scale bar unit is in metres Distribution of maximum tsunami elevation and inundation show the inundation areas along Taramea, Mitchells and Henderson Bays and to the Golf course areas flow speed. Scale bar unit is in metres The tsunami flow speed distributions show a strong currents along the nearshore areas as well as at the estuary entrance and at tidal flat areas. Scale bar unit is in m/s Distribution of maximum tsunami elevation at New River Estuary shows a localised inundation at northern-end of the estuary through the Waihopai River. Scale bar unit is in metres At MHWS, the tsunami elevation along the Southland coast from this source scenario is increased and higher compare to tsunami elevation at MSL, even though their height distribution pattern almost similar Scale bar unit is in metres Extensive inundations occur on both sides of the estuary entrance over the township and Golf course and coastal areas along Taramea, Mitchells and Henderson Bays. Scale bar unit is in metres Strong inundation flow occurs on both sides of estuary as well as at tidal flat areas inside and outside of the estuary. Scale bar unit is in m/s Distribution of maximum tsunami elevation and inundation for this source scenario at MHWS show an increase of tsunami elevation and the extent of inundation at the northern-end of the estuary compare to scenario at MSL. Scale bar unit is in metres Distribution of maximum tsunami elevation from this scenario where the earthquake magnitude is increased from M w 8.6 to 8.7 (through increasing the vertical dislocation) shows an increase in tsunami elevation along the coast of Southland with almost similar distribution where the areas from Fiordland to New River Estuary GNS Science Consultancy Report Riverton Southland Modelling 4

7 and the west coast of the Stewart Island are the areas that had a significant impact compare to other places further east. Scale bar unit is in metres Figure Extensive inundation occurs on Riverton where most of the townships are inundated with maximum tsunami elevation up to 8.0 m above MSL. Scale bar unit is in metres Figure Strong flow speed occurs at the open coast as well as inside the estuary, especially along the intertidal flat areas. Scale bar unit is m/s Figure A localised inundation occurs along the coast of Oreti Beach. Inside the estuary, the inundation is quite extensive to the low-lying areas near to the Airport, Waihopai River and Woodend. Scale bar unit is in metres Figure Distribution of a maximum tsunami flow speed show high flow speeds of up to 8.0 m/s occur inside and outside the New River Estuary. Scale bar unit is in m/s Figure At MHWS, the tsunami elevation along the Southland coast from this source scenario is increased and higher compare to a similar source scenario at MSL. The extents of higher tsunami elevation along the coast and offshore were unequivocal. This result show the impact of a tsunami to the Southland coast is very significant. Scale bar unit is in metres Figure A large scale of inundation occurs at Riverton for this source scenario at MHWS. Most of the townships are completely inundated. Scale bar unit is in metres Figure Strong inundation flow and tsunami flow speed were obvious on the model results for this source scenario at MHWS. This high speed overland flow across the golf course and townships shows high degrees of possible impact on this area. Scale bar unit is in m/s Figure Extensive inundation occurs inside the New River Estuary at the northern-end and eastern part of the estuary. Relative large inundation also occurs along the Oreti Beach where the sand dunes not exist. Scale bar unit is in metres Figure The flow speed distributions for this source scenario at MHWS show a significant high speed flow occurs at the intertidal flat and inundated areas. These flow speeds illustrated the high degrees of tsunami impacts inside the estuary. Scale bar unit is in m/s Figure Distribution of maximum tsunami elevation for this distant source (Peru M w 9.1) show a moderate to significant impact of a tsunami along the Southland coast and the eastern coast of Stewart Island. Scale bar unit is in metres Figure A significant inundation occurs at Riverton township where the low-lying areas along the Taramea, Mitchells and Henderson Bays were completely inundated, and moderate inundation also occurs towards golf course areas. Scale bar unit is in metres Figure Strong currents occur inside and outside estuary and along the inundated areas. Scale bar unit is in m/s Figure A localised inundation from this source scenario occurs at the low-lying areas around the Waihopai River and Woodend. There is no inundation along the Oreti beach. Scale bar unit is in metres Figure Distribution of maximum tsunami elevation for this distant source (Peru M w 9.1) at MHWS show an increase of tsunami elevation along the Southland coast and the eastern coast of Stewart Island compare to the scenario at MSL. This provides a significant risk along the low-lying areas of the Southland coast. Scale bar unit is in metres Figure Significant inundation occurs at Riverton from this source scenario at MHWS. Most of the low-lying areas were inundated. Scale bar unit is in metres Figure High flow speed occurs across the inundation areas and the nearshore areas along Taramea, Mitchells and Henderson Bay as well as inside the estuary. These flow speeds illustrated the high degrees of tsunami impacts inside the estuary. Scale bar unit is in m/s Figure Significant inundation occurs inside the New River Estuary along the northern-end bay (Stead Street), and the low-lying areas of Waihopai River banks and Woodend. Scale bar unit is in metres Figure The flow speed distributions for this source scenario at MHWS show a significant high speed flow occurs inside the estuary at the intertidal flat and inundated areas. Scale bar unit is in m/s Figure Distribution of maximum tsunami elevation for this distant source (Peru M w 9.4) at MHWS show a significant increase of tsunami elevation along the Southland coast and the coast of Stewart Island compare to the source scenario M w 9.1. This provides a significant risk along the low-lying areas of the Southland coast. Scale bar unit is in metres Figure Significant inundation occurs at Riverton from this source scenario at MHWS. Most of the low-lying areas were inundated. Scale bar unit is in metres Figure High flow speed occurs across the inundation areas and the nearshore areas along Taramea, Mitchells and Henderson Bay as well as inside the estuary as what occur GNS Science Consultancy Report Riverton Southland Modelling 5

8 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure at Peru Mw 9.1 at MHWS source scenario. These flow speeds illustrated the high degrees of tsunami impacts inside the estuary. Scale bar unit is in m/s Significant inundation occurs inside the New River Estuary along the northern-end bay (Stead Street), and the low-lying areas of Waihopai River banks and Woodend. Scale bar unit is in metres The flow speed distributions for this source scenario at MHWS show a significant high speed flow occurs inside the estuary at the intertidal flat and inundated areas. Scale bar unit is in m/s Tsunami energy directivity and distribution from the source to the coast for the Fiordland fault rupture scenario (M w 7.9). Scale bar unit is in metres Tsunami energy directivity and distribution from the source to the coast for the Downes et al. (2005) Puysegur scenario Mw 8.6. Scale bar unit is in metres Tsunami energy directivity and distribution from the source to the coast for the Hayes and Furlong (2010) Puysegur scenario. Scale bar unit is in metres Tsunami energy directivity and distribution from the source to the coast for the Peru M w 9.1 scenario (Berryman et al. (2005) and Okal et al. (2006)). Scale bar unit is in metres Red contour lines show the tsunami arrival time after fault rupture. Contour unit is in minutes Red contour lines show the tsunami arrival time after fault rupture. Contour unit is in minutes Red contour lines show the tsunami arrival time after fault rupture. Contour unit is in minutes Red contour lines show the tsunami arrival time after fault rupture. Contour unit is in minutes Red contour lines show the tsunami arrival time after fault rupture. Contour unit is in minutes TABLES Table Manning roughness coefficient Table Fault parametres for Fiordland as suggested by Downes et al. (2005) with modification on strike angle Table Fault parametres for Puysegur, as suggested by Downes et al. (2005) with modification on strike angle Table Fault parametres for Puysegur of Hayes and Furlong (2010) Table 7.2 Fault parametres for Peru scenario (Okal et al. 2006) GNS Science Consultancy Report Riverton Southland Modelling 6

9 EXECUTIVE SUMMARY Emergency Management Southland contracted GNS Science to undertake a tsunami inundation assessment for the Riverton and New River Estuary area, Southland, for hazard mitigation purposes. This report presents the results of a tsunami hazard assessment at regional scale, with special emphasis on inundation assessment for Riverton and the New River Estuary from each source scenario developed, and provides recommendations in relation to tsunami hazard mitigation and actions. Tsunami Source Scenario A scenario based approach is used to model the potential tsunami hazards and inundation for the Southland region. Scenarios from both local and distant earthquake sources were defined based on available published literature. For distant sources, the source scenarios from off the Peru coast of South America with M w 9.1 and M w 9.4 are chosen. The trans-oceanic tsunami propagation shows that the outgoing tsunami generated off the Peru Coast of South America is oriented to the Southwest Pacific, hence is more effectively directed towards New Zealand (Berryman et al. 2005, Power et al. 2007). The sources further south along the Chilean Coast show the tsunami energy concentration toward the Northwest Pacific (i.e. Japan). Berryman et al. (2005) indicated that tsunamigenic earthquakes from South America with magnitude less than M w 8.5 will have a minimal impact and risk to New Zealand. The recent Chilean M w 8.8 event (27 February 2010) demonstrated this; although a 0.27 m wave was recorded at Dog Island, the impacts to Southland, especially to the New River Estuary, were barely observable to most people (Telford, 2010). Local source scenarios were selected and based on source definitions from available published literature (Berryman et al. 2005; Downes et al. 2005; Greenslade et al. 2007, 2009; Bathgate et al. 2008; Hayes et al and Hayes and Furlong 2010). These source regions are associated with the Alpine Fault system (off the coast of Fiordland) and the Puysegur Trench to the south. The most recent event occurred on 15 July 2009 (M w 7.8) known as the Dusky Sound earthquake and tsunami. While for the Puysegur Trench, the last earthquake event occurred in 1979 (M w 7.3), but no tsunami was reported. The study area for inundation modelling is divided into two model domains. The first model domain is for a distant source that covers the whole South Pacific region, and the second is for local sources that cover the southern ocean of the Southland region, with all local sources sited well inside the model domain. A nested grid setup for inundation modelling is made with different grid resolutions from 1.8 km at the open ocean to 10 m within and around Riverton and New River Estuary. Various data sets (GEBCO-08, NZ Topography and navigational charts, and an RTK survey data for Riverton) were used in setting up the model grid. Inundation models were conducted using COMCOT v.1.7 (Wang, 2009) at a range of tide levels (Low Water Level (LWL), Mean Sea Level (MSL), and Mean High Water Spring (MHWS). For local sources, the potential uplift or subsidence on land and the seabed caused by earthquakes is taken into account in the model simulation. GNS Science Consultancy Report Riverton Southland Modelling 7

10 Regional Modelling Results Results of regional hydrodynamic modelling of tsunami from event scenarios for Southland coast indicate: Event scenarios from Puysegur Trench and Peru provided a moderate to significant tsunami risk to the Southland region. While tsunami from the Fiordland source scenario affected most of the Fiordland region and less threat along the eastern part of the Southland coast. Tsunami from the Puysegur Trench source scenarios affected most of the areas along the Southland coast from Fiordland to Tiwai Peninsula, and the west coast of Stewart Island. Further east, their effects become less as tsunami being blocked by Stewart Island for Hayes and Furlong scenarios (2010). However, modified Downes et al.2005 scenarios showed the effects of the shallow part of Campbell Plateau that refracted the tsunami towards the east coast of Southland region that provide a significant threat to the areas from Tiwai Peninsula to Curio Bay. Tsunami from Peru Scenarios affected most of the Southland coast from the east to west. Detailed Modelling Results for Riverton and New River Estuary Detailed modelling for Riverton had been carried using a combination of Real Time Kinematic (RTK) DGPS survey data sets and Shuttle Radar Topographic Mission (SRTM) data with 90 m resolution, while for New River Estuary, the topography data is based on the existing topographic maps and in combination with SRTM. The results as follow: The Fiordland scenarios provide less threat to the Southland coast, except for the Fiordland region; there is almost no inundation on land. Tsunami only inundated the beach front areas (i.e. up to the high tide level). The event scenarios that provide both land and marine threats to most of the Southland coast are occurred during Mean High Water Spring Scenario (MWHS) as follows: i. Puysegur event scenario of Downes et al. (2005) (M w 8.6). The first tsunami wave arrives ~70-90 minutes after fault rupture. Maximum tsunami elevation varies from 1.0 m to 8.0 m. ii. Puysegur event scenario of Hayes and Furlong (2010) (Mw 8.3) as well as modified scenarios of Hayes and Furlong (2010) (Mw 8.4, 8.6 and 8.7). These scenarios show the first tsunami wave arrives minutes after fault rupture. Maximum tsunami elevation for Mw 8.3 varies from 1.0 m to 4.5 m; Mw 8.4 varies from 1.0 m to 6.0 m; Mw 8.6 varies from 1.0 m to 8.0 m, and Mw 8.7 varies from 1.0 m to 10.0 m. iii. Peru event scenario of Berryman et al. (2005) and Okal et al. (2006) (M w 9.1). The first tsunami wave arrives minutes (15-16 hours) after fault rupture. Maximum tsunami elevation varies from 1.0 m to 10.0 m. iv. Peru event scenario of Berryman et al. (2005) (M w 9.4) at High Tide (MHWS). This event had a similar impact to the M w 9.1 scenario. Maximum tsunami elevation varies from 1.0 m to 10.0 m. GNS Science Consultancy Report Riverton Southland Modelling 8

11 Tsunami Hazard Mitigation and Warnings In reducing the possible impact from tsunami, a common response internationally (especially in Japan) is to opt for structural mitigation measures such as building seawalls or breakwaters. However, Horikawa and Shuto (1983) based on experience in Japan, stated that it is quite dangerous to believe that the violent attack of tsunami can be completely prevented by man-made structures, and it is incorrect to depend too much on the functioning of coastal defence structures. However, generally coastal defence structures can minimise the possible impact when they are designed properly and involve a combination of soft (non-structural measures) and hard (engineering/structural measures) solutions. The combination of coastal sand dunes and forest offer an existing mitigation option along the low-lying areas of Riverton and New River Estuary. An elevated beach front and drainage stop gate could be an option for inundation from smaller events that comes through the river or access channel, especially for the New River Estuary. The elevated beach front can be designed as a combination of artificial sand dunes and riprap or seawall. Further assessment needs to be done on what type of protection is appropriate to prevent inundation of Riverton and New River Estuary. After the 2004 Boxing Day tsunami, the effectiveness of coastal forest along the shoreline in reducing tsunami energy became fashionable, even though it has been recognised previously by Shuto (1993) with affirmative views where it could: Stop the floating debris; Reduce water flow velocity and inundation height; Provide a life-saving means by catching persons carried by tsunami; and Collect wind-blown sands and raise dunes that act as a natural barrier against tsunami. However, it was noted that some trees could be up-rooted by tsunami and become destructive floating debris, therefore careful design should be considered before it comes into practice (Prasetya et al., 2008). Safety measures for ships and boats will provide protection to the vessels themselves, as well as preventing secondary damage caused by drifting vessels. The first tsunami waves arrive ~80 and 90 minutes after the fault rupture for local sources, and 16 to 17 hours for distance sources (Peru). In the case of locally-sourced tsunami it may be dangerous to evacuate if the time required is more than an hour, as strong currents will soon occur at the harbour entrance, but for distant sources there is enough time. A combination of loose mooring and anchorage can reduce the risk of vessels drifting inland. Keeping the mooring and anchor cables loose could prevent vessels from being damaged by the collision of the first wave or strain buoyancy, but this becomes more difficult when the strength of the current increases. Tsunami mitigation works may affect the quality of daily life, including inconvenience and efficient use of the waterfront. However, they involve choices, trade-offs and risk; they also involve adjustment. Periodic simulation for emergency preparedness programs and procedures are an essential learning exercise, and regular information and instructional GNS Science Consultancy Report Riverton Southland Modelling 9

12 materials should be provided and kept updated for those occupying tsunami hazard areas (Wiegel, 2006). Limitations of the current work In the present study, no erosion or sedimentation is considered or taken into account in the modelling processes. Therefore, no morphology changes to beaches and sand dunes that may occur due to strong currents and wave breaking during inundation processes are included in the modelling. The topography data being used are based on the RTK survey from the major access road within Riverton in combination with SRTM 90 m resolution. The New River Estuary topography data are based on SRTM 90 m resolution and available topography map. Most the bathymetry data are combination of digitized nautical chart and GEBCO 30 arcsecond. Most of the scenarios used in this report assume earthquake ruptures with uniform slip. However, in real events, the slip on the fault plane has a variety of spatial scales. The pattern of slip distribution can affect the resulting tsunami for near-field (or local sources), but not so significantly for distant sources (Geist, 1998). Uniform slip distributions used here may represent an approximation for average events, compared to the real earthquakes of the same magnitude which may have larger slips in some areas and lesser slips in others. Recommendations For preventing loss of life, it is clear that preparation of coastal resilient community is essential, as is an early warning system. There is need for integration of life-long efforts to educate the population about the tsunami hazards, and preparedness. Tsunami signage within identified tsunami hazards zone need to be put in place where the community can easily recognised and understand the information in it. As the model results show, the sand dunes provide an effective barrier or protection from the tsunamis. Thus the sand dunes along the beach front should be preserved and any activities that deteriorate the dune systems should be avoided. If the dune systems are degraded due to storm waves or other extreme events, necessary actions need to be carried out to return them to their condition before the event. Strong currents are generated inside and outside the estuary. These strong currents are dangerous for ships and boats within the estuary and or harbour. Detailed planning should be done on how to create guidelines to evacuate any boats and ships within the harbour as well as land use planning including development setback. Detailed topography (i.e. from LIDAR survey) and nearshore bathymetry (i.e. from multibeam survey) data are required to produce the inundation map of Level 4 based on MCDEM guidelines. GNS Science Consultancy Report Riverton Southland Modelling 10

13 1.0 INTRODUCTION Emergency Management Southland contracted GNS Science to undertake a tsunami inundation assessment for Riverton and New River Estuary. A scenario based approach is used in modelling the potential tsunami inundation through a set of tsunami scenarios potentially caused by both local and distant earthquake sources. These sources were defined based on available published literature (i.e. Berryman et al. 2005; Downes et al ; Greenslade et al. 2007, 2009; Bathgate et al. 2008; Okal et al. 2006; Hayes et al and Hayes and Furlong 2010). For distant sources, the trans-oceanic tsunami propagation shows that the tsunami propagation generated off the Peru Coast of South America is oriented to the Southwest Pacific, in such a way that the tsunami energy is more effectively directed towards New Zealand (Berryman et al. 2005, Power et al. 2007). For sources further south along the Chilean coast, the tsunami energy is concentrated towards the Northwest Pacific (i.e. Japan). Berryman et al. (2005) indicated that a tsunamigenic earthquake from South America with magnitude less than M w 8.5 will have a minimal impact and risk to New Zealand. The recent Chilean M w 8.8 event (27 February 2010) demonstrated this; although a 0.27 m wave was recorded at Dog Island, tsunami impacts to Southland, especially to the New River Estuary, were barely observable to most people (Telford, 2010). Local source scenarios were selected and based on source definitions from available published literature (Berryman et al. 2005; Downes et al. 2005; Greenslade et al. 2007, 2009; Bathgate et al. 2008; Hayes et al and Hayes and Furlong 2010). These source regions are associated with the Alpine Fault system (off the coast of Fiordland) and the Puysegur Trench to the south. The most recent event occurred on 15 July 2009 (M w 7.8), known as the Dusky Sound earthquake and tsunami. While for the Puysegur Trench, the last earthquake event occurred in 1979 (M w 7.3), but no tsunami was reported. The study area for inundation modelling is divided into two model domains. The first model domain is for a distant source that covers the whole South Pacific region, and the second is for local sources that cover the southern ocean of the Southland region with all local sources sited well inside the model domain. A nested grid setup for inundation modelling is made with different grid resolutions from 1.8 km at the open ocean to 15.0 m within and around the Riverton and New River Estuary. Various data sets (GEBCO-08, NZ Topography and navigational charts) and an RTK survey data of Riverton and surrounding areas are used in setting up the model grid. Inundation models were conducted using COMCOT v.1.7 (Wang, 2009) at a range of tide levels (Low Water Level (LWL), Mean Sea Level (MSL), and Mean High Water Spring (MHWS). For local sources, the potential uplift or subsidence on land and the seabed due to the earthquakes is taken into account in the model simulation. 2.0 BACKGROUND A tsunami is a progressive wave with a long wavelength and period, generated by a disturbance of the seafloor associated with various geologic processes such as submarine fault movements accompanying earthquakes; submarine volcanism; landslides; or a combination of these sources. The tsunami s size and effect on the shoreline are often GNS Science Consultancy Report 2010/293 11

14 decomposed by several factors: the amplitude of the wave on the source, which is primarily controlled by the size and geometry of the source and the response of the coastal features, such as bays and harbours (which are capable of resonance, thereby amplifying selected wave frequencies) and waves, and the nonlinear hydrodynamics of the breaking wave as it rolls up shore. In order to predict detailed and quantitative information, the modelling of tsunami behaviour is essentially required. Scientific terms such as runup height, tsunami wave height and inundation often confuse the general public and result in misleading information about what happened. This situation had an impact on the subsequent rehabilitation, reconstruction and mitigation efforts as to what happened during the Indian Ocean Tsunami in The run up height depends upon the slope of the ground inland, and the ability of the topography to concentrate the wave energy or reflect it. For the same incoming tsunami waves, those areas with a gentle slope inland will experience lower runup heights but longer inundation distances compared to the areas that have steeper slopes. Therefore, in reporting the properties of tsunamis, the location of measurements should be mentioned, as is standard practice in tsunami surveys (Synolakis and Okal, 2005). The flow depth is the depth of water under the tsunami wave as it flows inland, and if already referenced to Mean Sea Level (MSL), it is termed tsunami elevation. At the inland limit of the flow, the height above MSL is termed the runup height, and the horizontal distance from the shoreline is the inundation distance. The runup height was frequently confused with the tsunami wave height in the media coverage (Prasetya et al., 2008). Coastal areas in the Indian and Pacific Oceans have suffered damage from tsunamis for a long time. However, their effects are often underestimated in comparison with earthquake hazard, possibly because of their infrequent occurrence. Based on historical data of tsunami events in the Pacific Ocean, tsunamis affecting the New Zealand coast have been generated by a variety of different mechanisms. Some sources can be categorised into far-field tsunamis which are generated at some distance from New Zealand and propagate through the deep water of the Pacific Ocean before reaching the coast, while some are near-field tsunamis generated close to New Zealand s coastline and propagated through relatively shallow water. The descriptive accounts of the 1820 s event and studies by Downes et al. (2005) and Hayes & Furlong (2010) indicate that the Southland coast poses significant tsunami hazard and risk. An event in the 1820 s that drowned people along the coast near Orepuki has been attributed to a local source by Downes et al. (2005), as there is no known South American source during the period In addition, paleotsunami reconnaissance has considered a possible tsunami that impacted the Southland region during the 14 15th centuries, but the exact origin or sources are undefined. Three historical tsunami events were generated by large earthquakes off the South American coast in 1868 (Peru ~M9.1), 1877 (northern Chile ~M9.0) and 1960 (central Chile ~M w ) and caused significant, widespread damage and disruption along the east coast of the North and South Islands of New Zealand. Tidal disturbances were observed at Bluff and Riverton, Southland, for the 1868 Peru, 1877 and 1960 Chile events (Downes, pers. comm.). GNS Science Consultancy Report 2010/293 12

15 MSL A s α s I s Steep slope R s A S and A G : tsunami wave height α S : angle for steep coastal zone α G : angle for gentle coastal zone R S : Runup height at steep slope R G : Runup height at gentle slope I S : Inundation distance at steep slope I G : Inundation distance at gentle slope MSL A G Shoreline α G Gentle slope R G For: A s = A G ; α s > α G Then: R s > R G ; I s < I G I G Runup height Flow depth Inundation distance Figure Schematic diagram for runup height and inundation for different beach slopes (simplified from Prasetya et al. 2011). For the same incoming tsunami height at the coast, those areas with a gentle slope inland will experience lower runup heights but longer inundation distances compared to the areas that have steeper slopes. 3.0 OBJECTIVE The main objective of this project is to provide an estimate of tsunami inundation for Riverton and New River Estuary, as well as a regional tsunami threat along the Southland coast from event scenarios of local (Fiordland and Puysegur) and distant sources (Peru). 4.0 METHOD 4.1 Numerical model In this study the tsunami model COMCOT (Cornell Multi-grid Coupled Tsunami model) was used to simulate the propagation, runup and inundation of tsunamis from both local and distant sources. Developed at Cornell University, USA, in the 1990 s, this model has become publicly available and has been widely used by researchers to study different aspects of tsunami impacts. It has been systematically validated against analytical solutions (Cho, 1995), experimental studies (Liu et al., 1994; Liu et al., 1995; Cho, 1995) and benchmark problems (Wang, Orfila and Liu, 2008) and has consistently shown its satisfactory accuracy. Some of its applications include the study of the 1960 Chilean Tsunami (Liu et al., 1994), the 1986 Taiwan Hualien Tsunami (Liu et al., 1998), the 2005 Algerian Tsunami (Wang and Liu, 2005), the 2004 Indian Ocean Tsunami (Wang and Liu, 2006, 2007, 2008), and the recent 2009 Samoa tsunami (Beavan et al., 2010). It has also been applied to study flooding and GNS Science Consultancy Report 2010/293 13

16 evaluate the tsunami forces on structures in the cities of Galle, Matara and Hambantota in Sri Lanka during the 2004 Indian Ocean Tsunami for building code suggestions (Wijetunge et al., 2008). Using a modified staggered finite difference scheme to solve linear/nonlinear shallow water equations, COMCOT was developed to investigate the evolution of long waves in the ocean, particularly tsunami, including their generation, propagation, runup and inundation. To account for the shallowness of water depth and ensure enough spatial resolution in nearshore regions, a nested grid configuration was implemented in COMCOT, through which the model can use a relatively larger grid resolution to efficiently simulate the propagation of tsunami in the deep ocean and then apply finer grid resolutions in coastal regions. In this approach, the computational efficiency and the numerical accuracy will also be well balanced. To calculate the initial disturbance of sea surface in a submarine earthquake event, an elastic, half-space dislocation modelling approach has been used to estimate the co-seismic seabed uplift for each scenario, using the equations of Okada (1985); this co-seismic uplift provides the initial condition for the tsunami model. The scenarios assume uniform slip on one or more rectangular fault segments. The assumption of uniform slip is widely used in tsunami modelling but, in actual ruptures, the slip on the fault plane varies in dislocation in complicated ways on a variety of spatial scales, and this heterogeneity can affect the subsequent tsunami (Geist, 1998). The assumed uniform slip distribution may represent an approximate average event compared to real earthquakes of the same magnitude which may have larger slip in some areas and less slip in others. It should be noted that variations in slip may additionally affect the wave period of the tsunami and thus could alter how the tsunami interacts with the coast. The amount of co-seismic uplift in the source areas is also sensitive to the distribution of slip on the nearby region of the plate interface. In the scenarios developed for this study, the effect of tsunamis occurring at different sea level conditions (i.e. MHWS) is modelled. This was approximated by adding a fixed increment (i.e. half the tidal range of 2.21 m for Tiwai Point) to the baseline water-level (i.e., MSL). In these scenarios, the generated tsunami propagates and interacts with coastal areas at this increased water level. 4.2 Model grids As a tsunami approaches coastal regions from the deep ocean, its wave length becomes shorter and the amplitude becomes larger due to the shallowing of water depth. To account for this effect, a nested grid configuration with four resolution levels of 900 m, 180 m, 45 m and 15 m was implemented for the numerical simulations (Figure ). The bathymetric/topographic data was derived from the raw data sets including digitization of LINZ charts, Shuttle Radar Topographic Mission (SRTM) satellite imagery, GEBCO database and those obtained from the RTK field survey (Section 5). The first-level grids (layer 01) cover the south part of New Zealand s South Island as well as the entire Puysegur subduction trench where future earthquakes could generate tsunami potentially threatening the study area, ranging from to in longitude and from to in latitude, with the resolution of 900 m (Figure ). GNS Science Consultancy Report 2010/293 14