Roy A. Walters

Ocean tides around New Zealand

Abstract The distribution of amplitude and phase for eight ocean tidal constituents (M2, S2, N2, K2, K1, O1, P1, Q1) is presented as tidal maps for the New Zealand area. The distribution was calculated using a barotropic tidal model driven by TOPEX/ Poseidon data on the outer ocean boundaries. The maps exhibit the known features of the tides in this area such as a complete rotation of the semi‐diurnal tides around New Zealand and the reduced spring‐neap variations on the east coast. They also point out several new features for which there are few or no observations, such as diurnal trapped waves and shelf waves. A comparison of the model results with observations shows that sea level errors are within 0.1 m in amplitude and 10° in phase for the largest constituents at all locations, including sites where the data are of low quality and where the geometry is not adequately resolved. For locations where the geometry is adequately represented and the observations are of high quality, sea level errors are within 0.02 m in amplitude and 7° in phase. These results represent the most accurate and highest resolution calculations of tides and currents yet attained for this area.

Locally generated tsunami along the Kaikoura coastal margin: Part 2. Submarine landslides

Abstract An examination of the underwater landscape along the northeast coast of the South Island, New Zealand, identified a substantial potential for a submarine landslide in Kaikoura Canyon. A numerical model was applied to calculate runup and inundation arising from a local tsunami generated by such a landslide. The model is based on the Reynolds‐averaged Navier‐Stokes (RANS) equation and used a finite element spatial approximation, implicit time integration, and a semi‐Lagrangian advection approximation. The results indicate that a landslide‐generated tsunami represents a large potential hazard to the area from South Bay to Oaro, South Island, New Zealand, and has the potential to generate large tsunami runup heights along this section of coast. In addition, the tsunami events are characterised by a short time interval between generation and runup.

Locally generated tsunami along the Kaikoura coastal margin: Part 1. Fault ruptures

Abstract Tsunami are generated by sudden movements of the ocean bed or by objects such as subaerial landslides and bolides falling into the ocean. An examination of the geophysical setting for the northeast coast of the South Island, New Zealand, identified a substantial potential for submarine fault ruptures and submarine landslides. To examine possible effects of a tsunami, a numerical model was applied to calculate runup and inundation arising from locally‐generated tsunami along this section of the coast. The specific events considered were fault ruptures on the Kekerengu Bank Fault and two lesser faults, and a submarine landslide in Kaikoura Canyon. The model is based on the Reynolds‐averaged Navier‐Stokes (RANS) equation and used a finite element spatial approximation, implicit time integration, a semi‐Lagrangian advection approximation, and a simple method for treating non‐hydrostatic pressure variations. The results indicate that the different generation events have significant effects on different parts of the coastal margin. For the fault‐rupture events, the northern coast of the study area is very exposed to damage from a potential rupture of the Kekerengu Bank Fault. A concern is that the highest waves near Kaikoura would be a result of late arrival of coastally‐trapped waves and would be delayed by up to 1.5h after the fault‐rupture event.

Ocean-tide loading and Earth tides around New Zealand

Abstract Cotidal charts of ocean‐tide loading and Earth tides for the New Zealand region are presented for eight constituents (M2, S2, N2, K2, K1, O1, P1, Q1). Ocean‐tide loadings were calculated by evaluating a convolution integral between tide models (global and local) and a Greens function that describes the response of the Earth to tide loading. Earth tides were calculated from the tide‐generating potential. The ocean‐tide loadings are a maximum to the north of New Zealand for M2 and N2 and in the western Cook Strait region for S2 and K2. The diurnal ocean‐tide loadings are dominated by amphidromes east of the Chatham Islands (K1 and P1) and north of Bay of Plenty (O1 and Q1). Diurnal Earth tides are a maximum at 45°S and vary only slightly in amplitude and phase over New Zealand; semidiurnal Earth tides increase from south to north.

Verification of RiCOM for Storm Surge Forecasting

RiCOM is an unstructured-grid finite-element coastal ocean model. It is used to provide storm surge forecasts as part of a larger suite of environmental forecasting models known collectively as EcoConnect. RiCOM is forced with surface pressure and with 10 m winds forecast by the weather prediction model, NZLAM-12. Our objective is to evaluate the RiCOM forecasts, to understand the strengths of the model, and to identify improvements that can be made. The verification process involves comparison of the predicted sea level with data gathered from the New Zealand-wide sea level network managed by NIWA. Predicted time series for each site are built up from successive 48-hour forecasts. Different forcing and frequency components of the real and model time series are then compared. Historical mooring data are also used to evaluate RiCOMs ability to capture flows. Tidal and low frequency components of RiCOM are seen to compare well with sea level network data. There are some situations where RiCOM does not perform as well. Velocity forecasts have only been basically evaluated due to a lack of current data. Future additions and improvements to the model have been identified such as improving the wind stress formulation, extending the model to three dimensions, and adding baroclinic circulations and coupling the model with a wave model.

Numerical Simulation of Tsunami Generation, Propagation and Runup

A numerical model is applied to calculate cleanup and inundation along the east coast of New Zealand arising from tsunami generated locally along the New Zealand coastal margin. In general, tsunami can be generated by a suddenly movement of the ocean bed or by objects such as subaerial landslides and bolides falling into the ocean; however, this study is restricted to fault ruptures and submarine landslides. The model is based on the Reynolds-averaged Navier-Strokes (RANS) equation and uses a finite element spatial approximation, implicit time integration, a semi-Largrangian advection approximation, and several different methods for treating pressure variations. These methods include the hydrostatic approximation, a simplified pressure interpolation scheme, and a full solution with pressure Poisson equation. The different methods of approximation are being evaluated against test problems for wave runup and submarine avalanches. Although these results are preliminary, the results with a simplified pressure model are encouraging in that they provide a realistic approximation to non-hydrostatic effects while remaining competitive with the efficiency of depth-averaged models.