Terry R. Healy

New Zealand tsunamis 1840-1982

Abstract An extensive search of newspaper reports, archival material, and the literature has revealed that many more tsunamis have affected the New Zealand coast than hitherto realised. 32 tsunami events are listed, including their probability of occurrence, the maximum runup height, as well as the epicentre and Richter magnitude for those events associated with earthquakes. Most coastal regions of New Zealand are reported as experiencing tsunamis. Generally these events have been associated with earthquakes, although the tsunami source mechanisms have also been attributed to large rotational slumps, submarine slumping along the Chatham Rise, and submarine mud volcanism associated with diapiric intrusions on the continental shelf off Poverty Bay. Tsunami waves and seiching accompanying the Krakatoa eruption of 1883 appear to have been induced by pressure coupling between the atmosphere and oceans. Most tsunamis have affected the east coast. This largely reflects both exposure to pan-Pacific origins and th...

Expansion Dynamics of Monospecific, Temperate Mangroves and Sedimentation in Two Embayments of a Barrier-Enclosed Lagoon, Tauranga Harbour, New Zealand

Abstract In recent years, mangrove expansion has become a coastal management issue in the North Island of New Zealand. Little is known about the spatial evolution and forest structure of temperate mangrove forests in New Zealand or about the associated rates of sedimentation. The extent of mangrove colonization in both a large (Waikareao Estuary) and small (Welcome Bay) embayment within Tauranga Harbour were documented. Forest structure and growth rates were described using tree height, stem density, pneumatophore density, and seedling establishment. Surface elevation changes within mangrove habitat were determined from erosion pin measurements and, on bare intertidal flats, using buried baseplates. Sediment texture and availability was also assessed using sediment traps. Results show that in 1943, mangrove vegetation covered <1% of either embayment, with ongoing expansion resulting in mangrove habitat occupying 9% of Welcome Bay and 6% of Waikareao Estuary in 2003. Mean plant heights were all <1.5 m, with vertical plant growth of 3 cm y−1 to 7.5 cm y−1. Pneumatophore densities ranged from ∼200 per m2 to ∼540 per m2, and seedling densities of <1 per m2 to 14 per m2, with seasonal variation, were documented. Surface elevation changes within mangrove habitat ranged from 0 to +21 mm y−1, compared with −16 to +15 mm y−1 on adjacent bare flats. Surface sediment within the mangrove habitat was mostly muddy, whereas bare flats were dominated by fine sand. Sediment-trap results provided accumulation rates of up to 32,000 g m−2 mo−1 on bare flats, and close to 29,000 g m−2 mo−1 within the fringing mangrove vegetation. In this study, mangrove vegetation was found to successfully trap and hold muddy sediments, resulting in increased surface elevation. Rising surface elevation from fine sand deposition is also occurring on bare tidal flats in front of mangrove vegetation at some sites.

Research-Based Surfing Literature for Coastal Management and the Science of Surfing—A Review

Abstract Incorporating recreational surfing into coastal management practices is required to protect the seabed features and oceanographic processes that create surfing waves. A review of research-based surfing literature is undertaken to provide a summary of information available to assist in coastal management decision making around surfing breaks. The different categories of research-based surfing literature are identified as artificial surfing reef (ASR) design, ASR monitoring, ASR construction, ASR sediment dynamics, biomechanics, coastal management, economics and tourism, industry, numerical and physical modeling, surfers and waves, sociology, and physical processes. The majority of this research has been undertaken in the last decade, making it a relatively young research area. As a background for nonsurfing coastal researchers and managers, the characteristics of surfing waves and surfing breaks are described, referring to relevant literature. Wave height, peel angle, breaking intensity, and section length are identified as essential parameters to describe surfing waves. Existing surfer skill and maneuver categorization schemes are presented to show the relationship between surfers and surfing waves. The geomorphic categories of surfing breaks are identified as headland or point breaks, beach breaks, river or estuary entrance bars, reef breaks, and ledge breaks. The literature discusses the various scale bathymetric components that create these surfing breaks. Examples of modeling offshore wave transformations at Mount Maunganui, New Zealand, as well as the measurement microscale wave transformations at “The Ledge,” Raglan, New Zealand, are presented to demonstrate surfing wave transformations.

A “blind-folded” test of equilibrium beach profile concepts with New Zealand data

Abstract Methodology for calculating equilibrium beach profiles for uniform sand size characteristics is extended to the case of an arbitrary distribution of sediment characteristics across the profile. The application of this method and comparison with actual profiles is posed as a means of interpreting whether the profile contains a deficit or excess of sediment and thus whether long-term shoreline recession or advancement can be anticipated. Various types of profile disequilibrium are reviewed and the significance discussed. The methodology is applied using measured profiles, sediment sizes and beach face slopes for ten sites on the Northern Island of New Zealand. One profile was documented in this study, whereas the data for the other nine were obtained from published sources. The number of sediment samples available for each profile varied from three to twelve. The agreement between the actual and calculated profiles differs considerably for the ten sites. The degree of disequilibrium is quantified by calculating the shoreline adjustment, Δy , required for the actual profile to equilibrate for depths less than 7 m, which represents the near-maximum depth available on all profiles. These shoreline adjustments ranged from − 105 m (recession) to + 159 m (advancement) with four of the ten sites having positive values. Three of the sites with negative shoreline adjustment have been, or are presently, sites of substantial sand extraction from the beach or in the nearshore waters. However, the differences between the actual and equilibrium profiles are not consistent with anticipated profile forms and/or volumes and it is thus concluded that sand mining is not responsible for most of the observed deficits. At this stage, it is not possible to state with confidence whether differences between actual and (calculated) equilibrium profiles are due to true disequilibriums or to limitations in the equilibrium beach profile methodology. Studies of the type reported here when applied to many different areas will advance the methodology and contribute to the confidence in the resulting interpretations.

Holocene sediment lithofacies and dispersal systems on a storm-dominated, back-arc shelf margin: The east Coromandel coast, New Zealand

Abstract Holocene sediment lithofacies and dispersal systems on the east Coromandel shelf, New Zealand, are mainly characterised by an accommodation-dominated regime in which autochthonous siliciclastic sediments were reworked through erosional shoreface retreat during the post-glacial marine transgression (12.0–6.5 ka). Stabilisation of sea level at its present position ca. 6.5 ka initiated onshore reworking of the autochthonous deposits into fine-grained regressive barrier and shoreface sands, while coarser sands remained offshore to form an erosional-lag inner-shelf deposit. Modern episodes of shoreface erosion rework fine and coarse autochthonous sands offshore and northwards into very large (η = 0.5–2.5 m; λ = 250–1500 m) submarine dunes. The submarine dunes are similar in form to sand ridges on the North American Atlantic shelf, but with crests striking perpendicular to both the shoreline and generating flows. Allochthonous siliciclastic lithofacies are also important aspects of east Coromandel shelf sedimentation, and are transported offshore from infilled estuary systems to form very fine-grained upper shoreface and muddy mid-shelf sands. The regional geology has a strong influence on shelf lithofacies and dispersal systems off the east Coromandel coast. Southern shelf regions are associated with deep back-arc basins which have formed major late Cenozoic depocentres for sediments sourced from the Taupo Volcanic Zone. Further north, a shallow volcanic platform extends out across the shelf and forms a barrier to any large-scale along-shelf dispersal of sediments. Consequently, lithofacies in northern shelf regions tend to be highly variable and reflect local catchment lithologies.

Modelling of tsunamis generated by pyroclastic flows (Ignimbrites)

Pyroclastic flows entering the sea played a major role in generating the largest tsunamiwaves, arising from the 1883 eruption of Krakatau, Indonesia, which caused a considerabledeath toll, most deaths resulting from the tsunamis. The potential exists for similar eventsto occur in Indonesia and New Zealand.Processes leading to tsunami generation by pyroclastic flows, especially those associatedwith Krakatau-type eruptions, are reviewed. The major processes include:1. Deposition at the shoreline causing a lateral displacement as the zone of depositionmoves offshore.2. Upward and lateral displacement of water caused by the propagation of a watersupported mass-flow.3. Downward and lateral displacement of water caused by the sinking of debris from a segregated flow travelling over the water surface.4. Upward displacement of a large volume of water due to the deposition of acaldera-infill ignimbrite or pyroclastic flow deposit.The pyroclastic flow is modelled as a horizontal piston forcingwater displacement. The flow behaves as a wedge of material displacingseawater horizontally and vertically as it moves outwards from the source.Individual pyroclastic flows are treated as linear features that travel alonga specific direction from the volcano, exhibiting limited lateral spreading.The event duration for the formation of a large pyroclastic flow and thedeposition of the ignimbrite is taken as 200–400 s, with flow velocitiesdependent on the volume of material erupted.For simulations it is assumed that the ignimbrite deposit is elliptical with relativelyuniform thickness and the principal axis orientated along the flow direction. Therefore the tsunami is generated by defining an elliptical source region and defining an effective displacement behaviour at each node within that region. The effective displacement is defined by a start time, a finish time and a vertical velocity. These three parameters determine when the seafloor starts to rise and how far it travels during a model time step. The result is a seafloor disturbance that propagates away from the source.The major difficulty with this approach is determination of the appropriate verticalvelocity. With a real pyroclastic flow the effective vertical velocity at any point isvery high. However the model needs to average the displacement spatially andtemporally. Accordingly we apply the model to pyroclastic flows from Mayor Island, New Zealand to examine the influence of model parameters. To further calibrate the numerical model this study is being undertaken in conjunction with physical modelling of the Krakatau 1883 eruption at the Indonesian Tsunami Research Center, BPPT, Jakarta. Historical data will also be used to refine and calibrate the pyroclastic flow model.

Holocene coastal depositional sequences on a tectonically active setting: southeastern Tauranga Harbour, New Zealand

Abstract More than 70 cores, a high-resolution seismic survey, and SCUBA observations provide the basis for the interpretation of depositional environments that accumulated during the Quaternary in the southeastern Tauranga Harbour area of the North Island, New Zealand. Three lithofacies comprise this sequence; in ascending order they are pumiceous sand and gravel, shelly mud and shelly sand. The pumiceous sand is interpreted as fluvial and fan deposits of Pleistocene to early Holocene age with a radiometric date of 9420 ± 100 yr BP near the top of the unit. The shelly mud represents low-energy estuarine deposition of essentially normal marine salinity in a valley-like setting. This unit dates at 8100 ± 80 yr BP. The extensive overlying shelly sand thickens seaward and represents wave-dominated shoreface conditions much like the present nearshore environment. Radiometric dating of samples within the present harbor are all between 6000 and 7000 yr BP and those seaward of the spit to Mt. Maunganui are less than 3370 ± 100 yr BP. The barrier spit that has attached to the volcanic headland began accumulating about 4000–5000 years ago.

Beach erosion at Waihi Beach Bay of Plenty, New Zealand

Abstract Since the 1940s, severe dune erosion has threatened property at Waihi Beach. This paper investigates beach erosion and dune recession in relation to wave climate, sedimentoldgy and mineralogy of the beach sediments, cyclical beach changes and sediment budget, and the littoral drift. Average dune recession along the entire beach between 1948 and 1977 was 27 m, although maximum recession recorded to 1977 was 83 m. Sand loss rates 1948–1977 average 3.4 m3 per metre of beach per year. The sediment budget of the entire beach is about 850 × 103 m3 and up to 74% of this was recorded as being ‘cut’ from the beach in one erosive episode. The cut and fill cycles are dominantly controlled by wave steepness, which has a critical value of 2.1 × 10−3 for breaking waves on Waihi Beach. Net littoral drift is towards the south‐east, and the basic reason for beach erosion at Waihi Beach is the lack of sediment to supply the littoral drift.

Coastal oceanography and sedimentology in New Zealand, 1967–91

Abstract This paper reviews research that has taken place on physical oceanography and sedimentology on New Zealands estuaries and the inner shelf since c. 1967. It includes estuarine sedimentation, tidal inlets, beach morphodynamics, nearshore and inner shelf sedimentation, tides and coastal currents, numerical modelling, short‐period waves, tsunamis, and storm surges. An extensive reference list covering both published and unpublished material is included. Formal teaching and research programmes dealing with coastal landforms and the processes that shape them were only introduced to New Zealand universities in 1964; the history of the New Zealand Journal of Marine and Freshwater Research parallels and chronicles the development of physical coastal science in New Zealand, most of which has been accomplished in last 25 years.

Ebb-Jet Dynamics and Transient Eddy Formation at Tauranga Harbour: Implications for Entrance Channel Shoaling

Abstract In 1992 the entrance channel through the tidal inlet to Tauranga Harbour, which is located along the Bay of Plenty littoral drift system, was deepened from 10 m to 14 m. The deepened channel has become a sediment trap for littoral drift bypassing and tidal current driven sediment transport through the inlet. Since 1992, there has been an increase in maintenance dredging requirements at the inlet, because of sand accumulation along the southeastern border of the entrance channel. Previous studies have identified an ebb tide–induced eddy operating on the eastern side of the ebb-jet as it exits the tidal gorge. In this article, the eddy system has been simulated with a validated two-dimensional hydrodynamic model, detailing time-varying current patterns over the ebb-tidal delta. Particular emphasis is placed on defining the trajectory of the eddy and evaluating its influence on the observed sedimentation patterns. The model results indicate the formation of opposing eddies on either side of the entrance channel, both of which are transient in nature. The centre of the eastern eddy propagates seaward along the downdrift margin of the entrance channel as the ebb-jet lengthens. Bathymetric survey residuals between 2004 and 2006 confirm significant accumulations of sediment along this downdrift margin. The evidence is consistent that the eddy system exerts a directional control over transport of sediments entrained by waves over the ebb-tidal delta.