Richard M. Gorman

Wave hindcast for the New Zealand region: nearshore validation and coastal wave climate

Abstract Historically, wave data coverage of New Zealands coast has been poor, particularly for directional records. With very few data sets available of more than 1 years duration, it has been difficult to establish accurate wave climatologies. To help fill in the gaps in our wave records, the wave generation model WAM (WAve Model) has been implemented over a domain covering the south‐west Pacific and Southern Oceans. The model has been used to hindcast the generation and propagation of deep‐water waves incident on the New Zealand coast over a 20‐year period (1979–98), using winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF). The resulting synthetic climatology is expected to provide a valuable tool for researchers and coastal planners. The hindcasts were compared with data from wave buoy deployments at eight representative sites around the New Zealand coast. With appropriate interpolation and correction for the effects of limited fetch and sheltering by land, the hindcast was found to provide a satisfactory simulation of wave conditions at sites on exposed coasts. Regression between measured and hindcast significant heights at the four deep‐water sites (100–120 m) achieved scatter indices (ratio of root mean square error to mean) averaging 0.28. At the four shallower sites (30–45 m), the corresponding scatter index averaged 0.49, indicating that for regions of complex coastal topography, deep‐water spectra do not represent inshore conditions well. Wave spectra can be considerably modified by the processes of refraction and shoaling. To address these effects, nearshore wave transformations in the outer Hauraki Gulf were investigated using the shallow water model SWAN (Simulating WAves Nearshore), which was used to derive wave statistics at nearshore locations from deep‐water wave spectra obtained from the hindcast. The simulations were validated using data from an inshore site in 30 m water depth at Mangawhai on the north‐east coast of the North Island. Use of the nested model improved the agreement between model and measured significant wave height, decreasing the scatter index from 0.50 to 0.26. The suite of tools provided by the hindcast and localised, shallow water models can provide accurate new wave information for most of New Zealands coastline.

Wave hindcast for the New Zealand region: Deep‐water wave climate

Abstract The wave evolution model WAM (WAve Model) has been implemented for the New Zealand region and used to simulate the generation and propagation of deep‐water waves over a 20‐year period (1979–98). The model extends to include the relevant generation areas of the south‐west Pacific and Southern Oceans. Input winds for the model were derived from the European Centre for Medium‐Range Weather Forecasts (ECMWF). The resulting synthetic wave climatology will provide a valuable tool for researchers and coastal planners, as it will help fill gaps in the available wave information for New Zealand waters. In this paper the hindcasts are described, and comparisons are made with wave height data from the altimeters flown on the TOPEX/ Poseidon and ERS1 and ERS2 satellite missions. Long‐term mean significant wave heights from the hindcast were generally 0.3–0.5 m lower than values from “buoy‐equivalent” altimeter data throughout the comparison region (150°E‐170°W, 60°S‐20°S). Hindcast distributions of significant wave height occurrence matched satellite data at the lowest wave heights and above the peaks of the distributions, but tended to overestimate occurrence below the peak and underestimate the occurrence of the largest wave events. The hindcast was then used to characterise the wave climate of the New Zealand region. Some prominent features noted were the large mean heights (3.6 m) in the high latitudes of the Southern Ocean, associated with strong prevailing westerlies. North of this band, waves largely propagate towards the north‐east, with diminishing mean heights, further attenuated by the blocking effect of the New Zealand landmasses. This results in mean wave heights of c. 2 m in offshore waters north‐east of New Zealand. Annual cycles of mean wave height with a range of c. 1 m were identified throughout the region. These were found to have minima in summer (December/January), and either unimodal maxima in winter (June/July) for tropical and temperate latitudes, or bimodal maxima (May and August) in southern waters. Longer‐term variations were also noted in the form of correlations with the El Niño/Southern Oscillation. Positive correlations (R +0.2) were found off the north‐east coast of the North Island, indicating a moderate tendency for increased wave heights there during La Niña conditions, whereas negative correlations were found south and south‐west of New Zealand (R ‐0.2), and in the Fiji/Vanuatu region (R +0.4), reflecting wave height enhancements in the El Niño phase.

Spectral Estimates of Dissipation Rate within and near the Surf Zone

Abstract Three-dimensional point velocity measurements from within and near the surf zone are used to examine changes in turbulent dissipation rate with location relative to the breakpoint and wave conditions. To separate turbulence from wave orbital currents, dissipation rate is derived using the inertial subrange of the wavenumber spectrum. The measured frequency spectrum is transformed into a wavenumber spectrum by generalizing Taylors hypothesis to advection of turbulence past a sensor by monochromatic water waves in arbitrary water depth. The resulting dissipation rate is compared with that obtained by averaging the dissipation rates calculated from frequency spectra of very short segments of the time series, over which Taylors hypothesis for steady currents can be used. The dissipation rate calculated using the latter compares well, in the averaged sense, to the dissipation rate calculated using the whole time series, even though the whole time series includes segments that violate Taylors assump...

Modelling shallow water wave generation and transformation in an intertidal estuary

An experiment is described in which wave growth was measured in Manukau Harbour, a New Zealand estuary with relatively large fetches and extensive intertidal flats. Wave spectra were obtained from pressure sensors and current meters placed at six sites across the estuary. The SWAN third-generation spectral model was then used to simulate wave transformation during a part of the study period during which consistent south-westerly winds blew along the instrument transect. The simulations incorporated refraction by currents using output from a circulation model of the estuary. Measured wave variance spectra were compared with the model results, and the contributions of the various processes represented by source terms within the model were compared. It was found that, along with whitecapping, bed friction and exponential growth from wind input, four-wave nonlinear interactions played a dominant role. Some limitations were noted in the discrete interaction approximation which the SWAN model uses to compute the four-wave nonlinear interaction term.

Bars formed by horizontal diffusion of suspended sediment

Abstract The mechanism responsible for the ubiquitous presence of convex beach profiles and shoreward migration of linear bars is examined using numerical circulation and sediment transport models. The models are validated against laboratory measurements and observed natural beach cross-sections. While not discounting the importance of infragravity and advective horizontal circulation or bed-return flow mechanisms, a robust diffusive process explains the convex profile shape and bar formation. In the presence of concentration gradients across the surf zone, a diffusive sediment flux from high to low concentration results in the transfer of sediment outwards from the breakpoint, both onshore and offshore, and the subsequent formation of a “diffusion bar” and “diffusion profile”. The profiles are characterised by single- and double-convex dome-like shapes, developing during shoreward migration of the bars by the diffusion mechanism. The mechanism explains several phenomena observed on natural beaches, including (i) convex beach profiles; (ii) shoreward migration of the bar with concomitant beach accretion under narrow-band swell; (iii) reduced propensity for bar formation on low-gradient, fine-sand beaches or under wide-band wave spectra (even though multiple bars are common on some low-gradient beaches) and (iv) offshore migration of the bar during periods of increasing wave height. The diffusion mechanism can be dependent on orbital motion alone and, as such, requires no frequency selection or strong correlation between multiple processes for bar formation.

Port redesign and planned beach renourishment in a high wave energy sandy-muddy coastal environment, Port Gisborne, New Zealand

Redesign of Port Gisborne for the 21st century has encompassed a broad interdisciplinary approach. This procedure has taken into account the operational requirements of the port, effects of dredging and construction upon the benthic fauna, and wave activity within the port confines after the proposed development. Added amenity value of the development to the local community is an important ancillary redesign consideration. Initially, a major research project into the environmental impacts of the developments has been undertaken.The project, which commenced in 1996 and is still continuing, involves an iterative approach integrating the initial design and development options with the operational feasibility, construction constraints, environmental constraints, social acceptability, and economic practicality of the port; all of these require in-depth assessment to obtain the necessary planning and development approvals. This requires close liaison between the professional environmental research scientists, port management, port operation staff (pilots), construction engineers, planners, and the community interest groups.Numerical modelling of the hydrodynamics of Poverty Bay, simulating waves and current effects on the various initial designs options, and calibrated against data from a substantial field program, has been a fundamental tool. It was applied experimentally to determine the best option for the port layout, as well as to assess sedimentation impacts. Modelling results indicated a significant increase in maintenance dredging expected as a result of deepening the navigation approach channel. Because this may have an impact on the nearby sandy beach by inducing erosion, the best option for disposal of the sandy dredged material was determined to be disposal in the surf zone for subtidal beach profile renourishment. Textural analysis of the sediments trapped in the navigation channel demonstrated that they were suitable for this purpose. D 2002 Elsevier Science B.V. All rights reserved.

Wind‐wave development across a large shallow intertidal estuary: A case study of Manukau Harbour, New Zealand

Abstract Locally generated wind‐waves in estuaries play an important role in the sediment dynamics and the transport of biota. Wave growth in estuaries is complicated by tidally varying depth, fetch, and currents. Wave development was studied at six sites along a transect across Manukau Harbour, New Zealand, which is a large intertidal estuary with a tidal range of up to 4 m. Three meteorological masts were also deployed across the measurement transect to measure wave forcing by the wind. A spatial variation in wind speed by up to a factor of 2 was observed which has a significant effect on wave development at short fetches. The wind variation can be explained by the extreme change in surface roughness at the upwind land‐water boundary. The tidally varying depth results in non‐stationary wave development. At the long fetch sites wave development is dictated by the tidally varying depth with peak frequencies continuing to decrease after high water, whereas wave height is attenuated by bottom friction. The non‐dimensional energy and peak frequency parameters commonly used to describe wave growth, clearly exhibit depth limiting effects, but with wider scatter than in previous studies in simpler environments. The peak frequency predictions of Young & Verhagen (1996a) fit our data well. However, the wide variability of energy limits the usefulness of standard growth prediction curves in such situations, and highlights the requirement for a validated, shallow‐water numerical model.

Extreme wave predictions around New Zealand from hindcast data

Abstract A recently implemented wave hindcast for the New Zealand region was used in conjunction with wave‐buoy data to evaluate extreme significant wave height at multiple sites around New Zealand, for the first time. Hindcast storm wave heights were under‐predicted compared with wave‐buoy measurements at three inshore sites, and a method for scaling the hindcast data to improve the comparison of predicted extreme wave heights was explored. Different statistical methods for predicting extreme wave heights were also compared. Offshore, extreme wave heights displayed a north‐south and an east‐west gradient that is in keeping with the mean wave climate, with larger waves in the south and in the west. However, the variation of extreme wave heights between sites was less than the mean wave climate would suggest, because mid‐latitude depressions generate comparatively large waves on the generally more sheltered northeast coast. At the most energetic site to the southwest of the South Island, a 1 in 100‐year return significant wave height Hs (100) of 19.3 m and maximum wave height H max(100)> of 45 m were predicted. At the least energetic site to the northeast of the North Island, estimates of HS (100) = 13.9m and H max(100) = 33m were obtained.

Regional influence of climate patterns on the wave climate of the southwestern Pacific: The New Zealand region

This work investigates how the wave climate around New Zealand and the southwest Pacific is modulated by the Pacific Decadal Oscillation (PDO), El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Zonal Wave-number-3 Pattern (ZW3), and Southern Annular Mode (SAM) during the period 1958–2001. Their respective climate indices were correlated with modeled mean wave parameters extracted from a 45 year (1957–2002) wave hindcast carried out with the WAVEWATCH III model using the wind and ice fields from the ERA-40 reanalysis project. The correlation was performed using the Pearsons correlation coefficient and the wavelet spectral analysis. Prior to that, mean annual and interannual variabilities and trends in significant wave height (Hs) were computed over 44 years (1958–2001). In general, higher annual and interannual variabilities were found along the coastline, in regions dominated by local winds. An increasing trend in Hs was found around the country, with values varying between 1 and 6 cm/decade at the shoreline. The greatest Hs trends were identified to the south of 48°S, suggesting a relationship with the positive trend in the SAM. Seasonal to decadal time scales of the SAM strongly influenced wave parameters throughout the period analyzed. In addition, larger waves were observed during extreme ENSO and IOD events at interannual time scale, while they were more evident at seasonal and intraseasonal time scales in the correlations with the ZW3. Negative phases of the ZW3 and ENSO and positive phases of the IOD, PDO, and SAM resulted in larger waves around most parts of New Zealand.

The “Wahine storm”: Evaluation of a numerical forecast of a severe wind and wave event for the New Zealand coast

Abstract The “Wahine storm” was one of the two worst regenerating tropical cyclones to hit New Zealand last century. Ex‐tropical systems of this type are the major source of coastal hazards in the form of severe winds and waves in the New Zealand region. At the time, the likely weather associated with the storm was well forecast, although its expected path was not. Taking advantage of recently produced reanalysis data from the National Centres for Environmental Prediction (NCEP) and using the Regional Atmospheric Modelling System (RAMS) atmospheric model and WAve Model (WAM) wave model, we have simulated the atmospheric and wave conditions associated with the Wahine storm. Using modern computing methods, the path and storm intensity were quite well forecast although uncertainties in the initial positions of the low level tropical vortex and the upper level mid‐latitude trough lead to a 3‐h delay in the forecast arrival of the associated southerly change at Wellington. This illustrates the need for forecasters to realise the limitations of the tools they are using. The waves forecast around the coasts of New Zealand based on the winds from this atmospheric forecast were very good between North and East Cape and down the Canterbury coast, but the model failed to produce the 10–12 m seas experienced in Wellington at the time of the sinking of the TEV Wahine. The westward error in the storm track, though a relatively modest 50–100 km, is likely to have contributed to this under‐prediction. A comparison of 20 years of model hindcast waves round the New Zealand coasts with shorter periods of Waverider observations near Wellington indicated that the wave model at 1.125° resolution generally under‐predicts the full degree of variability of wave height seen during extreme wave events in the Wellington region. It is likely that limited spatial and temporal resolution of the wind and wave models is a limiting factor in this respect. It was found that significant improvement in the timing and heights of the forecast waves was achieved by interpolating between the 2.5° spatially resolved and 6‐h time resolved NCEP atmospheric data using a sophisticated weather prediction model. Provided the resolution of the winds driving the wave model was better than 1° and the frequency better than 1 h then large gains were made. Increasing the resolution to 0.333° increased the peak waves by a further metre but going to 0.1° made little difference except near sheltered coastal regions.