Lb Mason

The structure of reef fish metapopulations:modelling larval dispersal and retention patterns

An improved understanding of the dispersal patterns of marine organisms is a prerequisite for successful marine resource management. For species with dispersing larvae, regional–scale hydrodynamic models provide a means of obtaining results over relevant spatial and temporal scales. In an effort to better understand the role of the physical environment in dispersal, we simulated the transport of reef fish larvae among 321 reefs in and around the Cairns Section of the Great Barrier Reef Marine Park over a period of 20 years. Based on regional–scale hydrodynamics, our models predict the spatial and temporal frequency of significant self–recruitment of the larvae of certain species. Furthermore, the results suggest the importance of a select few local populations in ensuring the persistence of reef fish metapopulations over regional scales.

Modelling wind driven circulation One Tree Reef, Southern Great Barrier Reef

A vertically integrated numerical model of wind-driven circulation at One Tree Reef is presented. The model is the numerical model SURGE developed originally to study tropical cyclone surge. Current data collected in the reef lagoon and over the reef flats is used to test the models applicability. The reef topography has been modelled explicitly, rather than using an assumed reef shape, with a grid spacing of 150 m. The model corresponds well to the measured current behaviour. The greatest drawback to use of the model is that, at low tide, currents reverse with depth due to lagoon enclosure and a depth integrated model cannot produce water velocity depth profiles. The model can be used to predict current behaviour in strong wind conditions, such as during a cyclone, and to estimate net flows into and out of the lagoon.

A wave model for the Great Barrier reef

A new wind wave generation model, WAMGBR, is presented that has been adapted from WAM especially for use in the complex geometry of the Great Barrier Reef. A technique (reef parameterization) has been presented that incorporates sub-grid scale dissipation caused by coral reefs. Three other improvements to WAM have been proposed. An explicit/implicit finite difference scheme has been implemented that allows for more efficient modelling (longer time steps) while maintaining diffusive characteristics that are at least as good as those of WAM. An offset in discrete angles creates more uniform diffusive characteristics. And, a transformed spherical coordinate system allows for more efficient grid sizes and smaller grid dependent refraction. Comparisons between modelling techniques and between model and measured data show that WAMGBR produces very good results in the difficult challenge of modelling both non-cyclonic and tropical cyclone waves in the geographically complex environment of the Great Barrier Reef.

Reef parameterisation schemes with applications to tidal modelling

A variety of analytical models is used to investigate the effects on tidal propagation of a barrier reef system. These models specify reef geometry by two parameters. They can accommodate cases where water flows over reefs, as well as through inter-reef gaps, and also incorporate quadratic bottom friction. Although based on a one-dimensional approach, adaptations of a solution by Huthnance are used to account for the additional blockage effects associated with two-dimensional flow patterns near reef barriers. The present work adopts the philosophy that only a numerical approach can cope with the wide variations in reef geometry that are encountered in areas such as the Great Barrier Reef (GBR) region of Australia. Moreover, since typical model grids cannot resolve inter-reef gaps and other features with sufficient accuracy, a parameterised approach is needed to accommodate the conflicting demands of reef geometry and an economically feasible model resolution. The formulation of the analytical models is such that they can be applied immediately to standard numerical algorithms. Numerical experiments for flow in a channel, with a reef barrier across its centre, are used to test the parameterisation schemes. Comparison of the results for parameterised reefs with those obtained using extremely fine grids, shows convincing evidence of the success of the schemes. A separate method for automatically generating reef parameters has simplified the task of applying the methodology to real reefal systems. A tidal model of the Southern GBR, a region which exhibits unusual tidal behaviour, but which also has ample field data available for model testing, is used to demonstrate the accuracy that can be attained with the parameterised approach. Although tides are considered specifically in the present work, the formulation should be applicable with equal ease to the many other significant classes of low frequency motions in the GBR.

Modelled response of Gulf St Vincent (South Australia) to evaporation, heating and winds

In Gulf St Vincent, Australia, the salinity of the head waters can exceed 42 in summer when evaporation is maximum and the rainfall is minimal. A depth-integrated implicit finite difference model is extended to simulate the summer–autumn evolution of salinity, temperature, and density distributions, with climatological evaporation, rainfall, air temperature, and wind stress as inputs. Advection of salt and heat by the density- and wind-driven circulation is modelled by the QUICK scheme, whereas horizontal mixing by tidal circulation is parameterised by a dispersion coefficient related to the oscillatory vertical shear. Simulated distributions and seasonal variations compare well with available observations, which feature the flow of highly saline water along the eastern side of the gulf, while the western side is bathed by less saline shelf water. Model results show that, despite the increasing salinity gradients in summer, opposing temperature gradients can stifle the shelfward density currents in the southern parts of the region. Autumn cooling intensifies these density currents so that at the end of the season the flushing of highly saline water, accumulated in the gulf throughout the summer, is enhanced. It was found that the variability in the general circulation brought about by the directional variability in the prevailing winds is an important factor in maintaining the observed salinity distributions in the region.

Resilient reefs may exist, but can larval dispersal models find them?

The Great Barrier Reef (GBR) is an interconnected system of thousands of coral reefs and shoals spanning more than 2,000 km of the eastern Australian coastline. Anthropogenic pressures—principally climate change—pose an existential threat to this iconic marine ecosystem [1]. Management actions are urgently required to halt and reverse the degradation, but the GBR’s enormous size creates logistical and budgetary challenges. In a recent research article, Hock and colleagues [2] offer a solution to this predicament: They argue that a tiny fraction of the GBR’s reefs—fewer than 1%—act as its ‘life-support system’ [3]. The reefs are primarily identified by their larval connectivity, the movement of juvenile individuals between reefs on ocean currents. A dispersive larval stage is common to many species on coral reefs—notably fishes and reef-building corals—and is an essential source of new recruits following mortality and disturbance. Unfortunately, because larvae are difficult and expensive to follow during their pelagic dispersal phase, empirical data are not available at management-relevant scales [4]. Instead, Hock and colleagues base their decisions on ‘biophysical models’—computer simulations that integrate individual-based models of larval behaviour with hydrodynamic ocean current models. In recent years, biophysical models have become a standard tool for investigating the effects of larval connectivity on marine ecology, evolution, and conservation [4–6]. Because their biophysical model predicts that a small number of reefs contribute disproportionately to the GBR’s persistence and resilience, Hock and colleagues claim that these reefs should be a primary focus of management resources. In this comment, we show that the identity of these reefs should be treated with caution, since different numerical models of larval connectivity select different reefs as priorities. More generally, we argue that the current generation of biophysical models should not be used to guide management actions at the scale of individual reefs. The last decade has seen rapid advances in the sophistication of biophysical models. Highperformance computing allows these models to simulate billions of larval releases across thousands of kilometres, while their biological components now incorporate experimentally measured sensory and swimming capabilities. However, despite these strengths, accurate numerical modelling of shallow coastal flow fields with highly variable bathymetry—conditions typical of coral reefs—remains an immense challenge. Critical near-reef hydrodynamics often vary at substantially smaller scales (1 m–1,000 m) than the resolution of biophysical models (100 m–10,000 m; [7]). Such difficulties are exacerbated by uncertainty about the parameters that describe larval development, behaviour, and mortality, as well as adult spawning behaviour [6]. These limitations have been debated extensively [8]; given this level of uncertainty, we have strong reservations about whether existing biophysical models are

CORRESPONDENCE Comments on ''Estimation of Tropical Cyclone Wind Hazard for Darwin: Comparison with Two Other Locations and the Australian Wind-Loading Code''

Cook and Nicholls recently argued in this journal that the city of Darwin (Northern Territory), Australia, should belocated inwindregion D rather than in the currentregion C in the Australian/New Zealand Standard AS/NZS 1170.2 wind actions standard, in which region D has significantly higher risk. These comments critically examine the methods used by Cook and Nicholls and find serious flaws in them, sufficient to invalidatetheirconclusions.Specificflawsinclude1)invalidassumptionsintheiranalysismethod,includingthat cyclones are assumed to be at the maximum intensity along their entire path across the sampling circle even after they have crossed extensive land areas; 2) a lack of verification that the simulated cyclone tracks are consistent with the known climatological data and in particular that the annual rate of simulated cyclones at each station greatly exceeds the numbers recorded for the entire Australian region; and 3) the apparent omissionofkeycycloneswhencomparingtheriskatDarwinwithtwootherlocations. Itisshownherethatthe number of cyclones that have affected Port Hedland (Western Australia), a site in Australia’s region D, greatly exceeds the number that have influenced Darwin over the same period for any chosen threshold of intensity. Analysis of the recorded gusts from anemometers at Port Hedland and Darwin that is presented here further supports this result. On the basis of this evidence, the authors conclude that Darwin’s tropical cyclone wind risk is adequately described by its current location in region C.

SS:Metocean "Extreme Hurricane Design Criteria for LNG Developments: Experience Using a Long Synthetic Database"

Tropical cyclones (hurricanes) dominate the extreme wave climate off North West (NW) Australia between latitudes 5 to 25 degrees south, with a storm occurrence rate of 4.5 storms per season. On average, two of these storms attain Category 5 intensity. The NW Australian region of interest has an area equal to the northern half of the Gulf of Mexico, and it appears that hurricane return period wave height criteria in the southern part of the NW Australian region (approximately 20 degrees south), produce return period wave heights of the same magnitude as for the central region of the Gulf of Mexico. A reliable historical hurricane database for that region exists only for the post-satellite era (since 1970), consequently the historical storm database is relatively short. Use of short historical storm databases for forecasting storm wind and wave fields, and application of extrapolation methods (e.g. peak-over-threshold) for determining return period estimates out to say the 100 year return period are well founded. However extension of this method to obtain estimates at very low probabilities (e.g. to the 1 in 10,000 year return period) involves extrapolating the tail of the distribution. It is recognized this provides highly uncertain estimates at low probability levels. To overcome the problem of extrapolation, a synthetic database has been developed consisting of approximately 450,000 individual synthetic storms (100,000 years) to represent the hurricane population of the study region. This paper describes development of the synthetic tropical cyclone (hurricane) storm database, and the distinct advantages that it offers as the associated time histories of the wind and wave fields are available at all return periods. This allows examination of storm and wave characteristics at say the 100, 1000 and 10,000 year levels, and application of the storm time histories to obtain response-based criteria. Introduction Generally metocean design criteria are established using a database of historical hurricane tracks with central pressures and use of wind and wave field model simulations. Wind and wave maxima from each storm are then subject to extrapolation in logarithmic space. This method has a series of deficiencies. Use of different extreme value methods (e.g. Weibull, Pareto), different fitting techniques (e.g. method of moments, maximum likelihood), and choice of data truncation level, result in different return period estimates, for the same data set at long return periods. Here, the primary deficiency of this method is that it involves extrapolation of the tail of the distribution, where there are few data points. This results in large confidence intervals on the extrapolated return period curves. In addition, implicit in the extrapolation method is that the behavior of the wind and wave fields do not change from the population of historical storms to that of rare storms. For example, in continental shelf water depths, the effects of wave refraction and water depth wave height limitations present in the hindcasts of historical storms, are not taken into proper account in the extrapolation process. This paper reports on the results and application of a synthetic tropical cyclone study which largely removes difficulties associated with extreme value analysis methods. The 10 -4 Waves Study (synthetic hurricanes) has been successfully applied to set metocean design criteria for numerous offshore oil and gas facilities in the NW Australia region. The study results have shown significant advantage in a number of areas:  Selection of independent extremes without resort to extreme value analysis  Selection of robust joint wind and wave factors (relationships)

Generating Synthetic Tropical Cyclone Databases for Input to Modeling of Extreme Winds, Waves, and Storm Surges

The attack of a severe tropical cyclone at any location is a rare event; therefore, a long data record is necessary in order to determine the characteristics of the population of storms that can affect a location. Unfortunately, reliable and complete data of tropical cyclone tracks and central pressures are not nearly long enough to define the severe end of the distributions. To mitigate this problem of the lack of data in the two Australian tropical cyclone regions a state-of-the-art modeling system has been developed and deployed in three projects, two in the Coral Sea (Hardy et al., 2003, 2004) and one in the Northwestern Australia waters. The Coral Sea studies produced a set of 3,000 years of synthetic tropical cyclones and then simulated the winds, waves, and storm tides. Three climate change scenarios were also modeled. The Northwestern study was much more ambitious, modeling 100,000 years of tropical cyclones to obtain robust measures of the 100-10,000 year return periods of wind and wave conditions. The modeling system required development and/or adaptation of a series of models: (a) synthetic tropical cyclone model, (b) parametric wind field model, (c) wave model, and (d) storm surge and current model. This modeling technique could be applied to any tropical cyclone region to provide input to wind, wave, storm surge, erosion, rainfall, and flood routing models.

SIMULATION OF TIDES AND STORM SURGES IN THE GREAT BARRIER REEF REGION

This report will update the coastal zone practitioner on the National Flood Insurance Program (NFIP) as it affects the implementation of manmade changes along the coastline. It is our intent to place in proper perspective this fast-changing and often difficult to interpret national program. Readers will achieve an overall understanding of the NFIP on the coast, and will be in a position to apply the programs requirements in their efforts. We will begin with a history of the application of the NFIP to the coastal zone. The history of the problems encountered will lead into current regulations, methodologies, and the changes the Federal Emergency Management Agency plans for the future.The spatial variability of the nearshore wave field is examined in terms of the coherence functions found between five closely spaced wave gages moored off the North Carolina coast in 17 meters depth. Coherence was found to rapidly decrease as the separation distance increased, particularly in the along-crest direction. This effect is expressed as nondimensional coherence contours which can be used to provide an estimate of the wave coherence expected between two spatial positions.Prediction of depositional patterns in estuaries is one of the primary concerns to coastal engineers planning major hydraulic works. For a well-mixed estuary where suspended load is the dominant transport mode, we propose to use the divergence of the distribution of the net suspended load to predict the depositional patterns. The method is applied to Hangzhou Bay, and the results agree well qualitatively with measured results while quantitatively they are also of the right order of magnitude.