Our modelling group has also recently completed small-scale modelling in Cockburn Sound, Bass Strait, Port Phillip Bay, and the Derwent Estuary. In these cases, model results are being used to estimate the impact of coastal construction on flushing time, compare the distribution of sewer effluent for various outfall designs, and drive ecological models. For these applications, we often use particle tracking to determine concentrations of nutrients, toxicants, suspended-sediments, or other tracers. In the Port Phillip Bay application, the calibrated hydrodynamic model used to determine daily exchanges among 59 boxes for 10 years. These exchanges were used to efficiently force complex (and computationally expensive) ecological models. This approach enabled us to make many more runs of the ecological models to explore model parameters and management scenarios.

Internal waves are seen as both semi-diurnal internal tides and nonlinear wave forms such as bores and internal solitary waves.

• form from large amplitude internal tide
• produce strong currents and possibly mixing
• Korteweg de-Vries modelling used to study the evolution of the internal tide into internal solitary waves
• predicted waveforms are dependent on background stratification, shear flow, initial wave amplitude, rotation and frictional dissipation

• forms over the slope region where internal wave characteristics have similar slopes to that of the topography and where the barotropic tide flows up and down slope
• strong currents form, often intensified near the seabed
• difficult to obtain comprehensive data sets showing spatial and temporal variability of the internal tide
• results from 1995 Franklin cruise show internal tide in the North Rankin region as most energetic over upper slope between 100 and 400 m depth

• first modelling for 2D cross sections over the shelf
  comparisons to data for the North Rankin region tend to underestimate amplitude
  of the vertical displacements
• 3D regional model is under development
• shows increasing baroclinic energy moving from NW Cape towards the Timor Sea
• model predicts a complex 3D field of energy flux with changes from onshore to offshore over relatively short distances
• predictions for the North Rankin region show strong 3D dependence.

Work is underway modelling sediment transport processes by internal tides.

ESA ERS1&2 and NASA TOPEX Extended Mission projects, ESA ENVISAT-1 project proposed.

NW Shelf transect between Scott Reef and Adele Is., and shelf between Rowley shoals and King Sound.

Intensive internal wave observations during 1994/5 summer with deployment of extra instruments at the long-term shelfbreak current mooring site. A thermistor chain and several high capacity S4 CM accompanied the long-term mooring with enhanced sampling rates to resolve super-tidal internal waves, internal solitons and surface waves. Shipboard hydro-survey data prior, post and during deployment from a succession of AIMS cruises on board the RV Lady Basten and O.R.V Franklin (led by Miles Furnas).

Time series of thermistor chain and S4 CM data over 3-month period during summer; sampling rates of 2-5 minutes showing recurring internal tide and internal soliton wave train features. A variety of 100 x 100 km Synthetic Aperture Radar images have been acquired both during the intensive deployment and at other times. These show both linear and radially-symmetric internal wave packets evidently generated by the interaction of the barotropic tide with the shelfbreak and reef/island features. Thermistor data show water is locally homogenised during the passage of larger wave groups. The SAR imagery suggests the internal tide evolves into soliton groups then possibly an internal tidal bore. A numerical hydrodynamic model has been implemented in a 2d vertical slice mode (with realistic topography) and in a fully 3d mode (with idealised topography). The results are being compared with the features seen in the SAR imagery.

The topographic complexity of the Scott Reef area (particularly the slope) demands a fully 3d approach to modelling, and inclusion of deep topography. It could also yield multiple generation sites (near critical slopes) and produce interfering wave patterns. Scott Reef appears to block shoreward-propagating internal tidal waves generated seaward of the reef; so scattering is an issue. Wavelengths predicted by the model differ from those in SAR images for reasons not yet determined. A nonhydrostatic model appears to be required to adequately represent the dynamics and evolution of the soliton wave trains from the internal tidal waves. We have insufficient CTD data to fully specify temporal changes in the stratification.

A 2D vertical slice (x-z) numerical hydrodynamic model has been implemented in collaboration with Prof. Kerry Black (NIWA, NZ). The model captures the internal tide generation process (it can reproduce results obtained by Holloway on his Dampier transect). It has good resolution (Horiz: 750 m, Vert: 2.5-5 m), but cannot reproduce the evolving solitons. A 5-layer 3d model with an idealised flat bottom and a bump have been used to demonstrate radial spreading of internal tidal waves, but again the solitons are not present.

Analyse SAR imagery from a variety of sites, including the Sahul shelf and Dampier- Ningaloo areas and compare with historical current meter data (with Holloway et al.). Implement 2 and 3d quasi- and fully non-hydrostatic models to capture the soliton group dynamics. Use ENVISAT-1 to expand the SAR database and map the distribution and transformation of energy residing in the barotropic and baroclinic tide.

A unique field study was used to trial the application of the model "MUDMAP" to investigate the dispersion of produced formation water (PFW) from the offshore petroleum platform "Harriet A" on Australia’s Northwest Shelf. In September 1995, scientists from the Australian Institute of Marine Science and the Australian Geological Survey Office collected extensive and detailed field data on the fate and dispersion of (PFW) discharges from "Harriet A. The survey measured critical hydrographic conditions in conjunction with fine resolution measurements of both the horizontal and vertical concentrations of produced water elements such as benzene and toluene. These datasets provided a unique opportunity to calibrate and verify advanced dispersion models such as MUDMAP for Australian environmental and operational conditions of produced water discharges.

From this data set, it was possible to define both the horizontal diffusion parameter and the vertical diffusion parameters that characterise mixing for the region. This was achieved by comparing the model predictions with the field distributions of benzene at different phases of the ebb tidal cycle. Consequently the horizontal diffusion parameter was estimated to be 0.5m²/s and the vertical diffusion parameter was estimated to be 0.005 m²/s. The model, using these parameters, were then used to successfully validate a flood tide plume, lending support to the relevance of these parameters in describing the mixing of PFW in this region.

MUDMAP was also run using (a) actual point source current meter data near the platforms and (b) with predicted current data from a fine-resolution and verified 3 dimensional hydrodynamic model (GCOM3D) over the whole region. This comparison revealed that both techniques provided good predictions of the near platform PFW distributions, but that the point source current meter data was not useful for farfield predictions, since non-uniform current fields often occur in coastal waters.

The MUDMAP simulations, using the "GCOM3D" 3 dimensional hydrodynamic model, were validated for the plume position and concentration. This validation exercise provided measurable tolerances or error bars to plume predictions using a predicted current field that changes with space and time. These tolerances were measured against the field data to be within a factor of 2 for estimates of concentrations near the platform and within 400 m in terms of plume position overall. It is important to note that the accuracy of the predictions was made possible due to the high spatial resolution of benzene concentrations measured in the field survey.

Given the ability of the MUDMAP system to predict the field observations over a range of current speeds, the model was used to look at mechanisms leading to the overall variability of the plume distribution and concentration. The MUDMAP system predicted that nearfield plume concentrations may vary significantly as a function of the current speed, and hence concentrations will vary throughout the tidal cycle. Further, due to the oscillatory nature of tidal flows, previously dispersed PFW water may return in later tidal cycles; thus ‘second dosing’ of this water is possible. Given this finding, it is predicted that PFW concentrations will be measurably higher and localised around the platform during the neap tides and light wind conditions.

MUDMAP was used to help understand the mechanisms for creating the observed surface micro-layers or ‘slicks’. The tests with the MUDMAP Near Field Model indicated that the low dilutions of PFW measured in the surface layer occurred due to water being discharged with minimal momentum. This would be the case for the spray that occurs with above water discharge of PFW. The MUDMAP model predicted that the spray from an above water PFW discharge pipe, as was observed from "Harriet A" platform during the study period, would not mix with the same vigour as the main discharge water due to the loss of its discharge momentum. Hence the model predicted that the PFW spray would remain on the surface with the 1:100 to 1:500 dilutions reported in chapter 2.

Overall, this study highlighted that by validating the key variable, that is, the plume concentration and position in both the horizontal and vertical dimension, makes MUDMAP an effective management tool for defining potential impacts. This is achieved by using MUDMAP to investigate the range of potential PFW dispersion patterns possible around "Harriet A". Collectively, these simulations would provide measures of best case and worst case scenario’s which can be used for understanding potential ecological impacts. Further, MUDMAP can readily estimate the influence of changing discharge practices, such as: altering pipe width and/or depth, improving oil reduction prior to release, etc. Such features, in a modelling system such as MUDMAP, are essential for decision support to platform managers in their efforts to minimise the environmental impacts of offshore platform operations.

Oceanographic field studies were carried out, in August-October 1993, of the water circulation around Scott Reef. Macro-tides generated 60 m internal waves around Scott Reef mainly at the semi-diurnal frequencies (Wolanski and Deleersnijder, in press). Modelling and the field data suggested that the internal waves were locally generated in the absence of island wakes and rotated counter-clockwise around the island while radiating energy. The wave frequencies were not restricted to the diurnal and semi-diurnal tidal frequencies that dominated the sea surface fluctuations, the energy at the inertial frequency was negligible. This suggests that oceanic islands are internal wave generators and may contribute to the oceanic variability even far away. This study will be extended to compare the performance of the classical 3-D numerical models of oceanographic circulation (Princeton POM, Hamburg HAMSOM, AIMS-AIMS, NOAA MECCA) for baroclinic flows around an island, in parallel to an earlier study at AIMS of their performance for 3-D barotropic flows around islands (Galloway et al. 1996). The results will be used to evaluate recruitment of fish larvae following the technique of Wolanski and Sarsenski (1997) and Wolanski et al. (1997). These waves are believed to be important for upwelling nutrients to the corals of Scott Reef provided the upward motions generated at depth by the internal waves continues in the top 40m. Such a mechanism may be the pumping along the reef slope resulting from the breaking of surface waves at the reef crest, as in oceanic islands of French Polynesia (Wolanski and Delesalle 1995). To evaluate this process PHRI-AIMS collaborative laboratory studies are presently evaluating the circulation in the spur-and-groove system of coral reefs.

Oceanographic studies were carried out in October 1997 of the water circulation and fine sediment dynamics in King Sound. Further field studies are planned in September 1998. Modelling follows the techniques of Wolanski et al. (1996) and Guan et al. (1998) with new processes included to accommodate the huge tides of King Sound. Modelling and the field data suggest that the colloidal mud dynamics are primarily controlled by the tidal asymmetry and flocculation processes. These generate a turbidity maximum zone in the Derby area. This model is extended to chemicals in interacting dissolved and particulate phases following the technique of Wolanski et al. (in press).

References

Wolanski E, King B, Spagnol S (in press). The implication of oceanographic chaos for coastal management. In Salomons W (Ed), "Integrated Coastal Management", Springer-Verlag.

Ningaloo reef is the largest fringing coral reef system in Australia, extending 300 km from Gnarraloo Bay to Point Murat. Shallow (2-4m depth, on average) lagoons are protected from the full force of oceanic swells, seas and currents by the fringing reef flat, where waves break and much of their energy is dissipated. The proximity of the fringing reef to the coast ranges from 200m to 7 km. The Ningaloo Reef study was undertaken to obtain a comprehensive data set on the physical oceanographic processes that influence the circulation and flushing of the reefs in the Ningaloo region. The data obtained will be utilised to verify theoretical and numerical models of these oceanographic processes, such that these models can be generically applied to the Ningaloo Reef region as a whole.

The study site was chosen such that it fulfilled two criteria:

1. the reef system had to be typical of the other reefs in the area: a series of reefs running approximately parallel to the coast separated by deep, narrow channels, and separated from the coast by wide lagoons open at both the north and south ends.
2. the area had to be sufficiently small to allow adequate coverage by the observational instrument array.

The site chosen was the fringing reefs between S21° 48'.5 E114° 05'.4 and S21° 49'.8 E114° 04'.4.

This location consisted of two reefs, each approximately 1km long, separated by a 100m wide channel. A shallow, 500m wide lagoon separated the reefs from the beach. Both reef faces had slopes of approximately 1 to 25 and rose from a depth of 10 m to between 0 m and 1 m on the reef top.

Instrument locations were chosen to observe all physically significant, short-term oceanographic processes that occur in, and influence the study region. An observational instrument array consisting of three Acoustic Doppler Current Profilers, six S4 Vector Averaging Current Meters (four of which were capable of wave measurements), one SeaPac 2100 Wave Gauge and four Water Level Recorders was deployed in the study region for a fourteen day period from November 26 until December 11, 1997. Additionally, one Waverider buoy was deployed well offshore and approximately 2.6 nautical miles north of the study region.

Instruments were deployed as follows:

• An S4 with an internal water level sensor was located in the northern entrance to the northern lagoon.
• An S4 and Water Level Recorder were located in the southern entrance to the southern lagoon.
• An S4 Current meter/Wave gauge was deployed in the channel between the two reefs.
• A Water Level Recorder was located within the lagoon, but in line with the channel between the two reefs, to provide water level data within the lagoon relative to the water levels observed at both the Northern and Southern entrances.
• In the deeper water immediately seaward of both the Northern and Southern reefs, two tide gauges and two Acoustic Doppler Current Profilers were located to observe the water level and current structure in front of the reefs, and give some indication of the flow of water towards the reef front.
• Current meter/Wave gauges were deployed near the reef break, and slightly landward of the reef break.
• The waverider buoy was deployed offshore to observe swell incident on the study area.

Bathymetry data was recorded using a downward looking Acoustic Doppler Current Meter mounted on the transom of a small boat, and a DGPS positioning system. Meteorological data was obtained from AIMS’ existing automatic weather station at the Milyering Visitor Centre, 30km south of the study region. 

Results

A consistent flow over the reef top into the lagoon was evident from the reef top data. Currents through the deep channel and through the southern opening of the lagoon were directed out of the lagoon, regardless of the phase of the tide. Current direction through the northern lagoon entrance were more variable, and rarely into the lagoon (towards the west south-west). In particular, flow through the southern opening of the lagoon was invariably against the prevailing strong south-westerly winds (consistently >20km/hr). In contrast, currents in front of the reef display some periodic reversing and amplitude modulation consistent with tidal flow. Current meter data shows no significant evidence of inflow into the lagoon other than via flow over the reef top. Although a water mass balance for the reef-lagoon system, calculated from observed currents, is not presented here, it is apparent from the combined flux of water out of the lagoon that wave pumping over the reef top is a very significant process for reef circulation. The continual outflow of water from the lagoon, independent of tidal phase, would indicate that wave pumping is a more important process than tidally driven circulation in reef flushing.

Luciano Mason¹, Stan Massel², Lance Bode¹, Richard Brinkman²
(¹Dept Civil & Environmental Engineering, James Cook University, Townsville
²Australian Institute of Marine Science, Townsville)

Collaboration between AIMS and JCU

The study region is confined to Exmouth Gulf. The modelling domain extends off the continental shelf to move open boundaries away from the immediate vicinity of the Gulf mouth.

To perform a basic modelling study which identified the predominant hydrodynamic characteristics of the Gulf. This will help in planning future work and give a preliminary indication of pathways of water borne materials.

• Tidal analysis of water height data collected (AIMS) in the Gulf,
• Brief analysis of available wind data,
• Currently results include a model calibrated for the major semi-diurnal and diurnal tidal constituents, and a hindcast simulation of tidally- and wind-driven currents, using a 3D sigma coordinate model.

Further calibration and verification of the model using ADCP and other data collected in the region.