Tidal Currents in a Microtidal Estuary - Port Stephens, Australia part of Vignettes:Vignette Collection
Port Stephens is a popular tourist destination located on the mid-north New South Wales (NSW) coast, 230 km north of Sydney, Australia (Figure 1). Despite the regions popularity, it is not protected from the forces of nature and the wide-spread problem of coastal erosion that threatens the recreational value and infrastructure of the area (Figure 2). To successfully manage coastal erosion it is necessary to understand the processes responsible for the erosion, transportation and deposition of sediment. Port Stephens is an estuary, which is a term used to describe an area that is the interaction of freshwater and marine systems. In this particular setting the freshwater or fluvial influences are largely confined to the western basin and the marine influences dominate the eastern basin (Figure 1). Marine influences refer to the supply of sediment to the estuary and the processes (waves and tidal currents) which are responsible for eroding, transporting and depositing this sediment. A study is being undertaken to better understand these processes. The first step of this process is to record the tidal currents at representative locations throughout the eastern basin. This is achieved using two Acoustic Döppler Current Profilers (ADCP). These instruments are mounted on the sea floor and are capable of simultaneously recording the water velocity and direction at different depths above the instrument. This has been done at nine locations over a period of 13 months (Nov. 2007 to Dec. 2008). This data can then be used to characterise tidal flow patterns and infer sediment transport pathways. To characterise the tidal flow, the current velocity data has been depth-averaged and separated based on its flow direction. Currents flowing into the estuary (in this case, flowing to the west) are associated with a rise in the water level in the estuary and are referred to as 'flood' phase. Currents flowing out of the estuary (flowing to the east) and associated with a lowering of the water level in the estuary are referred to as 'ebb' phase. If we assume the amount of sediment transported by moving water is proportional to the cube of the water velocity, then it can be inferred that the tidal phase with the greatest mean velocity will transport a greater amount of sediment (Fitzgerald and Nummedal, 1983). The current velocities associated with each tidal phase were averaged over the duration of the instrument deployments. The mean flood current velocity was then subtracted from the mean ebb current velocity to give a residual current velocity (Figure 3). This residual indicates the phase (current direction) that has the greater mean current velocity and is also referred as the dominant phase. Finer sediments exhibit a lag between the flow velocity dropping below that needed to transport the sediment and the sediment being deposited. Thus, the duration that a current flows in a particular direction is also of significance when inferring sediment transport. The duration of each flow phase have been processed in the same way as the current velocity to produce a residual phase duration. Since both current velocity and duration play a role in the transport of sediment, the residuals of these two variables were combined to produce an index value. This index value represents the residual direction and magnitude of the tidal current flow. This can be visualised (Figure 4) and used to infer net sediment transport patterns prior to more detailed analysis. Some interesting results were observed from this study, most notably the occurrence of 'double dominance', whereby a greater mean velocity in a particular direction (phase) is paired with longer flow durations in the same direction (phase) (Figure 3). There is a prevalence of flood dominated flows over the shallower regions of the basin and ebb dominated flows at the entrance. These results indicate a complex interaction between the tidal circulation patterns and the estuarine morphology evident in the large sand shoals or 'flood-tide delta' that can be seen in the eastern region of the aerial photograph (Figure 4). Whilst these observations are preliminary, they are contributing to improving the understanding of the processes that drive changes in the morphology of estuaries. The morphology of the eastern estuary affects the processes that are responsible for the coastal erosion that is evident in Figure 2. This research is contributing to the understanding of the relationship between the processes and morphology of estuaries and how these affect the processes and morphology of related estuarine beaches. This understanding will better facilitate the successful management of these systems.
Effects of 4-Wheel Drive Vehicles on Beach Sediment Transport part of Vignettes:Vignette Collection
The use of 4-Wheel Drive (4WD) vehicles on coastal beaches is an activity that attracts considerable controversy amongst beach users. 4WD vehicles are currently permitted access to hundreds of beaches throughout Australia. The ecological and physical impact of 4WD vehicles on beaches is an area of steadily growing knowledge. The effects on dune vegetation and vertebrates have been the focus of many studies with fewer studies directed towards the effects on invertebrates and the physical disturbance to the beach. 4WD vehicles directly physically alter beaches by affecting the beach surface with tyre tracks. The distribution of these tracks is principally between the lower swash zone (extent of wave run-up) and the foredune. This zone is the source of sand for the creation, replenishment and growth of coastal dunes. Coastal dune systems play a substantial role in protecting and nourishing the beach and the areas behind it during and after erosion events. It was hypothesized that the tracks created as a result of driving a 4WD vehicle on a beach causes a change in the surface roughness that significantly disrupts the transport of sand from the beach to the dunes by the wind. It was thought that the vehicle tracks form a 'micro-catchment', trapping sand being transported across the beach and change the beach surface roughness, affect the airflow over the beach and the subsequent transport of wind-blown (aeolian) sand. A study was undertaken to quantify the effect of 4WD vehicle tracks on the amount of sand transported from the beach to the dunes. The first experiment involved simultaneously measuring the amount of sand transported on an unaffected section of beach and an adjacent section of beach with a single vehicle track (Figure 1). This experiment was repeated, increasing the number of times the vehicle drove along the track in 25 pass increments, up to 100 vehicle passes. The results of this experiment indicated that the vehicle track did affect the amount of sand transported (Figure 2). The mean weight of sand trapped on the section with vehicle tracks was consistently lower than the unaffected section. This experiment however did not allow the comparison between the number of times the vehicle drives along the track as the measurements were taken at different times, and therefore under different conditions. This lead to a second experiment being conducted. The second experiment was designed in such a way that the results would be statistically comparable. This involved artificially creating the wind using a garden blower and using replicate study plots (Figure 3). An analysis of variance (ANOVA) was conducted on the measurements taken and it was determined that vehicle tracks result in a significant reduction in the amount of sand transported compared to an unaffected section of beach (Figure 4). There was not a significant difference between the number of times the vehicle drove along the track. This experiment concluded that the tracks caused by 4WD vehicles resulted in a significant reduction in the amount of sand transported when compared to an unaffected section of beach. This experiment is however limited in two aspects. The first is the duration over which the measurements were taken. Due to the size of the sand traps used, the experiments could not be conducted for greater than 5 minute intervals because the traps would reach maximum capacity. It was observed that after longer periods of time the vehicle tracks in-filled, thus reducing their impact (Figure 5). Secondly, the experiments were conducted across a single vehicle track, with multiple passes, however it is commonly observed that under normal use, multiple vehicle passes result in multiple tracks. These limitations are acknowledged, but the significant results obtained indicate the potential implications of vehicle tracks on the transport of sand from the beach to the dunes. Further research is required into the physical impacts of 4WD vehicles on beaches not only to address the limitations of this study, but to address other gaps in the current knowledge in order to insure the successful management and sustainable use of coastal beaches by 4WD vehicles.