Associate Professor Hannah Power

Understanding swash zone dynamics is of crucial importance for coastal management as the swash motion, consisting of the uprush of the wave on the beach face and the subsequent downrush, is responsible for driving changes in the beach morphology through sediment exchanges between the sub-aerial and sub-aqueous beach. Improved understanding of the probabilistic characteristics of these motions has the potential to allow coastal engineers to develop improved sediment transport models which, in turn, can be further developed into coastal management tools. In this paper, novel descriptors of swash motions are obtained by combining field data and statistical modelling. Our results indicate that the probability distribution function (PDF) of shoreline height timeseries (p(¿)) and trough-to-peak swash heights (p(¿)) measured at a high energy, sandy beach were both inherently multimodal. Based on the observed multimodality of these PDFs, Gaussian mixtures are shown to be the best method to statistically model them. Further, our results show that both offshore and surf zone dynamics are responsible for driving swash zone dynamics, which indicates unsaturated swash. The novel methods and results developed in this paper, both data collection and analysis, could aid coastal managers to develop improved swash zone models in the future.

We present the result of a collaborative priority setting exercise to identify emerging issues and priorities in coastal geoscience and engineering (CGE). We use a ranking process to quantify the criticality of each priority from the perspective of Australian CGE researchers and practitioners. 74 activities were identified across seven categories: Data Collection and Collation, Coastal Dynamics and Processes, Modelling, Engineering Solutions, Coastal Hazards and Climate Change, Communication and Collaboration, and Infrastructure, Innovation, and Funding. We found consistent and unanimous support for the vast majority of priorities identified by the CGE community, with 91% of priorities being allocated a score of = 3 out of 5 (i.e., above average levels of support) by = 75% of respondents. Data Collection and Collation priorities received the highest average score, significantly higher than four of the other six categories, with Coastal Hazards and Climate Change the second ranked category and Engineering Solutions the lowest scoring category. Of the 74 priorities identified, 11 received unified and strong support across the CGE community and indicate a critical need for: additional coastal data collection including topographic and bathymetric, hydrodynamic, oceanographic, and remotely sensed data; improved data compilation and access; improved understanding of extreme events and the quantification of future impacts of climate change on nearshore dynamics and coastal development; enhanced quantification of shoreline change and coastal inundation processes; and, additional funding to support CGE research and applications to mitigate and manage coastal hazards. The outcomes of this priority setting exercise can be applied to guide policy development and decision-making in Australia and jurisdictions elsewhere. Further, the research and application needs identified here will contribute to addressing key practical challenges identified at a national level. CGE research plays a critical role in identifying and enabling social, environmental, and economic benefits through the proactive management of coastal hazard impacts and informed planning to mitigate the potential impacts of growing coastal risk, particularly in a changing climate. The prevalence and commonalities of the challenges faced by coastal communities globally due to increasing pressures from coastal hazards in a changing climate suggest that our findings will be applicable to other settings.

Understanding of breaking and broken waves is key for the prediction of nearshore sediment transport and coastal hazards, however the difficulty of obtaining measurements of highly unsteady nearshore waves has limited the availability of field data. This paper reports on a novel field experiment designed to capture the time-varying free-surface throughout the surf and swash zones was conducted on a dissipative sandy beach using an array of 2D LiDAR scanners. Three scanners were deployed from the pier at Saltburn-by-the-Sea, UK for a 6 day period to monitor the surface elevation of nearshore waves from the break point to the runup limit at temporal and spatial resolutions (order of centimetres) rarely achieved in field conditions. The experimental setup and the procedure to obtain a continuous time series of surface elevation and wave geometry is described. A new method to accurately determine the break point location is presented and compared to existing methodologies.

Rip currents are a significant hazard on global surf beaches and are a factor in hundreds of drowning fatalities each year. Contemporary rip current safety information often idealises rip currents as part of a Transverse Bar Rip (TBR) morphology with rip channels bound by shallow, shore-connected bars. Real-world conditions frequently differ from this model, with potential implications for rip current escape strategies promoted to, and undertaken by, the general public. This study describes outcomes of rip current escape strategies conducted at North Cronulla Beach, NSW, Australia, over two distinct morphologies; a mixed Low Tide Terrace/Transverse Bar Rip (LTT/TBR) and a Rhythmic Bar Beach (RBB) system lacking shore-connected bars. Swimmers attempted to escape by adopting one of three pre-determined strategies: Stay Afloat, Swim Parallel and Swim Onshore. A total of 100 escape attempts were conducted, with the RBB system experiencing longer duration (t¯¿=¿2.4¿min) escapes than the LTT system (t¯¿=¿0.8¿min). The RBB system was associated with a higher rate of action failure, particularly for Stay Afloat, due to a lack of shore connectivity of adjacent bars. Swim Parallel was of lower duration (t¯RIP1¿=¿0.66, t¯RIP2¿=¿2.68¿min) in both systems, but durations and distances to safety in the RBB system often exceeded swimming abilities of weaker bathers. Although Swim Onshore was more successful (t¯RIP1¿=¿0.22, t¯RIP2¿=¿1.65¿min) than Swim Parallel, promotion of such a strategy is strongly discouraged in conventional safety advice. Results suggest that contemporary rip current escape strategies may be inappropriate in non-TBR rip current systems and that alternative strategies should be considered, including Swim Onshore and a greater focus on preventative strategies, particularly in relation to bathers with limited swimming ability.

Waves are the main hydrodynamic force acting on coral reefs and are crucial in driving the evolution of reef systems. Previous research has mainly focused on wave breaking and transformation on the reef flat but often neglected the spatial variation of reformed wave characteristics once they have propagated into the back-reef environment. This study examines wave conditions on a reef flat and the adjacent back-reef sand apron specifically focusing on the transformation of wave height, period and spectra (including changes to long-period and incident wave height). The ability of reformed waves in the back-reef environment to entrain sediment is also investigated. Wave conditions were found to be distinctly different on the sand apron compared to the reef flat, with the majority of wave energy dissipated during initial breaking and transformation. Almost all incident waves are dissipated on the reef flat under a depth threshold of 0.5. m before reaching the back-reef with long-period motions dominating in the back-reef during these tidal stages. At higher tidal stages incident waves are capable of propagating into the back reef but they are very low energy with under 1% of all waves capable of entraining sediment. This suggests that higher energy events, such as high frequency storms, are required to significantly transport sediment and change reef geomorphology. Smaller scale spatial changes in wave height were observed on the sand apron that shows the influence of both cross- and along-reef attenuation processes. A distance parameter (Xpd) is introduced that combines the cross-reef distance from the reef crest (Xd) and the temporally specific along-reef distance from the first point of wave breaking on the reef rim (Xp, where Xpd=. Xd+. Xp). Xpd is shown to accurately describe the changes in wave height and sediment entrainment if deep water significant wave height, wave direction, and depth over the reef flat are known. The results in this study shows that wave conditions, sediment entrainment, and longer term trends in sediment characteristics can be predicted in back-reef environments from a few simple geomorphic inputs.

A new method to obtain information on individual waves as they travel shoreward is described. The wave-tracking method involves analysis of synchronous water level records obtained from a shore-normal transect of pressure transducers (PTs). The method is illustrated here using data from field deployments inside the surf zone, although the method could equally be applied outside the surf zone. As a wave travels shoreward along the instrument transect it appears in the PT records at a progressively later time. Serial cross-correlation techniques are used to shift each record in time so that each wave in the first record aligns approximately with the corresponding wave (if it exists) in subsequent records from along the transect. A local minima analysis [Power et al., 2010] is used to delimit individual waves in each record and then parameters of interest can be determined for each wave at each PT location. The wave tracking method provides the evolution of these parameters for a single wave as it travels to shore, and they can be analysed as a function of distance or local water depth. The utility of the method is illustrated with case studies including the evolution of wave height and the evolution of the ratio of wave height to water depth for surf zone waves.

In this paper we synthesise a large data set gathered from a wide variety of field deployments and integrate it with previously published results to identify the spectral signatures of swash from contrasting beach types. The field data set includes the full range of micro-tidal beach types (reflective, intermediate and dissipative), with beach gradients ranging from approximately 1:6 to 1:60 exposed to offshore significant wave heights of 0.5-3.0. m. The ratio of swash energy in the short-wave (f > 0.05. Hz) to long-wave (f < 0.05. Hz) frequency bands is found to be significantly different between the three beach types. Swash energy at short-wave frequencies is dominant on reflective and intermediate beaches and swash at long-wave frequencies is dominant on dissipative beaches; consistent with previously reported spectral signatures for the surf zone on these beach types. The available swash spectra were classified using an automated algorithm (CLARA) into five different classes. The ordered classes represent an evolution in the spectrum shape, described by a frequency downshifting of the energy peak from the short-wave into the long-wave frequency band and an increase in the long-wave swash energy level compared to a relatively minor variation in the short-wave swash energy level. A universally common feature of spectra from all beach-states was an f-4energy roll-off in the short-wave frequency band. In contrast to the broadly uniform appearance of the short-wave frequency band, the appearance of the long wave frequency band was highly variable across the beach-states. We incorporate the results presented here and previously published observations into the morphodynamic beach-state model, and propose an ordered sequence of swash spectra under increasing and decreasing incident wave energy level. This extension of the beach-state model to include the swash zone leads to the following propositions for morphodynamic controls on the nature of the swash spectrum. (1) The short-wave part of the swash spectrum is relatively constant in form across all beach-states (f-4 energy roll-off) and the energy density per unit frequency is controlled by the beach face gradient alone. (2) The spectral bandwidth of the energy roll-off varies directly with offshore wave energy level and inversely with beach face gradient (or beach-state), in a manner consistent with the non-linear wave breaking criterion. (3) The infragravity part of the swash spectrum is highly variable in form across all beach-states and the energy level is related to the offshore wave energy level and surf zone morphology. © 2014 Elsevier B.V.

This study describes the first comprehensive measurements of nearshore current patterns across the entire extent of an embayed beach bounded by headland rip currents. A field experiment at Whale Beach, NSW, Australia provides valuable insights into: (i) embayment-wide spatial and temporal flow behaviour; (ii) rates of cross- and alongshore water exchange; and (iii) the influence of geological control by headlands on morphology and circulation. Lagrangian flow data was obtained using 34 GPS drifters with 293 individual deployments, over a single ebbing tidal cycle during moderate-low energy (Hs=1m) oblique wave forcing. In-situ wave and current data, and bathymetric data were also collected. Beach morphology was dominated by a large mid-beach rip channel with lesser headland rip channels. Mean flow rates were 0.6ms-1 in the mid-beach channel and 0.4ms-1 in the headland channels, with the majority of cross-shore water volume flux (~60%) through the central channel. A weak alongshore current O (0.1ms-1) was forced by the oblique offshore wave angle. Rip current velocities, flow variability, and rate of surfzone exits by Lagrangian drifters increased as water level decreased. Transient currents on a planar bar along the northern half of the beach, with mean speeds velocity standard deviation up to 0.2ms-1, were not tidally modulated. Lagrangian time series were used to differentiate four current regimes (rip cell, rip head, planar bar and offshore low energy zone) based on mean velocity, velocity variability and degree of tidal modulation. An increase in surfzone exit rates by drifters was observed from south (upwave) to north (downwave), with exit rates per drifter deployment of 22% at the south headland rip, 65% at the mid-beach open rip, and 80% at the north headland rip. The high rate of drifter exits contrasts previous observations on open coast beaches. Observed flow behaviours are attributed to wave shadowing at the upwave (protected) end of the beach, and longshore currents forced by oblique waves deflected offshore at the downwave headland. These field observations are in good agreement with recent numerical modelling. A relationship between bathymetric variability and current intensity was determined, with cross-shore average mean velocity correlating with a parameterisation of bathymetric alongshore non-uniformity. This study demonstrates that flow behaviour and exchange rates can vary along the length of an embayed beach due to geological control. This research has implications for transport of organisms, nutrients and pollutants, is relevant to beach safety practitioners, and can be used in calibration of numerical models. © 2014 Elsevier B.V.

This paper investigates the mechanics of sediment transport on a subtidal sand apron located on a coral reef environment. In this environment 100% of the sediment is carbonate bioclasts generated in situ. The sand apron is located on the back reef and only affected by waves during high tides. It is commonly accepted in the literature that sand aprons are features that prograde lagoonwards and that most of the progradation occurs during high-energy events. Measurements of water depths, waves, currents and near bed suspended sediment concentrations (all at 10. Hz) on the sand apron were undertaken over a nine day intensive field campaign over both spring and neap tides; waves and tides were also measured in the lagoon. The topography and bathymetry of the sand apron were measured and mixing depth was obtained on three transects using depth of disturbance rods. We found that sediment transport on sand aprons is not solely restricted to high-energy events but occurs on a daily basis during spring tides. The main factor controlling the sediment transport was the water depth above the bed, with depths of 2-2.3. m allowing waves to promote the most sediment transport. This corresponds to a depth over the reef crest of 1.6-1.9. m. The second most important control was waves; transport was observed when Hs on the apron was 0.1. m or greater. In contrast, current magnitude was not a controlling mechanism for sediment entrainment but did affect sediment transport. The morphology of the sand apron was shown to affect the direction of currents with the currents also expected to influence the morphology of the sand apron. The currents measured during this field campaign were aligned with a shallow channel in the sand apron. Mixing depths were small (< 2.5. cm) yet they were larger than the values predicted by empirical formulae for gentle siliciclastic ocean beaches. © 2013 Elsevier B.V.

The details of flow at the tip of a viscous swash front are important to describe the propagation of the wave, the bed shear and to estimate material transport rates and impact forces. This paper presents novel experimental data illustrating the convergence of fluid at swash fronts generated by dam-break flows. Very viscous fluids (detergents) were used to slow the flow sufficiently to enable video tracking of particles on the free surface and within the interior of the flow. The experiments were performed both up a slope and on a horizontal bed. The particle tracking shows that surface particles travel faster than the mean flow, converge on the swash tip and then rapidly decelerate, a process that will induce a high bed shear stress at the swash tip as observed in recent experiments. Particles also converge on the wall boundaries because of the no-slip condition. A simple analytical model is developed to estimate the ratio of the velocity of surface particles and the wave front. For laminar flows, this ratio is found to be 3/2, independent of the bed slope and flow depth, and is in good agreement with the experimental data. The same model approach suggests a ratio of 8/7 for turbulent flows. This flow convergence does not appear to be included in either analytical modeling of the tip region or in basal resistance laws for the swash front and would modify the momentum equation at the swash tip [c.f. Hogg and Pritchard, 2004] and the kinematic boundary condition at the shoreline. The flow convergence is consistent with observations of the behavior and build-up of buoyant debris at the leading edge of tsunami wave front and can be observed in natural swash flows on beaches. © 2014 Elsevier B.V.

This study aims to develop new attenuation rules for tsunamis in NSW estuaries. The results of the study will be presented in full at the 2015 Coasts and Ports conference in Auckland, New Zealand.

This study has assessed the current state of knowledge regarding the potential for tsunami impact on the NSW coastline from submarine mass failure events. 2D modelling suggests that there is the potential for sizeable coastal flow depths, however, the lateral extent of the impact is not estimated using these methods. Preliminary 3D modelling results suggest that models are highly sensitive to model input factors resulting in significant variation in the initial sea surface waveform. This then results in varied inundation footprints. Full 3D modelling results will be presented at the conference.

Swash zone boundary conditions are investigated using field observations of the inner surf zone and the swash zone for a range of micro-tidal, predominantly swell-dominated, sandy beaches. The boundary conditions are characterized in terms of a terminal bore height and the asymmetry between uprush and backwash flows. Terminal bore heights are calculated by examining wave evolution across the surf zone. This is investigated in terms of wave height to water depth ratios (?) and waves are shown to be unsaturated. These observations are used to approximate the terminal bore height at the surf swash boundary as Hb¿0.12Ho, which is consistent with recent laboratory data sets. Wave transformation modelling is used to identify conditions where the surf zone is predicted to be unsaturated and this is found to occur for steep beaches and large wave periods. A normalised beach slope parameter is derived to separate these two regions. The asymmetry between uprush and backwash flows is investigated using remote sensing techniques. The time of flow reversal in the swash zone was found to occur at approximately 40-50% of the duration of the swash cycle, which is significantly different from previous theory.