Hannah E. Power

Nearshore wave height variation in unsaturated surf

The nearshore evolution of wave height is presented from field observations during unsaturated surf conditions from 10 different beaches characterized by microtidal conditions and predominantly swell-dominated wave climates. Wave evolution is presented in terms of wave height to water depth ratio (gamma) for comparison with previous data from saturated surf. Both conventional time-averaged (gamma(rms)) and a new wave-by-wave analysis (gamma(w)) are performed. Values of gamma increase with increasing offshore wave height, indicating unsaturated surf. The observations show a variation in gamma values from near constant values in the mid surf zone to rapidly and asymptotically increasing gamma values in the inner surf zone. In contrast to previous data from saturated surf, gamma shows no dependence on either the absolute beach slope or the relative beach slope beta/k (h) over bar. The skewness of the distributions of gamma(w) is consistent with waves that are not depth limited. The inner surf zone wave heights are approximately equally dependent on the water depth and offshore wave height. The previous observations of gamma from saturated surf are shown to be consistent with a terminal bore height at the shoreline which is in excellent agreement with a previously derived value for the Miche parameter. In contrast, for the present unsaturated surf conditions, the terminal bore height at the shoreline can be approximated by H-b approximate to 0.12H(o), which is consistent with recent laboratory data sets.

Evolution of Coral Rubble Deposits on a Reef Platform as Detected by Remote Sensing

Abstract: An investigation into the evolution of coral rubble deposits on a coral reef platform is assessed using high-resolution remote sensing data and geospatial analysis. Digital change detection analysis techniques are applied to One Tree Reef in the southern Great Barrier Reef by analysing aerial photographs and satellite images captured between 1964 and 2009. Two main types of rubble deposits were identified: (1) rubble flats that are featureless mass accumulations of coral rubble; and, (2) rubble spits that are shore-normal linear features. While both deposits prograde in a lagoon-ward direction, rubble spits move faster (~2 m/yr) than rubble flats (~0.5 m/yr). The volume of rubble, the underlying substrate, the energy regime, and storm frequency control the rate of progradation. Rubble flat occurrence is restricted to the high-energy (windward) margin of the coral reef platform, while rubble spits are distributed reef wide, both in modal high energy and modal low energy regions of the reef. Rubble spit deposition is considered to be a result of enlarged spur and groove morphology of the forereef, whereby wave energy is focused through the enlarged groove formations causing the preferential deposition of coral rubble in particular zones of the adjacent reef flat. One last control is thought to be the elevation of the reef crest whereby lower areas are more prone to rubble flat development. A vertical and ocean-ward accumulation of rubble is occurring on the windward margin of the reef leading to a build-up and build-out of the reef, governing the expansion of the reef footprint. This study shows for the first time the evolution of a coral reef rubble flat and rubble spits over decadal time scales as detected through remotely sensed images spanning 45 years.

Coral reef sediment dynamics: evidence of sand-apron evolution on a daily and decadal scale

ABSTRACT Vila-Concejo, A. Harris, D.L., Shannon, A.M., Webster, J.M., and, Power, H.E., 2013. Coral reef sediment dynamics: evidence of sand-apron evolution on a daily and decadal scale This paper investigates sand apron progradation on decadal and daily scales on a platform reef (One Tree Reef, OTR) located in the southern Great Barrier Reef. The decadal scale is addressed by analysing sand apron progradation using remotely sensed images (aerial photos and satellite imagery) coupled with wind data and cyclone events. The daily scale is addressed through a field campaign that was undertaken in September-October 2011. The campaign consisted of hydrodynamic measurements in three stations over the southern sand apron in OTR. It was found that while there was a small overall progradation over the last 31 years, the progradation had not occurred continuously or consistently along the entire sand apron. Additionally, the effect of cyclones was not clear on the decadal scale. On the daily scale, it was found that currents are generally weak (<0.4 m/s) and that currents during conditions at which suspended sediment is maximized are ocean-ward directed on the central part of the sand apron and lagoon-ward directed on the easternmost end. As such, daily sediment transport does not represent a gross contribution to lagoon infilling by sand apron progradation. Our results show that sand apron progradation does not occur continuously on the decadal or the daily scale.

Coral reef structural complexity provides important coastal protection from waves under rising sea levels

If coral reefs continue to degrade, waves on coastlines may substantially increase, leading to greater coastal erosion. Coral reefs are diverse ecosystems that support millions of people worldwide by providing coastal protection from waves. Climate change and human impacts are leading to degraded coral reefs and to rising sea levels, posing concerns for the protection of tropical coastal regions in the near future. We use a wave dissipation model calibrated with empirical wave data to calculate the future increase of back-reef wave height. We show that, in the near future, the structural complexity of coral reefs is more important than sea-level rise in determining the coastal protection provided by coral reefs from average waves. We also show that a significant increase in average wave heights could occur at present sea level if there is sustained degradation of benthic structural complexity. Our results highlight that maintaining the structural complexity of coral reefs is key to ensure coastal protection on tropical coastlines in the future.

Surf zone states and energy dissipation regimes-a similarity model

A similarity parameter is derived to describe surf zone dissipation using the classical energy dissipation model for surf zone bores. This parameter can also be interpreted as a relative beach slope parameter, βγ, and, for shallow water sinusoidal waves, is the ratio of the local beach slope and the local wave steepness (H/L). βγ = 1 defines the boundary between two different energy dissipation regimes. Conditions with βγ < 1 represent over-dissipative conditions, where the classical bore dissipation model provides more dissipation than that required for depth-limited waves (constant H/h) to be maintained. Conditions with βγ > 1 represent under-dissipative conditions, where the bore model provides insufficient dissipation for depth-limited conditions to occur. Conditions with βγ = 1 at the breakpoint lead to locally saturated but not depth-limited surf. Hence, the new similarity parameter distinguishes between saturated and unsaturated surf conditions. Based on this bore dissipation model, an analytical model for the wave height transformation of monochromatic waves on planar beaches is derived. The cross-shore variation in wave height obtained from this model show different functional forms; concave upward for over-dissipative conditions and convex upward for under-dissipative conditions. Further, the analytical model shows that depth-limited conditions within the inner surf zone are not possible with this bore dissipation model and the model assumptions. Additional work is required to determine if this parameter is a useful predictor of other surf zone characteristics. Highlights: We derive a similarity parameter to describe surf zone dissipation. The similarity parameter defines the boundary of two dissipation regimes. The parameter distinguishes between saturated and unsaturated surf conditions. The classical bore dissipation model cannot predict depth-limited wave heights on a plane beach.

Predicting Breaking Wave Conditions Using Gene Expression Programming

The forces and loading resulting from shallow water breaking waves are one of the most important drivers in coastal engineering design and morphological change. The importance of accurately and precisely predicting breaking wave conditions cannot be overstated. Using a novel dataset of laboratory and field scale breaking wave conditions, this study assesses the performance of widely applied empirical relationships for breaking waves and uses newly available artificial neural networks and gene expression programming (GEP) numerical methods to develop an accurate and easily applied predictor of breaking conditions for coastal engineers and planners. A novel GEP model is developed and shown to: provide excellent predictive ability at all scales, greatly improve prediction compared with previous works at laboratory scale, and clearly identify the relevant importance of seafloor slope and the water depth to wavelength ratio.

Seamless bathymetry and topography datasets for New South Wales, Australia

This paper describes three datasets of seamless bathymetry and coastal topography for Sydney Harbour (Port Jackson), Botany and Bate Bays, and the Hawkesbury River. The datasets used to form these compilations were the most recent and highest quality available to the authors and were originally collated using the software ESRI ArcGIS. The original compilation of this data was undertaken to support tsunami modelling research by the authors of this paper. Before processing, all data were adjusted and/or reprojected to conform to the vertical datum Australian Height Datum (AHD) and horizontal projection WGS84 UTM zone 56. Data resolution and density was highly variable and grid resolutions of the final datasets were selected as the highest resolutions possible using the most sparse data in the compilation in question. For areas where no data were available, the ESRI ArcGIS interpolation tool, Topo to Raster, was used to provide a best estimate. These dastasets of three important Australian waterways provide a useful tool for coastal research and scientific interest.

114. REMOTE SENSING OF SWASH ZONE BOUNDARY CONDITIONS USING VIDEO AND ARGUS

A method to determine swash zone boundary condition sis developed using remote sensing by video and ARGUS. Guard and Baldock (2007) proposed new numerical solutions for swash hydrodynamics, where the flow field varies according to a free parameter, k. k=0 in the Shen and Meyer swash solution but k=1 appears more realistic for real swash. This study has developed a semi-automated method to obtain values for k from multiple waves in field conditions using ARGUS video date and image analysis techniques. The results indicate that k is greater than 0 and close to 1 for natural swash. No trends were observed between k and run-up length, offshore significant wave height or tidal phase, consistent with the self-similarity of the swash hydrodynamics. In conjunction with modelling, the method offers the possibility of remote sensing of the asymmetry in the swash zone hydrodynamics.

The Tsunami Threat to Sydney Harbour, Australia: Modelling potential and historic events

Tsunami modelling of potential and historic events in Australia’s Sydney Harbour quantifies the potentially damaging impacts of an earthquake generated tsunami. As a drowned river valley estuary exposed to distant source zones, these impacts are predominantly high current speeds (>2 m/s), wave amplification and rapid changes in water level. Significant land inundation only occurs for scenarios modelled with the largest waves (9.0 MW source). The degree of exposure to the open ocean and the geomorphology of locations within the Harbour determine the relative level of these impacts. Narrow, shallow channels, even those sheltered from the open ocean, create a bottleneck effect and experience the highest relative current speeds as well as elevated water levels. The largest maximum water levels (>8 m) occur in exposed, funnel-shaped bays and wave amplification is greatest at locations exposed to the open ocean: >7 times deep water wave heights for 9.0 MW source waves. Upstream attenuation rates of runup and maximum water level show a linear correlation with wave height parameters at the 100 m depth contour and may provide some predictive capabilities for potential tsunami impacts at analogous locations. In the event of a tsunami in Sydney Harbour, impacts may threaten marine traffic and infrastructure.

Steps to improve gender diversity in coastal geoscience and engineering

Robust data are the base of effective gender diversity policy. Evidence shows that gender inequality is still pervasive in science, technology, engineering and mathematics (STEM). Coastal geoscience and engineering (CGE) encompasses professionals working on coastal processes, integrating expertise across physics, geomorphology, engineering, planning and management. The article presents novel results of gender inequality and experiences of gender bias in CGE, and proposes practical steps to address it. It analyses the gender representation in 9 societies, 25 journals, and 10 conferences in CGE and establishes that women represent 30% of the international CGE community, yet there is under-representation in prestige roles such as journal editorial board members (15% women) and conference organisers (18% women). The data show that female underrepresentation is less prominent when the path to prestige roles is clearly outlined and candidates can self-nominate or volunteer instead of the traditional invitation-only pathway. By analysing the views of 314 survey respondents (34% male, 65% female, and 1% ‘‘other’’), we show that 81% perceive the lack of female role models as a key hurdle for gender equity, and a significantly larger proportion of females (47%) felt held back in their careers due to their gender in comparison with males (9%). The lack of women in prestige roles and senior positions contributes to 81% of survey respondents perceiving the lack of female role models in CGE as a key hurdle for gender equality. While it is clear that having more women as role models is important, this is not enough to effect change. Here seven practical steps towards achieving gender equity in CGE are presented: (1) Advocate for more women in prestige roles; (2) Promote high-achieving females; (3) Create awareness of gender bias; (4) Speak up; (5) Get better support for return to work; (6) Redefine success; and, (7) Encourage more women to enter the discipline at a young age. Some of these steps can be successfully implemented immediately (steps 1–4), while others need institutional engagement and represent major societal overhauls. In any case, these seven practical steps require actions that can start immediately.