Peter J. Cowell

Morphodynamics of reflective and dissipative beach and inshore systems: Southeastern Australia

Abstract Field experiments involving direct measurement of surf and inshore current spectra, inshore circulation patterns, and depositional morphology have been replicated under different energy conditions and in several environmentally contrasting beach localities on the high-energy coast of New South Wales, Australia. The region exhibits compartmentalized beach systems and is dominated by a highly variable wind-wave climate superimposed on persistent high-energy swell (T = 10–14 sec). Two general types of beach system occur: (1) predominantly reflective systems in which much of the incidentwave energy is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent energy dissipation. Reflective systems are characterized by steep, linear beach faces, well-developed berms and beach cusps, and surging breakers with high runup and minimum setup; rip cells and associated three-dimensional inshore topography are absent. Wave height and current spectra from reflective beaches consistently have their dominant peaks at incident wave and subharmonic frequencies, and cross-spectra indicate the existence of low-mode edge waves at those frequencies. Infra-gravity peaks are negligible. Under low-energy conditions subharmonic peaks are low relative to incident-wave peaks; however, increasing breaker height tends to be accompanied by increasing subharmonic dominance. Analyses of shore-normal currents near the bed show that under all conditions the strongest shoreward motions are induced by the incident waves; however, seaward motion near the beach face is subharmonic-dominated. Dissipative systems characterize the exposed open coast and are fronted by concaveupward nearshore (seaward of break) profiles and wide flat surf zones. Waves break 75–300 m seaward of the beach and dissipate much of their energy before reaching the beach, creating significant radiation-stress gradients and setup. Topography is much more complex and varied than in the case of reflective beaches; one or more bars, three-dimensional inshore topography and different scales of rip cells are normally present. The commonly occurring time-and-environment-dependent morphologic states can be classified into six general types. The greatest total dissipation is associated with Type 1 which prevails in the regions of most abundant inshore sediments or during and immediately after severe storms. Setup is highest and runup is lowest (relative to incident-wave height) with this type and the dominant energy near the beach is in the surf-beat part (80–120 sec) of the spectrum. As the bar migrates shoreward (Types 2 and 3) and beach face steepens and a deep trough develops within which the partially dissipated waves reform. Although the outer surf zone remains dissipative, synchronous and subharmonic resonance occurs near the beach face and, as with reflective beaches, low-mode edge waves form beach cusps. A conspicuous feature of Types 2 and 3 is the occurrence of pronounced resonant spectral peaks at 4T ( ⋍ 40–50 sec ) within the trough and on the bar. Edge waves at this frequency may be responsible for the development of the crescentic bar forms (Type 3). Lower frequency surf beat peaks also remain present but are secondary. The peak at 4T attenuates as Type 4 develops and is not present with Type 5 topography but the first subharmonic (2T) becomes more pronounced, particularly with high tide. Type 6 morphology represents the fully accreted beach state and occurs only after prolonged periods of low swell. This type is a reflective beach with a steep linear beach face and a very high berm which remains continuous for long distances alongshore; rips are always absent. Wave and current spectra are also similar to those described for reflective beaches.

Simulation of large-scale coastal change using a morphological behaviour model

Abstract Quantitative simulation of large-scale coastal behaviour (LSCB) is possible using a model based only on principles of sand-mass conservation and geometric rules for shoreface and active sand-body morphology. Dynamic processes are subsumed into a set of parameters that define the geometry of the active cross-shore profile which in turn can represent an entire coastal cell. The parameters, together with a set of local behaviour rules governing adjustments at discrete points along the profile, provide a simple formulation that is constrained by sediment mass balance to yield emergent, numerical solutions for profile kinematics. The emergent qualities of this shoreface-translation model are exemplified in simulation experiments on changes to sand deposits on the southeastern Australian coast and shelf during the post-glacial marine transgression. In another example, the model is used to simulate LSCB over engineering time scales at an idiosyncratic site. Both examples illustrate the over-riding importance of small residual sand movements that accumulate through time to cause LSCB. In comparison to gross sand movements, the residuals are too small to measure directly or predict on the basis of available theory. Exploratory (inverse) simulation provides a quantitative method for side stepping these limitations. The shoreface-translation model therefore is used as a tool to explore possible forms of coastal behaviour through an approach that incorporates available data along with theoretical considerations. Validation is not strictly possible for this type of inverse approach, so the degree to which a simulation reproduces observed features in nature is a measure of its success.

Management of Uncertainty in Predicting Climate-Change Impacts on Beaches

Abstract Management of uncertainty in model predictions of long-term coastal change begins by admitting uncertainty. In the case of geometric mass-balance models, the first step is to relax restrictive assumptions to allow for open sediment budgets, time-dependent morphology, effects of mixed sediment sizes, and variable resistance in substrate material. These refinements introduce new uncertainty regarding the choice of parameter values. The next step is to actively manage uncertainty using techniques readily available from information science. The final step requires a shift in coastal management culture to accept decision making based on risk-management protocols. Stochastic simulation was applied to manage predictive uncertainty in cases involving complications resulting from open sediment budgets, rock reefs, and seawalls. In these examples, the respective effects caused between 20% and 60% difference from conventional predictions based solely on equilibrium assumptions and substrates comprised entirely of sand. Stochastic simulation makes it possible to establish confidence limits and determine the statistical significance of differences caused by varying effects such as substrate resistance and shoreface geometry. It also enables the likelihood of critical impacts to be specified in terms of probability. Moreover, probabilistic forecasts provide a transparent basis for coastal management decisions by revealing the consequences if quantitative estimates prove to be wrong.

Shoreface translation model: computer simulation of coastal-sand-body response to sea level rise

The response of coastal sand bodies to sea level rise can be modelled using computer simulation of large scale coastal behaviour based only on principles of sand-mass conservation and geometric rules for shoreface and barrier morphology. The model simulates horizontal and vertical translation of coastal sand bodies over the pre-existing coastal substrate which undergoes reworking as a consequence. This produces changes in position of the coastline as well as reconfiguration of the backshore and inner-continental shelf morphology and stratigraphy. Application of the model is illustrated with examples from mineral exploration of the continental shelf and assessment of coastal erosion risks in the face of sea-level rise due to the enhanced greenhouse effect.

Integrating Uncertainty Theories with GIS for Modeling Coastal Hazards of Climate Change

Prediction of coastal hazards due to climate change is fraught with uncertainty that stems from complexity of coastal systems, estimation of sea level rise, and limitation of available data. In-depth research on coastal modeling is hampered by lack of techniques for handling uncertainty, and the available commercial geographical information systems (GIS) packages have only limited capability of handling uncertain information. Therefore, integrating uncertainty theory with GIS is of practical and theoretical significance. This article presents a GIS-based model that integrates an existing predictive model using a differential approach, random simulation, and fuzzy set theory for predicting geomorphic hazards subject to uncertainty. Coastal hazard is modeled as the combined effects of sea-level induced recession and storm erosion, using grid modeling techniques. The method is described with a case study of Fingal Bay Beach, SE Australia, for which predicted responses to an IPCC standard sea-level rise of 0....Prediction of coastal hazards due to climate change is fraught with uncertainty that stems from complexity of coastal systems, estimation of sea level rise, and limitation of available data. In-depth research on coastal modeling is hampered by lack of techniques for handling uncertainty, and the available commercial geographical information systems (GIS) packages have only limited capability of handling uncertain information. Therefore, integrating uncertainty theory with GIS is of practical and theoretical significance. This article presents a GIS-based model that integrates an existing predictive model using a differential approach, random simulation, and fuzzy set theory for predicting geomorphic hazards subject to uncertainty. Coastal hazard is modeled as the combined effects of sea-level induced recession and storm erosion, using grid modeling techniques. The method is described with a case study of Fingal Bay Beach, SE Australia, for which predicted responses to an IPCC standard sea-level rise of 0.86 m and superimposed storm erosion averaged 12 m and 90 m, respectively, with analysis of uncertainty yielding maximum of 52 m and 120 m, respectively. Paradoxically, output uncertainty reduces slightly with simulated increase in random error in the digital elevation model (DEM). This trend implies that the magnitude of modeled uncertainty is not necessarily increased with the uncertainties in the input parameters. Built as a generic tool, the model can be used not only to predict different scenarios of coastal hazard under uncertainties for coastal management, but is also applicable to other fields that involve predictive modeling under uncertainty.

Shoreface Sand Supply to Beaches

The possibility of sand supply from the shoreface to beaches was evaluated based on a variety of methods involving field data and modeling results obtained from five coasts on three continents representing a wide range of coastal environments. The field data include wave-current measurements, historical seabed soundings and geological surveys. Cross-shore transport estimates from modeling on the annual time scale were compared against scaled-down inferences from the seabed-change and geological data. The results are all consistent with there being net onshore transport over the long term from the lower shoreface to beaches in each of the environments. These environments typify settings that occur commonly (probably predominantly) along the worlds coasts. So net shoreface sand supply to beaches may be a widespread and common but little appreciated factor in coastal stability. The effect of this net supply is to offset other factors causing shoreline recession, such as positive gradients in littoral transport Moreover, shoreline progradation occurs if sand supply from the shoreface dominates over littoral sediment losses. Implications are clearly significant for coastal engineering and coastal management, despite the processes not being immediately apparent: long-term shoreface sand supply to beaches is masked by more rapid cyclical changes. Rates of shoreface sand supply to beaches indicated from various lines of evidence are typically on the order of 10 0 m 3 a –1 per meter of shoreline. This volume corresponds to a lowering of the shoreface by only a few grain diameters per year.

GIS-Based Coastal Behavior Modeling and Simulation of Potential Land and Property Loss: Implications of Sea-Level Rise at Collaroy/Narrabeen Beach, Sydney (Australia)

Rising sea level potentially poses a threat to many coastal areas, thereby possibly affecting coastal environments, including human assets. Taking into account the precau--tionary principle demanded at the Framework Convention for Climate Change in Rio de Janeiro in 1992, coastal managers and planners are required to evaluate the possibility of both physical and economic impacts of sea-level rise. However, long-term and cost-intensive data capture is often not affordable for a first estimation of general trends. To determine physical and economic impacts on a spatial scale of less than 10 km, a rapid and low-cost method is required. A Geographic Information System (GIS), in combination with readily available data and two coastal behaviour models (the Bruun-GIS Model and the Aggradation Model) was applied to simulate shoreline recession caused by a rise in sea level. In addition, the potential impacts of a 50-year design storm were considered in conjunction with sea-level rise. The monetary vulnerability was assessed and combined with the simulated recession rates. This procedure provides a first estimate on the potential risk a locality (here Collaroy/Narrabeen Beach) may face due to the impacts of sea-level rise and/or coastal storms. Overall, the modelling outcome suggests that long-term erosion problems associated with rising sea level are less significant in comparison with those impacts associated with short-term coastal storm events for Collaroy/Narrabeen Beach.

Contemporary hydrodynamics and morphological change of a microtidal estuary: a numerical modelling study

Contemporary hydrodynamics and morphological change are examined in a shallow microtidal estuary, located on a wave-dominated coast (Port Stephens, NSW, Australia). Process-based numerical modelling is undertaken by combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. Model results suggest that the complex estuarine bathymetry and geometry give rise to spatial variations in the tidal currents and a marked asymmetry between ebb and flood flows. Sediment transport paths correspond with tidal asymmetry patterns. The SE storms significantly enhance the quantities of sediment transport, while locally generated waves by the westerly strong winds also are capable of causing sediment entrainment and contribute to the delta morphological change. The wave/wind-induced currents are not uniform with flow over shoals driven in the same direction as waves/winds while a reverse flow occurring in the adjacent channel. The conceptual sediment transport model developed in this study shows flood-directed transport occurs on the flood ramp while ebb-directed net transport occurs in the tidal channels and at the estuary entrance. Accretion of the intertidal sand shoals and deepening of tidal channels, as revealed by the model, suggest that sediment-infilling becomes advanced, which may lead to an ebb-dominated estuary. It is likely that a switch from flood- to ebb-dominance occurs during the estuary evolution, and the present-day estuary acts as a sediment source rather than sediment sink to the coastal system. This is conflictive to the expectation drawn from the estuarine morphology; however, it is consistent with previous research suggesting that, in an infilling estuary, an increase in build-up of intertidal flats/shoals can eventually shift an estuary towards ebb dominance. Thus, field data are needed to validate the result presented here, and further study is required to investigate a variety of estuaries in the Australian area.

Shoreface Controls on Barrier Evolution and Shoreline Change

Barriers exist in a continuum of forms, which are fundamentally governed by processes that shape the shoreface to determine the envelope available for sediment accommodation. This envelope is contained between the shoreface and underlying surface defined by the continental shelf and coastal plain (i.e., the substrate). Barrier form also depends on coastal change that is constrained, following reasonably well-established principles, by the volume and type of sediment supply (or loss) and rates of change in sea level that modify the shoreface and associated accommodation potential. While the shoreface is therefore significant to barrier form and behavior, processes that shape the shoreface itself remain poorly understood. In particular, systematic long-term evolution of the shoreface, which is evident in geological data, indicates not only a time-varying morphology, but also a lagged response to environmental change. Such shoreface evolution has implications for barrier evolution (and vice versa). In this chapter, we review (1) relations between shoreface and barrier form, (2) limits to knowledge on shoreface behavior and insights from depositional records from which systematic changes over time can be inferred, and (3) exploratory experiments on the morphodynamic timescale of shoreface change. The third part of the review derives from results of experimental modeling of combined shoreface and barrier evolution constrained by geologic data. The numerical experiments demonstrate that, on intermediate timescales (decades to centuries) that are most relevant to coastal management and planning, adjustments are dominated by sediment exchanges between the beach and shallower portions of the shoreface in response to rapid changes in boundary conditions, especially sea level. Significant morphodynamic hysteresis can be expected from the partial adjustment of lower shoreface geometry during sea-level change, resulting in ongoing barrier evolution and shoreline migration after the stabilization of boundary conditions.

Tidal Asymmetry of a Shallow, Well-mixed Estuary and the Implications on Net Sediment Transport: A Numerical Modelling Study

Abstract Tidal asymmetry plays a pivotal role in the transport of sediment and morphological change in shallow inlet/estuarine systems, particularly those with extensive tidal flats and channels. This study examined tidal circulation patterns of a shallow and well-mixed estuary on the central coast of New South Wales, Australia. A numerical modelling study was carried out by applying a depth-averaged flow model (MIKE 21 HD). The model was calibrated and successfully validated against recently acquired hydrodynamic data. Model results reveal the presence of a number of eddies within the estuary, which may have substantial influence on net sediment transport when combined with wave effects. Tidal phase duration and magnitude asymmetries indicate that, in contrast to typical flood/ebb dominance, double ebb-dominance is in deep channels and the entrance cross-section, whereas the flood ramp area shows double flood-dominance. The implications of tidal asymmetry on net bed-load sediment transport were inferred from the duration and magnitude asymmetries of the above threshold velocity. The ebb-directed net transport occurs at main tidal channels and may result in further deepening channels, whereas a flood-directed net transport in the flood ramp area may cause an accumulation of sediment on the delta or a landward progradation of the tidal flat. The present-day estuary is characterised as ebb-dominated, tends to flush bed-load sediment seaward more effectively and may represent more stable geometries.