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The article focuses on the response towards the report "Management of Uncertainty in Predicting Climate-Change Impacts on Beaches." The authors aim to represent the practice in coastal science that attempts to organize qualitative understanding of complex coastal-change problems through a systematic framework. Such method identifies relevant factors involved in the development and selection of tools for quantitative exploration of potential coastal changes in certain cases. They also argued on the issue of improving the capacity of models to predict coastal change.

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. [ABSTRACT FROM AUTHOR]

The coastal-tract approach to coastal morphodynamics, described in the companion paper (The Coastal-Tract Part 1), provides a framework for aggregation of process and spatial dimensions in modeling low-order coastal change (i.e., evolution of the shoreline, continental shelf and coastal plain on time scales of 10² to 10³ years). Behavior-oriented, coastal-change models encapsulate aggregate dynamics of the coastal tract. We apply these models in a coastal-tract framework to illustrate the use of the concept, and to explore low-order morphological coupling under different environmental settings. These settings are characterized by data-models that we have constructed from four contrasting continental margins (NW Europe, US Pacific, US Atlantic, and SE Australia). The gross kinematics of the coastal tract are constrained and steered by sediment-mass continuity. The rate of coastal advance or retreat is determined quantitatively by the balance between the change in sediment accommodation-space, caused by sea-level movements, and sediment availability. If the lower shoreface is shallower than required for equilibrium (negative accommodation), then sand is transferred to the upper shoreface (NW Europe, US Pacific, and SE Australian cases modelled) so that the shoreline tends to advance seaward. This tendency also occurs when relative sea level is falling (coastal emergence). Coastal retreat occurs when the lower shoreface is too deep for equilibrium (positive shoreface accommodation). This sediment-sharing between the upper and lower shoreface is an internal coupling that governs first-order coastal change. The upper shoreface and backbarrier (lagoon, estuary or mainland) also are coupled in first-order coastal change. Sediment accommodation-space is generated in the backbarrier by sea-level rise (and reduced by sea-level fall), but the amount of space is also moderated by influx of fine sediments from the coast, or sand and mud from fluvial sources. Remaining space can then be occupied by sand transferred from the upper shoreface causing a retreat of the latter (transgressive phases modelled for NW Europe, US Atlantic, and SE Australian cases). [ABSTRACT FROM AUTHOR]

Existing principles for perched-beaches and rock-truncated shorefaces were used to modify Bruun-type concepts in developing principles for prediction of atoll-island response to sea-level rise. The principles incorporate the effects of a non-erodable reef substrate and how these influence profile movements, with an appropriate modification to the key concept of closure depth. This influence is exerted through sediment-continuity constraints: sediment-displacement volumes are limited due to shoreface truncation by the reef. The elevation of the reef thus defines profile closure, because of which the closure-depth problem involves less uncertainty than on continental siliclastic coasts. The revised principles were incorporated numerically into an existing profile-based computer model: the Shoreface Translation Model (STM). The modified STM was applied to idealized settings for islands with different types of substrates, but ignoring sedimentsupply effects and other factors such as changes in the wave climate. Simulation results confirm the extreme sensitivity of shoreline recession on islands due to barrier-rollover effects in the absence of sediment supply. Previous ideas that islands located on raised conglomerate platforms have enhanced stability prove unfounded with respect to sea-level rise, with such islands responding in simulations by the same amount as islands located directly on the reef flat. However, simulations of islands located on prograded sediment terraces were more stable under conditions of sea-level rise: the sediments from the underlying terrace are reworked into the retreating island, thus providing an additional sediment source. Overall, the critical importance of carbonate-sediment production and supply can be inferred from the ultrasensitivity of islands to sea-level rise evident in simulations. Because of this, secondary effects of sea-level rise are likely to be of first-order importance to island response. These secondary effects involve changes in littoral sediment-transport patterns caused in turn by changes to wave transformation due to increased water depths over reefs. These effects may be compounded by directional shifts in regional wave regime: like sea-level rise, a potential by-product of global climate change. Profile-response models, such as the STM, incorporate these effects by including sediment gains and losses as boundary conditions in the mass balance solution. [ABSTRACT FROM AUTHOR]

The morphological response of atoll islands to sea-level rise is of paramount importance to low-lying atoll nations, which have limited land area. These islands are situated on horizontal non-erodable reef platforms backed by deep lagoons. To date, there have been few attempts to model atoll island response to changes in sea level. This paper uses the modified shoreline translation model (STM) for non-erodable and horizontal substrate surfaces to examine the likely mode of island response to changing sea level using morphological surveys from two islands on Tarawa atoll, Kiribati. Model simulations using a range of sea-level rise scenarios show that atoll islands can undergo a range of morphological responses from the standard Bruun response to Barrier rollover. The mode and relative magnitude of response was shown to be sensitive to island morphology. Low elevation and narrow islands were shown to be most sensitive to changes in sea level. These islands undergo barrier rollover and migrate across the reef flat toward the lagoon and become increasingly unstable as sea-level rise increases. That is, since islands are low in elevation and have lagoons behind them, beach sediments will be displaced to the top of islands or through inlets into lagoons. Higher elevation islands are shown to be more stable and undergo a combination of Bruun and barrier recession responses as sea level increases. Simulations indicate that washover processes allow the island surface to keep pace with sea level rise. Under total washover barrier rollover also implies no change in island width although islands become very unstable rapidly migrating across the reef flat. Under restricted washover conditions (wide islands) simulations indicate upgrowth of the island margin and island narrowing as the barrier recedes. All simulations indicate recession of the island shoreline away from the reef edge. Relative differences in magnitude of change indicate that while low narrow islands experience significant recession, higher islands are relatively stable and are able to cope with sea level rise. Significantly results dispel a number of misconceptions regarding possible atoll island response to sea level showing that: atoll islands will respond in a variety of ways depending on the physical characteristics of islands; conglomerate platform does not necessarily provide stability for islands; and that total loss of atoll islands is unlikely as islands will be able to morphologically adjust to changes in sea level ensuring their future persistence. [ABSTRACT FROM AUTHOR]

Rip currents are an integral component of intermediate beaches as described by the WRIGHT and SHORT (1984) model, but their morphologic character and behaviour at various states has yet to be suitably quantified. This study utilises field measurements encompassing an almost complete cycle of low-energy beach state evolution to describe morphometric approaches for quantifying rip current behaviour. First, a subjective analyses of the relative planimetric areal extent of rip feeder, rip-neck and non-rip (bar) components showed that active rip areas (AR) were never greater than 50% of the total system area (AT) with AR decreasing from 40-45% during the longshore bartrough (LBT) state to less than 20% during the low tide terrace (LTT) state. Second, the concept of overfit and underfit rip currents (SHORT, 1985) is examined by describing topographically confined and unconfined flow conditions through the use of the Rip Fit Parameter; RF = (M-O)/Ar, where M = morphologic cross-sectional rip channel area; O = overbank cross-sectional rip area, and Ar = total cross-sectional rip area. In general, low-energy rips are unconfined at high tide for all intermediate states and are confined when RF > 0.5 during transverse bar-rip and low-tide terrace conditions at low-tide. Confined rips are also associated with maximum rip flow velocities and can be related to planimetric rip area. Both morphometric approaches provide new and useful quantification of intermediate beach state rip current morphology during accretionary conditions. [ABSTRACT FROM AUTHOR]

Discusses the influence of antecedent topography on coastal evolution of the Rio Grande do Sul (RS) in Brazil during the Holocene as tested by the shoreface translation-barrier model (STM). Holocene barrier system of the RS; Regional setting of the RS; Procedure for evaluating the role of the former land topography in the evolution of the RS coast.