Bruce G. Thom

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 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.

Last glacial “coastal” dunes in Eastern Australia and implications for landscape stability during the Last Glacial Maximum

Abstract The Last Glacial Maximum or LGM (25,000–15,000 yr B.P.) has been recognized in Australia as a period of increased dryness, coolness, continentality and windiness compared to earlier and later period Enhanced aeolian activity during the LGM occurred in arid and semi-arid regions of western, central and eastern Australia. The east coast has been considered to have been better watered and vegetated. However, at a number of sites from Tasmania to North Queensland, there is evidence for extensive aeolian instability of coastal sand deposits. Dating by radiocarbon and thermoluminiscence techniques has supported morphologic and stratigraphic evidence of dune formation during the LGM under the influence of westerly (offshore) winds in the southern sector of the east coast (i.e. south of 31°S latitude) and southeast winds to the north. It is now apparent that vegetation cover on sandy surfaces was quite patchy during the LGM. Sand surface instability under conditions of strong west or southeast winds promoted linear and/or parabolic dune development. This suggests greater concentration of forests in more discrete, protected sites along the eastern escarpment than was previously considered by palaeoecologists. More widespread drier and cooler climatic conditions operated even in coastal regions on expanded continental shelves at this time. Stabilization of areas of active dunes became more likely as sea levels rose, reduced windiness occurred, and precipitation increased as sea surface temperatures began to rise in the Holocene.

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.

Australia: An Unstable Platform for Tide-Gauge Measurements of Changing Sea Levels: A Discussion

The recent detailed analyses by Aubrey and Emery (1986) of Australian sea level trends continues their efforts to define tectonic and climatic factors worldwide that dominate longand short-term fluctuations respectively in sea level records. These factors have included sediment and water loading on the adjacent shelf, the tectonic behaviour of plates, fluctuations in the Southern Oscillation, behaviour of currents impinging on the shelf, and river runoff. We do not object to these efforts; however we are disturbed by misrepresentations in their recent paper on Australian sea levels regarding (1) the interpretation of the nature of sea-level records, (2) the use of tectonic explanations to account for low-frequency sea level trends and (3) the somewhat incomplete and sometimes inaccurate consideration of the effects of climatic variables upon sea level fluctuations and trends.

East Australian marine abrasion surface

Abstract Almost one-third of the seabed off the coastline north and south of Sydney comprises a planated bedrock surface, evident from sidescan surveys over the inner continental shelf. In seismic records, this rock surface extends up to 23 km offshore from the sea cliffs along 300 km of the coast. The rock surface dips offshore to as much as 180 m below sea level, where it merges with a major unconformity in the shelf sediment wedge. The surface is eroded into Mesozoic and Palaeozic rocks and is heavily dissected by sediment-filled, palaeo-valley incision and structural jointing. The sediment-fills comprise sand wedges that thicken landwards to form beaches and estuarine flood-tide deltas, respectively, in smaller and larger palaeo-valleys incised to below present sea level. At the base of the cliffs, the planated surface is buried by shelf sand bodies up to 30 m thick in places. The seaward edge of the surface is everywhere buried by the onlapping continental-shelf sediment wedge. The contiguity of the abrasion surface with the unconformity in the shelf sediment wedge suggests that marine planation began in the Mid-Oligocene, indicating time-average rates of gross cliff retreat at about 1 mm a−1.

The man/land theme in geography: a Sydney University perspective

SUMMARY Geography as practised at Sydney University over the years has been multi‐faceted. One branch of the discipline is the man/land (or people/environment) theme which was nurtured by the founding fathers of the department, Griffith Taylor and Macdonald Holmes. Numerous staff members have taught and researched aspects of this theme. This paper reviews some of the contributions from the time of Griffith Taylor to the present.

Reply to: Pilkey, O.H. and Cooper, A.G., 2006. Discussion of: Cowell et al., 2006. Management of Uncertainty in Predicting Climate-Change Impacts on Beaches, Journal of Coastal Research, 22(2), 232–245; Journal of Coastal Research, 22(6), 1577–1579

The discussion of our paper by Pilkey and Cooper highlights the existence of two contrasting approaches to the assessment of coastal change. We represent that tradition in coastal science that seeks to organise qualitative understanding of complex coastal-change problems through a systematic framework that sorts relevant factors on the basis of scale to guide development and selection of appropriate tools for quantitative exploration of potential coastal changes in specific cases (COWELL and THOM, 1994; COWELL et al., 2003; STIVE, 2004; WRIGHT and THOM, 1977). Pilkey and Cooper take a different perspective. To them the nature of shoreline response is too complex to model deterministically or probabilistically for accurate quantitative predictions or forecasts. They, and other colleagues, have been forceful in their criticism of the application of coastal models in general (COOPER and PILKEY, 2004a; THIELER et al., 2000). Alternatives to the purely modelling approach have been offered by these authors (see also COOPER and PILKEY 2004b). The alternative approach involves an examination of past patterns of accretion and erosion and the use of expert judgment in assessing likely future change at any given site. We note that Pilkey and Cooper recognise our contribution to coastal management with the use of probabilistic answers to coastal responses. What they cannot accept is the conceptual basis of the modelling that underpins our stochastic approach to problems of coastal change. In our response to their criticisms, we offer four arguments. The first is very straightforward. There is no way coastal scientists will turn their backs on the challenge of improving the capacity of models to predict coastal change, not only for

Promoting Resilience of Tomorrow’s Impermanent Coasts

There are three categories of actions that humans need to take in order to minimize the detrimental impacts of global change on tomorrow’s coastal systems. The first, of course, is to cause less harm by reducing our carbon footprint and ceasing to do destructive things like polluting, dredging, severing sediment supply, withdrawing groundwater, overdeveloping etc. Much has been written and spoken about this even though we have said relatively little about it in this book. The second category of actions, which has received minimal attention from the popular media but has been the motivating theme of this book, involves promoting deep enough understanding of the myriad complex interconnections of coastal processes to allow long-term predictions of what may lie ahead. Such predictions are essential to evolving effective strategies for adapting and remaining resilient. The third action is to ensure that to the extent possible we embed coastal science, including matters related to future impacts of climate change, into state and federal policies and law. The aim must be to ensure that regional coastal strategies are based on the best available science to reduce risk to built and natural assets from the adverse effects of short-term practices driven by local vested interests.

Future Adaptive Coastal Management

Many coastal regions constitute “contested spaces”. Potential consequences of climate change is bringing another set of pressures for communities and decision-makers to understand as they review and deliberate on options and set priorities for management action. Underpinning the decision-making process is a recognition of the values of those with particular interests at stake in any given location. Social conflicts arise from these different interests such as between landowners seeking to protect private property and those with a passion to ensure public good values are retained. Governments should be in a position to develop legislation that address all these interests in the context of long-term changes to coastal environments driven by natural forces knowing that frequently these forces are modified by human interventions. It is vital for science to have an input into the various stages of adaptive coastal management to ensure the consequences of existing and future human actions do not have adverse environment, social and economic consequences. Scientists should be willing to engage at all these stages from policy, law-making, implementation and enforcement and to monitor outcomes of actions so that in future improvements can be made. Their experience with field observations of coastal change, experiments in coastal processes, and modelling of long-term impacts are seen as important inputs in alerting governments and communities to those likely consequences of operating in a climate changing coastal world. This means that for adaptive coastal management in many countries to be effective coastal scientists should be prepared to move beyond the comfort zone of their disciplinary confines no matter how frustrating and painful it may be at times.