Abstract
Coastal and estuarine landforms provide a physical template that not only accommodates diverse ecosystem functions and human activities, but also mediates flood and erosion risks that are expected to increase with climate change. In this paper, we explore some of the issues associated with the conceptualisation and modelling of coastal morphological change at time and space scales relevant to managers and policy makers. Firstly, we revisit the question of how to define the most appropriate scales at which to seek quantitative predictions of landform change within an age defined by human interference with natural sediment systems and by the prospect of significant changes in climate and ocean forcing. Secondly, we consider the theoretical bases and conceptual frameworks for determining which processes are most important at a given scale of interest and the related problem of how to translate this understanding into models that are computationally feasible, retain a sound physical basis and demonstrate useful predictive skill. In particular, we explore the limitations of a primary scale approach and the extent to which these can be resolved with reference to the concept of the coastal tract and application of systems theory. Thirdly, we consider the importance of different styles of landform change and the need to resolve not only incremental evolution of morphology but also changes in the qualitative dynamics of a system and/or its gross morphological configuration. The extreme complexity and spatially distributed nature of landform systems means that quantitative prediction of future changes must necessarily be approached through mechanistic modelling of some form or another. Geomorphology has increasingly embraced so-called ‘reduced complexity’ models as a means of moving from an essentially reductionist focus on the mechanics of sediment transport towards a more synthesist view of landform evolution. However, there is little consensus on exactly what constitutes a reduced complexity model and the term itself is both misleading and, arguably, unhelpful. Accordingly, we synthesise a set of requirements for what might be termed ‘appropriate complexity modelling’ of quantitative coastal morphological change at scales commensurate with contemporary management and policy-making requirements: 1) The system being studied must be bounded with reference to the time and space scales at which behaviours of interest emerge and/or scientific or management problems arise; 2) model complexity and comprehensiveness must be appropriate to the problem at hand; 3) modellers should seek a priori insights into what kind of behaviours are likely to be evident at the scale of interest and the extent to which the behavioural validity of a model may be constrained by its underlying assumptions and its comprehensiveness; 4) informed by qualitative insights into likely dynamic behaviour, models should then be formulated with a view to resolving critical state changes; and 5) meso-scale modelling of coastal morphological change should reflect critically on the role of modelling and its relation to the observable world.
Highlights
Morphodynamic complexity arises in several ways, including the residual influence of previous states; the interplay between self-regulation and selfforcing, and the non-linear nature of many of the functional linkages between system components
Several issues arose from this type of analysis: i) the understanding of what drift- and swash-alignment meant in terms of coastal behaviour was not widely understood beyond academic geomorphology; ii) the interaction of estuaries with the open coast was not given real prominence, given the difficulty of developing pathways for finer non-beach grade sediment (Cooper and Pontee, 2006); and iii) sediment pathways, as articulated by supply changes, were not recognised as the major time and spatial scale determinant of coastal evolution
Whilst there is certainly scope for changing the kind of questions that we demand of models there remains a real demand for quantitative prediction of coastal morphodynamic behaviour, even if this is translated into semi-qualitative outcomes — such as the probabilistic assessment of potential changes in state
Summary
Predictions of morphological change at the coast are increasingly important, since coastal landforms provide a physical template that accommodates diverse ecosystem functions and human activities (Murray et al, 2008), and mediates flood and erosion risk (Sayers et al, 2002; Narayan et al, 2012). This position paper arises from a need to formulate an overarching theoretical framework for a programme of mesoscale coastal behaviour model development being undertaken in the Integrating Coastal Sediment Systems (iCOASST) project (Nicholls et al, 2012). We identify a set of requirements for what might be termed ‘appropriate complexity modelling’ of quantitative coastal morphological change at a mesoscale that is commensurate with contemporary management and policy-making requirements
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