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7 - Coral atolls
- Edited by R. W. G. Carter, University of Ulster, C. D. Woodroffe, University of Wollongong, New South Wales
- Foreword by Orson van de Plassche
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- Book:
- Coastal Evolution
- Published online:
- 06 July 2010
- Print publication:
- 05 January 1995, pp 267-302
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Summary
Introduction
Coral atolls consist of an annular reef rim surrounding a central lagoon. On the atoll rim there may be reef islands either sandy cays or shingle motu (Stoddart & Steers, 1977). They appear particularly fragile constructions, exposed to a range of oceanographic, atmospheric and anthropogenic processes, and yet the prototypes of present day sea-level atolls have endured in a variety of forms for millions of years. Of the 425 atolls in the world (Stoddart, 1965), most are in the Indian and Pacific Oceans, though isolated atolls do occur in the Caribbean (e.g. Stoddart, 1962a)
The evolution of atolls is of particular relevance in the context of coastal evolution studies, because one of the most central theories of atoll evolution dates back to Charles Darwin and the voyage of HMS Beagle. This voyage gave rise to the concept of natural selection and evolution of species, with the publication of On the Origin of Species in 1859, but Darwin's view on evolution of atolls, was published much earlier with his observations on the geology of reefs (Darwin, 1842).
Darwin had formulated his theory of reef development shortly after leaving South America, and before seeing the reefs of the Pacific. He refined his ideas as a consequence of visiting reefs in the Society Islands, and had completed a draft of his theory before reaching New Zealand (see Stoddart, 1962b). The only atoll that Darwin landed on was the Cocos (Keeling) Islands in the Indian Ocean, and at that stage he was keen to verify his intuition.
1 - Coastal evolution: an introduction
- Edited by R. W. G. Carter, University of Ulster, C. D. Woodroffe, University of Wollongong, New South Wales
- Foreword by Orson van de Plassche
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- Book:
- Coastal Evolution
- Published online:
- 06 July 2010
- Print publication:
- 05 January 1995, pp 1-32
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Summary
‘if the environment is the theatre, then evolution is the play’
G. Evelyn HutchinsonStudies of coastal evolution examine and explore the reasons why the position and nature of the shoreline alter from time to time. Although this type of approach has been practised for generations – by geomorphologists, geologists and engineers – events over the last two decades have brought a new immediacy to the subject. Generally there has been realisation that many of the world's coastlines are under ‘threat’ (see, for example the recent US Geological Survey Publication Coasts in crisis (Williams, Dodd & Gohn, 1990) or the Intergovernmental Panel on Climate Change (IPCC) report Global climate change and the rising challenge of the sea (1992)) and that environmental change is the consequence of human occupation of shorelines, to which adjustment (of some kind) is inevitable. Specifically, the spectre of rising sea levels has induced a strong political response as well as raising inevitable questions among scientists as to the state of our knowledge and understanding (processes that are not always convergent). It would not be hard to conclude that our knowledge is woefully thin. Despite many excellent studies from a wide range of environments, we are still well short of understanding how a coastline will respond or react to secular variations in forcing functions such as sea-level rise, storm intensity and magnitude variations or shifts in the sea state pattern. The commonest conclusion is to predict flooding or coastal erosion, yet such processes are clearly only part of much broader responses, which need to be viewed over a range of scales, in both space and time.
5 - Macrotidal estuaries
- Edited by R. W. G. Carter, University of Ulster, C. D. Woodroffe, University of Wollongong, New South Wales
- Foreword by Orson van de Plassche
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- Book:
- Coastal Evolution
- Published online:
- 06 July 2010
- Print publication:
- 05 January 1995, pp 187-218
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Summary
Introduction
Macrotidal estuaries can be viewed within a continuum of deltaic-estuarine coastal depositional settings, influenced by riverine processes, wave regime and tidal energy (Wright & Coleman, 1973; Wright, 1985; Boyd, Dalrymple & Zaitlin, 1992; see Fig. 1.6). The morphodynamics of individual macrotidal estuaries are a function of sea-level changes and prior or inherited topography. The response of estuaries to sea-level changes, past and future, is affected by tidal range, nearshore wave climate, river inflow and the nature and supply of sediment. All estuaries assumed their present form during the rise of sea level that followed the last glacial maximum, about 18000 years ago. In areas that are relatively stable, such as northern Australia, estuaries have had similar sealevel histories, whereas in areas of very rapid crustal uplift or glacioisostatic response, estuaries are more likely to have experienced highly individual relative sea-level histories. The variety of estuaries reflects the range of submerged prior landforms, from relatively straight, steep coastlines through valleys in differing stages of infill to rock-barred basins. These different types of prior topography, which are manifest in coasts such as Has and dendritic drowned valley harbours, influence the interplay of processes which redistribute sediment to produce estuarine channels, tidal basins, backwaters and floodplains. Vegetation can modify estuarine morphodynamics through its effects upon sediment trapping and through its influence on the shear strength of channel banks.
This chapter is concerned with macrotidal estuaries, mostly in northern Australia which tectonically is a very stable region, with low sediment yield (about 5–15tkm−2a−1) and broad continental shelves.