|Working Group II: Impacts, Adaptation and Vulnerability|
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6.4.2. Beaches, Barriers, and Cliff Coasts
Sandy coasts shaped and maintained primarily by wave and tidal processes occupy about 20% of the global coastline (Bird, 1993). A smaller proportion consists of gravel and cobble-boulder beaches and related landforms, occurring in tectonically active and high-relief regions and in mid- to high-latitude areas of former glaciation. Coral rubble beaches and islands are common in low-latitude reefal areas. Any analysis of climate-change impacts on the coastal zone should include beaches and barriers of sand and/or gravel as well as coastal cliffs and bluffs.
Over the past 100 years or so, about 70% of the world's sandy shorelines have been retreating, about 20-30% have been stable, and less than 10% have been advancing. Bird (1993) argues that with global warming and sea-level rise there will be tendencies for currently eroding shorelines to erode further, stable shorelines to begin to erode, and accreting shorelines to wane or stabilize. Local changes in coastal conditions and particularly in sediment supply may modify these tendencies, although Nicholls (1998) has indicated that accelerated sea-level rise in coming decades makes general erosion of sandy shores more likely.
Previous discussions of shoreline response to climate change have considered the well-known simple relations between sea-level rise and shoreline retreat of Bruun (1962). This two-dimensional model assumes maintenance of an equilibrium nearshore profile in the cross-shore direction as sea level rises. Some papers have supported this approach for long-term shoreline adjustment (Mimura and Nobuoka, 1996; Leatherman et al., 2000); others have suggested various refinements (Komar, 1998a). Although the model's basic assumptions are rarely satisfied in the real world (Bruun, 1988; Eitner, 1996; Trenhaile, 1997), its heuristic appeal and simplicity have led to extensive use in coastal vulnerability assessments for estimating shoreline retreat under rising sea levels, with varying degrees of qualification (Richmond et al., 1997; Lanfredi et al., 1998; Stewart et al., 1998). Erroneous results can be expected in many situations, particularly where equilibrium profile development is inhibited, such as by the presence of reefs or rock outcrops in the nearshore (Riggs et al., 1995). Moreover, Kaplin and Selivanov (1995) have argued that the applicability of the Bruun Rule, based on an equilibrium approach, will diminish under possible future acceleration of sea-level rise.
Few models of shoreline response incorporate large-scale impacts of sea-level rise coupled to changes in sediment availability. Efforts to address this shortcoming have been pioneered by Cowell and Thom (1994) for sandy barrier-dune complexes and Forbes et al. (1995) for gravel barriers. Although these parametric models incorporate sediment supply as well as sea-level change, they are still in the early stages of development and are useful primarily to indicate general patterns of response. A multifaceted approach is needed to incorporate other factors such as longshore and cross-shore variability in shore-zone morphology, sediment supply, texture and composition, nonlinear shore-zone response to storms and storm sequences (Forbes et al., 1995), tectonic history of the site, and the presence or absence of biotic protection such as mangroves or other strand vegetation.
Impact assessment, adaptation actions, and other management decisions must consider all of these factors within a coastal systems context. Temporal variation in storminess and wind climate can produce significant coastal adjustments (Forbes et al., 1997a). Another important component of analysis involves historical trends of shoreline change, including variability caused by storms or other anomalous events (Douglas et al., 1998; Gorman et al., 1998). This analysis can provide essential baseline data to enable comparisons in the future, albeit prior to anticipated climate-change impacts.
Field studies and numerical simulation of long-term gravel barrier evolution in formerly glaciated bays of eastern Canada (Forbes et al., 1995) have revealed how sediment supply from coastal cliffs may be positively correlated with the rate of relative sea-level rise. In this case, rising relative sea level favors barrier progradation, but the system switches to erosional retreat when the rate of sea-level rise diminishes, cliff erosion ceases, and no new sediment is supplied to the beach. Along the South American coast, El Niño events are linked to higher-than-average precipitation causing increased sediment discharge to the Peruvian coast, leading to the formation of gravel beach-ridge sequences at several sites (Sandweiss et al., 1998).
In assessing coastal response to sea-level rise, the relevant sedimentary system may be defined in terms of large-scale coastal cells, bounded by headlands or equivalent transitionstypically one to several tens of kilometers in length and up to hundreds of kilometers in some places (Wijnberg and Terwindt, 1995). Within such cells, coastal orientation in relation to dominant storm wind and wave approach direction can be very important (Héquette et al., 1995; Short et al., 2000), and sediment redistribution may lead to varying rates and/or directions of shoreline migration between zones of sediment erosion and deposition.
Changes in wave or storm patterns may occur under climate change (Schubert et al., 1998). In the North Atlantic, a multidecadal trend of increased wave height is observed, but the cause is poorly understood and the impacts are unclear. Changing atmospheric forcing also has been suggested as a process contributing to increases in mean water level along the North Sea coast, independent of eustatic and isostatic contributions to relative sea level. Changes in large-scale ocean-atmospheric circulation and climate regimes such as ENSO and the Pacific Decadal Oscillation have implications for coastal beach and barrier stability (see Box 6-4).
Erosion of unlithified cliffs is promoted by rising sea levels but may be constrained or enhanced by geotechnical properties and other antecedent conditions (Shaw et al., 1998a; Wilcock et al., 1998). Bray and Hooke (1997) review the possible effects of sea-level rise on soft-rock cliffs over a 50- to 100-year planning scale. They evaluate different methods of analyzing historical recession rates and provide simple predictive models to estimate cliff sensitivity to sea-level rise in southern England. Historical observations of cliff erosion under an accelerating sea level suggest, however, that the results of such methods must be interpreted carefully.
If El Niño-like conditions become more prevalent (Timmermann et al., 1999), increases in the rate of cliff erosion may occur along the Pacific coasts of North and South America (Kaminsky et al., 1998; Komar, 1998a,b). For example, El Niño events raise sea level along the California coast and are marked by the presence of larger than average, and more damaging, waves and increased precipitation. These conditions and the changed direction of wave attack combine to increase sea-cliff erosion on the central California coast, particularly on southerly or southwesterly facing cliffs. An increase in El Niño-like conditions with global warming would very likely increase sea-cliff erosion along this section of coast and endanger infrastructure and property (Storlazzi and Griggs, 2000).
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