Box 62 Examples of extreme water level simulations for impact studies

Although inundation by increases in mean sea level over the 21st century and beyond will be a problem for unprotected low-lying areas, the most devastating impacts are likely to be associated with changes in extreme sea levels resulting from the passage of storms (e.g., Gornitz et al., 2002), especially as more intense tropical and extra-tropical storms are expected (Meehl et al., 2007). Simulations show that future changes are likely to be spatially variable, and a high level of detail can be modelled (see also Box 11.5 in Christensen et al. (2007).

Chittagong Water Level

Figure 6.4. Increases in the height (m) of the 50-year extreme water level. (a) In the northern Bay of Bengal under the IS92a climate scenario in 2040-2060 (K - Kolkata (Calcutta), C - Chittagong) (adapted from Mitchell et al., 2006). (b) Around the UK for the A2 scenario in the 2080s (L - London; H - Hamburg) (adapted from Lowe and Gregory, 2005).

Figure 6.4. Increases in the height (m) of the 50-year extreme water level. (a) In the northern Bay of Bengal under the IS92a climate scenario in 2040-2060 (K - Kolkata (Calcutta), C - Chittagong) (adapted from Mitchell et al., 2006). (b) Around the UK for the A2 scenario in the 2080s (L - London; H - Hamburg) (adapted from Lowe and Gregory, 2005).

Figures 6.4 and 6.5 are based on barotropic surge models driven by climate change projections for two flood-prone regions. In the northern Bay of Bengal, simulated changes in storminess cause changes in extreme water levels. When added to consistent relative sea-level rise scenarios, these result in increases in extreme water levels across the Bay, especially near Kolkata (Figure 6.4a). Around the UK, extreme high sea levels also occur. The largest change near London has important implications for flood defence (Figure 6.4b; Dawson et al., 2005; Lavery and Donovan, 2005). Figure 6.5 shows the change in flooding due to climate change for Cairns (Australia). It is based on a combination of stochastic sampling and dynamic modelling. This assumes a 10% increase in tropical cyclone intensity, implying more flooding than sea-level rise alone would suggest. However, detailed patterns and magnitudes of changes in extreme water levels remain uncertain (e.g., Lowe and Gregory, 2005); better quantification of this uncertainty and further field validation would support wider application of such scenarios.

Current climate

2050 climate Cairns road network

2050 climate Cairns road network

Figure 6.5. Flooding around Cairns, Australia during the >100 year return-period event under current and 2050 climate conditions based on a 2xCO2 scenario. The road network is shown in black (based on Mclnnes et al., 2003).

heights have enhanced erosion rates of bay shorelines, tidal creeks and adjacent wetlands (Stone and McBride, 1998; Stone et al., 2003). The impacts of accelerated sea-level rise on gravel beaches have received less attention than sandy beaches. These systems are threatened by sea-level rise (Orford et al., 2001, 2003; Chadwick et al., 2005), even under high accretion rates (Codignotto et al., 2001). The persistence of gravel and cobble-boulder beaches will also be influenced by storms, tectonic events and other factors that build and reshape these highly dynamic shorelines (Orford et al., 2001).

Since the TAR, monitoring, modelling and process-oriented research have revealed some important differences in cliff vulnerability and the mechanics by which groundwater, wave climate and other climate factors influence cliff erosion patterns and rates. Hard rock cliffs have a relatively high resistance to erosion, while cliffs formed in softer lithologies are likely to retreat more rapidly in the future due to increased toe erosion resulting from sea-level rise (Cooper and Jay, 2002). Cliff failure and retreat may be amplified in many areas by increased precipitation and higher groundwater levels: examples include UK, Argentina and France (Hosking and McInnes, 2002; Codignotto, 2004b; Pierre and Lahousse, 2006). Relationships between cliff retreat, freeze-thaw cycles and air temperature records have also been described (Hutchinson, 1998). Hence, four physical features of climate change - temperature, precipitation, sea level and wave climate -can affect the stability of soft rock cliffs.

Soft rock cliff retreat is usually episodic with many metres of cliff top retreat occurring locally in a single event, followed by relative quiescence for significant periods (Brunsden, 2001; Eurosion, 2004). Considerable progress has been made in the long-term prediction of cliff-top, shore profile and plan-shape evolution of soft rock coastlines by simulating the relevant physical processes and their interactions (Hall et al., 2002; Trenhaile, 2002,2004). An application of the SCAPE (Soft Cliff and Platform Erosion) model (Dickson et al., 2005; Walkden and Hall, 2005) to part of Norfolk, UK has indicated that rates of cliff retreat are sensitive to sea-level rise, changes in wave conditions and sediment supply via longshore transport. For soft cliff areas with limited beach development, there appears to be a simple relationship between long-term cliff retreat and the rate of sea-level rise (Walkden and Dickson, 2006), allowing useful predictions for planning purposes.

6.4.12 Deltas

Deltaic landforms are naturally shaped by a combination of river, wave and tide processes. River-dominated deltas receiving fluvial sediment input show prominent levees and channels that meander or avulse2, leaving abandoned channels on the coastal plains. Wave-dominated deltas are characterised by shore-parallel sand ridges, often coalescing into beach-ridge plains. Tide domination is indicated by exponentially tapering channels, with funnel-shaped mouths. Delta plains contain a diverse range of landforms but, at any time, only part of a delta is active, and this is usually river-dominated, whereas the abandoned delta plain receives little river flow and is progressively dominated by marine processes (Woodroffe, 2003).

Human development patterns also influence the differential vulnerability of deltas to the effects of climate change. Sediment starvation due to dams, alterations in tidal flow patterns, navigation and flood control works are common consequences of human activity (Table 6.1). Changes in surface water runoff and sediment loads can greatly affect the ability of a delta to cope with the physical impacts of climatic change. For example, in the subsiding Mississippi River deltaic plain of south-east Louisiana, sediment starvation and increases in the salinity and water levels of coastal marshes due to human development occurred so rapidly that 1565 km2 of intertidal coastal marshes and adjacent lands were converted to open water between 1978 and 2000 (Barras et al., 2003). By 2050 about 1300 km2 of additional coastal land loss is projected if current global, regional and local processes continue; the projected acceleration of sea level and increase in tropical storm intensity (Section 6.3.2) would exacerbate these losses (Barras et al., 2003). Much of this land loss is episodic, as demonstrated during the landfall of Hurricane Katrina (Box 6.4).

Deltas have long been recognised as highly sensitive to sea-level rise (Ericson et al., 2006; Woodroffe et al., 2006) (Box 6.3). Rates of relative sea-level rise can greatly exceed the global average in many heavily populated deltaic areas due to subsidence, including the Chao Phraya delta (Saito, 2001), Mississippi River delta (Burkett et al., 2003) and the Changjiang River delta (Liu, 2002; Waltham, 2002), because of human activities. Natural subsidence due to autocompaction of sediment under its own weight is enhanced by sub-surface fluid withdrawals and drainage (Table 6.1). This increases the potential for inundation, especially for the most populated cities on these deltaic plains (i.e., Bangkok, New Orleans and Shanghai). Most of the land area of Bangladesh consists of the deltaic plains of the Ganges, Brahmaputra and Meghna rivers. Accelerated global sea-level rise and higher extreme water levels (Box 6.2) may have acute effects on human populations of Bangladesh (and parts of West Bengal, India) because of the complex relationships between observed trends in SST over the Bay of Bengal and monsoon rains (Singh, 2001), subsidence and human activity that has converted natural coastal defences (mangroves) to aquaculture (Woodroffe et al., 2006).

Whereas present rates of sea-level rise are contributing to the gradual diminution of many of the world's deltas, most recent losses of deltaic wetlands are attributed to human development. An analysis of satellite images of fourteen of the world's major deltas (Danube, Ganges-Brahmaputra, Indus, Mahanadi, Mangoky, McKenzie, Mississippi, Niger, Nile, Shatt el Arab, Volga, Huanghe, Yukon and Zambezi) indicated a total loss of 15,845 km2 of deltaic wetlands over the past 14 years (Coleman et al., 2005). Every delta showed land loss, but at varying rates, and human development activities accounted for over half of the losses. In Asia, for example, where human activities have led to increased sediment loads of major rivers in the past, the construction of upstream dams is now seriously depleting the supply of sediments to many deltas with increased coastal erosion a widespread consequence (see Chapter 10, Section 10.4.3.2). As an example, large reservoirs constructed on the Huanghe River in China have reduced the annual sediment delivered to its delta from 1.1 billion metric tons to 0.4 billion metric tons (Li et al., 2004). Human influence is likely to continue to increase throughout Asia and globally (Section 6.2.2; Table 6.1).

Sea-level rise poses a particular threat to deltaic environments, especially with the synergistic effects of other climate and human pressures (e.g., Sánchez-Arcilla et al., 2007). These issues are especially noteworthy in many of the largest deltas with an indicative area >104 km2 (henceforth megadeltas) due to their often large populations and important environmental services. The problems of climate change in megadeltas are reflected throughout this report, with a number of chapters considering these issues from complementary perspectives. Box 6.3 considers the vulnerability of delta systems across the globe, and concludes that the large populated Asian megadeltas are especially vulnerable to climate change. Chapter 10, Section 10.6.1 builds on this global

2 Avulse: when a river changes its course from one channel to another as a result of a flood.

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