6.4.1.2 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).
Box 6.3. Deltas and megadeltas: hotspots for vulnerability
Deltas, some of the largest sedimentary deposits in the world, are widely recognised as highly vulnerable to the impacts of climate change, particularly sea-level rise and changes in runoff, as well as being subject to stresses imposed by human modification of catchment and delta plain land use. Most deltas are already undergoing natural subsidence that results in accelerated rates of relative sea-level rise above the global average. Many are impacted by the effects of water extraction and diversion, as well as declining sediment input as a consequence of entrapment in dams. Delta plains, particularly those in Asia (Chapter 10, Section 10.6.1), are densely populated and large numbers of people are often impacted as a result of external terrestrial influences (river floods, sediment starvation) and/or external marine influences (storm surges, erosion) (see Figure 6.1).
Ericson et al. (2006) estimated that nearly 300 million people inhabit a sample of 40 deltas globally, including all the large megadeltas. Average population density is 500 people/km2 with the largest population in the Ganges-Brahmaputra delta, and the highest density in the Nile delta. Many of these deltas and megadeltas are associated with significant and expanding urban areas. Ericson et al. (2006) used a generalised modelling approach to approximate the effective rate of sea-level rise under present conditions, basing estimates of sediment trapping and flow diversion on a global dam database, and modifying estimates of natural subsidence to incorporate accelerated human-induced subsidence. This analysis showed that much of the population of these 40 deltas is at risk through coastal erosion and land loss, primarily as a result of decreased sediment delivery by the rivers, but also through accentuated rates of sea-level rise. They estimate, using a coarse digital terrain model and global population distribution data, that more than 1 million people will be directly affected by 2050 in three megadeltas: the Ganges-Brahmaputra delta in Bangladesh, the Mekong delta in Vietnam and the Nile delta in Egypt. More than 50,000 people are likely to be directly impacted in each of a further 9 deltas, and more than 5,000 in each of a further 12 deltas (Figure 6.6). This generalised modelling approach indicates that 75% of the population affected live on Asian megadeltas and deltas, and a large proportion of the remainder are on deltas in Africa. These impacts would be exacerbated by accelerated sea-level rise and enhanced human pressures (e.g., Chapter 10, Section 10.6.1). Within the Asian megadeltas, the surface topography is complex as a result of the geomorphological development of the deltas, and the population distribution shows considerable spatial variability, reflecting the intensive land use and the growth of some of the world’s largest megacities (Woodroffe et al., 2006). Many people in these and other deltas worldwide are already subject to flooding from both storm surges and seasonal river floods, and therefore it is necessary to develop further methods to assess individual delta vulnerability (e.g., Sánchez-Arcilla 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 view and examines the Asian megadeltas in more detail. Chapter 5, Box 5.3 considers the threats to fisheries in the lower Mekong and associated delta due to climate change. Hurricane Katrina made landfall on the Mississippi delta in Louisiana, and Box 6.4 and Chapter 7, Box 7.4 consider different aspects of this important event, which gives an indication of the likely impacts if tropical storm intensity continues to increase. Lastly, Section 15.6.2 considers the specific problems of Arctic megadeltas.