Geological hazards in loess terrain, with particular reference to the loess regions of China
Introduction
Drylands cover vast areas of central, east and south Asia, including about 30% of the total area of China (Fig. 1). Many of China's deserts are of the warm type, with July temperatures >24°C. However, most have severe winters, with mean minima for the coldest month <−8°C, with extensive areas <−12°C. Most of Asia's deserts lie at moderate to high altitudes. For example, the great Plateau of Tibet (Qinghai-Xizang), at an altitude of ca. 5000 m above sea level, is a cold desert. Loess terrain in central and eastern Asia is generally peripheral to the deserts and piedmonts, as shown in the well-known map by Rozycki (1991), Fig. 2.
The past and present distributions of the drylands of central and eastern Asia are fundamentally related to the Asian monsoon and its fluctuations. Over much of the region, cold, dry winds from between north and west dominate the region under the influence of the Mongolian–Siberian high-pressure system during winter (Fig. 3). In summer, with the breakdown of the Mongolian–Siberian High, warm, moist oceanic air is drawn into the heart of the continent from the Indian Ocean and South China Sea, reversing the regional flow. There is accumulating evidence that uplift of the Tibetan Plateau and the Himalaya has played a crucial role in the development of the Asian monsoon, a recent numerical climate-modelling experiment suggesting that three main stages in the evolution of the Asian monsoon (ca. 9-8, 3.6-2.6, and post 2.6 Myr ago) reflect phases in Himalayan-Tibetan Plateau uplift as well as successive Northern Hemisphere glaciation (An et al., 2001). Uplift has not only diversified the climates across this great region, but it has also impeded significantly the ingress of moisture into central Asia. Whether the critical threshold occurred at the beginning of the Quaternary (e.g. Li et al., 1979, Li et al., 1995), around 3–3.5 Ma ago (Qiang et al., 2000) or as long ago as 7–8 Ma BP Harrison et al., 1992, Prell and Kutzbach, 1992, Molnar et al., 1993 remains contentious. The first input of loess into China is now put at 7–8 Ma BP Ding et al., 1998, Sun et al., 1998 or 8.35 Ma (Qiang et al., 2000), but the rate of accumulation greatly accelerated between 3.5 and 3.1 Ma, with further intensification after about 2.6 Ma BP. Moreover, the loess cover became progressively more extensive as the Quaternary progressed (Liu and Ding, 1984), suggesting that conditions in central and east Asia became progressively drier, especially in the late Quaternary.
The considerable morphodynamic energy provided by the continuing tectonic evolution of Asia is expressed in high erosional potentials and very high rates of sediment production. Sand and silt grains are generated by tectonic crushing, glacier grinding, freeze–thaw action, salt-weathering and cyclic hydration. They are reworked and exported from the region by some of the world's greatest rivers, including the Hwang He (Yellow River), Yangtze, Indus, Ganges, Syr Darya and Amu Darya (Fig. 2), which carry greater sediment loads than any other major rivers (Ferguson, 1984). The juxtaposition of high mountains and plateaus with frost action and glaciers, deep desert basins, and great rivers has produced a distinctive landform assemblage and substantial sedimentary accumulations, including the exceptional accumulations of loess up to ca. 500 m thick found in North China (e.g. Liu, 1958, Liu, 1985, Liu and Chang, 1964, Liu et al., 1964, Liu et al., 1965, Liu et al., 1966, Liu et al., 1987, Derbyshire et al., 1997). The Loess Plateau of China (Fig. 4) lies in the middle reaches of the Hwang He (Yellow River), between the Tibetan Plateau to the southwest and the deserts to the north and west (Fig. 2), the thickest loess series being found within the major structural basins (see Fig. 8.1 in Derbyshire et al., 1993).
Southeastwards across western and central China, rocky desert and gobi (Mongolian=stony desert) grades into sandy desert, then into areas of sandy loess and, finally, into thick loess (Fig. 5), with progression from arid, through semi-arid to sub-humid climatic regimes. The average grain size of the loess also diminishes from NW to SE across the Loess Plateau, reflecting in a general way the direction and strength of the dominant winds (Liu et al., 1966).
Section snippets
The Chinese written record of loess terrain hazards
Given its distinctive combination of strong seismicity, high relative relief, steep slopes and strongly seasonal (monsoonal) climates, central and eastern Asia is one of the earth's most dynamic landscapes. It is a harsh and challenging environment for its multi-million population. The remarkable Chinese written record of the last 2000 years is packed with accounts of a wide range of events including floods and droughts, locust swarms, shifting desert margins, dust storms and landslides.
Loess and loess terrain
Loess mantles about 631 000 km2 of China, equivalent to about 6.6% of the total area of the country Liu et al., 1964, Liu, 1985. About 317 000 km2 forms the Loess Plateau, the greatest bulk accumulation of loess on earth. The distinctive physical properties of loess have here resulted in a unique landscape type that is highly susceptibility to erosion.
Conclusion
The mountainous drylands of the earth, and particularly those in High Asia, are vulnerable to both natural (i.e. climatically and tectonically driven) and artificially enhanced environmental changes that reduce the stability of slopes and soils and threaten essential water supplies.
The populations occupying the loess regions of north China are subject to several types of hazard arising from the distinctive properties and bulk behaviour of loess, a tectonically active landscape, and a diverse,
Acknowledgements
The material on which this review is based was accumulated over a quarter of a century, so that the list of colleagues to whom the reviewer is indebted is legion, making full acknowledgment impracticable. The late Professor Wang Jingtai (Gansu Academy of Sciences) was a leading light in that period of collaboration, and Dr. Xingmin Meng's considerable knowledge and appreciation of the Chinese loess has been a source of invaluable help and insight right up to the present in both scientific and
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