China
Africa
Explore chapters and articles related to this topic
Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
Rates of denudation depend on climatic, geostructural, geomorphological, and soil cover conditions. Human land development, in forms of land use, including agricultural and urban development, is considered a significant factor in erosion and sediment transport. Corbel has calculated that a mean world denudation rate of 30 mm/1,000 years, corresponding to 30 meters in a million years or even 300 meters in ten million years, is sufficient to produce considerable volumes of sediments, and bring about major landscape changes. A useful measure of these slow processes is the Bubnoff unit (B): 1 B = 1 mm/ky = 1 µ/y; Corbel's figure becomes then 30 B. Lake Mead during the years 1935–4198 received 2,000 million tons. The Mississippi drainage basin, which covers an area of about 4.38 million square kilometers, annually receives about 1.15 tons of soil from each square kilometer. Approximately, two-thirds of this material are carried in suspension. It has been estimated that about 710 million tons of materials are removed from the USA each year (Schumm, 1963).
Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Land areas are continually being reduced and their shape modified by weathering and erosion, and the general term for this is denudation. Rocks exposed to the atmosphere undergo weathering from atmospheric agents such as rain and frost. Chemical weathering, or decomposition, is the break-down of minerals into new compounds by the action of chemical agents; acids in the air, in rain and in river water, although they act slowly, produce noticeable effects especially in soluble rocks. Mechanical weathering, or disintegration, breaks down rocks into small particles by the action of temperature, by impact from raindrops and by abrasion from mineral particles carried in the wind. In very hot and very cold climates changes of temperature produce flaking of exposed rock surfaces. In areas of intense rainfall soil particles may be dislodged and the surface of the soil weakened by raindrops. In arid areas landforms are shaped by sand blasted against them during storms. Biological weathering describes those mechanical and chemical changes of the ground that are directly associated with the activities of animals and plants. When present, microbial activity can change the chemistry of the ground close to ground level. Burrowing animals and the roots of plants penetrate the ground and roots produce gasses which increase the acidity of percolating rain water.
Soil erosion and desertification
Published in F.G. Bell, Geological Hazards, 1999
Soil erosion is part of the process of denudation along with the other agents that are responsible for changing the landscape. According to Kirby (1980), soil erosion becomes a more effective agent of denudation once slopes are reduced to gradients that are relatively stable as far as mass movements are concerned. Unfortunately, however, this process can be accelerated by the activity of man: witness the disasters in the Dust Bowl and Tennessee Valley of the United States prior to the Second World War. It is difficult however, to separate natural from human-induced changes in erosion rates. Although accelerated, human-induced soil erosion does not figure as prominently among environmental issues now as it did then, this does not mean that the problem no longer exists. It most certainly does, and in some countries it is as serious as it ever was, if not more so.
Critical runoff depth estimation for incipient motion of non-cohesive sediment on loose soil slope under heavy rainfall
Published in Geomatics, Natural Hazards and Risk, 2019
Xingyu Yuan, Neng Zheng, Fei Ye, Wenxi Fu
Soil erosion, as an extremely serious global environment problem, has been of great concern worldwide (Pimentel 2006, Trimble and Crosson 2000). For the triggering mechanisms, it is generally recognized that soil erosion is mainly influenced by exogenic forces, such as wind, freezing-thaw, overland flow and rainfall (Abdulkadir et al. 2019, Geng et al. 2017). Meanwhile, it could also be influenced by materials, such as organic matter (Liu et al. 2018, Liu et al. 2017). Soil erosion induced by overland flow and rainfall, known as water erosion, is widespread in the world, especially in hilly region. During water erosion process, transport and denudation of soil or other slope sediment would occur under hydraulic conditions, and they could provide provenance for geological hazards, such as landslides and debris flows, seriously threatening local people’s life and property. Therefore, it is necessary to focus on slope failure mechanism from the perspective of water erosion. In nature, soil and other sediment could be classified into cohesive sediment and non-cohesive sediment. Compared with cohesive sediment, non-cohesive sediment would be more easily driven away by water flow as there is no bond between particles.
Sources and downstream variation of surface water chemistry in the dammed Waitaki catchment, South Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Vincent Pettinger, Candace E. Martin, Christina R. Riesselman
The denudation of mountain belts takes place via chemical and physical erosion, the relative intensities of which reflect both climatic (e.g. precipitation, temperature and runoff) and tectonic (e.g. exhumation, landscape surface age, mechanical weathering and sediment transport) factors. The chemical weathering of silicates is generally considered to be responsible for regulating the carbon cycle on geological time scales, which in turn has control on global average temperature (e.g. Berner et al., 1983). However, in settings where the residence time of fresh waters in a catchment is short, carbonate weathering becomes an important control on their chemical and isotopic composition. Furthermore, the relationship between physical and chemical weathering in a catchment depends on the rates of uplift and weathering. For example, previous work on the Southern Alps of New Zealand has shown that c. 90% of total weathering is physical, whereas the remainder is chemical (Lyons et al.,2005).
Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis
Published in Australian Journal of Earth Sciences, 2018
Slow erosion over extended time periods (denudation) is a well-established geomorphological process. Rates of contemporary and long-term denudation in the Sydney Basin are low, varying between 5.5 and 21 mm/ka (Tomkins et al., 2007). When cliff tops are eroded along horizontal bedding planes and vertical joints, blocks can become detached and subsequently slide to rest close to the original outcrop level (Cox et al., 2010). This process of ‘slow collapse’ is non-catastrophic, and can result in imbrication of detached blocks (e.g. Figure 2b) but would be unlikely to result in extensive overturn and randomisation of individual blocks in contrast to rapid collapse from high cliff falls.