s: The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for ...s: The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.展开更多
Soils are under threat around the world. Erosion, pollution, salinization and acidification degrade soils irreversibly. Evidently, the rate of soil degradation greatly exceeds the rate of soil formation. In some count...Soils are under threat around the world. Erosion, pollution, salinization and acidification degrade soils irreversibly. Evidently, the rate of soil degradation greatly exceeds the rate of soil formation. In some countries, soil lost is almost 100 times faster than the rate of its formation. Therefore, soil needs to be considered as a nonrenewable resource. Incessant construction activities along with unsustainable mining operations and energy-related undertakings in recent years have accelerated the degradation of soil in Turkey. Highway travellers can witness the mindless plunder all over the land. As one travels smoothly on newly constructed highways, looming along the roads are piles of leftover destruction： heaps of broken asphalt from previous roads; newly excavated topsoil dumped into adjacent streams; eroded hillsides. In the age of global climate change, these so-called development efforts sound incomprehensible since such anthropogenic activities exacerbate the change. Clearly, climate change increases the erosion rates and reduces the soil quality. This fact alone requires each country to adopt and implement sound conservation practices to protect their soils, and hence, social stability and security. The most effective way to conserve soil is to establish and maintain ground cover vegetation. Avoiding unnecessary soil sealing also conserves soil.展开更多
文摘s: The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.
文摘Soils are under threat around the world. Erosion, pollution, salinization and acidification degrade soils irreversibly. Evidently, the rate of soil degradation greatly exceeds the rate of soil formation. In some countries, soil lost is almost 100 times faster than the rate of its formation. Therefore, soil needs to be considered as a nonrenewable resource. Incessant construction activities along with unsustainable mining operations and energy-related undertakings in recent years have accelerated the degradation of soil in Turkey. Highway travellers can witness the mindless plunder all over the land. As one travels smoothly on newly constructed highways, looming along the roads are piles of leftover destruction： heaps of broken asphalt from previous roads; newly excavated topsoil dumped into adjacent streams; eroded hillsides. In the age of global climate change, these so-called development efforts sound incomprehensible since such anthropogenic activities exacerbate the change. Clearly, climate change increases the erosion rates and reduces the soil quality. This fact alone requires each country to adopt and implement sound conservation practices to protect their soils, and hence, social stability and security. The most effective way to conserve soil is to establish and maintain ground cover vegetation. Avoiding unnecessary soil sealing also conserves soil.
