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
甲病毒是感染人类和动物的重要病原体,该病毒属成员可分为两大类:一类可致关节炎,如基孔肯雅病毒;另一类可致脑炎,如委内瑞拉马脑炎病毒和东部马脑炎病毒。尽管对于甲病毒的研究已开展多年,但甲病毒的侵染机制和宿主抗病毒机制尚未清楚...甲病毒是感染人类和动物的重要病原体,该病毒属成员可分为两大类:一类可致关节炎,如基孔肯雅病毒;另一类可致脑炎,如委内瑞拉马脑炎病毒和东部马脑炎病毒。尽管对于甲病毒的研究已开展多年,但甲病毒的侵染机制和宿主抗病毒机制尚未清楚,亦未研制出有效的疫苗和抗病毒药物。近年来,基孔肯雅病毒和委内瑞拉马脑炎病毒的受体基质重塑关联蛋白8(matrix remodeling associated 8,MXRA8)和低密度脂蛋白受体结构域蛋白3(low density lipoprotein receptor class A domain containing 3,LDLRAD3)相继被发现,鉴于这两种受体为最新发现,时效性较强,因此本文就这两种已知受体的研究进展进行总结概述,以期为甲病毒致病机制的进一步研究及相关疾病的预防控制提供新的思路。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.
文摘甲病毒是感染人类和动物的重要病原体,该病毒属成员可分为两大类:一类可致关节炎,如基孔肯雅病毒;另一类可致脑炎,如委内瑞拉马脑炎病毒和东部马脑炎病毒。尽管对于甲病毒的研究已开展多年,但甲病毒的侵染机制和宿主抗病毒机制尚未清楚,亦未研制出有效的疫苗和抗病毒药物。近年来,基孔肯雅病毒和委内瑞拉马脑炎病毒的受体基质重塑关联蛋白8(matrix remodeling associated 8,MXRA8)和低密度脂蛋白受体结构域蛋白3(low density lipoprotein receptor class A domain containing 3,LDLRAD3)相继被发现,鉴于这两种受体为最新发现,时效性较强,因此本文就这两种已知受体的研究进展进行总结概述,以期为甲病毒致病机制的进一步研究及相关疾病的预防控制提供新的思路。
