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581.
Alan D. Howard 《地球表面变化过程与地形》1982,7(4):303-325
Equilibrium is defined as a single-valued, temporally invariant functional relationship between the values of an output variable and the values of the input variable(s) in a geomorphic system. Disequilibrium occurs if the output deviates from the functional relationship by more than a consensual degree. Natural geomorphic variables are characterized by a relaxation time. Output variables are insensitive to cyclical inputs with frequencies much greater than the relaxation time, but can respond completely for sufficiently low frequencies. Rapid trends, recent step changes or pulse inputs, and intermediate frequency inputs can cause disequilibrium. The gradient of sand-bed alluvial channels (the output variable) is determined by sediment and water delivery from slopes (the input variables), and changes in this hydraulic regime require regrading by erosion and deposition. Initial stages of adjustment to changed regime in a long, unbranched channel with sediment and water delivery only at the upstream end propagate downstream, but later stages of adjustment occur simultaneously throughout the reach. In a dendritic channel network the gradient responds rather uniformly throughout the network to changes in regime during all stages of adjustment. The time scale of adjustment to changed regime depends upon the size of the channel network (or stream length), the sediment and water discharges, and to a lesser degree upon the magnitude of the change. Grade as defined by Mackin (1948) is synonymous with equilibrium as used in this paper if ‘a period of years’ is replaced by ‘a time period commensurate with the relaxation time of the gradient’. The use of the term grade is best restricted to a single-valued relationship between channel gradient and the hydraulic regime. 相似文献
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Abstract An array of moored current meters was deployed over a canyon system on the British Columbia continental shelf in the spring of 1985 and maintained for 110 days. The array was specifically designed to monitor the various terms in the vorticity budget. The observations indicate that the substantial variations in vorticity with time are being driven by vertical velocities originating from the head of a small canyon. These vertical velocities are in turn being directly driven by deep northward flows along the axis of the canyon and these latter flows are closely related to the southward shelf‐edge‐flow. 相似文献
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Integration of a record of Holocene landscape evolution with the cultural record from a multicomponent archaeological site in northern Minnesota has fostered the development of a regional model of human interaction with the changing physical environment. Excavation of 21-CA-169 near Nushka Lake revealed two components, an aceramic Archaic zone where processing of large mammals was the major activity and an upper zone with artifacts suggestive of wild rice processing. Seasonal occupation of the lower zone occurred during a period of falling regional lake levels resulting from natural stream network evolution. The upper zone represents human occupation in a physical and biological setting that was quite different than during earlier occupation. Integration of the geomorphic history of the region along with an interpretation of the types of cultural activities occurring on the changing landscape have provided a model to understand land use and to predict the location of other sites. © 1993 John Wiley & Sons, Inc. 相似文献