The Genesis, Transportation and Accumulation of the Bed Drift of Mountain Rivers

摘要

The genesis, transportation and accumulation of the mountain rivers bed drift in time and space, are casual processes, which depend on such values, as river water discharge (Q, m~3/sec), river inclination (I, ‰), geology and other parameters of the basin. The genesis and amount of the river drift is a function of a climate and depending on its cycles, is more intense in warm periods and reduces when it gets cold. The major drift-forming natural factors are glaciers, erosion, landslides and torrents. Drift transportation, i.e. the process of the drift transfer is a function of many variables, which is mainly described by the laws of Chezy-Maning and Airy. In the upstream of the reservoirs, this process, besides the said laws, is subject to the regularities of the ratio between the drift with the greatest diameter (d_(max)) and velocity of the river (V, m/sec). At some locations, a river may cross an uplifting ridge, or "epirogenic border" and subsiding plain. Such a border is sawn down by the river bed to form so called "balance bed" with relevant properties. If the speed of the threshold elevation is more than the bed cutting speed, a depression is formed, in which the river permanently precipitates the drift to form the bed of the right inclination, or "balance" bed. Some rivers use a significant portion of drift to compensate the stagnations formed by a torrent talus train, the "torrent dam". The result of these processes is the formation of Chiori floodplain and Saglolo and Chrebalo plain-accumulative terraces in the river Rioni basin and Tianeti Plain in the upper reaches of the river Iori and many other events. A significant portion of the drift is used to form the drift prisms of water reservoirs. The river, uses a portion of the remained bed drift to form a balance bed located on the subsiding plains and neutralize the sea eustasy effect. Another part of the drift is used to form a delta, permanently fill the coastal beaches and create the accumulative forms in the sea. A mountain water reservoir conflicts with the sea coastal protection and safety of the upstream population and infrastructure. Therefore, when selecting its location, the genesis of the riverbed drift, sites of natural accumulation and grain size of the drift must be considered. An optimal site to build a reservoir dam is epirogenic borders, as the dam rises higher there and consequently, the exploitation phase of the dam is longer.