| The formation and development of natural alluvial river is a long and complicated process. There are few previous studies on the development law of alluvial rivers in initial stage; this article is preliminary study on the influence of water, sand and erosion base level on the riverbed evolution by generalized flume experiment. The experimental tank full length is24.6m; width is3m and high is0.6m. For the convenience of analysis, defined the area from initial point between-150cm and-50cm is the left side area, the area from initial point between-50cm and100cm is the center area, and the area from initial point between100cm and150cm is the right side area. These experiments used plastic sand and natural sand to study the formation and development of delta deposition and the process of river channel evolvement when the water flow and sediment from the mountains and come into the open waters of river channel. The plastic sand is experimented on the one single constant flow discharge. The natural sand is experimented on two various constant flow discharges, that the latter situation were base studied on the former situation. The main results are shown as follows:1. The formation and development of delta depositionIn plastic sand experiment, the formation and development of delta deposition with water flow and sediment from the mountains into the open waters of river channel can be divided into three stages. First stage, the delta deposition formed small sand wave, banded sand wave, fan delta deposition and tongue morphogenetic development gradually, without main channel on the surface of the deposition and no string ditch on both sides. Second stage, river channel gradually formed on the surface of deposition. A straight micro-bend river channel bias on the left (right) side of the center area formed firstly, then water flow and sediment soon moved forward and silted up on the left (right) side area which cause water flow and sediment were blocked and turn to the right (left) side area diversion of discharge and sediment. Then deposition formed on the right (left) side area, as a result, water flow and sediment moved from the right (left) side area to the left (right) side area, which led to the deposition deposited on the left (right) side of area. Finally, the process of water flow and sediment movement repeated the above cycles. The last stage, river channel get longer with the advance of deposition, which leads to the curve shape of river channel. Due to the influence of the circulation flow in bend, local bend channel appeared sharp bend, the phenomena of the cut-bank occurred in the curve channel, then flow and sediment carved out a new channel on the surface of deposition. A smaller river channel formed on the transition section of upper corner of river channel because of the influenced of water flow and sediment erosion. The meandering river channel existed on the surface of deposition for the diversion of discharge and sediment.In natural sand experiment, the formation and development of delta deposition with water flow and sediment from the mountains into the open river channel mainly divided into two stages. At the first stage, the delta deposition formed tongue morphogenetic, fan delta deposition and dendrites’deposition development which have main channel on the surface of the deposition and countless string ditches on both sides. At the second stage, water flow and sediment formed the straight channel, branching channel, straight channel and slightly curved channel gradually, which led to the mainstream moved forward in three different ways on the surface of deposition. One is that water with sediment swing forward in the middle of the area when the straight channel formed on the surface of deposition. The other is that water flow and sediment moved along the branching channel direction to the both sides of the wall, and then moved forward along the middle of the area when branching channel formed on the surface of deposition. The last is water and sediment moved forward through three ways, from the wall to the middle of the area, from the middle of the area to the side wall and along the single side wall when slightly curved channel formed on the surface of deposition.By contrast, in the field of longitudinal launching rate, the initial rate of plastic sand is less than natural sand, and plastic sand and natural sand showed a trend of gradual increasing and decreasing with the time going on, respectively. Finally, the two kinds sand moved forward with the same rate. In the laws of vertical deposition, plastic sand and natural sand are the same, that the rate of silting up was large in initial duration, and then the rate of silting up decreased gradually and the rate tend to constant finally. In the laws of lateral broadening, water and sediment moved to the sides because of the influence of sediment at the bottom of the river channel elevation in initial duration, lateral sediment transport occurred along the river channel mainstream due to the development of deposition longitudinal length.2. The evolutionary changes of alluvial channelsIn plastic sand experiment, under the action of a constant water and sediment, there are three sages that which appeared in the delta deposition’s form and develop corresponded with the succession law of alluvial river channel, as flows:straight channel, slight curve channel and branching channel, meandering channel. The delta deposition longitudinal slope declined gradually which is benefit for the development of curved shape of river channel. At last, the meandering river channel formed.In natural sand experiment, before the flow discharge, the alluvial channels sharp went through two stages on the surface of delta deposition. At the first stage, straight channel and slight curve channel were appeared alternately. At the second stage, there’s mainly as the single slight curve channel occurred with slight curve channel concave bank continuous scour regressive, and finally formed the meandering river channel. After the flow discharged, the alluvial channels sharp went through four stages on the surface of delta deposition: straight and branching channel, slight curve channel, branching channel and slight curve channel, and at last formed as the stable single slight curve channel.By contrast, the developments of river channel under different sand experiments are almost the same, that both appeared went straight channel, slight curve channel, branching channel and meandering (slight curve) channel, and sine curve form among the succession process. In the field of delta deposition sediment characteristics, the result that which is mainly studied on the natural sand after the flow changed, show that the ratio of sediment discharge at outlet to the inlet in river bed gradually increased and tended to1with time going on, which proved that the delta deposition’s change of sediment discharge is gradually tend to balance. |
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