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在美国怀俄明东部羚革溪的河曲凸岸一侧所形成的砂坝,其外表与点砂坝相似,但不具有长年性蛇曲河流中点砂坝的成因和内部结构。根据对这种季节性浅水砂质河流中的沉积物的观察结果,可以认为这种砂坝是由“湍流”和大的宽-深比造成的,它们阻止了产生与强劲侧流有关的螺旋流。显然,沉积物的搬运方向与顺流的速度矢量是平行的。在丰水期,河曲处由于流量增大而有大量沉积物在那里堆积。当水流继续淹没砂坝顶端时,就会因垂向加积作用而进一步发生堆积。随后当枯水期时,低流量导致了深泓线弯曲和流槽发育,从而引起了砂坝的侵蚀,细粒物质沉积在流槽中和砂坝边缘上。这些作用导致了形成一种内部结构以涨水沉积充填为主体並具有垂向加积的顶积层和少许边缘前积层的边坝。在外砂坝的边缘和流槽内可出现浅水细粒沉积薄层。我们认为应该将这些砂坝与真正的点砂坝和其它类型的边坝加以区别,並建议采用“凸岸砂坝”(Convex bar)这个术语。
The sand bar formed on the bend side of the Okinawa River in the eastern part of Wyoming in the United States is similar in appearance to the point sand bar but does not have the genesis and internal structure of a midpoint sand bar in the long serpentine river. Based on the observations of sediments in this seasonal shallow, sandy river, it is believed that this type of dam is caused by “turbulence” and a large width-to-depth ratio that prevents the creation of a spiral associated with a strong lateral flow flow. Obviously, the sediment transport direction is parallel to the downstream velocity vector. During the wet season, a large amount of sediment accumulates there as the flow increases. When the water flow continues to submerge the top of the sand dam, it will accumulate further due to vertical accretion. Subsequent low flow rates led to deep bend lines and runner development during the dry season, causing erosion of the sand bar and deposition of fine-grained material in the runner and at the edge of the sand bar. These effects led to the formation of an internal structure of the upwelling sediment filling as a main body with vertical accretion of the top ply and a little margin of the front ply. In the outer edge of the dam and runner can appear shallow fine sediment layer. We think we should differentiate these sand bars from true point sand bars and other types of side bars and suggest the term “Convex bar.”