网状河流多重河道形成过程的实验模拟

Experimental Simulation of Anastomosing Multiple Channel Formation

网状河流是受到人们关注的新型冲积河流,其水文特征、地貌特征和沉积特征已经不同程度地被揭示。然而,网状河流的水槽实验迄今仍是空白,而水槽模拟实验是在时间和空间都大大缩小之后的自然界河流演变过程的再现。本文报道的是在实验水槽中通过原河道决口后网状河流的发生和演变过程。实验初始条件为:水槽辅助区的待决口河道,目标区为轴部略微下凹的长方形泛滥平原(4.5 m×16.5 m),其上下层分别为1.5 cm厚、中值粒径为0.0132 mm的高岭土层和0.5 m厚、中值粒径为0.188mm的天然细砂层,从5.5 m-17 m区段的平均纵比降为0.0058,17m-22m区段的平均纵比降为0.0077。采用的定常流量为3 L/s,悬移质输沙量在前3小时为4.5g/min,其后为1.2 g/s。实验总历时50小时。实验初期,目标区的上游段以垂向加积作用为主,中游以随机侵蚀为主,下游以溯源侵蚀为主,在13.5小时左右,相互连通的多重河道的网状河流体系的雏形基本展现。此后至25.5小时,网状河道的演变以下蚀为主演变为以适度的侧蚀,但河岸的后退幅度很小,标志网状河道逐步过渡到成熟期。从25.5小时至50小时,个别河道的局部废弃和决口是该成熟期网状河道演变的新特点。实验成功地模拟了天然网状河流的形成、发展和演变过程,同时也证明了它是不同于分汉河流的河型。这不?

Anastomosing river is a new fluvial river pattern that gets more attention from re- searchers. The hydrological, geomorphological and sedimentological characteristics of some anastomosing rivers have been revealed. But the experiment simulation of anastomosing river is vacant hitherto. The flume simulation experiments are the reappearance of natural rivers under the condition of decreasing time and space. This work reports a formation and evolution of anastomosing river simulated in a flume. The experimental beginning condition is: an artificial meandering channel in the assistant area of flume; a rectangle floodplain (4.5 mx×16.5 m) with concave cross sections is in the objective area and its stratum has a structure of double layers. The upper layer with a thickness of 1.5 cm consisted of Gaolin whose median grain size is 0.0132 mm;below layer with a thickness of 0.5 m consisted of fine sands whose median grain size is 0.188 mm. The mean longitudinal gradient of the floodplain is 0.0058 from 5.5 m to 17 m and 0,0077 from 17 m to 22 m along the ordinate. Adoptive constant discharge is 3 cm3/s. Suspended sediment load is 4.5 g/min in the first 3 hours and 1.2 g/s thereafter. The total time span of flume experiment is 50 hours. In the initial stage of simulation the sedimentation mainly occurred in the upper reach, the random erosion occurred at the middle reach and the backward erosion mainly occurred in the lower reach of the objective area. A rudimental anastomosing river with multiple channels connected each other basically appears about 13.5 hours. Hereafter up to 25.5 hours the evolution fashion of the anastomosing river changed from downcutting erosion to lateral erosion, but the fall back of channel banks is not evident. It indicates that the anastomosing channels change into their autumn. From 25.5 hours to 50 hours the abandonment or avulsion of individual channel is the new evolution characteristics of the simulated anastomosing river. The whole flume experiment simulates successfully the formation and evolution of natural anastomosing rivers and proves that the anastomosing river is a new river pattern which is different from anabranched river. This is a useful attempt to reveal the formation of anastomosing river via flume simulation and helpful to understand the characteristics of anastomosing river.