Experimental Study On Development Of Alluvial-fans Under Lateral Confinement

Alluvial fans are common in mountainous areas.Under continuous sedimentation over thousands of years,an alluvial fan may develop into a valley plain.Valley plains are different from delta plains as they are confined by mountains located on both sides.As a result,valley plains have higher progradation rate,higher aggradation rate and higher river evolution rate.As population in valley plain areas increased,plain morphological evolution and human activities are more closely related.The research of laterally confined alluvial fan not only improves relative theories,but also provides suggestions for prevention and reduction of natural disasters.A 183h long flume experiment is carried out to study the development of alluvial fan under lateral confinement.River pattern on the fan varies periodically with time.When the fan surface is steep and the flow velocity is relatively high,river bed is scoured and forms a straight channel.With development of the river bed erosion,channel slope and flow velocity are reduced,and therefore sedimentation within the channel is enhanced.When the sediment deposit emerges,flow is bifurcated.In this process,water level keeps rising and finally exceeds the height of levee.After that,overbank flow is observed and caused widespread deposition on the upper fan.The fan surface becomes steep again and is ready for the coming channel bed scouring.This periodical process has been observed in many previous studied:However,in this experiment,due to the confinement of lateral walls,the channel length is smaller,and the river pattern transformation is faster.During the unconfined flow stage,the radial tiny flows may converge along the side wall and pose influence on river evolution.When the flow along side wall is unstable,it may shorten the fluvial cycle.However,if the flow along side walls lasts for a longer time,it may lengthen the fluvial cycle.The space for sediment deposition is reduced due to lateral confinement.As a result,progradation rate of the fan grows larger.The excess sediment deposition near side walls makes sediment lateral distribution more even.The flow along side walls delivers more sediment to the shoreline,producing slender lobes on the shoreline and increasing progradation rate of the fan.This paper proposed a geometric model to simulate the depositional mode of the confined fan.Comparing with the geometric model for unconfined fan,the influence of lateral confinement on fan development can be studied.In addition to the long period experiment,27 short period experiments have been carried out to further study the fan development under different flume width and different boundary conditions.These boundary conditions include water discharge,sediment feed rate,ponding water depth,flume slope and sediment grain size.Experiment results show that the decease of flume width increases progradation and aggradation rate of the fan.The channel pattern also varies more frequently in narrower flumes.Water depth increases as the flume width decreases,therefore widening process of the fan becomes slower.When flume width decreases to an adequately small value,its water depth increases dramatically and the whole fan is submerged under water.Channel morphology and the fluvial cycle disappeared.Due to the increased flow velocity,fan surface erosion is intensified,and therefore upstream fan aggradation rate is reduced.In order to quantify the influence on fan development caused by lateral confinement,we calculated the relative change rate of fan length and siltation thickness between experiments with different flume widths.Results show that the relative change rate increases nonlinearly when flume width decreases uniformly.It indicates that when carrying out river channel planning and management,the sensitivity of river sedimentation to channel narrowing is different within different channel width range.When the sensitivity is adequately high,the selection of channel width should be more cautious.The influence of different boundary conditions on alluvial fan development is analyzed through comparisons of fan geomorphology and river channel transformation in different experiments.Under various boundary conditions,the influencing mechanism of side walls on fan growth process is also investigated.It is concluded that when the flow discharge and sediment feed rate grow larger,the influence of lateral confinement on fan development is more significant.Besides,when the flume slope is steeper,the impact of side walls on fan is more prone to be interfered by the variation of flow discharge and sediment feed rate.In the later period of some representative experiments,flow discharge and sediment feed rate were changed to analyze alluvial fan’s delayed response to external disturbances.The authors used rate law to simulate the response process and compute the response rate.Calculation results demonstrate that the speed of fan’s delayed response depends on three factors:external disturbance intensity,state of the fan before the disturbance and alluvial fan’s response pattern.In addition,it is observed that when the fan develops into a new state of equilibrium after the external disturbance,its fluvial cycle and sedimentation process become much different from the fan before being disturbed.