入湖三角洲形成过程与淤积形态变化的实验研究

入湖三角洲的形成和发展过程是冲积河流演变的重要组成部分,三角洲尾闾水道生成、地貌演变机理更是河流动力学研究中的重点问题。利用自然模型法对入湖三角洲形成与形态发展过程进行模型试验,通过改变上游来沙和下游水位探究不同边界条件对三角洲平面形态、纵向推进、横向展宽及垂向抬高的影响。三角洲堆积体先呈舌状推进,随后随朵体的生长而变化,其长宽比先迅速增大后逐渐减小。上游来沙量越大,三角洲长宽比越小,下游水位越高对三角洲尺寸抑制越明显。三角洲纵向推进和横向展宽都呈现先迅速增长,后进入相对平稳期,并出现"台阶式"增长的趋势。来沙越多,横纵推进速度越快,越早出现突变式的增长。水位越高,三角洲越早以较小尺寸进入平稳期,突变周期越长,增长值越小。泥沙主要落淤在三角洲中下游,但较多来沙量会促使泥沙淤积在上游,并使上游河槽深度减小,中下游横截面坡度增大,三角洲整体纵向坡度也增大。

Experimental study on formation and morphologic evolution of alluvial lake deltas

Formation and evolution of shallow lake deltas are an important alluvial process of a river system, and research on the waterway and morphology over such deltas has become a key issue of river dynamics. In this study, a tank experiment based on the natural model method is conducted to simulate the formation and evolution of a shallow lake delta, focusing on the influence of boundary conditions, i.e. different sediment inflows and different lake stages, on its plane shape and stretching in longitudinal, vertical and horizontal directions. Results reveal that the delta first stretches forward like a tongue and then evolve as a developing lobe. Its aspect ratio of length to width increases rapidly first but approaches a relatively stable stage; the more the sediment inflow, the smaller the aspect ratio; a higher lake stage suppresses more the size of the delta. Both its longitudinal stretching and lateral broadening manifest a rapid growth at the initial stage, and then it enters a relatively stable mode of stepped growth. Higher sediment concentration leads to faster advancing longitudinally and laterally, resulting in an earlier start of the stepped growth mode; a higher lake stage leads to a similar earlier start with a smaller delta size, a longer mutation period, and a smaller delta size increment in one step. Sediment often silts over the lower delta. However, more incoming sediment will cause silting over the upper part. This brings about a decrease in the upper channel depth, an increase in the lateral bed slope of the middle and lower reaches, and an increase in the delta’s overall longitudinal slope.