从水动力学角度看三角洲前缘的沉积特征

Sedimentary characteristics of delta front from a hydrodynamic perspective

三角洲前缘发育过程的深入剖析对沉积环境判断具有重要意义,有必要从水动力学角度深入探讨其形成特点与产生的机制。为此,从水动力学、水槽实验及水下地貌观察等多个角度,研究了河道水体的流速垂向分布、三角洲前缘沉积体发育的特点。研究发现,河流流速随深度增加单调降低的现象未被足够重视;中下游河床顺流向地形起伏增大,发育垂直于流向的水下沙波,而垂直于流向的河道断面地形起伏会越来越平缓;河流入海过程中,在水下一般以一条主流河道入海,水下不会出现分叉河道,且原先起伏的河床也会逐步填平补齐,由尖锐的V形或W型转变为U形。得出:三角洲前缘大多呈席状、扇状,而非顺流向带冲刷面的条带状;三角洲前缘亚相不发育水下分流河道、水下分流间湾等微相,在现代三角洲前缘亚相中找不到这些微相,且无水槽实验支持;提出了河道、三角洲沉积的水深—流速共同作用模式。

The in-depth analysis of the development process of the delta front is of great significance for the judgment of the sedimentary environment,and it is necessary to deeply explore the formation characteristics and mechanism of the delta front from the perspective of hydrodynamics.In this study,the vertical distribution of the flow velocity in river water bodies and the development characteristics of delta front sedimentary bodies are investigated,from multiple perspectives of hydrodynamics,flume experiments,and underwater geomorphological observations.The results show that the phenomenon of river flow velocity,exhibiting a monotonic decrease with depth,has not been given sufficient attention.The terrain undulation of the middle and lower reaches of the riverbed increases with the flow direction,resulting in the underwater sand waves perpendicular to the flow direction,while the terrain undulation of the river section perpendicular to the flow direction becomes more and more gentle.During the process of rivers entering the sea,there will be a mainstream channel without branching channels underwater.Additionally,the originally undulating riverbed will gradually be filled up,changing from a sharp V-shaped or W-shaped to a U-shaped.It can be concluded that most of the front edge of the delta are sheet-like or fan-shaped,rather than a strip of erosion surface along the flow direction.Microfacies such as underwater distributary channels and bays do not develop in the delta front subfacies,which cannot be found in the modern delta front subfacies and are not supported by flume experiments.Furthermore,a water depth velocity interaction model for river channel and delta sedimentation is proposed.