Control of Seasonal Lake Level Changes on the Size Characteristics and Growth Process of Bar Fingers at Shallow-Water Delta Front

Key words: 
• Keywords：water level change /
• shallow-water delta /
• bar fingers /
• numerical simulation of sedimentation /
• sedimentary architecture /
• sedimentary evolution

Abstract: Abstract：[Objective] In this paper, the effect of seasonal lake level changes on the sedimentary characteristics, growth process, and sedimentary architecture of the bar fingers in shallow delta fronts is clarified. [Methods] Based on the modern mud and sand and hydrological data of the bar fingers in the Ganjiang Delta, the sedimentary numerical simulation software Delft3D, which is commonly used in China and abroad, is used to carry out numerical simulation of the sedimentation of the bar fingers under seasonal lake level changes and constant lake level conditions, and to compare the differences in sedimentary architectures. [Results] The study shows that under the condition of seasonal lake level changes, the bar fingers exhibit the following sedimentary architecture and growth evolution characteristics: (1) There are few bar fingers, small curvature, with an average value of about 1.42, long length, with an average value of about 9.2 km, thick in the middle, thin at the edge, with a thickness difference of 15.4 m, there is no obvious confluence between the bar fingers, presenting a plane combination style dominated by bird-foot shape; (2) The bar fingers develop deep distributary channels, the natural levees are wide and thick, and the thickness of the single-stage accretionary body in the mouth bar is large; (3) Under seasonal lake level changes, the bar fingers grow stably. During the period of lake level drop, the bar fingers undergo progradation, the distributary channels are mainly eroded-extended, and mouth bar deposits are formed at the front edge of the distributary channels. The distributary channels are also affected by the mouth bars and will undergo diversion and breach, forming multiple terminal distributary channels, while the natural levees are almost undeveloped; during the period of lake level rise, the bar fingers undergo retrogradation, the erosion of the distributary channels is weak, the distributary channels are abandoned and optimized, and the natural levees on both sides of the distributary channels accumulate and continue to thicken. Under the condition of constant lake level, the bar fingers show the following sedimentary architecture and growth evolution characteristics: (1) There are many bar fingers, with large curvature (average value is about 1.53), short length (average value is about 7.2 km), large overall thickness (maximum and minimum thickness differ by only 4.8 m), no obvious topographic difference, multiple bar fingers are constantly bifurcated and merged locally, forming a complex the bar fingers network, and the plane combination style of the bar fingers is mainly interlaced; (2) Deep distributary channels are also developed in the bar fingers, and the single-stage accretion body of the estuary bar is thick, but the most important difference is that there is no natural levee deposition; (3) The growth process of the bar fingers is relatively simple, mainly multiple the bar fingers grow simultaneously, intertwine with each other, and finally form an interlaced plane combination style. [Conclusion] Therefore, seasonal lake level change is an important formation condition for the bar fingers at the front of shallow deltas, which promotes the development of natural levees, improves the stability of distributary channels and the bar fingers, and plays an important role in controlling the sedimentary architecture and growth process of the bar fingers.