Low-angle dunes (LADs) are recognized as the most common roughness element in large fluvial rivers and estuaries. Comprehending their development is vital to understand the flow and sediment dynamics, and paleo-environmental reconstruction of the geological record. However, our understanding of LADs evolution under unsteady flows is poorly understood. Here we investigate dune adaptation under the combined effect of discharge and tides at the tidal current limit of the Changjiang Estuary. Our observations show that compound dunes exist in the late flood season, while only simple dunes are found in the late dry season after five-month decay. The effect of tides enhances bedload transport, thereby accelerating the decay of compound dunes. The increasing deformation rate of superimposed small dunes from the trough to the crest of primary dunes results in a decrease in dune size in the neap tide, as superimposed dunes over the higher part of primary dunes decay more significantly. We find that the size of superimposed small dunes in the late flood season and simple dunes in the late dry season are significantly controlled by fortnightly tides when discharge is between 20,000 and 30,000 m³/s. It implies that compound dunes decay with crest erosion via frequent migration and deformation of superimposed small dunes between neap and spring tides. This finding could potentially help identify and interpret paleo-hydraulic environments that were influenced by tides from sedimentary records. Additionally, given the discharge of the lower Changjiang River is increasingly regulated by dams, figuring out dune adaptation to the regulated hydrograph could be used to advance our prediction of the evolution of river morphology.

低角度沙丘 (LAD) 被认为是大型河流和河口中最常见的粗糙度元素。了解它们的发展对于了解流动和沉积动力学以及地质记录的古环境重建至关重要。然而，我们对不稳定流动下 LAD 演化的理解知之甚少。在这里，我们研究了长江口潮汐流极限下流量和潮汐共同作用下的沙丘适应。我们的观察表明，复合沙丘存在于汛期后期，而在经过五个月的衰退后，在旱季后期只发现了简单的沙丘。潮汐的作用增强了底泥运输，从而加速了复合沙丘的腐烂。叠加小沙丘从海槽到原始沙丘顶部的变形率增加导致小潮中沙丘尺寸减小，因为原始沙丘较高部分的叠加沙丘衰减更显着。我们发现，当流量在 20,000 至 30,000 m 之间时，汛期后期叠加小沙丘和旱季后期简单沙丘的大小受每两周一次的潮汐控制显着。³ /秒。这意味着复合沙丘通过小潮和大潮之间叠加的小沙丘的频繁迁移和变形而随着波峰侵蚀而腐烂。这一发现可能有助于识别和解释受沉积记录潮汐影响的古水力环境。此外，鉴于长江下游的流量越来越多地受到大坝的调控，确定沙丘对受调控的水文过程线的适应性可用于推进我们对河流形态演变的预测。