An estuarine turbidity maximum (ETM) is a region of elevated suspended sediment concentration (SSC) resulting from residual transport mechanisms driven by river flow, tides, and salinity-induced density gradients (SalDG). However, in energetic and highly turbid environments such as the Yangtze Estuary, SedDG may also substantially contribute to the formation and maintenance of the ETM. Since this mechanism is relatively poorly understood, we develop a three-dimensional model to explore the effect of SedDG on tidal dynamics and sediment transport. By running sensitivity simulations considering SalDG and/or SedDG, we conclude that the longitudinal SedDG leads to degeneration and landward movement of the ETM. Moreover, two effects of the vertical SedDG are identified to be responsible for sediment trapping: One by enhancing the vertical sediment concentration gradients, and another by additionally affecting hydrodynamics including the water levels, velocities and salinities. The longitudinal and vertical SedDG leads to seasonal and spring-neap variations of upstream migration of the salt wedge: Vertical SedDG is more pronounced at neap tides in the wet season due to stronger stratification effects, whereas longitudinal SedDG is more pronounced at intermediate tides in the dry season due to weaker mixing and limited deposition. These findings imply that the SedDG contributes substantially to channel siltation and salt intrusion in highly turbid systems, and need to be accounted for when numerically modeling such phenomena.

中文翻译：

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沉积物密度梯度对长江口河口浊度最大值的影响

河口最大浑浊度（ETM）是由河流，潮汐和盐度引起的密度梯度（SalDG）驱动的残余运移机制导致的悬浮泥沙浓度（SSC）升高的区域。但是，在充满活力和高度浑浊的环境（例如长江口）中，SedDG也可能对ETM的形成和维护做出重大贡献。由于对该机理的了解相对较差，我们开发了一个三维模型来探讨SedDG对潮汐动力学和泥沙输送的影响。通过考虑SalDG和/或SedDG进行灵敏度模拟，我们得出结论，纵向SedDG导致ETM的退化和向地面移动。此外，确定了垂直SedDG的两种作用是造成沉积物捕集的原因：一种是通过增加垂直沉积物浓度梯度来实现的，另一种是通过另外影响包括水位，速度和盐度在内的流体动力学。纵向和垂直SedDG导致盐楔上游迁移的季节变化和春季变化：由于分层效应更强，垂直SedDG在潮湿季节的潮汐时更为明显，而纵向SedDG在中间潮汐时更为明显。干燥季节，因为混合较弱且沉积物有限。这些发现表明，SedDG在高度混浊的系统中对通道淤积和盐分侵入有重大贡献，并且在对此类现象进行数值模拟时需要加以考虑。纵向和垂直SedDG导致盐楔上游迁移的季节变化和春季变化：由于分层效应更强，垂直SedDG在潮湿季节的潮汐时更为明显，而纵向SedDG在中间潮汐时更为明显。干燥季节，因为混合较弱且沉积物有限。这些发现表明，SedDG在高度混浊的系统中对通道淤积和盐分侵入有重大贡献，并且在对此类现象进行数值模拟时需要加以考虑。纵向和垂直SedDG导致盐楔上游迁移的季节变化和春季变化：由于分层效应更强，垂直SedDG在潮湿季节的潮汐时更为明显，而纵向SedDG在中间潮汐时更为明显。干燥季节，因为混合较弱且沉积物有限。这些发现表明，SedDG在高度混浊的系统中对通道淤积和盐分侵入有重大贡献，并且在对此类现象进行数值模拟时需要加以考虑。而纵向SedDG在干燥季节的中间潮汐时更为突出，这是因为混合较弱且沉积物有限。这些发现表明，SedDG在高度混浊的系统中对通道淤积和盐分侵入有重大贡献，并且在对此类现象进行数值模拟时需要加以考虑。而纵向SedDG在干燥季节的中间潮汐时更为突出，这是因为混合较弱且沉积物有限。这些发现表明，SedDG在高度混浊的系统中对通道淤积和盐分侵入有重大贡献，并且在对此类现象进行数值模拟时需要加以考虑。