Retrospect and perspective of the estuarine turbidity maximum zone researches

Document Type


Publication Date



Geography and Geology

Publication Title

Kexue Tongbao/Chinese Science Bulletin


Turbidity maximum zone (TMZ) is the dynamic turbid water areas within limits in an estuary, where the concentrations of total suspended solids (TSS) are steadily and significantly higher than in the upstream river and the downstream sea. TMZ is a special phenomenon of the process of suspended sediment movement and transport in estuaries, which reflects the interaction of particulate matter and turbulent energy between rivers and oceans. TMZ has long been a hot topic for scientific inquiries and engineering innovations among a broad spectrum of scholars, government agencies, engineering corporations, and communities. The studies of TMZ has a long history of over 80 years since it was proposed in the last century. Based on a comprehensive review, the paper summarizes the main findings and progress of the TMZ studies and provides prospects of TMZ research in the future. The previous studies have focused on the basic characteristics of TMZ systematically and comprehensively, such as the TSS concentrations, and the sources and compositions TSS in estuaries. Although the formation and development of TMZ are affected by various factors and rather complex, estuarine gravitational circulation and tidal effects are considered as the primary mechanisms of TMZ formation and the main control factors of TMZ in different types of estuaries, including weakly, partially, or well-mixed estuaries. Besides, sediment re-suspension also plays an important role in TMZ formation. Additionally, dynamic evolution characteristics of TMZ from short to medium terms have been analyzed and discussed in great detail. It has been commonly agreed that these past and current studies of TMZ have made great progress in many aspects, and the findings and outcomes have made significant contributions to our theoretical understanding and practical management of TMZ. However, several areas warrant further exploration to advance the TMZ research, to improve our understanding of the spatial and temporal variations of TMZ, and to contribute to the sustainable management of TMZ for improving its surrounding natural eco-environment. The first one is to accurately detect, extract, and measure the size, depth, and volume of TMZ in estuaries. Where a TMZ is located in an estuary is a fundamental question and an important task for studying TMZ. However, current studies of detecting TMZ were mainly based on relatively rough description or some types of thresholds of TSS concentrations, or certain levels of turbidity criteria, which had low accuracies, were not consistent in different estuaries and times and showed weak applicability. Therefore, future works should focus more on the exploration of new methods and techniques for precisely detecting and extracting the spatiotemporal distributions of TMZ. Secondly, to comprehensively understand the whole features of TMZ, detection, and measurement of 3D distribution characteristics of TMZ in different conditions should be rigorously carried out in the future. Last but not least, future studies should pay more attention to the studies of the long term impacts of human activities and global environmental changes on spatial and temporal variations of TMZ to obtain a more profound understanding of the change dynamics of TMZ for better management policies.

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