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The morphological evolution of estuarine areas occurs at multiple time scales. The Modaomen Estuary (ME) in Pearl River Delta (PRD), China, is one of the main outlets of freshwater and sediment into the South China Sea. A long-term morphodynamic model based on TELEMAC numerical modeling system was applied to the ME to explore input reduction of the river boundary condition, discuss the effect of wave, and analyze the classification of the ME and its long-term morphodynamic equilibrium. Four input reduction scenarios were tested in the model: annual average runoff discharge, bed formation discharge, hourly water levels in a representative year, and combined seasonal discharge (flood season, intermediate season and dry season). The model results show that the general patterns of erosion and deposition at the mouth bar and channels are represented reasonably well when combined seasonal river discharge is used. Sediment was silted onto the external slope of the central mouth bar due to runoff, and was transported landward and westward under the effect of waves. Based on the modeled long-term morphological evolution results, the general morphological pattern can be reproduced under the control of runoff and tides without considering the effect of waves, showing that classification of the ME is runoff dominant. At the seasonal scale, morphology is adjusted through silting in the flood and intermediate seasons and scouring in the dry season. At the interannual scale, from 1994 to 2017, the net volume changes decreased gradually, indicating that erosion and deposition tend to be balanced in the ME. The results of this study provide a valuable basis for long-term modeling and estuarine regulation. [ABSTRACT FROM AUTHOR]

Estuarine morphodynamic processes caused by the combined effects of global and coastal environmental processes and human activities significantly influence the sustainable development of estuaries. Based on the bathymetric and boundary data extracted from multi-year digital charts for the Lingdingyang estuary (LE), South China, and in situ observations of the main hydrological stations, the spatiotemporal variations of depocentres in the LE and possible contributing factors are analysed. The results indicate that the spatial evolution of the morphology of the LE features shoal-deposition and channel erosion, and that the temporal evolution (1955–2015) presents a general pattern of alternating erosion and sediment deposition. A pattern of multi-depocentres exists in the LE, and the evolution of the depocentres can be divided into three general stages: i) a stage dominated by natural processes (1955–1989), ii) a stage dominated by highly-intensive reclamation (1989–2006), and iii) a stage dominated by large-scale channel dredging or sand mining (2006–2015). The transfer path of depocentres in the eastern and middle shoals traversed mainly from a north-westerly to south-easterly direction, while the transfer path of depocentres in the western shoal shows a clockwise spiral transfer feature. The large reduction of sediment supply from upstream river after the 1980s, resulted in an overall decrease in the mean annual sediment deposition volume in the LE from ~0.61 million m3/year (1989–1998) to ~0.29 million m3/year (2012–2015). The mean annual sediment deposition rate of the LE over time (i.e. 1955–2015) fluctuates in an oscillating manner rather than a linearly declining trend. Large to extreme flood (e.g. flood events of '94.6', '98.6' and '08.6') led to higher sediment deposition rates and larger deposition volumes in the LE during the periods 1989–1998 and 2006–2008. The position of the four outlets were significantly pushed outwards by highly-intensive reclamation between 1989 and 2006, thus resulting in the bifurcation of runoff (in the Jiaomen and Hengmen outlets) and the formation of new artificial 'outlets' in the new channels. These great variation of the outlets induced the depocentres to transfer southeast or southwest during the period 1989–2006. A large number of temporary depocentres formed as a result of large-scale sand mining, were located in the middle shoal from 2006 to 2015. In addition, the enhancement of tidal dynamics combined with the great reduction of sediment supply from upstream river, actuated the continued transfer of the depocentres in the eastern, middle, and western shoals to the northwest or northeast after 2006. It is expected that our findings can provide a scientific foundation for port engineering construction, channel development, and the use of the tidal flat zone in the LE. Our conclusions provide a reference for other estuaries that are subject to similar anthropogenic activities. • A pattern of multi-depocentres and three stages of depocentres evolution in the LE from 1950s to 2010s. • The transfer paths of depocentres in the western shoal presented a clockwise spiral feature between 1955 and 2015. • After the 1980s, the construction of upstream dams generally decreased the sediment deposition volume in the LE. • The great variations of outlets positions outwards in 1989–2006, induced the depocentres to transfer south or southeast. • Large-scale sand mining in the middle shoal after 2006 led to a large number of deep pits and temporary depocentres. [ABSTRACT FROM AUTHOR]

Siltation is of primary concern for the construction and maintenance of the waterways worldwide. Lingding Channel (LC) and Hengmen Channel (HC) are two major seagoing waterways in the Lingding (LD) Bay, Pearl River Delta, South China. TELEMAC-2D numerical modeling was performed to elucidate the characteristics and mechanism of siltation in the two seagoing waterways under usual and extreme flood conditions. The deposition thickness during usual flood conditions was 0.22 m in the LC and 0.16 m in the HC; during extreme flood conditions, the thickness reached 0.32 m in the LC and 0.44 m in the HC. In the LC, the main sediment deposition area was in the lower section of the LC, south of Neilingding Island. In the HC, the main deposition areas were in the Lanshan, Ermao, and Qi'ao Shoals. Under extreme flood conditions, the sediment loads into the LD Bay from the Humen, Jiaomen, Hongqili and Hengmen outlets reach 16.65 × 106 t in total, compared with 4.54 × 106 t under usual flood. The certain amount of sediment in the upper and middle parts of the estuary is resuspended by high bottom shear stress and transported downstream, deposited in the lower sections of the HC and LC. High sediment load combined strong resuspension contribute to the sudden and substantial sedimentation in the navigation channels during extreme floods. Siltation intensity varies with the velocity change along the navigation channel, and the main deposition appears in areas with low velocity. The angle between the current direction and channel axis is favorable to the sediment deposition location and intensity, which results in serious deposition in the lower sections of HC and LC. The stagnation point moves around the middle of estuary under usual and extreme flood conditions, so, the middle and lower parts of the LD Bay is heavily silted. The impacts of extreme floods on the navigation channels can not be overlooked and mitigation measures such as bathymetric monitoring, flood control and dredging should be taken. • A coupled model is established to examine the waterway sedimentation in the Lingding Bay under usual and extreme flood events. • High sediment transport rates and strong resuspension contribute to the sudden and substantial sedimentation in the navigation channels during extreme floods. • The current variation, the combined effects of tide and runoff and the jets associated with outlets results in spatial deposition difference along the channel. [ABSTRACT FROM AUTHOR]

• Mud samples in the LD estuary were measured using U-GEMS, and average τ ce of 0.26 N/m2 was obtained. • The simultaneous paradigm with τ ce of 0.26 N/m2 well explained the measured SSCs during the dry season. • The simultaneous paradigm with τ ce of 0.15 N/m2 agreed better with observations during the wet season. • The simultaneous deposition paradigm was better for SSC modelling during both the wet and dry seasons. Suspended sediment and the surficial sediment in most estuaries throughout the world consist of fine particles. Modelling cohesive sediment transport is an important way to explore the morphological evolution of estuaries where cohesive sediment is dominant. Various values for critical shear stress and deposition paradigms for cohesive sediment transport were evaluated here by analysis of field samples combined with numerical simulation in the Lingdingyang (LD) estuary in China. A UMCES-Gust Erosion Microcosm System (U-GEMS) experiment using field samples suggests that the experimental average value of critical shear stress for erosion (τ ce) is 0.26 N/m2. Two conflicting paradigms, i.e. mutually exclusive versus simultaneous erosion and deposition, are commonly used for cohesive sediment transport. Simulations of tidal currents and sediment transport during the wet season of 2007 and dry season of 2016 in the LD estuary were conducted under these two paradigms using the TELEMAC-2D hydrodynamic module and the sediment module SISYPHE in the TELEMAC model. Model verification under these two paradigms was carried out, and the τ ce parameters in the TELEMAC model were revised based on a U-GEMS experiment. The results showed the following. (1) The simulation results of the model in the LD estuary were satisfactory when an experimental average τ ce value of 0.26 N/m2 was used. (2) The simultaneous paradigm using a τ ce value of 0.26 N/m2 well explained the changes in the field-observed depth-averaged suspended sediment concentrations under all tidal regimes during the dry season, whereas the simultaneous paradigm using a τ ce of 0.15 N/m2 was more similar to observations in the LD estuary during the wet season. [ABSTRACT FROM AUTHOR]