Impact of river discharge on hydrodynamics and sedimentary processes at Yellow River Delta

During the Anthropocene, regulating river discharge by high dams may have met the need for water demands in river basins, but resulted in carrying less freshwater and sediment to the sea, inducing land degradation and shoreline retreat in worldwide mega-river deltas. In land-ocean interaction, tide response to water discharge changes plays an important role and is crucial for the river-laden sediment transfer and dispersal, affecting both nearshore and estuarine deposits. The Yellow River Delta (YRD), which is under an increasing pressure of the new discharge regime of the Yellow River, has undergone drastic changes in terms of sediment dynamics and morphologic evolution. To gain a better understanding of the overall fluvial and marine hydrodynamics and morphodynamic processes in the YRD, in this study, a full-scale numerical model is built to investigate the interaction and impacts of changing environmental forcing and dynamics on flow and sediment transport in the estuary of YRD and its adjacent coasts. The results show that the river discharge strongly affects the tidal dynamics and morphology of the delta, particularly in the close vicinity of the outlet and the intertidal zone. Tidal constituents M2 and K1, which are the most significant ones in the YRD, are found to be noticeably affected with a decreasing trend when the river discharge increases. The model results also indicate that river discharge affects the location and intensity of the shear front that occurs in the nearshore areas of the YRD. Increasing the river discharge can induce a seaward movement of the shear front, reduce its width and concentrate its shear intensity. It is found that the reverse of the flow direction at each side of the shear front and strong longshore tidal current can act as a barrier for the sediment dispersal process by keeping suspended sediment in the inner zone, thus to form a particular sediment deposition zone and the depo-center

Impacts of topography change on saltwater intrusion over the past decade in the Changjiang Estuary

Saltwater intrusion in estuaries is mainly controlled by tides and river discharge, as well as by topography and other factors. The Changjiang estuary has been seen a significant change in its topography from the data obtained in 2007 and 2017. In this study, a well-validated 3D numerical model was used to simulate and analyze the residual water and salt transport, water diversion ratio (WDR) in bifurcated channels and water resources in the Changjiang Estuary in 2007 and 2017. The comparisons of the model results showed that due to the North Branch becoming much shallower and narrower over the period from 2007 to 2017, the overall salinity in the North Branch decreased and the intensity of saltwater spillover (SSO) from the North Branch into the South Branch weakened. In the North Channel, the simulated residual or net transection water flux (NTWF) and WDR decreased during spring tides, resulting in increased saltwater intrusion. During neap tides, the saltwater intrusion was weakened despite the decreased NTWF and WDR because the water depth at the river mouth became shallower. The changes of topography during that period also resulted in changes of DWR, NTWF, salt transport across the tidal flats and dykes in the North Passage, South Passage and the South Channel, as well as overall dynamic mechanism. The results indicated that the salinity at the water intakes of the three reservoirs in the estuary slightly decreased, indicating that the time that reservoirs can take water from the estuary become longer in dry seasons. In the scenario of complete silt-up of the North Branch, the saltwater intrusion was weakened in the South Branch because of the disappearance of the SSO, which was favorable for the utilization of freshwater resources, but enhanced in the North Channel, North and South Passages. The overall influence from the topographic change over the period is that the saltwater intrusion is weakened in the North Branch, and enhanced during spring tides and weakened during neap tides in the North Channel, North and South Passages. Sediment accretion in the North Branch is favorable for utilization of freshwater resources

Impact of storm surge on the Yellow River Delta: Simulation and analysis

Storm surges can lead to serious natural hazards and pose great threats to coastal areas, especially developed deltas. Assessing the risk of storm surges on coastal infrastructures is crucial for regional economic development and disaster mitigation. Combining in situ observations, remote sensing retrievals, and numerical simulation, storm surge floods in the Yellow River Delta (YRD) were calculated in different scenarios. The results showed that NE wind can cause the largest flooding area of 630 km2, although the overall storm surge risk in the delta is at lower levels under various conditions. The coastal oilfields are principally at an increasing storm surge risk level. E and NE winds would result in storm surges of 0.9–1.4 m, increasing the risk of flooding in the coastal oilfields. Nearshore seabed erosion in storm events resulted in a decrease in inundation depths and inundation areas. To prevent and control storm surge disasters, we should adapt to local conditions. Different measures should be taken to prevent the disaster of storm surges on different seashores, such as planting saltmarsh vegetation to protect seawalls, while the key point is to construct and maintain seawalls on high-risk shorelines

Storm-induced hydrodynamic changes and seabed erosion in the littoral area of Yellow River Delta: a model-guided mechanism study

Morphological evolution of large river deltas is highly vulnerable to extreme storm events due to insufficient sediment supply. As an abandoned delta lobe, the coasts along the northern Yellow River Delta (YRD) and Gudong Oil Field have recently suffered serious erosion due to extreme storm events and become increasingly vulnerable. In this study, a well validated and tested Delft 3D module by the observing hydrodynamic and sediment data to simulate the hydrodynamics and seabed erosion during a storm event in the littoral area of YRD. Observed wave, current and sediment data under both fair-weather and storm conditions were collected in the study area and used to validate the model. The results indicated that the model can reproduce well the hydrodynamic and sediment transport processes. A series of numerical experiments were carried out to examine the hydrodynamic changes and sediment transports. In the numerical experiment of normal condition, there is hardly any sediment transport off the YRD. The numerical experiment of storm condition showed that storms enhanced tidal residual currents, weakened tidal shear front, and significant wave heights up to 2 m, considerably intensified the sediment resuspension and dispersal. The local sediment resuspension due to the increased wave-induced bottom stress promoted the sediment plume to expand to the central area of Laizhou Bay, which seemed to provide sediment source for offshore and southward transport. During the storm, the active nearshore sediment resuspension provided sediment source for offshore and southward transport. The intensive dynamics and sediment transport under storm conditions caused significant changes in seabed erosion and siltation. The main erosion occurred off the Gudong and northern YRD, while the main siltation appeared in the central area of Laizhou Bay. No significant recovery after a storm and frequent strong winds have an accumulative effect on the erosion, which is very likely to dominate the erosive states of the YRD coast in the future