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The Huanghe River (Yellow River) is known by its high suspended sediment concentration (SSC) in its river mouth tidal flat. However, the factors controlling the high SSC over there are not well understood. Therefore, we conducted 7-d hydrodynamic observations (water depth, wave height, and current velocity) and SSC measurements on the tidal flat off the Huanghe River Mouth. The data shows that in most of time, under the calm sea condition, the SSC ranges 0.1–3.5 g/L, and sediment discharge from the river is the main source. However, when hydrodynamics are enhanced in a tidal cycle and large-scale erosion occurs on the seafloor, resuspended sediment becomes the main source, and the SSC in the water column reaches 17.3 g/L. We find the suspended sediment flux is mainly controlled by the tidal current and Stokes drift, while the wave-induced shear stress could also affect the variation of suspended sediment flux. During the observation period, when sea under calm-rippled conditions, the current-induced resuspended sediment concentration (RSC) was greater than the wave-induced RSC. In contrast, in smooth-wavelet sea conditions, the wave-induced RSC was greater than the current-induced RSC, for instance, a single wave event was found to cause 11.8 cm seabed erosion within 6 h. This study reveals different controlling factors for the high SSC near a river-influenced tidal flat, and helps us get a better understanding of a delta’s depositional and erosional mechanisms.

The objectives of this study are carried out a series of controlled large wave flume experiments using fine-grained sediment from the Huanghe River Delta, exploring the complete sequence of sediment behavior in the bottom boundary layer (BBL) during wave-induced liquefaction. The results show that: (1) The BBL in silty seabed is exposed to a progressive wave, goes through a number of different stages including compaction before liquefaction, sediment liquefaction, and compaction after liquefaction, which determines the range and thickness of BBL. (2) With the introduction of waves, first, the sediment surface has settled by an amount S (S=1–2 cm) in the course of wave loadings with an insufficient accumulation of pore water pressure. And a thin high concentration layer formed the near-bed bottom. (3) Once the liquefaction sets in, the liquefied sediment with an ‘orbital motion’ and the sub-liquefied sediment form a two-layer-sediment region. The range of BBL extends downwards and stopped at a certain depth, subsequently, develops upwards with the compaction process. Meanwhile, re-suspended sediments diffuse to the upper water column. (4) During the dynamics process of the BBL beneath progressive waves, the re-suspended sediment increment ranked as sediment liquefaction > erosion before liquefaction > compaction after liquefaction.

Submarine sediment failures or landslides triggered by storm waves in river delta areas pose a significant risk to coastal infrastructure. Due to the limitations of in situ monitoring technology, existing investigations are mostly carried out with geophysical techniques to provide some basic characteristics (e.g. location, size, runout distance, volume, and potential triggers) of existing submarine landslides. However, it is of equal importance to identify the starting criterion and the in situ evolutionary process of the initial stage of seafloor instabilities—sediment failure, which naturally relies heavily on long-term field observations. A field monitoring system was developed for observing sediment failure, which successfully recorded the wave-induced seabed deformation in the subaqueous Yellow River Delta for the first time. Once sediment failure is initiated, the shallow soil undergoes periodic and reciprocating oscillations under alternating action of wave crests and troughs. The evolution of the maximum deformation depth interface moves from shallow to deep, and then migrates upward from deep to shallow layers (the shallow-deep-shallow pattern), which indicates a stability recovery process following the wave-induced seabed failure, and these processes were found to occur multiple times within one storm event. Laboratory wave flume experimental results reproduced and verified the field observations, while also providing pore pressure data which explains the initiation of sediment failure and the deformation process. Finally, a development pattern of the seafloor instabilities in the subaqueous Yellow River Delta is proposed. The in situ observation methods proposed and the knowledge acquired by field monitoring and flume testing could benefit the investigation of costal seafloor instabilities.

In situ observations of suspended sediment concentrations (SSCs), bed elevations, and concurrent hydrodynamics were conducted with an instrumented tripod on an abandoned lobe of the Yellow River Delta in the winter of 2014–2015. Four typical winter storms were recorded, but no significant local seabed erosion was observed at the observation site. Nevertheless, the sediment dynamic processes in this area were inferred from the data. The bottom sediment is eroded by the waves induced by strong northerly winds, and the erosion source area is located northwest of the observation point. Therefore, during strong wind and wave conditions, the observation point forms a horizontal SSC gradient, which generally decreases from west of to east of the observation point. The gradient mainly passes the observation site through diurnal tidal horizontal advection and is intermittently influenced by advection by the residual current driven by sea‒land wind and semidiurnal tides. Additionally, local resuspension of a “fluffy layer” on the seabed by the semidiurnal tide occurs. Flood tides transport the SSC gradient southeasterly, while ebb tides push the gradient back periodically. An asymmetry in the suspended sediment transport rate exists and is estimated to be 1.77 kg m−2 s−1 to the southeast. A new parameter, the net loss rate of suspended sediment, is defined and is estimated to be 0.0109 g L−1 h−1. These findings provide a better understanding of the erosion mechanism of the Diaokou lobe and some new ideas/methods for analysing hydrodynamics and sediment dynamics with a single-point measured dataset.

In marine engineering, the strength of a submarine sediment is an indispensable parameter for assessment of construction. In this study, a free-fall cone penetrator named IPen was developed to realize a rapid and efficient measurement of sediment strength. The equipment is characterized by modular design and self-contained data acquisition. It is equipped with an acceleration sensor, a water pressure sensor, and a piezocone penetration test (CPTu) probe. It is designed to be released from near seabed surface with a releaser and then fall freely to provide a higher penetration velocity. Its maximum working depth is approximately 2500 m and maximum penetration depth is approximately 3 m. To derive the correlation between penetration resistance and sediment strength, a calibrator was devised to determine the penetration-rate factor. In addition, the factor applicable to in situ test points was determined in laboratory experiments. In June 2016, the IPen was tested in situ in the South Yellow Sea, China, during a shared voyage funded by the National Science Foundation. Meanwhile, undisturbed column samples were collected for laboratory tests. Based on the in situ test results, it was demonstrated that the IPen could accurately record the working states of various sensors during the freely falling course. IPen test results reliably reflected the sediment strength at all the testing points when compared with laboratory calibration tests, in situ vane tests and penetration tests, laboratory penetration tests, and unconsolidated and undrained triaxial compression tests.

The objective of this study was to investigate that the effects of different hydrodynamic conditions on sediment resuspension on a tidal mudflat in the Yellow River Estuary. A field experiment was ...

Sediment resuspension is commonly assumed to be eroded from the seabed surface by an excess bottom shear stress and evolves in layers from the top down. Although considerable investigations have argued the importance of wave-induced seabed fluidization in affecting the sediment resuspension, few studies have been able to reliably evaluate its quantitative contribution till now. Attempt is made to preliminarily quantify the contribution of fluidization to resuspension using a series of large-scale wave flume experiments. The experimental results indicated that fluidization of the sandy silts of the Huanghe Delta account for 52.5% and 66.8% of the total resuspension under model scales of 4/20 and 6/20 (i.e., relative water depth: the ratio of wave height to water depth), respectively. Some previously reported results obtained using the same flume and sediments are also summarized for a contrastive analysis, through which not only the positive correlation is confirmed, but also a parametric equation for depicting the relationship between the contribution of fluidization and the model scale is established. Finally, the contribution of fluidization is attributed to two physical mechanisms: (1) an attenuation of the erosion resistance of fluidized sediments in surface layers due to the disappearing of original cohesion and the uplifting effect resulting from upward seepage flows, and (2) seepage pumping of fines from the interior to the surface of fluidized seabed.

A scientific hypothesis is proposed and preliminarily verified in this paper: under the driving of seepage flows, there might be a vertical migration of fine-grained soil particles from interior to surface of seabed, which is defined as ‘sub-bottom sediment pump action’ in this paper. Field experiments were performed twice on the intertidal flat of the Yellow River delta to study this process via both trapping the pumped materials and recording the pore pressures in the substrate. Experimental results are quite interesting as we did observe yellow slurry which is mainly composed of fine-grained soil particles appearing on the seabed surface; seepage gradients were also detected in the intertidal flat, under the action of tides and small wind waves. Preliminary conclusions are that ‘sediment pump’ occurs when seepage force exceeds a certain threshold: firstly, it is big enough to disconnect the soil particles from the soil skeleton; secondly, the degree of seabed fluidization or bioturbation is big enough to provide preferred paths for the detached materials to migrate upwards. Then they would be firstly pumped from interior to the surface of seabed and then easily re-suspended into overlying water column. Influential factors of ‘sediment pump’ are determined as hydrodynamics (wave energy), degree of consolidation, index of bioturbation (permeability) and content of fine-grained materials (sedimentary age). This new perspective of ‘sediment pump’ may provide some implications for the mechanism interpretation of several unclear geological phenomena in the Yellow River delta area.