Subaqueous Yellow River Delta Research Articles

Risk zonation of submarine geological hazards in the Chengdao area of the Yellow River subaqueous delta

Submarine geological disasters occur frequently in the Chengdao area of the Yellow River subaqueous delta, which seriously threaten the safe operation of marine engineering in the region. Therefore, it is of great significance to carry out risk zonation for this region. The current study presents a detailed assessment of risk zones related to submarine geological hazards in this region based on analytical hierarchy process (AHP). The system of assessment index for risk zonation was established with four aspects of hydrodynamic condition, engineering geological environment, disaster geological conditions and human engineering activities. Eight geological hazard evaluation factors were selected and the distribution characteristics of each evaluation factor were discussed in detail by combining qualitative analysis and quantitative calculation. The risk level of submarine geological hazards in the Chengdao area are divided into four types: low risk, relatively low risk, relatively high risk, and high risk. The results show that the areas with a high geological hazard risk in this region are mainly distributed in the areas with water depths of 9–12 m, where the hydrodynamic effect is strong, with many human engineering activities, and seriously suffered from geologic hazards. The study results can provide scientific basis for engineering construction and hazard prevention in the Chengdao area.

Subaqueous silt ripples measured by an echo sounder: Implications for bed roughness, bed shear stress and erosion threshold

Bedforms like ripples are widely distributed in the coastal zone. They influence the bed roughness thus the estimation of bed shear stress and associated sediment transport. An echo sounder was mounted on a bottom-supported tripod intended to measure the erosion and deposition of seabed in the subaqueous Yellow River Delta, China. However, variations in bed elevation are found to be not the net erosion or deposition at the observation site, but the migration of silt ripples which were generated by waves and pushed back and forth or flattened off by the tidal currents. Ripple heights were observed to be within 0.1–0.7 cm and were used for testing the model of Nielsen (1981). The model overestimated the height of silt ripples as it was developed for sands, but the deviation can be well addressed by incorporating a linear modification (R2 = 0.79). Alternatively, a new model specifically for silt ripple height was regressed from the field data with R2 up to 0.78. The existence of silt ripples increases the bed shear stress by 4 times due to the additional bed roughness. A “fluffy layer” overlies the consolidated seabed, therefore, net seabed erosion occurs after the “fluffy layer” is resuspended. A representative critical bed shear stress for net seabed erosion in the study area was found to be 0.8 Pa. The echo sounder can be an alternative tool for observing silt ripples in coastal regions like the Yellow River Delta where the water is too turbid for underwater videos. The proposed model infers silt-ripple features from bed grain size and flow condition and provides a quick estimate for bed roughness improving the understanding on sediment transport.

Importance of Bed Liquefaction‐Induced Erosion During the Winter Wind Storm in the Yellow River Delta, China

AbstractData obtained during a 5‐month field observation of the Yellow River Delta show that the storm wave‐related oscillatory flows are mainly in an intermittently turbulent regime (400 < ReΔ < 1,200). Wave‐induced seabed liquefaction causes changes to the bed erodibility and increases the sediment entrainment rate, resulting in the formation of fluid mud layer (FML) during winter wind events. The strong NE winter winds lead to most of the wave‐induced liquefaction events in the study area. It was found the sediment erosion rate is underestimated when we solely focus on the wave‐induced bottom shear stress. To parameterize wave‐induced excess pore pressure buildup and sediment liquefaction, a series of experiments were conducted in a large wave flume and a new liquefaction‐erosion method is proposed to calculate the sediment erosion flux. Analysis of model results shows that the FML thickness is about 2–12 cm at the storm‐arrival stage during the entire observation period. The thickness of FML significantly increases with the increase of wave height and decrease of water depth. Once the FML is formed, strong currents can remove fluid mud 10 hr after the storm‐arrival stage, while the background currents on calm days can hardly cause suspension. The erosion rate in liquefaction zones can increase 5–10 times compared with the erosion rate in nonliquefaction conditions. The whole Bohai Sea is highly turbid after intense NE winter wind, and the liquefaction zones during winter winds would be the major sediment source. Enhanced sediment erosion can cause the subsequent degradation of the subaqueous Yellow River Delta.

Morphology and origin of liquefaction-related sediment failures on the Yellow River subaqueous delta

Subaqueous slopes of many actively prograding deltas are heavily modified by widespread submarine sediment failures, forming highly complex geomorphic features that undergo continuous evolution even over short time scales. However, the effects of wave-seabed interactions (especially liquefaction) on the evolution of seabed sediment failures are rarely studied. Here, we use side-scan sonar, sub-bottom profilers, multibeam bathymetry, and in situ and laboratory geotechnical tests to map the bottom morphology and sediment structure of the Yellow River subaqueous delta. Our results indicate that: 1) The morphology along the Yellow River subaqueous delta is highly complex and can be categorized as a moderately disturbed area and heavily disturbed area; 2) Heavily disturbed area shows notable lines of evidences of liquefication-related sediment failure and four separate morphological zones (Zones 1–4). Zone 1 consists of differently sized sediment highs, which considered to be scarp or monadnock features. Zone 2 undulates with widely distributed tabular sediment blocks, gullies, and small-scale seafloor irregularities. Zone 3 is characterized by irregular, elongate sediment ridges. Finally, Zone 4 is characterized by relatively flat terrain with some localized highs; 3) Sub-bottom profiles used to visualize failure features in Zone 1 show acoustically continuous high-amplitude parallel or subparallel reflection, and the geomorphic features in this area are mainly caused by erosion. In comparison, Zones 2, 3 and 4 lack internal structures and display chaotic/disturbed signatures in corresponding sub-bottom data, which might be an indication of liquefaction; and 4) A combination of preconditioning and triggering factors lead to sediment failures and the complex morphological features in this area; however, silt liquefaction under storm conditions is the most important controlling factor. Based on this evidence, we propose formation mechanism for the liquefaction-related sediment failure features in this area. Sediment liquefaction forms an initial depression and gully when fine particles are resuspended along the arc-shaped liquefaction boundary as sediment vents and re-solidification of the liquefied sediment leads to a stiffer bed. Gullies are then widened and deepened by the predominant currents, and the disturbed depressions develop into elongated highs because of the denser structure and higher strength following liquefaction. While disturbed by engineering activities, the sediment highs become fragmented due to intense localized erosion, having the potential for downslope transport and the formation of sediment ridges. The geometries and distributions of the sediment failure features on the Yellow River subaqueous delta provide insights into process mechanisms and causative factors, especially when compared with other, similar phenomena reported elsewhere.

Comparison of the Causes of Erosion–Deposition between Yellow River, Yangtze River, and Mekong River Subaqueous Delta l: Model Building

An estuary delta is an area with active human economic activities and a fragile natural ecological environment. With the intensification of global changes and human activities, coastal and seabed erosion around the world is becoming more and more serious. This study compared the hydrodynamic effect and sediment diffusion path at the Yellow River subaqueous delta (river control type) and the Yangtze River subaqueous delta (tide-river type) in the East Asian monsoon area using a Delft 3D numerical simulation to analyse the causes of the distribution of accumulation and erosion landform in the different types of subaqueous deltas. This study found that the river deltas were dominated by the river when the accumulation landforms concentrated in the direction of the Coriolis forces, which was due to hydrodynamic erosion driven by wind, and the sediment diffusion range was mainly concentrated in shallow water areas (water depth < 6 m), resulting the Yellow River subaqueous delta presenting the pattern of “northern erosion and south accumulation”. The Yangtze River subaqueous delta was controlled by tidal effects, making the southern part of the estuary more susceptible to erosion. The research results can not only further deepen the understanding of the physical process and mechanism of erosion and deposition in the subaqueous delta but also provide stronger scientific support for the deltas’ ecological environment protection, geological environment safety, and disaster geology prevention.

Characteristics and sedimentological significance of acoustic anomalies in silty seabed in the Yellow River subaqueous delta

Due to extraordinary natural geographical features, seabed topography, and sediment sources, wave-induced silt liquefaction is a common occurrence in the subaqueous delta of the Yellow River, China. When liquefaction occurs, the original sedimentary structure of silt is destroyed; after liquefaction, the sediment particles are rearranged and reconsolidated, resulting in distinct physical and mechanical differences between the liquefied material and the nearby adjacent strata that have not been subjected to liquefaction. Based on data from geotechnical testing (GT), scanning electron microscope (SEM) images, and sub-bottom profiling (SBP), the physical and acoustical characteristics of silt remoulded by wave-induced liquefaction (SRL) in the Yellow River subaqueous delta are determined. According to the GT results, liquefaction resulted in distinct decreases in the water content (2.6%), the void ratio (0.056), and the clay content (4.52%), as well as slight increases in the wet density (0.01 g/cm3), the dry density (0.05 g/cm3), and the grain size (d50) (0.015 mm). Per our SEM analyses, the SRL material is denser than the unremoulded silt, attributed to the rearrangement and reconsolidation of the silt particles. As can be determined from the high-resolution SBP data, that acoustically, the SRL is distinctly different from the adjacent intact strata. The smooth funnel-shaped features demarcate the extent of the region that has been subjected to a specific liquefaction and silt reconsolidation event. Due to the presence of a fault-like uplifted reflection event in the underlying strata, the SRL produces positive sound velocity anomalies on the order of 53–127 m/s. Analyses of the physical and acoustical characteristics of the SRL provides valuable insight into the engineering properties of seabed silt.

In Situ Observation of Silt Seabed Pore Pressure Response to Waves in the Subaqueous Yellow River Delta

In Situ Observation of Silt Seabed Pore Pressure Response to Waves in the Subaqueous Yellow River Delta

In‐Situ Observation of Storm‐Induced Wave‐Supported Fluid Mud Occurrence in the Subaqueous Yellow River Delta

AbstractTwo relatively comprehensive and long‐term observation campaigns of bottom sediment dynamics have been conducted at the subaqueous Yellow River delta in the winters of 2016 and 2018. High suspended sediment concentration (SSC) up to 10 g/L were observed in 2016 and active events of wave‐supported fluid mud (WSFM) were found to exist at the abandoned river delta during storms in 2018. A set of calibrated optical backscatter sensors measured the near‐bed SSC >30 g/L with a maximum WSFM thickness >60 cm. WSFMs emerging at the abandoned river delta were classified into three types: single‐peak, multi‐peak, and hybrid according to the time‐varying characteristics of WSFMs. The dominant factors triggering the WSFM generation and dissipation are storm‐wave actions and tidal effects, with limited contribution from current‐induced turbulence. In this study, several lines of evidence were put forward to prove the presence of seabed liquefaction to a depth of 10–16 cm, and thus we can infer that another significant factor enhancing the WSFM thickness scale may be wave‐induced liquefaction. Observations showed obvious signs of WSFM motion, or in other words, wave‐supported gravity flow (WSGF) moved downslope. The downslope velocity (around 1.9 cm/s) was estimated by solving a buoyancy‐friction force balance. This study observed frequent WSFM events at the abandoned Yellow River delta for the first time and demonstrates the need to attach more concerns about its roles in sediment transport and offshore engineering in the future.

A physics-informed statistical learning framework for forecasting local suspended sediment concentrations in marine environment

An in-situ monitoring of water quality (suspended sediment concentration, SSC) and concurrent hydrodynamics was conducted in the subaqueous Yellow River Delta in China. Empirical mode decomposition and spectral analysis on the SSC time series reveal the different periodicities of each physical mechanism that contribute to the SSC variations. Based on this physical understanding, the decomposed SSC time series were trained separately with a newly-proposed augmented lncosh ridge regression, in which (1) a lncosh function was incorporated in traditional ridge regression for handling outliers in original data, and (2) the temporal auto-correlation in the decomposed SSC series was used for augmented regression. Finally, the trained sub-series were added up as the final prediction. The advantages of this decomposition-ensemble framework is that it depends on SSC only, superior to the normal process-based models which need the concurrent hydrodynamics for estimating bed shear stress. This will not only reduce the measurement uncertainties of the input when training the data-driven model, but also save the prediction cost as no other parameters than SSC need to be measured and input for running the model. The framework realized 6-hour-ahead high-accuracy forecasting with mean relative errors of 5.80–9.44% in the present case study. The proposed framework can be extended to forecast any signal that is superposed by components with various timescales (periodicities) which is common in nature.

Environmental magnetic properties of core sediments in the Yellow River subaqueous delta and their chronologicalapplications

The Yellow River has shifted frequently in the downstream region since 1885; it has flowed into the sea through the Qingshuigou channel from 1976 onward, thus forming the latest delta lobe (1976–present). During the past ~40 years, the sediment load of the Yellow River has been greatly reduced because of human and natural activities, such as reservoir construction, soil and water conservation, and river avulsion. Beginning in 2002, the water-sediment regulation scheme (WSRS) has also changed the sediment loading patterns of the Yellow River. Several studies have reported on the erosion and deposition characteristics of the latest lobe of the Yellow River Delta over the past several decades, based on remote sensing methods and water-depth measurements. However, continuous sedimentary records of delta erosion and deposition and their responses to natural and human activities in the watershed are lacking. Here, we obtained three sediments cores, YD01, YD02, and YD03, from the subaqueous delta of the latest lobe of the Yellow River Delta. These cores contain sediments deposited from 1976 to 2018. We analyzed the environmental magnetism and lithological characteristics of these cores, and investigated their responses to natural and human activities in the watershed of the Yellow River, as well as possible applications to sediment dating in the Yellow River Delta. The results show that frequency-dependent susceptibility ( χ fd%) and anhysteretic remanent magnetization susceptibility ( χ ARM) values were high in core sediments deposited from 1976 to 1996, indicating abundant contents of fine single-domain (SD) and superparamagnetic (SP) magnetic particles, and therefore a dominant sediment supply from the Chinese Loess Plateau. Small decreases in χ fd% and χ ARM in sediments deposited after 1986 revealed slight reduction of the sediment load from the Yellow River, which was likely caused by sediment retention related to the construction of the Longyangxia Reservoir. Rapid rises in χ fd% and χ ARM occurred in core YD01 sediments deposited during 2002 and ~2007, likely in response to the implementation of the WSRS, which resulted in an increase in sediment transportation. Increases in hard isothermal remanent magnetization (HIRM) values, an indicator of high-coercivity magnetic minerals such as goethite and hematite, in cores YD02 and YD03 from 2002 to ~2007 may also represent the start of the WSRS. This is because the downstream riverbed of the Xiaolangdi Reservoir underwent intense erosion during the implementation of the WSRS, leading to large-scale transportation of coarse sediments enriched in goethite and hematite, which were deposited on the riverbed before the implementation of the WSRS. The lack of increased contents of fine SD and SP magnetic minerals in cores YD02 and YD03 compared with YD01 in the interval deposited between 2002 and ~2007 was attributed to erosion and redeposition in the areas around cores YD02 and YD03. Large drops in SD and SP contents after ~2007 were detected in all three cores, revealing intense erosion near the river mouth likely caused by river channel shift and reduced capability of the WSRS to remove sediments. Contents of SD and SP particles increased again in sediments deposited after 2014 in cores YD02 and YD03, likely related to erosion of the present (after 1996) Yellow River mouth and transportation of fine particles to the region near cores YD02 and YD03. In summary, the magnetic parameters χ fd% and χ ARM and the lithological characteristics of cores YD01, YD02, and YD03 recorded sediment delivery changes caused by four events in the Yellow River watershed: The main river channel shift in 1976, construction of the Longyangxia Reservoir in 1986, the beginning of the WSRS in 2002, and weakened capability of the WSRS to remove sediments after ~2007. The notable changes of χ fd%, χ ARM, and lithology at the 1976, 1986, 2002, and ~2007 horizons may be used as dating points in future research of sediment cores from the Yellow River Mouth.

Design and Application of an In Situ Test Device for Rheological Characteristic Measurements of Liquefied Submarine Sediments

Liquefied submarine sediments can easily lead to submarine landslides and turbidity currents, and cause serious damage to offshore engineering facilities. Understanding the rheological characteristics of liquefied sediments is critical for improving our knowledge of the prevention of submarine geo-hazards and the evolution of submarine topography. In this study, an in situ test device was developed to measure the rheological properties of liquefied sediments. The test principle is the shear column theory. The device was tested in the subaqueous Yellow River delta, and the test results indicated that liquefied sediments can be regarded as “non-Newtonian fluids with shear thinning characteristics”. Furthermore, a laboratory rheological test was conducted as a contrast experiment to qualitatively verify the accuracy of the in situ test data. Through the comparison of experiments, it was proved that the use of the in situ device in this paper is suitable and reliable for the measurement of the rheological characteristics of liquefied submarine sediments. Considering the fact that liquefaction may occur in deeper water (>5 m), a work pattern for the device in the offshore area is given. This novel device provides a new way to test the undrained shear strength of liquefied sediments in submarine engineering.

The modern Yellow River Delta in transition: Causes and implications

Sustainable development of megadeltas worldwide has recently been the focus of international socioeconomic and environmental concerns. Long-term geomorphic sustainability of deltas is crucial for sustaining coastal resources and marine ecosystems. The modern Yellow River Delta (YRD), which has been developed since 1855, is increasingly facing erosion threats under natural and human-induced changes. Utilizing long-term hydrological data, remote-sensing satellite images and bathymetric survey data, the transition processes of the modern YRD (including both the subaerial and subaqueous delta) during 1855–2015 from the perspective of morphodynamics are examined. The results show that over the past 160 years, the evolution of the Yellow River subaerial delta is under rapid accretion before 2000, and transitioned to slight erosion state after 2000, which is believed to be associated with the reduced sediment supply. Correspondingly, the Yellow River subaqueous delta is also believed to transition from the accretion into net erosion since 2000, with a net volume change rate of −1.5 × 108 m3/yr. As the primary source for deltaic deposits, at least 2.6 × 108 t/yr of sediment supply is required to sustain the erosion-accretion balance of both the subaerial and subaqueous delta. Compared with the previous research, we present that the 160-year morphological evolution of the modern deltaic system is strongly influenced by the reduction of sediment input. Within this time frame the changes in the partition of sediment load among frequent migrations of the deltaic channel could have a direct impact on variations of the long-term erosion-deposition patterns of the modern YRD. Extreme events of storm surges and frequent strong winds caused significant short-term fluctuations in the erosion/deposition of the subaqueous delta that would affect long-term changes. The insights gained from this study may help to set scientific guidelines for the sustainable development of other river deltas worldwide that are subject to climate change and human interventions.

Multiscale Superposition and Decomposition of Field‐Measured Suspended Sediment Concentrations: Implications for Extending 1DV Models to Coastal Oceans With Advected Fine Sediments

AbstractOne‐dimensional vertical (1DV) diffusion‐settling models are practical tools for sediment transport analysis of sands, which is mostly a local process. However, for fine sediments, the observed concentrations C(t) can be mixed via horizontal advection (HA) due to the lower settling velocity, which makes the C(t) not necessarily a local process, thus making 1DV models invalid. It is important to determine the qualitative significance of HA or quantitatively separate HA signals when applying 1DV models to environments with advected fine sediments. Here, novel methods are combined to (1) qualitatively identify the physical mechanisms underlying C(t) variations and determine whether HA is significant via multiscale frequency superposition and (2) quantitatively decompose C(t) components according to their physical mechanisms via spectral filter decomposition. In situ observational data of C(t) and concurrent hydrodynamics in the subaqueous Yellow River Delta is employed to analyze the methods' performance. The decomposed signals are reasonable because they can be interpreted in light of other observed physical processes. The results indicated that M2 tidal advection contributed 8.30% by carrying sediments from 1.6 km upstream of the flood tides, M4 and M6 + M8 tidal resuspension contributed 4.16% and 3.96%, respectively, by periodically resuspending a “fluffy layer.” Waves resuspended sediments from an erosion center 5 km upstream of the flood tides and contributed to 76.49% of the elevated C(t). The proposed methods can exclude HA signals from the measured C(t) to increase the applicability of 1DV models to environments with advected fines when C(t) and hydrodynamics are measured.

A temporal LASSO regression model for the emergency forecasting of the suspended sediment concentrations in coastal oceans: Accuracy and interpretability

In situ observations of suspended sediment concentration (SSC) and hydrodynamics were conducted in the subaqueous Yellow River Delta, China. With the dataset, a new least absolute shrinkage and selection operator (LASSO) regression model with temporal autocorrelation incorporated (temporal LASSO) is proposed for SSC prediction and mechanism investigation in coastal oceans. The model is concise and practical, effectively shrinking the interrelated variables into representative ones, while also achieving one-hour ahead forecasting with both higher accuracy and better interpretability than other data-driven methods. The model interpretability is further validated with direct data analysis from a physical perspective. Specifically, Empirical Mode Decomposition is employed to decouple the measured SSC into intrinsic mode functions (IMFs) and a residual. The periods of each subseries estimated from both zero-crossing and spectrum analysis show that IMF1 physically corresponds to the sediment resuspension by M4 tidal currents, IMF2 is the M2 tidal advection, IMF3-IMF5 are the resuspension by wind waves, IMF6 is the spring–neap tidal pumping of sediments. The contributions estimated with the ratio of variance are 12 %, 14 %, 63 %, and 10 %, respectively, over the observation period. The residual is the seasonal variations which can be taken as the background SSC thus not included for variance contribution. Waves make the dominant contribution which verifies the rationality of the LASSO shrinkage and confirms the model interpretability. The temporal LASSO model is shown to be a potential tool for emergency forecasting and mechanism explanation of SSC to benefit ocean environmental engineering management.

Derivation of settling velocity, eddy diffusivity and pick-up rate from field-measured suspended sediment concentration profiles in the horizontally uniform but vertically unsteady scenario

Accurate determinations of sediment settling velocity (ws), eddy diffusivity (Ds), and pick-up rate p(t) from available field data are vital for engineering calculations or numerical simulations of coastal processes. Existing methods to determine these parameters are normally based on the assumption of a steady equilibrium that ignores the “phase lag” and “amplitude damping” effects of a diffusing concentration in unsteady scenarios that are typical of many field settings. The present paper gives an analytical solution for a time-varying suspended sediment concentration profile C(z,t) in the horizontally uniform but one-dimensional vertical (1DV) unsteady scenario with a vertically constant Ds distribution. The model is used to estimate ws, Ds, and p(t) by determining best fits between the modelled and C(z,t) profiles measured with an instrumented quadripod in the subaqueous Yellow River Delta (YRD) during a local sediment resuspension event. Values of field-derived ws range from 0.01 to 0.073 mm/s, while Ds range from 2 × 10−5 to 1.46 × 10−4 m2/s, and p(t) range from 8.3 × 10−5 to 3.14 × 10−4 kg m−2 s−1 under mild and energetic wave conditions, respectively, which are consistent with our laboratory measurements and with the results reported in the literature; time variations of the derived parameters are also reasonable in response to the synchronous hydrodynamics, both confirm that the parameter optimizations are successful. The proposed analytical model is shown to be a practical tool for inferring ws, Ds, and p(t) from available measured C(z,t) profiles free of secondary prediction equations or unknown empirical parameters.

Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

The grain-size distribution of surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments (mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion; whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Wave-induced seafloor instabilities in the subaqueous Yellow River Delta—initiation and process of sediment failure

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.

Migration and Diffusion of Heavy Metal Cu from the Interior of Sediment during Wave-Induced Sediment Liquefaction Process

Sediments are an important sink for heavy metal pollutants on account of their strong adsorption capacity. Elevated content of Cu was observed in the Chengdao area of the Yellow River Delta, where the surface sediment is mainly silt and is prone to be liquefied under hydrodynamic forces. The vertical transport of fine particles, along with pore water seepage, during the liquefaction process could promote the migration and diffusion of Cu from the interior of sediment. The present study involved a series of wave flume experiments to simulate the migration and diffusion of Cu from the interior of sediment in the subaqueous Yellow River Delta area under wave actions. The results indicated that sediment liquefaction significantly promoted the release of Cu from internal sediment to overlying water. The variations of Cu concentrations in the overlying water were opposite to the suspended sediment concentrations (SSCs). The sediment liquefaction caused high initial rises of SSCs, but led to a rapid decline of dissolved Cu concentration at the initial period of sediment liquefaction due to the adsorption by fine particles. Afterwards, the SSCs slightly increased and then gradually decreased. Meanwhile, the dissolved Cu concentration generally kept increasing under combined effects of intensively mix of sediment and overlying water, pore water seepage, and desorption. The dissolved Cu concentration in the overlying water during sediment liquefaction phase was 1.5–2.2 times that during the consolidation phase. Sediment liquefaction also caused vertical diffusion of Cu in sediment and the diffusion depth was in accordance with the liquefaction depth. The results of the present study may provide reference for the environmental management in the study area.

Sediment Re-Suspension and Release of Dissolved Heavy Metal Cu under Wave Actions

Located in the semi-enclosed Bohai Sea, the Yellow River subaqueous delta faces serious pollution by the heavy metal Cu in seabed sediments and is frequently subject to storm surges which further incur sediment re-suspension or even liquefaction, and cause Cu to be released back into seawater. This study performed an indoor wave flume experiment to simulate the release of Cu into overlying water under different hydrodynamic conditions in three phases: static diffusion, seabed nonliquefaction and re-suspension, and seabed liquefaction and re-suspension. As indicated by the results, the concentration distribution of Cu in overlying water was closely related to that of suspended sediment and had consistent change trends. Sediment liquefaction clearly promoted the release of Cu into overlying water, and the dissolved Cu concentrations in the overlying water during the seabed liquefaction and re-suspension phase were 18 times those in the static diffusion phase, and 11 times those in the seabed non-liquefaction and re-suspension phase. In addition, seabed liquefaction caused Cu in sediments to diffuse towards deeper levels, and the diffusion depth of was about half of liquefaction depth.

Post-liquefaction re-compaction effect on the cyclic behavior of natural marine silty soil in the Yellow River delta

Re-compaction process after previous liquefaction exerts significant effects on subsequent cyclic behaviour of soil. Due to the complexities in environmental loading conditions, the relevant studies on this topic are limited. In this paper, we collected natural submarine silts from non-liquefied and post-liquefied seabed in the Yellow River subaqueous delta via offshore hydraulic sampling method, respectively. Monotonic and cyclic triaxial tests were conducted between them to investigate the re-compaction effect after previous wave-induced liquefaction on the undrained static and cyclic shear behaviours of seabed soil. Our experiments indicated that due to re-compaction process, the static and cyclic strengths of seabed silt in post-liquefied area are both enhanced in comparison to non-liquefied soil. A transformation in static shear failure mode from plastic to brittle after wave-induced liquefaction could be identified in seabed silt. Furthermore, the developments of pore water pressure and cyclic axial strain with number of cycles for seabed silts could be characterized by the hyperbolic and power models, respectively, and an empirical relationship between pore water pressure and cyclic axial strain is proposed for natural seabed silt. Finally, compared with non-liquefied seabed silt, the threshold cyclic stress ratio (CSR) for post-liquefied seabed silt is also elevated due to re-compaction effect.