Guangxue Li

Sedimentation in the shear front off the Yellow River mouth

Abstract Based upon in-situ measurements and Landsat scanning images, spatial–temporal changes in the shear front and associated sedimentation in the subaqueous delta slope of the Yellow River have been studied. The results show that the shear front is an important dynamic factor in controlling rapid accretion at the Yellow River mouth. Suspended sediment converges and is deposited rapidly along the shear front zone; this is because a low-velocity zone is formed between two inverse flow bodies. Some small eddies occur in the shear front zone, shown on (method of concentration–temperature ratio) mapping; this is conducive to rapid convergence and accumulation of the suspended sediment in the shear front. At the same time, some hyperpycnal plumes and gravity-driven underflow may be formed; these move away from the shear front zone, due to the high sediment concentrations there. The shear front is formed firstly at the prodelta area; it then moves to the river mouth, in response to the flood or ebb tidal controls and deep sea influences. It moves along the subaqueous delta slope for 1 2 – 1 3 of the tidal cycle, during the river flood season. The shear front zone of low velocity forms a wall of water that prevents dispersal of the river outflow. The temperature image from TM6 shows that a river–sea interaction zone, a transition between the river and sea water temperatures, occurs around the mouth. The shear front is formed within this zone. A four-stage dynamic process, related to the shear front during the tidal cycle, is discussed. The results show that the shear front occurs twice during a tidal cycle.

Sedimentation in the Yellow River delta. Part III. Seabed erosion and diapirism in the abandoned subaqueous delta lobe

Abstract Historical bathymetry, satellite remote sensing and river discharge data have been used to examine the evolution of an abandoned delta lobe in the northern area of the modern Yellow River delta. Results show three stages of evolution in the subaqueous delta slope after lobe abandonment: (1) rapid erosion stage; (2) slow erosion stage; and (3) erosion–accumulation adjustment stage. According to 1:10,000 bathymetry data and high-resolution seismic profiles, diapirism in the subaqueous abandoned delta lobe is closely related to seabed erosion, which mainly occurs in the middle part of subaqueous delta slope. Diapirism results from the deformation of soft, fine-grained prodeltaic sedimentary stratum when the accumulation body on the soft stratum is removed in rapid erosion stage. The maximum accumulation centre in the delta lobe during growth coincides with the maximum erosion centre in the delta lobe abandonment phase. The diapir body occurs there. Distinguishing from diapirism in the Mississippi delta, three stages of the soft stratum deformation and diapirism are established in the Yellow River delta. The mobile strata and its diapirism are destructive to the offshore engineering facilities.

Seasonal evolution of the Yellow Sea Cold Water Mass and its interactions with ambient hydrodynamic system

The Yellow Sea Cold Water Mass (YSCWM) is an important component of the hydrodynamic system in the South Yellow Sea (SYS). However, its intricate interactions with the ambient flows over long time scales are not fully understood. This paper presents the analysis of the data set obtained from a seabed‐mounted Acoustic Doppler Current Profiler (ADCP) deployed for nearly 1 year in the western SYS. It allowed us to study the evolution of YSCWM, including the seasonal changes of tidal currents, near‐inertial oscillations (NIOs), and the wind‐driven currents due to typhoons and winter storms. Strong NIOs were found near the bottom of mixed layer and in the pycnocline with nearly opposite current directions, with maximum velocity of nearly 20 cm·s−1 in summer. The YSCWM can also inhibit the direct downward energy transport in the water column due to typhoons. Conversely, the hydrodynamic system also feeds back to influence the change of YSCWM. A large current shear (S) of 20 cm·s−1·m−1 is generated near the top of pycnocline. Generally, the intensity and depth of the pycnocline determine Ss magnitude and vertical location, respectively. Based on the monthly averaged density profile data, the Richardson number and wavelet analysis, the NIOs are considered to be capable of inducing predominant shear instability around the pycnocline. However, the NIOs are not strong enough to influence the lower YSCWM. In addition, in autumn, each fortnightly spring tide corresponds with a bottom temperature increase of nearly 2°C, indicating that tidal currents are the leading hydrodynamic driving force to decline the YSCWM.

Hydrodynamic condition and suspended sediment diffusion in the Yellow Sea and East China Sea

Based on monthly averaged current, temperature, and salinity, we analyzed the changes of suspended sediment concentration (SSC) and the relationship with the warm current, coastal current, and cold water mass (CWM) in the East China Seas (ECSs). The result shows that the coastal current and surface diluted water are the route for transporting suspended sediment. The Kuroshio and its derived warm current branches play the important role of the continental shelf circulation system and control the diffusion of suspended sediment. High SSC has been mainly concentrated in coastal current and CWM. Two sedimentary dynamic patterns have been identified. The winter-half-year pattern lasts almost 7 months. The coastal currents off the Shandong Peninsula, northern Jiangsu, Zhejiang-Fujian coast are the main routes for diffusion and deposition of the suspended sediment from the Yellow River and Changjiang River. The summer-half-year pattern is characterized by the well-developed CWM. All CWMs have a unique function to trap suspended sediment under the thermocline due to weakening tidal current and residual current there. These CWMs in the Yellow Sea (YS) and north ECS are connected together. The layer above the thermocline is characterized by diluted water with low salinity, high temperature. Suspended sediment can be transported into the Okinawa Trough and the South Korea coast during this period. A strong eddy always occur nearby the Kuroshio bend at northeast Taiwan, which has promoted the exchange between the ECS shelf and Okinawa Trough, and the development of the shelf edge current and Taiwan warm current (TWC).

The marine dynamics and changing trend off the modern Yellow River mouth

Topography around the Yellow River mouth has changed greatly in recent years, but studies on the current state of marine dynamics off the Yellow River mouth are relatively scarce. This paper uses a two-dimension numerical model (MIKE 21) to reveal the tidal and wave dynamics in 2012, and conducts comparative analysis of the changes from 1996 to 2012. The results show that M2 amphidromic point moved southeastward by 11 km. It further reveals that the tides around the Yellow River mouth are relatively stable due to the small variations in the tidal constituents. Over the study period, there is no noticeable change in the distribution of tidal types and tidal range, and the mean tidal range off the river mouth during the period studied is 0.5–1.1 m. However, the tidal currents changed greatly due to large change in topography. It is observed that the area with strong tidal currents shifted from the old river mouth (1976–1996) to the modern river mouth (1996–present). While the tidal current speeds decreased continually off the old river mouth, they increased off the modern river mouth. The Maximum Tidal Current Speed (MTCS) reached 1.4 m s−1, and the maximum current speed of 50-year return period reached 2.8 m s−1. Waves also changed greatly due to change in topography. The significant wave height (H1/3) of 50-year return period changed proportionately with the water depth, and the ratio of H1/3 to depth being 0.4–0.6. (H1/3) of the 50-year return period in erosion zone increased continually with increasing water depth, and the rate of change varied between 0.06 and 0.07 m yr−1. Based on the results of this study, we infer that in the future, the modern river mouth will protrude gradually northward, while the erosion zone, comprising the old river mouth and area between the modern river mouth and the old river mouth (Intermediate region) will continue to erode. As the modern river mouth protrudes towards the sea, there will be a gradual increase in the current speed and decrease in wave height. Conversely, the old river mouth will retreat, with gradual decrease in current speed and increase in wave height. As more coastal constructions spring up around the Yellow River mouth in the future, we recommend that variation in hydrodynamics over time should be taken into consideration when designing such coastal constructions.

Stratigraphic variations in the Diaokou lobe area of the Yellow River delta, China: implications for an evolutionary model of a delta lobe

Abstract Characterization of the evolution of delta lobes has theoretical significance for the formation of entire deltas. The Diaokou lobe of the Yellow River delta provides a typical example. The Yellow River transported a great deal of sediment to the Diaokou area during the period of 1964–76 after the river changed its course to the Diaokou River and formed a new delta lobe – the Diaokou lobe. The Diaokou lobe reflects the evolution of the modern Yellow River delta and contains a record that represents the complete modern Yellow River delta depositional system. Using grain-size characteristics, magnetism, and the accelerator mass spectrometry (AMS) 14C dating of cores ZK10-3 and ZK30 in the northern part of the Diaokou lobe, combined with collected data from cores ZK227, ZK1 and ZK228, which are located further south on the Diaokou lobe, we analysed the Holocene and recent stratigraphy of the Diaokou lobe area and its evolution. The Holocene stratigraphy of the Diaokou lobe and the nearby area contains upwards-succession, shallow-marine, river and lake, salt-marsh, and delta facies. This area received deltaic deposits beginning in 1855 and experienced prodelta, delta-front (lateral) and delta-plain deposition. When the Diaokou lobe began to form, this area experienced four stages of deposition after 1964: (1) dispersed-flow deposition; (2) single-channel deposition; (3) diversion deposition; and (4) abandonment and erosion. Compared with the evolution of a lobe of the Mississippi delta, the Yellow River delta contains thick, laterally extensive deposits as a result of the higher sediment load.

Sediment records of environmental changes in the south end of the Zhejiang-Fujian coastal mud area during the past 100 years

Previous studies carried out in the East China Sea (ECS) mud area focused on long-term environmental changes in sedimentary records during the Holocene, especially during the mid-Holocene high-stand water levels period. These results indicate that sensitive grain size groups can be used as a sedimentary proxy to reconstruct the evolution of the East Asian Winter Monsoon (EAWM). The studies have been carried out mainly in the northern and middle portions of the Zhejiang-Fujian coastal mud, however, similar research in the southern portion and the comparison between sedimentary proxy and modern measured data of EAWM are lacking. In this paper, we focused on a sedimentary record of the past 100 years with an enhanced resolution of 1.8 years. Investigations of the southern end of the Zhejiang-Fujian coastal mud area were conducted on the basis of 210Pb chronology, grain-size analysis and chemical element analysis. The correspondence between the mean grain size (Mz) of sediment sensitive grain size and the measured EAWM was confi rmed for the fi rst time. We found that during the recent 100 years, the variation of the mean grain size of the sensitive population in the southern portion of the Zhejiang-Fujian mud was mainly controlled by the EAWM intensity changes; and not directly related to changes in the sediment discharge from Datong station of the Changjiang River (DTSD). Finally, recent changes in the content of heavy metals in study area refl ect the impact of human activities on the environment.

Late Quaternary Strata and Carbon Burial Records in the Yellow River Delta, China

Sediment carbon sequestration plays an essential role in mitigating atmospheric CO2 increases and the subsequently global greenhouse effect. To clarify the late Quaternary strata and carbon burial records in Yellow River delta (YRD), detailed analysis of benthic foraminifera, total carbon (TC), organic carbon (Corg), sedimentary characteristics and moisture contents of sediments, was performed on core ZK3, 30.3 m in length and obtained from YRD in 2007. Eight depositional units (designated U1-U8 in ascending order) were identified. A comprehensive analysis method of historical geography and sedimentary geology was used to determine the precise depositional ages of the modern Yellow River delta (MYRD), from which pre-MYRD ages were deduced. The results indicates that the maximum burial rates of TC, inorganic carbon (IC) and Corg occurred in the delta front (U5), and the minimum in the shallow sea (U3). Remarkable high sedimentation rates in the MYRD are responsible for burial efficiency of carbon, with an average rate of Corg burial reaching 2087±251 g (m2 yr)−1, and that of IC reaching 13741±808 g (m2 yr)−1, which are much higher than those of other regions with high contents of Corg. Therefore, YRD has a significant burial efficiency for carbon sequestration.

Seasonal Suspended Particles Distribution Patterns in Western South Yellow Sea Based on Acoustic Doppler Current Profiler Observation

An Acoustic Doppler Current Profiler (ADCP) observation site was set up in the Western South Yellow Sea from 2012 to 2013 to study the local suspended particle matters (SPM) distribution pattern. The SPM concentration could be semi-quantitatively represented by backscatter intensity (Sv), converted by the echo intensity (EI) of ADCP. Results show two types of SPM in the water column: the quasi-biological SPM and quasi-mineral SPM. The quasi-biological SPM mainly exists in summer half year and is concentrated above the thermocline. It has periodically diurnal variations with high concentration at night and low concentration in the daytime. The quasi-mineral SPM is located in lower part of the water column, with similar relation to monthly tidal current variation all year round. However, the daily quasi-mineral SPM distribution patterns vary between summer and winter half year. The sunlight is thought to be the origin factor leading to the diurnally vertical motion of the biological features, which might cause the diurnal Sv variation. Unlike in winter half year when tidal current is relatively single driving force of the monthly SPM pattern, the high speed current near the thermocline is also responsible for the concentration of quasi-mineral SPM in summer half year. The sediment input difference between summer and winter half year contribute to the varied daily variation of quasi-mineral SPM with re-suspended SPM in winter and sediments from Yellow Sea Mud Area (YSMA) in summer. The seasonal variations in hydrodynamics, water structure and heavy-wind incidents are the primary factors influencing the differential seasonal SPM distribution patterns.

Primary study on the diapir in the north of the Yellow River delta

Based on the measuring data of landforms, high-resolution seismic profiles, drilling cores, etc, a diapir body was found in the north of the modem Yellow River delta. The diapir body with a length of 5 km and a width of 1 km is distributed on the middle to low part of the slope of an abandoned delta lobe. Its formation is related to the deformity of the soft stratum which is deposited in the prodelta and covered by the stratum of the mouth bar sediments. Research results show that its formation is very different from the Mississippi River delta’s diapirs but related to the erosion of the seabed and occurs on the location with a large eroded quantity. The soft stratum and its diapir body can result in a great hazard to marine buildings.