Heqin Cheng

Tidal currents, bed sediments, and bedforms at the South Branch and the South Channel of the Changjiang (Yangtze) estuary, China: Implications for the ripple-dune transition

Measurements were made of tidal currents, bed sediment particle sizes, and bedform dimensions at the South Branch and the South Channel of the Changjiang estuary, China, during the dry season in 1997 and the flood season in 1998. The near bottom current speed and direction were measured by a mechanical current meter for 10 h in 1997. The near surface current speed and direction were measured by a Current Meter of Endeco/YSI Inc. 174 SSM for 14 h in 1997 and 1998. Nine bed sediment samples were taken and their particle sizes were analyzed with sieves and siphons. The bedforms were nautically detected by an echo sounder and a side scan sonar. Results show that the ebb tides had larger near-bottom and near-surface current speeds and longer durations than the flood tides, in which the former occurred during the flood season in 1998 and not in the dry season in 1997. The bed sediments were composed of coarse silts and very fine sands during the dry season but of fine sands during the flood season. Bedforms were dominated by ebb tidal currents, the height∶length ratios of dunes and lee face angles were low, and heights and lengths were larger during the flood season in 1998. The ebb and flood tidal currents, bed sediment sizes, and dune morphology were largely controlled by the seasonal runoff variations. A new tentative boundary might be proposed for natural dunes in very fine sand with the availability of additional field data in the future.

Ecosystem model predictions of fishery and conservation trade-offs resulting from marine protected areas in the East China Sea

The East China Sea (ECS) supports a highly productive fishery and is rich in biodiversity, but economic development in China and peripheral countries has led to intensifying anthropogenic impacts in the ECS. In response to this the Chinese government has introduced a range of marine spatial management measures. A spatial ecosystem model (Ecospace) of the ECS was developed to examine (1) the likely nature of trade-offs between fishery and conservation goals resulting from the marine protected areas (MPAs) and (2) possible trade-offs within the fishery sector resulting from the MPAs. The results suggest that overall the fishery has benefited from all of the simulated MPAs, whereas, although they defy categorical interpretation, effects of the MPAs on biodiversity and ecosystem structure are variable. Simultaneous application of several metrics of ecosystem status indicates that the perceived effect of an MPA on ecosystem status can depend on which metrics for ecosystem status are used, and how these metrics are interpreted. The simulations indicate that a fisheries and conservation outcome beneficial to all is possible, but not guaranteed, with the creation of an MPA. Total landings and profitability are predicted to have increased as a result of each of the MPAs, albeit at the cost of reduced landings and profits to some sectors of the fishery. This study demonstrates the benefits of the additional information relating to biodiversity, ecosystem structure and within fishery dynamics available from spatial ecosystem models compared to the single species models typically used to examine MPA effects. However, the use of a more complex ecosystem model introduces additional uncertainty in model interpretation.

Discovery and implications of catenary-bead subaqueous dunes

Measurements of topography at the segment of bifurcation between the South Channel and North Channel in the Yangtze Estuary were conducted, and a new type of subaqueous dune was discovered. This structure, newly defined as a catenary-bead dune, consists of a catenary dune and its associated elliptical pit bedform. Based on this finding, the nomenclature of “morphology of dune associated with accompanying bedform” is first proposed. The measured data indicate a mean height and wavelength of 1.29 m and 31.89 m, respectively; wavelength/height ratio (L/H) of 14 to 56; and elliptical pits of mean and maximum depth 0.98 m and 1.98 m, respectively. Flow information was obtained using an Acoustic Doppler Current Profile (ADCP), and the bed material components were gathered with a bottom sampler. The results show mean flood and ebb velocities of 0.27 and 0.78 m s-1, respectively, with shorter duration of flood tide than ebb tide. The silt, very fine sand, and fine sand fractions were within the ranges 21.6–23.4%, 28.2–32.2%, and 39.7–41.6%, respectively, revealing complex bed material composition. Water depth at the study site varies from 13 to 17 m. This finding will enrich the study of dunes and provide important data for geomorphological research. Moreover, the results are significant for engineering applications to estuaries.

Effect of Riverbed Morphology on Lateral Sediment Distribution in Estuaries

ABSTRACT Yang, Z.; Cheng, H.; Cao, Z.; Guo, X., and Shi, X., 2018. Effect of riverbed morphology on lateral sediment distribution in estuaries. An idealized model is applied to a set of eight estuaries with various transverse depth cross-sections to examine the effect of riverbed morphology on suspended sediment concentration (SSC) distribution. Five estuaries consist of a single deep channel, and three estuaries consist of double deep channels. All estuaries share a set of boundary conditions, including tidal flow, density gradient, river discharge, in an attempt to represent a typical estuary. Under these conditions, model results suggest that both tidal and residual flow fields vary significantly among eight estuaries, resulting in significant differences on SSC distribution, e.g., maximum mean SSC (), location of (), and space uniformity of mean SSC. In most estuaries, the SSC is higher over the left channel than that over the right channel mainly due to the lateral density gradient-generated residual flow (). However, the space distribution of mean SSC is more uniform over the estuaries with a rightward deep channel than those estuaries with a leftward deep channel. Furthermore, the former estuaries get a higher value of , which is mainly caused by the decreasing lateral density gradient across the estuary. Lateral residual flow generated by nonlinearities would transport sediment from two sides of the estuary to the central channel (convergent transport). This transport component would play an important role in a narrow deep estuary, and would shift the location of to the central orientation. On the contrary, a much lower value of is examined in the broad wide estuary because of the extreme weak nonlinearity. Sensitivity analysis of tidal asymmetry indicates that both the amplitude and phase of M4 tidal flow would affect the transverse sediment distribution though the M4 tidal transport () and the influence of M4 tidal flow on the M2 tidal transport (), resulting in a significant variance of and over estuaries.

Riverbed erosion of the final 565 kilometers of the Yangtze River (Changjiang) following construction of the Three Gorges Dam

The world’s largest hydropower dam, the Three Gorges Dam (TGD), spans the upper Yangtze River in China, creating a 660-km long and 1.1-km wide reservoir upstream. Several recent studies reported a considerable decline in sediment load of the Lowermost Yangtze River (LmYR) and a rapid erosion in the subaqueous delta of the river mouth after the closure of the TGD in 2003. However, it is unknown if the TGD construction has also affected river channel and bed formation of the LmYR. In this study, we compared bathymetric data of the last 565 kilometers of the Yangtze River’s channel between 1998 and 2013. We found severe channel erosion following the TGD closure, with local riverbed erosion up to 10 m deep. The total volume of net erosion from the 565-km channel amounted to 1.85 billion m3, an equivalent of 2.59 billion metric tons of sediment, assuming a bulk density of 1.4 t/m3 for the riverbed material. The largest erosion occurred in a 100-km reach close to the Yangtze River mouth, contributing up to 73% of the total net eroded channel volume.

Fluctuations in the tidal limit of the Yangtze River estuary in the last decade

The tidal limit is the key interface indicating whether water levels will be affected by tidal waves, which is of great significance to navigation safety and regional flood control. Due to limitations in research methods, recent changes in the Yangtze River tidal limit, caused by sea level rise and large-scale engineering projects, urgently need to be studied. In this study, spectrum analysis was undertaken on measured water level data from downstream Yangtze River hydrological stations from 2007 to 2016. The bounds of the tidal limit were identified through comparisons between the spectra and red noise curves, and the fluctuation range and characteristics were summarized. The results showed that: (1) During the extremely dry period, when the flow rate at Jiujiang station was about 8440 m3 s−1, the tidal limit was near Jiujiang; whereas during the flood season, when the flow rate at Jiujiang station was about 66700 m3 s−1, the tidal limit was between Zongyang Sluice and Chikou station. (2) From the upper to lower reach, the effect of the Jiujiang flow rate on the tidal limit weakens, while the effect of the Nanjing tidal range increases. The tidal limit fluctuates under similar flow rates and tidal ranges, and the fluctuation range increases with increasing flow rate and decreasing tidal range. (3) With the continued influence of rising sea levels and construction in river basin estuaries, the tidal limit may move further upstream.

Impact of anthropogenic drivers on subaqueous topographical change in the Datong to Xuliujing reach of the Yangtze River

Changes of subaqueous topography in shallow offshore water pose safety risks for embankments, navigation, and ports. This study conducted measurements of subaqueous topography between Datong and Xuliujing in the Yangtze River using a SeaBat 7125 multi-beam echo sounder, and the channel change from 1998 to 2013 was calculated using historical bathymetry data. The study revealed several important results: (1) the overall pattern of changes through the studied stretch of the river was erosion–deposition–erosion. Erosion with a volume 700×106 m3 occurred in the upper reach, deposition of about 204×106 m3 occurred in the middle reach, and erosion of about 602×106 m3 occurred in the lower reach. (2) Dunes are the most common microtopographic feature, accounting for 64.3% of the Datong to Xuliujing reach, followed by erosional topography and flat river topography, accounting for 27.6% and 6.6%, respectively. (3) Human activities have a direct impact on the development of the microtopography. For instance, the mining of sand formed holes on the surface of dunes with lengths of 20–35 m and depths of 3–5 m. We concluded that the overall trend of erosion (net erosion volume of 468×106 m3) occurred in the study area mainly because of the decreased sediment discharge following the closure of the Three Gorges Dam. However, other human activities were also impact factors of topographic change. Use of embankments and channel management reduced channel width, restricted river meandering, and exacerbated the erosion phenomenon.

Using Multibeam Backscatter Data to Investigate Sediment‐Acoustic Relationships

Sediment properties are known to influence acoustic backscatter intensity. This sediment-acoustic relationship has been investigated previously through using physical geoacoustic models and empirical methods and found to be complex and nonlinear. Here we employ a robust machine-learning statistical model (random forest decision tree) to investigate the most likely nonlinear sediment-backscatter relationships. The analysis uses colocated sediment and acoustic backscatter data (collected from a 300-kHz multibeam sonar system) for 564 locations in four different areas on the Australian margin. Seven sediment grain size properties (%gravel, %sand, %mud, mean grain size, sorting, skewness, and kurtosis) were used to predict the acoustic backscatter responses at individual incidence angles. The modeling results demonstrate the effectiveness of this multivariate predictive approach for the investigation of sediment-acoustic relationship. Thus, we find that for incidence angles between 1° and 41°, the sediment variables explain around 70% of variance in the backscatter intensity. Sediment mud content was found to be the most important sediment variable in the model and has a significant negative relationship with backscatter intensity. Mean grain size was the second ranked sediment variable and found to have a positive relationship with backscatter intensity. The results also show that sediment mud content plays a key role in sorting-backscatter and sand-backscatter relationships. Using only two sediment properties, mud content and mean grain size, together it was possible to largely explain the sediment-acoustic relationship. The strongest backscatter return occurred with medium sediment mud content and large mean grain sizes (or muddy coarse sand).

Using multibeam acoustic remotely sensed data to investigate seabed sediment grain size characteristics

Acoustic remote sensing is the only effective technique to investigate the deep seabed. Modern high-frequency multibeam echo-sounders transmit and receive backscatter signals from hundreds of narrow-angle beams which generate small footprints on the seabed. They can produce bathymetry and backscatter data with a spatial resolution around 2% of water depth, which enables us to map the seabed with great detail and accuracy. After calibration, the backscatter intensity is largely controlled by three seabed physical properties: the acoustic impedance contrast (often called “hardness”), apparent interface roughness (relative to acoustic frequency) and volume inhomogeneity [6, 7, 11]. These seabed physical properties are directly related to sediment grain size.