Benwei Shi

Determination of Critical Shear Stresses for Erosion and Deposition Based on In Situ Measurements of Currents and Waves over an Intertidal Mudflat

ABSTRACT Shi, B.; Wang, Y.P.; Yang, Y.; Li, M.; Li, P.; Ni, W., and Gao, J., 2015. Determination of critical shear stresses for erosion and deposition based on in situ measurements of currents and waves over an intertidal mudflat. Accurate determination of the critical shear stress associated with the erosion and deposition of sediments is an important component of numerical models used to predict and quantify sediment behavior and transport across intertidal flats. In this study, water depth, wave parameters, near-bed turbulent velocity, suspended sediment concentration (SSC), and intratidal bed-level changes were measured to determine the erosion (τce) and deposition (τcd) thresholds of sediments on an intertidal mudflat at Jiangsu, China. Based on integrated field measurements of bed-level changes and hydrodynamics, the bed shear stresses of currents (τc), waves (τw), and combined current-wave action (τcw) were calculated, and the critical shear stress required for erosion (τce = 0.14 N/m2) and deposition (τcd = 0.08 N/m2) of these sediments was determined. Both values are in agreement with an estimate of τce (0.13 N/m2) that was based on water content and τcd values calculated by previous works, indicating that the value of τce estimated for these sediments is controlled primarily by the water content of the sediments. During field measurements, deposition occurred (τcw < τcd) when current action exceeded wave energy (τc > τw) during calm weather, whereas erosion occurred (τcw > τce) when wave action increased dramatically during rough weather. Our field data showed that high current velocities lead to low τc, possibly because high SSC reduced the drag coefficient, which is variable during a tide, and further caused low τc under high current velocities. Additionally, bedforms characteristic of intertidal mudflat (e.g., gullies, small creeks, ripples, or saltmarsh) has a significant influence on the drag coefficient of the bed. These observations suggest that the bed level responds strongly to changing hydrodynamic conditions, and also that waves are of great importance to erosion. This study demonstrates that the in situ determination of the parameters that control erosion and deposition is a useful approach to obtaining values of τce and τcd, which provide the basis for a mechanistic understanding of the morphological evolution and development of predictive sediment transport and erodibility models.

On the variability of near-bed floc size due to complex interactions between turbulence, SSC, settling velocity, effective density and the fractal dimension of flocs

Interactions between turbulence, suspended sediment concentration (SSC), settling velocity, effective density, fractal dimension, and floc size were studied on the tide-dominated, muddy coastal shelf of the southwestern Yellow Sea, China. The measurements were carried out in July 2013 at two sites located in water depths of 21.2 and 22.1 m. Negative correlations were observed between shear rate, SSC, effective density, and mean floc size, which supports the results of previous numerical, experimental, and field studies. A significant positive correlation was observed between near-bed SSC and shear rate, an indication that SSC variations are controlled by turbulence and re-suspension. In addition, significant linear relationships were found between settling velocity and other parameters (floc size, turbulence, SSC, effective density, and fractal dimension) at the two sites, indicating that the controlling factors on settling velocity are spatially variable. Principal component analysis was applied to determine the relative importance of turbulence, flocculation ability, and SSC as controls on floc size in situ. The relative contributions of turbulence, flocculation ability, and SSC to floc size (at both sites) were ~33.0%, 30.3%, and 29.7%, respectively, this being a new field-based quantitative analysis of the controls on floc size. The findings demonstrate that, in nature, flocculation ability affects floc size to the same degree as turbulence and SSC. Therefore, predictions of floc size in coastal marine environments require constraints not only on turbulence and SSC, but also on flocculation ability.

Role of wind in erosion-accretion cycles on an estuarine mudflat

Wind is an important regulator of coastal erosion and accretion processes that have significant ecological and engineering implications. Nevertheless, previous studies have mainly focused on storm−generated changes in the bed level. This paper aims to improve the understanding of wind−induced erosion–accretion cycles on intertidal flats under normal (non−stormy) weather conditions using data that relates to the wave climate, near−bed 3D flow velocity, suspended sediment concentration, and bed−level changes on a mudflat at the Yangtze Delta front. The following parameters were calculated at 10−minute intervals over 10 days: the wind wave orbital velocity (Uδ), bed shear stress from combined current–wave action, erosion flux, deposition flux, and predicted bed−level change. The time series of measured and predicted bed−level changes both show tidal cycles and a 10−day cycle. We attribute the tidal cycles of bed−level changes to tidal dynamics, but we attribute the 10−day cycle of bed−level changes to the interaction between wind speed/direction and neap−spring cyclicity. We conclude that winds can significantly affect bed−level changes in mudflats even during non−stormy weather and under macro−mesotidal conditions and that the bed−level changes can be predicted well using current–wave–sediment combined models. This article is protected by copyright. All rights reserved.

Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

Understanding erosion and accretion dynamics during an entire tidal cycle is important for assessing their impacts on the habitats of biological communities and the long-term morphological evolution of intertidal mudflats. However, previous studies often omitted erosion and accretion during very shallow-water stages (VSWS, water depths  0.2 m (i.e., probe submerged) are considered. These findings suggest that the magnitude of bed-level changes during VSWS should not be neglected when modeling morphodynamic processes. Our results are useful in understanding the mechanisms of micro-topography formation and destruction that often occur at VSWS, and also improve our understanding and modeling ability of coastal morphological changes.

Assessing the vulnerability of changing coasts, Hainan Island, China

Knowledge of coastline changes and vulnerability is of great importance to local government departments that are responsible for the management and development of coastal zones. To study the nature of change and vulnerability along the coasts of the Hainan Island, we collected a large number of sediment samples through the last few years, and reconstructed the changes of the coastline by combining the data of sediment grain-size analysis and the nautical charts/TM RS imaginary. Contrary to being almost free from erosion (as expected from the findings that the coastlines are in a relatively stable state), four major cities in Hainan (i.e., Haikou, Wenchang, Sanya and Changjiang) turned out to be suffered from a moderate coastal vulnerability primarily because of the large populations that impose considerable pressure on the coastlines. Thus, the assessment methodology utilized in this study, including both anthropogenic and natural factors, serves as a useful tool to obtain a comprehensive understanding of coastline vulnerability for local government, in terms of coastal management and adaptation.

Analysis of the characteristics of offshore currents in the Changjiang (Yangtze River) estuarine waters based on buoy observations

A buoy of 10 m in diameter was used to record the current speed and direction in a vertical profile in the offshore area of the Changjiang (Yangtze River) Estuary (with an average water depth of 46.0 m) for one year. The results include: (1) the currents rotate clockwise and the current direction is consistent in a vertical profile without clear seasonal variations. (2) The horizontal current speeds are generally high, with a maximum of 128.5 cm/s occurring in summer and 105.5 cm/s appearing in winter commonly close to the surface. The average current speeds in the vertical profile fall in the same range (the differences are less than 8.0 cm/s), with the maximum of 47.0 cm/s occurring in summer and 40.8 cm/s in winter. The average current speed during spring tides is twice that during neap tides (26.5 cm/s). (3) Significant differences of speeds are observed in the vertical profile. The maximum current speed occurs at either surface (spring and winter) or sub-surface (summer and autumn), with the minimum current speed appearing at the bottom. The maximum average current speed of all layers is 57.9 cm/s, which occurs in the 18-m layer during summer. (4) The average speed of the residual currents ranges from 7.5 cm/s to 11.3 cm/s, with the strongest occurring in spring and weakest in winter. The residual currents of all layers are eastward during spring and winter, whereas northeastward or northward during summer and autumn. (5) The currents in the offshore of Changjiang Estuary are impacted collectively by diluted Changjiang River discharge, the Taiwan Warm Current, monsoon and tides.