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Water exchange, featured by bottom inflow and surface outflow in a typical estuary, determines the transport and redistribution of salt, nutrients, pollutants, and suspended sediments and organisms. Water exchange in Chesapeake Bay, the largest estuary in the US, has been extensively studied, yet its long-term interannual variability and its relationship with the external forcings are not fully understood. Based on a long-term (1980–2011) numerical model simulation, this study examines the water exchanges between Chesapeake Bay and the adjacent coastal shelf, between different regions within the bay, as well as their relationships with river discharge, wind, and residence time. Through an EOF analysis of the bottom inflow and surface outflow at seven selected cross-bay sections, we found that over 90% of the spatiotemporal variations of water exchange can be explained by the first two EOF modes, which are highly correlated with the freshwater discharge and northwesterly wind, respectively. Unlike the outflow that increases linearly with river discharge as commonly expected, the inflow responds non-monotonically to river discharge. The relationship between the river discharge and inflow can be described by a combination of the Michaelis–Menten/Monod equation and a linearly decreasing function, i.e., the inflow initially increases with river discharge due to enhanced gravitational circulation and then levels, and gradually declines due to overwhelming seaward barotropic current. We found a locally enhanced water exchange in the lower-middle bay, which can be attributed to the persistent reflux of surface outflow due to the irregular geometry and rapid shoaling in the channel bathymetry. The water exchange and the mean residence time can be connected reciprocally through the bay volume, yet the validation of this relationship depends on the timescale to be considered since the residence time at a given time is controlled by the future hydrodynamics. A delay effect should be considered when using the relationship to estimate outflow interchangeably with the direct computation of the outflow. • Reflux contributes significantly to the water exchange in the lower bay. • River discharge and down-estuary wind dominate the water exchange in the bay. • Bottom inflow responds non-monotonically to the river discharge. • Water exchanges in the lower/upper bay respond oppositely to the down-estuary wind. • The time-lag effect should be considered to estimate the residence time by V/ Q out. [ABSTRACT FROM AUTHOR]

Wave height, pressure and orbital velocity statistics, as influenced by the factor of water depth, are analyzed on the basis of five data sets collected in situ from two intertidal sites with a mean water depth of 0.8–2.7 m and three inner shelf sites (water depths 14.6–27.6 m). Acoustic Doppler Velocimeters (ADVs) were used to measure instantaneous wave pressure and current velocity at 0.20–0.35 m above sea bed. The zero-crossing wave analysis method was applied to analyze the wave data to determine the wave pressure amplitude P and orbital velocity amplitude U of individual waves, together with the frequency of occurrence distributions of P and U , which are compared with the three commonly-used, classic Rayleigh, modified Rayleigh and Weibull distributions. It is found that the distribution of P is almost identical to that of U for each of the five study sites. The distributions of P and U for the intertidal flat sites fit the Weibull distribution better than the classic or modified Rayleigh ones, indicating a shallowness effect which is associated with finite-banded wind-wave spectra and possible wave breaking or near-bed turbulence. On the other hand, the distributions of P and U measured at the three inner shelf sites agree almost equally with the three distributions. With increasing water depth, the distributions of P and U are shown to reduce from the two-parameter Weibull distribution at the shallow intertidal sites to the classic Rayleigh at the deep-water sites. For coastal engineering applications, an empirical formula is proposed to estimate more accurately the significant wave height in intermediate coastal waters based on the surface water elevation data. • Study depth effects on statistical probability distributions of wave parameters. • Distribution of wave pressure is nearly identical to that of wave orbital velocity. • Weibull distribution is better than (modified) Rayleigh at very shallow water. • Considering the effects of strong turbulence and possible wave breaking. • An empirical relationship is derived to estimate significant wave height. [ABSTRACT FROM AUTHOR]

The definition of the threshold of sediment motion is critical for continental shelf sediment dynamics. The work by A. Shields laid the foundation for this research direction, leading to the well-known Shields curve. Here we review the most widely used threshold curves that have followed from the original Shields curve over the last 80 years, and propose that in terms of physical processes the threshold (critical Shields parameter) is a function of at least six variables, i.e. grain Reynolds number, grain size distribution, sphericity, roundness, particle cohesiveness and the scale effects of turbulence. Identifying these key factors, we paid a special attention to the role of the scale effects of turbulence. Turbulence was thought to be a random process, but the improvement of measurement techniques revealed that it has both temporal and spatial structures: the magnitude of instantaneous velocity fluctuations varies in time and in location, which can cause the deviation between in situ measurements and flume experiments. In coastal and shelf waters, in situ measurements of tidal currents and suspended sediment concentrations have revealed that resuspension takes place even though the bed shear stress is well below the Shields curve. Further process and mechanism studies are required to improve the theoretical framework regarding the turbulence structures and their interplay with sediment threshold. The scientific problems for future studies include the establishment of laboratory experiments, in situ measurements and process-based modelling under different water depths and hydrodynamic conditions to quantify the scale effects of turbulence; the development of new observation techniques for higher resolution and for extreme environments; development of new data processing methods, including big data methods to analyse turbulence structures; and the quantification of the effects of biological contributions and non-particle components on the family of Shields curves. • Measured threshold for initial sediment motion is well below the Shields curve. • The scale effects of turbulence cause the deviation between in situ measurements and flume experiments. • The threshold (critical Shields parameter) is now a function of at least six variables. [ABSTRACT FROM AUTHOR]