Sediment Dynamics in Wind Wave‐Dominated Shallow‐Water Environments

Abstract

Sediment dynamics driven by waves and currents in shallow-water estuarine environments impacts many physical and biological processes and is important to the estuary-wide sediment budget. Observational restrictions have limited our ability to understand the physics governing sediment entrainment and mixing in these environments. To this end, we use direct numerical simulation to simulate sediment transport processes in shallow, combined wave-and current-driven flows. Simulations are run with depth-averaged currents ranging from 0 to 9 cm/s, while wave conditions are held constant with a bottom orbital velocity and period of 10 cm/s and 3 s, respectively. Our results indicated that for wave-dominated conditions, waves reduce vertical momentum fluxes and the associated bottom drag, thereby accelerating mean currents. Conversely, currents do not significantly affect the wave velocity field. However, they increase the bed shear stress and change the timing and duration of sediment entrainment throughout the wave cycle. Counterintuitively, these effects lead to lower suspended sediment concentrations near the bed for a portion of the wave cycle. By analyzing sediment fluxes, waves are shown to drive near-bed sediment dynamics while currents control vertical mixing above the buffer layer, where downward settling is predominantly balanced by the current-generated vertical turbulent sediment flux. In the absence of currents, sediment concentrations are negligible above the wave boundary layer because mixing is weak. We show that the time-and phase-averaged sediment concentration profiles for wave and current conditions resemble the theoretical Rouse profile derived for equilibrium conditions in statistically steady, unidirectional turbulent channel flow. Plain Language SummaryThe transport of mass, such as nutrients and sediment, by fluid flows is fundamental to aquatic life and is crucial to many environmental and coastal engineering studies. Whether predicting the dispersion of shrimp larvae or assessing the mobilization of sediment-sorbed contaminants, the fluid mechanics governing the transport processes is the most important underlying physical phenomenon. Despite its importance, many mechanisms controlling the movement of sediment in estuaries are poorly understood. This is particularly true near the sediment bed where our ability to observe and measure properties relevant to the physics is limited. To this end, we apply state-of-the-art supercomputers to simulate sediment transport by fluid flow in environments with waves and currents. Contrary to popular belief within the fluid mechanics community, we find that currents can accelerate in the presences of waves. This acceleration can potentially affect how sediment and nutrients move within an estuary. Currents also affect the duration and magnitude of sediment erosion. Our results support the conceptual model that wind-generated waves strongly influence sediment erosion, but currents are required to mix sediment into the water column. Ultimately, our work gives b...

A Turbulence‐Resolving Numerical Investigation of Wave‐Supported Gravity Flows

“…At the bottom of the computational domain, the erodible/depositional boundary condition (Cheng et al, ; Nelson & Fringer, ) is implemented, which is written as where and are the erosional and depositional fluxes at the bottom, respectively. Following the continuous deposition formulation (Sanford, ), the depositional flux is modeled as .…”

“…We adopt the triple decomposition method (Reynolds & Hussain, ) to isolate the weak downslope gravity current and the organized variations in the turbulent fluctuating flow field. The triple decomposition is applied in a similar manner as other turbulence‐resolving numerical studies for a current‐wave‐fluctuation decomposition (Nelson & Fringer, ). We decompose a variable into a current component , a wave component , and a fluctuating component as follows …”

Observations of Near‐Bed Shear Stress in a Shallow, Wave‐ and Current‐Driven Flow

“…where z 0 is the bottom roughness and is the von Kármán coefficient. Recent work has suggested that = 0.38 is the most appropriate value for a boundary layer flow (Nagib & Chauhan, 2008), but we chose a constant = 0.41 for consistency with previous studies in South San Francisco Bay (e.g., Bricker et al, 2005;Cheng et al, 1999). The mean velocity profile can be measured with a profiling current meter, and equation (19) can be fit for optimal values of u * and z 0 .…”

“…Numerous models to describe the combined wave‐current bed shear stress have been proposed over the years, and many imply that wave‐current interactions increase the “apparent” bottom roughness and induce additional drag on the flow (Grant & Madsen, ; Styles et al, ; Styles & Glenn, ; You et al, ). However, one recent numerical study suggests that the opposite effect may be seen in estuarine environments with smooth beds; that is, the drag decreases with the addition of laminar waves (Nelson & Fringer, ).…”

Rapid shoreline flooding enhances water turbidity by sediment resuspension: An example in a large Tibetan lake

“…Rising water levels have been shown to impact the erosion of shorelines and low-lying areas, with direct consequences for sediment resuspension. Newly flooded areas along the shallow shores of lakes, sandy coastlines and shallow intertidal mudflats are particularly vulnerable to wind-wave action that can remobilize sediments and lead to erosion (Eadie et al, 2008;Fagherazzi and Priestas, 2010;Nelson and Fringer, 2018). Zhang et al (2004) indicated that sandy beach erosion increased by two orders of magnitude with sea level rise, with eroded sediment being resuspended by waves and carried offshore.…”