| On an intertidal mudflat in Central San Francisco Bay, we conducted four field experiments between February, 2001 and October, 2003 to investigate hydrodynamic and sediment transport processes. Not only tides and locally driven wind waves bring energy onto the mudflat, but also remotely forced ocean swell and infra-gravity seiching. These energy sources persist and dominate over different time scales, with variation occurring over minutes, hours, and days.; The different hydrodynamic forcing mechanisms superpose upon each other---and interact nonlinearly---in the water column, creating a dynamically varying interaction with the sediment bed. Wind waves produce the largest stresses; however, ocean swell often determines the virtual roughness experienced by the seiche and tidal currents. The boundary layer of the tide is modulated by seiching motions, and is better described by a log-linear profile than a logarithmic profile. However, an analytical log-linear model using acceleration and stratification length scales does not explain the observed velocity structure. Instead, a k-epsilon turbulence closure model (General Ocean Turbulence Model, GOTM) reproduces the unsteady boundary layer structure of the seiching motion. A simple analytical solution (Smith, 1977) suffices to model the boundary layer of ocean swell. Several methods of separating wave energy and turbulence are compared; results show that TKE and dissipation vary with the total energy climate.; Sediment erosion, deposition, and transport also vary on the tidal, seiche, and wave time scales. The tidal component of sediment flux has the largest magnitude, and is often determined by the timing and strength of wind events. Notable fluxes occur at the wave and seiche frequencies. Onshore transport occurs during an ebb tide at the seiche frequency. Sediment concentrations increase when the seiche opposes the ebb current, and decrease when they are aligned. Erosion is not occurring due to a critical stress, but when flow reverses over the rippled bed and causes sediment laden vortices to be ejected. When the seiche opposes the ebb current, waves are more likely to reverse the overall flow and sediment concentration increases. The interactions of multiple frequencies of motion are clearly vital to understanding sediment dynamics on an intertidal mudflat. |
