Measuring and Modeling Turbulent Bottom Boundary Layers in Estuaries

Parameterizations of sediment erosion in coastal models typically assume a turbulent wave boundary layer with wave-induced currents that exceed the mean currents, which is typical for beaches exposed to swell waves in the open ocean. When the wave-driven currents are strong, the addition of waves to a mean current leads to more bottom stress and sediment erosion because of wave-driven turbulent mixing. In estuaries like San Francisco Bay that are protected from open ocean swell waves, traditional wave-current parameterizations can be invalid because waves are generally much weaker since they are driven primarily by local winds. I will present numerical simulation results of wave-current boundary layers in such environments in which the wave-driven effects are relatively weak. The first is a flat-bed case with a laminar wave superimposed over a turbulent mean current. In this case, even though the wave-induced current exceeds the mean current, the bottom stress is reduced by the laminar wave-driven currents. In the second case, the wave-induced current is weaker than the mean current, but the bed consists of bumpy roughness elements. In this case, the addition of bottom roughness leads to an increase in the bottom stress with the addition of the weak waves. These results show that the addition of waves to mean currents in estuarine environments can produce surprising results that are often difficult to incorporate into sediment transport models.

Speaker: Oliver Fringer, Stanford University