Infragravity Frequency Wave-Driven Bottom Boundary Layer Turbulence in Shallow Estuaries

Abstract

We use turbulent dissipation measurements from a small estuary to determine how and when infragravity (IG) waves (periods $\sim$25–250 s) increase turbulence due to bottom friction. The frequency of IG waves leads to a larger wave boundary layer than for sea and swell waves. Current methods for predicting turbulence from mean currents rely on observations in regions of the water column where oscillating velocities either have a logarithmic or depth-uniform profile. We develop a new approach for predicting turbulent dissipation in the unsteady boundary layer regime that combines a quasi-steady regime at the bottom of the water column and an outer regime above the wave boundary layer. Using a numerical model and observations from Los Peñasquitos Lagoon, we find that the new approach allows for calculation of average turbulent dissipation rates throughout the entire water column and performs better than existing methods when calculating turbulent dissipation within the wave boundary layer. Our observations indicate IG waves increase turbulent dissipation across a substantial fraction of the water column when the mean current amplitude is less than $3/2$ of the current standard deviation. These conditions were typically observed during neap flood tides or at the very beginning and the end of spring flood tides. In addition, we find that the wave boundary layer height can be estimated from the instantaneous bottom stress, consistent with existing scaling approaches. Finally, we show that IG wave-induced increases in turbulence appear associated with sediment transport inside the estuary.