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Infragravity Frequency Wave‐Driven Bottom Boundary Layer Turbulence in Shallow Estuaries.

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Title: Infragravity Frequency Wave‐Driven Bottom Boundary Layer Turbulence in Shallow Estuaries.
Authors: Wheeler, Duncan C.1 (AUTHOR) dcwheele@ucsd.edu, Giddings, Sarah N.1 (AUTHOR), Merrifield, Mark1 (AUTHOR), Pawlak, Geno1,2 (AUTHOR)
Source: Journal of Geophysical Research. Oceans. Mar2025, Vol. 130 Issue 3, p1-31. 31p.
Subject Terms: *Water depth, *Ocean waves, Boundary layer (Aerodynamics), Orbital velocity, Territorial waters, Frequencies of oscillating systems
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 $3/2$ 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. Plain Language Summary: When an estuary meets the ocean inside the surfzone, normal ocean waves break and do not enter the estuary. Instead, long waves that do not break as easily in shallow water enter the estuary. In this work, we look at how these long waves increase the overall energy and random water movements within a specific estuary in Southern California, Los Peñasquitos Lagoon. Using observational data and a numerical model, we find that because these waves are long, they interact differently and more strongly with the bottom than shorter waves. Using a new formulation we develop to account for this impact, we can then determine when long waves significantly increase turbulence in the estuary and have the potential to affect other dynamics such as sediment movement. Key Points: In shallow water, infragravity frequency velocity oscillations produce wave boundary layers that occupy a large portion of the water columnWave boundary layer turbulence can be estimated using an exponential transition between quasi‐steady and outer flow regimesIn shallow estuaries, infragravity waves significantly contribute to turbulence when the mean current is ≤3/2 ${\le} 3/2$ of the orbital velocities [ABSTRACT FROM AUTHOR]
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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 $3/2$ 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. Plain Language Summary: When an estuary meets the ocean inside the surfzone, normal ocean waves break and do not enter the estuary. Instead, long waves that do not break as easily in shallow water enter the estuary. In this work, we look at how these long waves increase the overall energy and random water movements within a specific estuary in Southern California, Los Peñasquitos Lagoon. Using observational data and a numerical model, we find that because these waves are long, they interact differently and more strongly with the bottom than shorter waves. Using a new formulation we develop to account for this impact, we can then determine when long waves significantly increase turbulence in the estuary and have the potential to affect other dynamics such as sediment movement. Key Points: In shallow water, infragravity frequency velocity oscillations produce wave boundary layers that occupy a large portion of the water columnWave boundary layer turbulence can be estimated using an exponential transition between quasi‐steady and outer flow regimesIn shallow estuaries, infragravity waves significantly contribute to turbulence when the mean current is ≤3/2 ${\le} 3/2$ of the orbital velocities [ABSTRACT FROM AUTHOR]
ISSN:21699275
DOI:10.1029/2024JC021284