Quantifying Optically Derived Two-Dimensional Wave-Averaged Currents in the Surf Zone

Dylan Anderson,John Stanley,Nicholas Cohn,Rob A Holman,A Spicer Bak,Katherine L Brodie

doi:10.3390/rs13040690

Abstract

Complex two-dimensional nearshore current patterns are generated by feedbacks between sub-aqueous morphology and momentum imparted on the water column by breaking waves, winds, and tides. These non-stationary features, such as rip currents and circulation cells, respond to changing environmental conditions and underlying morphology. However, using fixed instruments to observe nearshore currents is limiting due to the high costs and logistics necessary to achieve adequate spatial sampling resolution. A new technique for processing surf-zone imagery, WAMFlow, quantifies fluid velocities to reveal complex, multi-scale (10 s–1000 s meters) nearshore surface circulation patterns. We apply the concept of a wave-averaged movie (WAM) to measure surf-zone circulation patterns on spatial scales of kilometers in the alongshore and 100 s of meters in the cross-shore. The approach uses a rolling average of 2 Hz optical imagery, removing the dominant optical clutter of incident waves, to leave the residual foam or water turbidity features carried by the flow. These residual features are tracked as quasi-passive tracers in space and time using optical flow, which solves for u and v as a function of image intensity gradients in x, y, and t. Surf zone drifters were deployed over multiple days with varying nearshore circulations to validate the optically derived flow patterns. Root mean square error are reduced to 0.1 m per second after filtering based on image attributes. The optically derived patterns captured longshore currents, rip currents, and gyres within the surf zone. Quantifying nearshore circulation patterns using low-cost image platforms and open-source computer vision algorithms presents the potential to further our understanding of fundamental surf zone dynamics.

Highlights

IntroductionWave-averaged currents are a fundamental driver of numerous nearshore processes, including rip currents that claim lives [1], alongshore currents impacting ecosystem health and pollutant transport [2], and flow gradients altering nearshore morphology [3]
The objectives of this work are (1) to define an alternate approach, referred to as WAMFlow, to remotely derive flow features at spatial and temporal scales larger than turbulent structures associated with individual waves (Section 2), (2) to validate the method with drifter deployments that contextualize the spatial variability in flow (Section 3), and (3) to highlight the potential for the technique to contribute new insights to surf zone hydrodynamics (Section 4)
Drifters were deployed by dropping them in the water from the Field Research Facility (FRF) pier at varying cross-shore locations and by collecting them after they had freely floated to shore and washed up in the shore break

Summary

Introduction

Wave-averaged currents are a fundamental driver of numerous nearshore processes, including rip currents that claim lives [1], alongshore currents impacting ecosystem health and pollutant transport [2], and flow gradients altering nearshore morphology [3]. Flows can be forced by breaking wave momentum [4], pressure gradients, and low-frequency motions [5]. Competition between forcing and dissipation yield varying velocities (decimeters per second) over large spatial scales (102 –104 m). Flows are typically stronger in the alongshore than cross-shore, except at rip currents where locally strong flows can exceed

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Discussion

Conclusion