Lagrangian observations of circulation on an embayed beach with headland rip currents

Abstract

This study describes the first comprehensive measurements of nearshore current patterns across the entire extent of an embayed beach bounded by headland rip currents. A field experiment at Whale Beach, NSW, Australia provides valuable insights into: (i) embayment-wide spatial and temporal flow behaviour; (ii) rates of cross- and alongshore water exchange; and (iii) the influence of geological control by headlands on morphology and circulation. Lagrangian flow data was obtained using 34 GPS drifters with 293 individual deployments, over a single ebbing tidal cycle during moderate-low energy ( H s = 1 m) oblique wave forcing. In-situ wave and current data, and bathymetric data were also collected. Beach morphology was dominated by a large mid-beach rip channel with lesser headland rip channels. Mean flow rates were 0.6 ms − 1 in the mid-beach channel and 0.4 ms − 1 in the headland channels, with the majority of cross-shore water volume flux (~ 60%) through the central channel. A weak alongshore current O (0.1 ms − 1 ) was forced by the oblique offshore wave angle. Rip current velocities , flow variability, and rate of surfzone exits by Lagrangian drifters increased as water level decreased. Transient currents on a planar bar along the northern half of the beach, with velocity standard deviation up to 0.2 ms − 1 , were not tidally modulated. Lagrangian time series were used to differentiate four current regimes (rip cell, rip head, planar bar and offshore low energy zone) based on mean velocity, velocity variability and degree of tidal modulation. An increase in surfzone exit rates by drifters was observed from south (upwave) to north (downwave), with exit rates per drifter deployment of 22% at the south headland rip, 65% at the mid-beach open rip, and 80% at the north headland rip. The high rate of drifter exits contrasts previous observations on open coast beaches. Observed flow behaviours are attributed to wave shadowing at the upwave (protected) end of the beach, and longshore currents forced by oblique waves deflected offshore at the downwave headland. These field observations are in good agreement with recent numerical modelling. A relationship between bathymetric variability and current intensity was determined, with cross-shore average mean velocity correlating with a parameterisation of bathymetric alongshore non-uniformity. This study demonstrates that flow behaviour and exchange rates can vary along the length of an embayed beach due to geological control. This research has implications for transport of organisms, nutrients and pollutants, is relevant to beach safety practitioners, and can be used in calibration of numerical models. • First embayment-wide measurements of currents on an intermediate embayed beach • First detailed morphological and hydrodynamic observations of headland rip currents • Geology and wave angle influence flow trajectories • High surf zone drifter exit rates complement recent modelling • New scaling relationship relates bathymetric non-uniformity to flow velocity