Macrotidal Rip Current Experiment: Circulation and Dynamics

Abstract

The macrotidal intermediate-dissipative beaches of the southwest UK display strong seasonality whereby extensive low-tide bar/rip systems form during the spring season. These features reach a state of maximum development during the summer months, when strong rip currents are present at various stages of the tide that switch on/off due to tidal translation. This research investigates for the first time the circulation and dynamics of macrotidal rip systems from the perspective of the flow dynamics and surf zone circulation. A field experiment was conducted on a macro-tidal (range 6.3 m) with low-tide bar/rip morphology to quantify the circulation and dynamics of the rip current systems. Measurements of flow velocity, water depth and suspended sediment concentration were collected over the channel and shoal of transverse bar/rip system using an in-situ sensor arry, whilst the surf zone circulation patterns were monitored with post-processed GPS drifter floats. The in-situ data show that currents within the rip system are strongly correlated with the stage of the tide and maximum current speeds are recorded during the 3-hour period immediately around low water. During this period, wave breaking is maximised on the adjacent shoal resulting in a longshore gradient in wave dissipation. The GPS drifters indicate that an extensive rotational surf zone circulation pattern exists, whereby the offshore flow within the rip channel returns landwards over the intertidal bar from the outer edge of the surf zone.