TEMPORAL OBSERVATIONS OF FLOW DYNAMICS AT PORTHTOWAN BEACH, CORNWALL

Porthtowan is a wave sheltered, macrotidal beach, located on the north coast of Cornwall (south-west England), and has been known to produce strong currents posing a significant threat to water users and bathers. Beaches that are formed in lee of shore-connected headlands are often popular recreation sites due to the wave sheltering effect of the headland. However, studies have shown that this wave sheltering and subsequent differences in wave set up could generate significant nearshore currents and potential hazardous rip currents along the sheltered face of the headland. The aim of this study was to provide an understanding of surf zone processes related to the generation of alongshore and cross-shore currents at Porthtowan beach, Cornwall, and to uncover the processes involved. A field experiment was carried out at Porthtowan beach to examine flow dynamics and surf zone circulation over a mid-tide headland constrained rip system. Measurements of flow velocity were recorded using two GPS- tracked lagrangian surf zone drifters, while inter-tidal beach topography were measured through RTK-GPS surveys. Argus video images were used to supplement data, allowing the location and extent of rip currents to be monitored. Data were collected over ten consecutive days, over a spring-neap tidal cycle with tidal ranges of 1-6.7m and offshore significant wave heights of 0.5-2.2m and peak periods of 4-15.5s. Results illustrate that large waves were generated on the southern end of the bay, while smaller wave heights occur in lee of the northern headland creating a south to north wave height gradient, driving longshore feeder current which collides with the north headland. This rotates the current offshore, where it gently diminishes into the rip head, extending up to 175m offshore. The trajectory of the rip, after passing the rip neck, varied a small amount, predominantly offshore into a diffuse, lobe shaped rip head. During high tide water depth was sufficiently deep to prevent waves breaking, inhibiting current generation, while low tide saw strong longshore currents dominate the surf zone. During mid tide the effect of coastal morphology becomes apparent with maximum wave heights breaking in lee of the southern headland, and minimum wave heights in lee of the northern headland, forming an alongshore wave height gradient, generating strong rip currents. Peak rip flow speeds of 3.49m/s were recorded at mid tide, when the effect of alongshore wave gradients were maximized. In light of the findings, Porthtowan beach could be classified as a temporally hazardous beach. During mid tide, when rip currents are at their strongest, extreme safety should be taken when entering the water, while high tide is the safest time to enter the ocean due to minimal current activity. Low tide was seen to be potentially dangerous to bathers, with temporary rips and strong longshore currents being generated.Faculty of Scienc