Sediment Dynamics on a Macrotidal Beach: Isle of Man, U.K

Abstract

This paper describes measurements of waves and nearbed velocities, beach morphology and sedimentology on a macrotidal beach on the northwest coast of the Isle of Man. The spring tide range was approximately 5.4 m, with mean wave heights of 0.56 to 1.35 m and periods of 3 to 8 seconds. The range of grain sizes available on the beach varied between -3.65φ and 2.13φ. The intertidal beach morphology exhibited three distinct zones: a steep, reflective high-tide zone, a moderately steep mid-tide zone and a flat, dissipative low-tide zone. An overall trend of shoreward-coarsening grain size was observed on this beach. Wave characteristics varied noticeably over a tidal cycle. The wave height increased with tidal height, and two wave zones were observed: a zone of broken waves, in which the wave field appeared to be saturated, and a zone of mixed unbroken and breaking waves. The surf scaling parameter increased and the breaker height decreased as the tide ebbed, indicating that conditions were moderately to strongly reflective over most of the measured tidal cycle. The macrotidal beach on the Isle of Man described in this paper does not fit easily into any previously identified macrotidal beach categories. Although it is generally exposed to only moderate wave energy, it does not exhibit the typical multi-barred profile. The Manx beach may represent an intermediate beach stage close to the reflective end of a continuum of macrotidal beach morphologies.