Beach profile change at St. Clair beach, Dunedin (1994)
AuthorsDyer, M. J.show all
This thesis examines the nature of changes that occur within the beach profile at St. Clair Dunedin on the south coast of the Otago Peninsula. The profile changes are linked to variations in the wave and wind environments, and the results are compared to both previous local work and to the relevant theory and models accepted in the literature. Erosion at St. Clair occurs during extended periods of strong southwest winds which are associated with increased wave heights, decreased periods and enhanced longshore currents. Under these conditions sand is transported both offshore and alongshore to the east, away from the western St. Clair corner, resulting in a lowering of the beach profiles. Erosion is accentuated at St. Clair by the presence of a sea wall, resulting in exceptionally low profiles which may allow fill to be eroded fom behind the concrete seawall face. While erosion is shown to be associated with predominantly steep waves, ( > 0.09) accretion at St. Clair was shown to often be unrelated to wave steepness. Within the significant accretion periods the longshore current direction was considered to be of greater importance. Thus currents moving to the west under the influence of easterly quarter swells transport sand in to the western St. Clair corner. The presence of the western headland blocking these currents results in rapid deposition, and accretion of the St. Clair profiles. Allen (1985) showed that the critical wave steepness equation of Dean (1973) applied for a medium sand beach. The equation was shown not to apply to inshore data for the south coast of the Otago Peninsula. This was considered to be due to the higher wave energy experienced on the Otago Peninsula. The failure to gain a critical wave steepness was considerd to be due to the inconsistent survey base, and the fact that at St. Clair the accretional periods were influenced primarily by longshore sand transport over wave steepness.