Geophysical monitoring of coastal erosion and cliff retreat of Monwabisi Beach, False Bay, South Africa

Abstract

Monitoring of the coastal zone is necessary to assess its vulnerability and help formulate coastal management plans. A predetermined stretch of beach along the northern rim of False Bay known locally as Monwabisi Beach was chosen to compare different monitoring techniques and from the data acquired, see if accurate comment could be made regarding sediment dynamics and its implications regarding any coastal encroachment on anthropogenic infrastructure. Digital elevation models of the study area were created from data acquired with a mobile laser scanner in April 2013, April 2014 and August 2014, chosen to cover a yearly and a seasonal cycle. Conventional beach profile data were acquired using a differential global positioning system (DGPS) in April 2014 and LiDAR data were acquired in November 2014. From the laser scanning datasets it has been calculated that a nett erosional trend exists for the study area with sediment moving towards the north. In the western portion of the study area, where a coastal road has been undercut and complete failure has occurred, the progress of cliff retreat has been accurately measured to reveal an average rate of retreat of 2.2m/yr. Although accurate figures were determined for sediment erosion and accretion, the rate of change of each could not be determined with any degree of confidence as the survey intervals were not regular enough to consider nett amounts; rather the gross amounts have been presented.