Trends in sandy beach variability EThekwini Municipality, South Africa

Abstract

It is well-known that the coastal zone attracts human populations like no other. There is ever increasing demand on the coastal zone by tourists, residents and developers alike; it is named as the fastest growing sector of the blue economy. Hence, coastal zone processes require better understanding to be effectively managed in the context of sustainable development of the asset. While sea level may be rising over the long-term, and many global-resolution studies lament the loss of sandy beaches as a result, coastal managers work over far shorter time-scales thus require site specific information to manage the coastal zone on a daily and monthly basis. In this contribution we discuss nine years of morphological change along a sandy beach in Durban, South Africa. The beach is managed by the eThekwini Municipality who are responsible for maintaining a coastal zone of ca. 100 km. The morphological data of this study were collected on a near monthly basis over 9.6 km of sandy beach between October 2011 and March 2020. From these data, beach volume and area are calculated and the variation is documented over time in conjunction with wave data recorded from a proximal directional wave buoy. Over the study period, the beach has experienced a net loss of 177885m3 and 29,375 m2 in volume and area respectively. However, the beach response has not been uniform throughout the study area. The southern three-quarters of the beach were affected by significant losses while the northern one-quarter gained in volume and area over the same period. The summer wave climate is characterised by increased variability in swell origin with greater easterly contributions than other seasons, and typically lacks the frequency of large swell (>3.5 m) events (7% of events) common to autumn (20%), winter (35%) and spring (38%). Winter, followed by spring then autumn seasons have more focused swell origins and southerly contribution, particularly in terms of large swell events. During periods of reduced event frequency allow for partial beach recovery, while erosion is associated with periods of increase event frequency. The sediment budget has been significantly reduced though impoundments on the proximal river catchment, compounded by un-managed sand mining. Rather than sea level, these near-field controls on sediment availability likely play a major role in beach volume and area in response to wave climate. There is less sediment available to nourish the beach naturally following erosional events; artificial nourishment will likely be required to maintain the sandy beach in the future.