PRESERVING DURBAN'S FRAGILE GOLDEN MILE
The current visible damage to Durban's central and northern beaches is a high profile example of what human activity and climate change can do to KwaZulu-Natal's fragile coastline.
Sand mining, darn building and harbour construction coupled with rising sea levels flowing from changing weather patterns are becoming increasingly destructive with costly outcomes. Further degradation of Durban's beaches will obviously have a negative impact on the fun lifestyle the city boasts about and - with sun, sea, sand and surf being such assets and attractions • possibly lead to a downturn in tourism and economic development. With the above in mind, understanding how to preserve or sustainably manage our coastline should be foremost on the agendas of planners and developers. Many factors affect shoreline change, including variations in sand supply, wave characteristics and sea level rises. The shoreline responds dynamically to these factors making it difficult to isolate one main driver of change. It is therefore important to have a systemic understanding of the physical processes that drive shoreline change and what can be done to mitigate the impact. Waves are generated by the interaction between wind and the ocean surface with major sea condition changes typically associated with low pressure weather systems that generate waves and swells which travel long distances before arriving at our beaches. Most of our waves tend to come from a south-easterly direction and a consequence of this is that the waves drive a dominant alongshore current from south to north which in turn transports a river of sand in the nearshore zone where the waves are breaking. Any obstruction to the flow within this zone will disrupt the sediment movement This includes the Durban Port's southern breakwater which blocks sediment moving up the coast resulting in the build-up of sand at the southern side of this breakwater. If this blocked sand is not moved to the area north of the harbour, the beaches there will slowly erode over the long term. A sand-bypassing scheme is operated in Durban to prevent this from happening, but it relies on the availability of dredging services to implement It. While the alongshore current impacts on the shoreline over the long term, large wave events associated with storms can rapidly erode a beach and cause significant damage to infrastructure in the short term. The severity of this damage depends on the height of the waves, the direction they come from, the duration of the storm, and other local factors. Wave height and duration control the magnitude of the current, whereas wave directions control the movement of the current. Waves that arrive perpendicular to the shoreline do not drive alongshore currents and since waves can come from many directions the alongshore current can reverse direction. Sand eroded from the beach face is deposited offshore just beyond where the waves start breaking but over time most of this sand is gradually moved back onshore during periods of smaller waves. Therefore, although Durban's beaches appear -unhealthy; given our complex coastal environment, it is easy to understand how such conditions have occurred. However, the encouraging news is that the beaches will recover, with the process taking as long as two years. Interventions to nourish the beaches with additional sand can speed the recovery process. Of greater concern over the longer term is the global problem of an overall reduction in the supply of sand to beaches due to sand-mining operations (e.g. for construction uses) and the building of darns that trap river sand before it reaches the coasts. About 80% of all beach sand comes from rivers. In addition, global sea level rise due to climate change will also have significant long term impacts on the stability of our beaches. Understanding the dynamic and systemic relationships between diverse causal factors is a core research interest at the University of KwaZulu-Natal's Centre for Research in Environmental, Coastal & Hydrological Engineering (CRECHE). CRECHE will continue to explore the province's vulnerability to storms and sea level changes, and develop new design solutions to address these problems.
The above was written by Professor Derek Stretch, Dr.Rudi Kimmie and Dr. Justin Pringle of the Afrihub and the Centre for Research in Environmental Coastal & Hydrological Engineering (CRCHE) in the School of Engineering at the Streets (SOS) initiative...