Contributing factors driving beach erosion

THE source of the sand on our beaches come from inland erosion moved by the Thugela, Mhlatuze and Lalazi rivers and washed onto our beaches by wave action.

The ‘building’ of our beaches results from the littoral active zone which drives this sand predominantly from south to north and is comprised of the near shore, beach and the dune system where sand is highly mobile.

The northerly flow dominates, but reversals do occur, especially in summer as a result of the greater number of easterly winds and swells.

The sand on our beaches range in size from 0.05mm to 2 mm and comprises mainly silica and quartz minerals and shell fragments.

Sand size is important as it determines the beach slope and therefore its susceptibility to erosion.

When this drift is interrupted by the port breakwater, accretion occurs on the updrift side (south) and erosion on the downdrift side (Alkantstrand) of the structure.

The nett yearly littoral drift is normally of the order of 0.25 to 0 .75 million cubic metres.

Read more about coastal erosion HERE

Thus in a number of years millions of cubic metres of sand may build up behind a breakwater and consequently a similar quantity of sand may be eroded unless expensive shore protection works are built.

Together with this, sand mining in the rivers and estuaries in the area has had a dramatic additional effect on coastal erosion.

According to the CSIR the impact of both dams and sand mining could result in mean coastal erosion of more than one metre per year.

Coastal erosion results from a combination of various factors – both natural and human-induced – which have different time and space patterns and have different nature (continuous or incidental, reversible or non-reversible).

In contrast to beach building, the beach erosion drivers include storm impacts (moves large quantities of sand); storm frequency (determines if beach recovery before next storm occurs); wind (effects wave height and Aeolian sand movement); astronomical effects (cyclical sea level and tides) and sea level rise (beach erosion).

The combination of storms (a pulse disturbance) and sea-level rise (a press disturbance) thus represents a doubly erosive force for beaches.

The consequence of such an eroding beach will be a landward migration as noticed recently.

This will result in a significant risk on infrastructure located too close to the beach.

If a beach is furthermore lined with development, walls or other hard structures, the beach cannot retreat. This will continue with massive erosion until our beaches re-establish equilibrium profiles in response to adjusted mean sea level.

Other drivers include the nature and source of the sediment (how easily the sand is eroded) and local coastal topography (affects the incident angle of wave attack).

Even the opening and closing of estuaries can impact on sand and water depths, which in turn affects waves, currents and sediment transport. Even water table level can change erosion/accretion tendency of our beaches.

Simonis is a professor at the University of Zululand’s Hydrology Department

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