Modelling the influence of varying sediment sources on coastlines.
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Date
2019
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Abstract
Coastal erosion is of concern to developed shorelines worldwide and has largely intensified due to anthropogenic influences. Sea-level rise, reductions in sediment supply
and changes to wave behaviour due to changes in climate were identified as potential
causes of chronic erosion. With climate change expected to increase the frequency and
intensity of storms, coastline management and planning will require greater attention.
A major obstacle of coastal planning is the lack of available models for predicting
long-term changes. Furthermore, reliable long-term wave data are often unavailable or
unreliable. Predicting long-term changes is essential for effective management of coastal
defence schemes. One-line models present a reduced-physics and reduced dimension
approach and provide an efficient and viable alternative to 2D and 3D models while
being less computationally intensive.
The long-term impacts of varying sediment inputs on the stretch of coastline
between uMhlanga and the uMngeni River mouth in Durban are explored using a
one-line model. Site selection was based on ongoing erosion and known operations
of sand-mining, damming and a sand-bypass scheme. Existing models are used as a
framework to develop a coastline model that uses statistically modelled wave climates
as the input source of wave data.
Results indicated that a minimum longshore sediment supply (460,961 m3/year)
required to maintain beach volume in the study region exceeds the estimate by Corbella
& Stretch (2012) of 418,333 m3/year. Observed beach erosion by eThekwini Municipality indicated a current longshore sediment supply of 410,276 m3/year. Furthermore,
volume conservation did not ensure beach width conservation along the entire coastline,
with a minimum sediment influx of 596,183 m3/year required for beach width and
beach plan area conservation.
Shore nourishment behaviour were analysed in the form of alongshore sand waves
with results showing that multiple, smaller nourishments results in more realistic sand
wave amplitudes that are required for diffusion dominant waves. Smaller nourishments
allow for more diffusive effects while maintaining a diffusive state whereas larger
nourishments tend to become advection dominant following rapid diffusion.
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An investigation of the advection-diffusion relationship of river sediment discharges
inferred that sand waves along the Durban coastline are advection dominated. A
critical aspect ratio of between 0.037 and 0.041 represented the equilibrium point
between advection and diffusion. River sediment discharges of this aspect ratio are
potentially significant in preventing erosion given the relatively high diffusive rate and
slow advection speed associated with the value. Furthermore, extreme river discharges
exceeding 200,000 m3
remained in coastal systems for between 3 and 4 years and are
potentially important mechanisms behind coastline recovery after storms.
Description
Masters Degree, University of KwaZulu-Natal, Durban.