Modelling the influence of varying sediment sources on coastlines.

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. vii 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.