Browsing by Author "Streuderst, Anchen"
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- ItemDesign of rubble-mound structure as scour protection for vertical seawalls: layer thickness, median rock mass and energy through the layers(Stellenbosch : Stellenbosch University, 2021-03) Streuderst, Anchen; Schoonees, J. S.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: This study contributesto the optimal design of a rubble-mound structure used as toe protection for a vertical seawall. A concrete seawall is placed on top of a screed layer. In front of the seawall, therubble-mound berm consisting of a core, filter layer and armour layer functions as a protection for the seawall andits foundation. Scouring of the screed layer is amongthe leading causes of seawall failure. To determine design guidelines to minimise the scouring of the screed layer, forty-two physical model testswere conducted at Stellenbosch University HydraulicsLaboratory.The (horizontal) erosion of the screed layer and scoured screedarea for each experiment was observed, measured and analysed. The scoured area was computed using a new method developed by the author using the Image Processing Toolboxin MATLAB. Wavecelerity increases as the wave period increases, leading to a rise in the rate of wave energy transmission through the structure. Subsequently, more scour of the screed develops as more wave energy reaches the screed layer.The scour areas for peakwave periods ranging between 6s and 12s were narrowly grouped, whereas the scoured areas for the 16s and 18s wave periods were significantly scattered and higher. In one of the most extreme cases tested, an 18s wave period caused 83% of the screed layer to be washed out.The rubble-mound structures with the highest crest provided the best protection. At an 18s peak wave period, the largest structure experienced a 20% scoured area, whereas the lowest structure experienced 80% scour of the screed layer. Increasingthe filter layer (underlayer) beneaththe armour layer proved to be effective and economical. Byadding two layers of rock to the filter layer (underlayer), a 19% increase in the total crest height led to a 50% decrease in the scoured screed area. A thicker layer generates an irregular surface resulting in better interlocking and increased porosity which improves wave energy dissipation and armour layer stability. Additionally, alarger median rock mass in the underlayers enhancedthe energy dissipation and structural stability. The filter criterion stating that the underlayer’s rock massshould be a tenth of the upper layer proved to be the most effective in themajority of experiments. As a first approximation, to determine the energy distribution, the dynamic pressure head was measured at different elevationsin the rubble-moundand converted into velocity. Even though the small-scale model produced high variability in the measurements,the general trendindicated that the outer layers contain the highest energy region, with limited energy penetrating the core (34% on average). The armour layer had the highest measured energy when the median rock mass of the filter layer was small since the flow was concentrated in the armour layer. When the median rock mass of the filter layer was larger, thewater was dissipated into the filter layer and became less violent in the armour layer, resulting in the highest energyregion being in the filter layer. Equations were developedusing dimensional analysis to predict the energy in the rubble-moundstructure layers based on identified factors affecting the flow through porous media.The results indicate that a well-designedrubble-mound berm can effectively dissipate the approaching wave energy and accordingly limit the energy penetrating the coreand so reduce scouring of the screed layer below the seawall.