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Design of Rubble-Mound Structure As Scour Protection

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Design of Rubble-Mound Structure As Scour Protection DESIGN OF RUBBLE-MOUND STRUCTURE AS SCOUR PROTECTION FOR VERTICAL SEAWALLS: LAYER THICKNESS, MEDIAN ROCK MASS AND ENERGY DISTRIBUTION THROUGH THE LAYERS BY ANCHEN STREUDERST Thesis presented in fulfilment of the requirements for the degree of Master of Engineering in the Faculty of Civil Engineering at Stellenbosch University. Supervisor: Prof JS Schoonees March 2021 Stellenbosch University https://scholar.sun.ac.za DECLARATION By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification. ________________________ Anchen Streuderst ________________________ Student number ________________________ Date Copyright © 2021 Stellenbosch University All rights reserved i Stellenbosch University https://scholar.sun.ac.za ENGLISH ABSTRACT This study contributes to 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, the rubble-mound berm consisting of a core, filter layer and armour layer functions as a protection for the seawall and its foundation. Scouring of the screed layer is among the leading causes of seawall failure. To determine design guidelines to minimise the scouring of the screed layer, forty-two physical model tests were conducted at Stellenbosch University Hydraulics Laboratory. The (horizontal) erosion of the screed layer and scoured screed area 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 Toolbox in MATLAB. Wave celerity 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 peak wave 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. Increasing the filter layer (underlayer) beneath the armour layer proved to be effective and economical. By adding 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, a larger median rock mass in the underlayers enhanced the energy dissipation and structural stability. The filter criterion stating that the underlayer’s rock mass should be a tenth of the upper layer proved to be the most effective in the majority of experiments. As a first approximation, to determine the energy distribution, the dynamic pressure head was measured at different elevations in the rubble-mound and converted into velocity. Even though the small-scale model produced high variability in the measurements, the general trend indicated 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, the water was dissipated into the filter layer and became less violent in the armour layer, resulting in the highest energy region being in the filter layer. Equations were developed using dimensional analysis to predict the energy in the rubble-mound structure layers based on identified factors affecting the flow through porous media. The results indicate that a well-designed rubble-mound berm can effectively dissipate the approaching wave ii Stellenbosch University https://scholar.sun.ac.za energy and accordingly limit the energy penetrating the core and so reduce scouring of the screed layer below the seawall. iii Stellenbosch University https://scholar.sun.ac.za AFRIKAANS OPSOMMING Die studie dra by tot die optimale ontwerp van ‘n ruklipstruktuur wat gebruik word as toonbeskerming vir ‘n vertikale seemuur. Die betonseemuur word geplaas bo-op ‘n vlaklaag. Seewaarts van die muur bestaan die ruklipstruktuur uit ‘n kern, filterlaag en bolaag. Uitskuring van die vlaklaag is een van die hoofoorsake van seemuurswigting. Twee-en-veertig fisiese model toetse is gedoen in Stellenbosch Universiteit se Hidroulika Laboratorium om te bepaal hoe om die uitskuring te verminder van die vlaklaag. Die horisontale uitskuring van die vlaklaag en die uitskuringsoppervlak vir die eksperimente is waargeneem, gemeet en geanaliseer. Die uitskuring oppervlak is bereken deur ‘n nuwe metode wat deur die skrywer ontwikkel is. Die metode maak gebruik van MATLAB se Image Processing Toolbox. Die golfsnelheid neem toe soos die golfperiode toeneem, dit lei tot ‘n toename in die oordragstempo van die golfenergie. Gevolglik ontwikkel ‘n groter uitskuringsoppervlak want meer energie bereik die vlaklaag. Die uitskuringsoppervlakte van die piekgolfperiodes tussen 6s en 12s is baie naby aan mekaar gegroepeer, maar die uitskuring van die 16s en 18s is aansienlik hoër. In een van die uiterste gevalle het ‘n 18s-golfperiode veroorsaak dat 83% van die vlaklaag uitgeskuur het. Die hoogste ruklipstrukture het die beste beskerming vir die vlaklaag gebied. By ‘n 18s piekgolfperiode het die grootste struktuur 20% uitskuring van die vlaklaag ervaar, waar die kleinste struktuur 80% uitskuring gehad het. Dit is bevind dat deur die filterlaag dikker te maak, in plaas van die bolaag, meer effektief en ekonomies is. Deur twee lae klip by die filterlaag te sit (19% toename in die totale hoogte), lei tot ‘n 50% afname in die uitskuringsoppervlak van die vlaklaag. ‘n Dikker laag genereer ‘n onreëlmatige oppervlak wat lei tot beter ineensluiting en ‘n toename in porositeit wat die golfenergieverspilling en strukturele stabiliteit verbeter. Addisioneel, dra ‘n groter gemiddelde klipmassa in die onderlae ook by tot golfenergieverspilling en bolaagstabiliteit. Die filterkriteria wat bepaal dat die onderlae ‘n tiende van die massa moet wees van die bolaag was die effektiefste in die meerderheid van eksperimente. As ‘n eerste benadering is die dinamiese druk op verskillende hoogtes in die ruklipstruktuur gemeet en omgeskakel na snelheid om die energieverspreiding te bereken. Al het die klein-skaalmodel hoë wisseling in die druklesings veroorsaak, het die algemene tendens nogtans getoon dat die bolae die hoogste energie bevat, en beperkte energie die kern bereik (gemiddeld 34%). Die bolaag het die hoogste energie wanneer die filterlaag se gemiddelde klip massa kleiner is want die vloei word dan gekonsentreer in die bolaag. Wanneer die gemiddelde klipmassa van die filterlaag groter is, kan die water na die filterlaag beweeg en word minder aktief in die bolaag. Dit veroorsaak dat die hoogste energie in die filterlaag is. Vergelykings is geskep deur die proses van dimensionele analise om die energie in die ruklipstruktuur lae te bereken gebaseer op geïdentifiseerde faktore wat die vloei deur poreuse media beïnvloed. Die resultate toon dat ‘n goeie ontwerpte ruklipstruktuur die energie wat die kern bereik beperk, en so ook die uitskuring van die vlaklaag onder die seemuur verminder. iv Stellenbosch University https://scholar.sun.ac.za ACKNOWLEDGEMENTS I am grateful for the following people: • My supervisor, Prof JS Schoonees for sharing his expertise in the coastal engineering field and for guiding me through my thesis. • The laboratory personnel; Johann Nieuwoudt, Iliyaaz Williams and Jody February, who provided practical solutions and was always willing to assist me. They created a fun working environment with their cheerfulness and stories. • Carl Wehlitz from the CSIR, who helped me obtain the necessary material for the screed layer and who was always willing to share his insight and knowledge. • My flatmate, Nicole Taylor, who came up with the idea to measure the scour area using MATLAB and who helped me write the necessary code, and for a fun two years of living and studying together. • My father, Johan Streuderst, for his advice, support and giving me the opportunity to study what I am passionate about. • My mother, Heilani Streuderst, for her continuous prayers and keeping me motivated and encouraged. • My fiancé, Pero Buys, for his insights and advice as a Civil Engineer. For his continued love and hard work that inspired me to finish my thesis and give my best. v Stellenbosch University https://scholar.sun.ac.za DEDICATIONS This thesis is dedicated to my Lord and Saviour, Jesus Christ. “Have you not known? Have you not heard? The Lord is the everlasting God, the Creator of the ends of the earth. He does not grow faint or weary; his understanding is unsearchable.” Isaiah 40:28 vi Stellenbosch University https://scholar.sun.ac.za TABLE OF
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