Understanding Flow Structure in a Groyne Field Themba Rutherford Ziqubu Engineering University of KwaZulu-Natal A thesis submitted in fulfilment of the requirements for the degree of Master of Science January 11, 2018 Supervisor: Prof D.D. Stretch i As the candidate’s Supervisor I agree/do not agree to the submission of this thesis. -------------------------- ------------------------ Prof. Derek Stretch Date ii DECLARATION I, Themba Rutherford Ziqubu declare that 1. The research reported in this thesis, except where otherwise indicated, is my original research. 2. This thesis has not been submitted for any degree or examination at any other university. 3. This thesis does not contain other persons’ data, pictures, graphs or other information, unless specifically acknowledged as being sourced from other persons. 4. This thesis does not contain other persons' writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then: a) Their words have been re-written but the general information attributed to them has been referenced b) Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced. 5. This thesis does not contain text, graphics or tables copied and pasted from the Internet, unless specifically acknowledged, and the source being detailed in the thesis and in the References sections. Signed ------------------------------------ iii Acknowledgements I would like to express my greatest gratitude to the following My family for the support and love during the period of this study My amazing partner Phumzile for her never ending belief in me and encouragement Vulindlela Zikhali for his vast contribution, I really appreciate your friendship and your help Most importantly Prof Stretch for never losing faith in me and always making me feel welcome, I truly value your mentorship and guidance. Ngiyabonga kakhulu. iv ABSTRACT Groyne structures are widely used all over the world for the purposes of controlling longshore currents and slowing down the rate of sediment transport along coast lines. The limited previous studies in this area have led to unfit designs and installations of these structures. The history of the Durban coastline highlights challenges concerning the use of these structures that engineers have been faced with. The groynes along the Durban coast are not sufficient to stabilize the coastline and they are used in conjunction with a sand nourishment scheme. This research aims to elucidate the flow field characteristics and patterns around groynes in a groyne field. A parametric study of three impermeable groynes in a groyne field was developed using numerical modelling software. The DHI MIKE 21 Spectral Wave and Hydrodynamic models were used for the study. Various wave and groyne field parameters were varied systematically. The main focus of the study was on the groyne tips as this is an area most prone to scour failure, especially with impermeable groynes. The simulations reveal how the flow patterns within the groyne compartments change with variation in wave parameters. It was found that under certain wave conditions, recirculation patterns develop within the groyne compartments. Storm conditions can have devastating effects on coastlines. Therefore, this study also aimed to understand how these conditions may affect the functioning of a groyne field. v Table of Contents 1. INTRODUCTION .......................................................................................................................... 1 1.1 Motivation ............................................................................................................................... 1 1.2 Study background ................................................................................................................... 2 1.3 Durban coastline climate ......................................................................................................... 5 1.4 Research question ................................................................................................................... 7 1.5 Aim ......................................................................................................................................... 7 1.6 Objectives ............................................................................................................................... 7 1.7 Dissertation Outline ................................................................................................................ 8 2. LITERATURE REVIEW ............................................................................................................... 9 2.1 Introduction ............................................................................................................................. 9 2.2 Types of groynes ..................................................................................................................... 9 2.2.1 Permeable groyne ............................................................................................................ 9 2.2.2 Impermeable groyne ..................................................................................................... 10 2.3 Coastal processes .................................................................................................................. 11 2.3.1 Wave induced current ................................................................................................... 11 2.3.2 Coastal erosion .............................................................................................................. 14 2.3.3 Longshore sediment transport ....................................................................................... 16 2.4 Hydrodynamic and morphodynamic driving forces ............................................................. 16 2.4.1 Breaking waves ............................................................................................................. 16 2.4.2 Radiation stress ............................................................................................................. 18 2.5 Residence time ...................................................................................................................... 20 2.6 Previous research on flow patterns and sediment transport within a groyne or groyne field 21 2.7 Summary ............................................................................................................................... 27 3. METHODOLOGY ....................................................................................................................... 29 3.1 Numerical modelling............................................................................................................. 29 3.1.1 Introduction ................................................................................................................... 29 3.1.2 Overview ....................................................................................................................... 29 3.1.3 Mike Mesh Generator ................................................................................................... 29 3.1.4 Spectral Wave Model .................................................................................................... 30 3.1.5 Hydrodynamic Flow Model .......................................................................................... 30 3.2 Durban Beach Profile ............................................................................................................ 30 3.3 Breaker zone ......................................................................................................................... 31 vi 3.4 Simulation Methodology ....................................................................................................... 31 3.4.1 Simulation overview ..................................................................................................... 31 3.4.2 Significant wave heights ............................................................................................... 32 3.4.3 Mean Water level .......................................................................................................... 32 3.4.4 Incident wave angle ...................................................................................................... 32 3.4.5 Groyne interspacing ...................................................................................................... 32 3.4.6 Storm condition ............................................................................................................. 32 3.5 Model configuration .............................................................................................................. 32 3.5.1 Flexible mesh model parameters ................................................................................... 33 3.5.2 Spectral wave model parameters ................................................................................... 35 3.5.3 Hydrodynamic model parameters ................................................................................. 36 3.5.4 Model discussion .......................................................................................................... 36 3.6 Numerical model compared to field data .............................................................................. 37 3.7 Dimensional Analysis ..........................................................................................................