Vessel wave wakes : new perspectives on their generation, propagation and shoreline impacts

The increasing prevalence of high-speed recreational and commercial craft utilising sheltered waterways has brought with it the commensurate degradation of the natural environment and public amenity. This is not new. Wave wake case studies for recreational craft in Australia were conducted as far back as the 1960s, but the growth in high-speed commuter ferry use since the 1980s to reduce urban transport pressures provided the impetus and requisite funding for more extensive research to be conducted. Initially, the premise of this study was to formulate a systematic methodology for the quantification of environmental impacts. After several decades of describing and reporting the problems, there is a distinct and growing disjuncture between the science and its practical application in vessel management and regulation. Vessel wave wakes are complex and not easy to qualify, let alone quantify. Shallow water wakes are known to be quite different to those in deep water, yet both are described using the same techniques. Very little of the science transmits well to the general public and it is open to misinterpretation and manipulation. Rather than continue towards a management and regulation methodology using science that has at times proved to be inadequate, if not questionable, the fundamental principles of wave wakes have been reviewed from the beginning to provide a more secure foundation for future application. A new method of ranking the erosive potential of wave wakes is proposed, subject to further validation. Apart from the general introduction and conclusions, this document is arranged in a somewhat different manner to a traditional thesis. The principle tenets of wave wake science are reviewed and renewed, supported by subject-specific appendices. Section 2: Literature Review. A novel form of literature review is presented. Rather than a standard review of the available literature, which often can read as abstract summaries, nine selected technical reports and journal papers are appraised in detail, highlighting perpetual misinformation, problems of interpretation, and the limitations of the science. Section 3: Waves. Waves are discussed from very basic concepts through to their propagation and interaction, but more in the context of how they are to be interpreted in a wave wake context rather than the principles of their existence. Section 4: Deep Water. A comprehensive and updated review of the generation and propagation of wave wakes in deep water is presented. Its apparently simple relationships in fact give rise to complex interactions that lead to consistent misinterpretation of wave wake phenomena. Section 5: Shallow Water. In the past, a shallow-water wake was analysed in the same way as a deep-water wake. The composition of the shallow water wake is analysed, showing that the leading wave has the attributes of a wave packet and not a single wave. Moreover, the existence of solitary waves that come to dominate the leading shallow water wave at high depth super-critical Froude numbers and the results of novel experiments are discussed. Section 6: Wave Energy and Power. The two composite parameters of wave energy and power are commonly used as indicators of erosion potential. The distribution of energy in a shoaling wake is discussed and the results of past erosion experiments analysed. Wave power is shown to have an intrinsic relationship with the wave wake itself, but wave energy and the form in which it is delivered are still believed the be two of the principal determinants of erosion potential. Section 7: Wave Height Decay. This contentious subject is shown to be complex. It is proposed that a definitive wave height decay equation probably doesn't exist, and a new method of determining wave height decay based on group celerity is proposed. Section 8: Severity of Erosion. A novel method is derived that determines a wave's propensity to entrain sediment, based on the summation of excess bottom shear stress above the threshold value, from threshold of sediment entrainment through to breaking. Promising correlation with field trials is demonstrated but based on a limited dataset. A total of eleven comprehensive appendices contain the results of investigations and experiments and are written in a stand-alone manner: Appendix A: Review of nine existing wave wake studies. Appendix B: Deep water wave height decay. Appendix C: Shallow water wave height decay. Appendix D: Shallow water wavefront propagation. Appendix E: First wave in very shallow water. Appendix F: Extremely shallow water. Appendix G: Very shallow water ‚Äö- depth transition. Appendix H: Wave propagation from shallow to deep water. Appendix J: Correlation between bed shear stress and turbidity. Appendix K: Gordon River turbidity correlation. Appendix L: Wind waves. Appendix M: Error analysis and uncertainty.