Reflection and Diffraction Around Breakwaters

Reflection and Diffraction Around Breakwaters

Boshek, M.R.

Verhagen, H.J. (mentor)
Van Vledder, G. (mentor)
Van Os, J. (mentor)
Stive, M.J.F. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2009-07-03

Diffraction diagrams are frequently utilized by engineers within the feasibility and early design phases of a project. These helpful graphics, based on directional spreading and wavelength, can be used for a number of layouts and angles of incidence. In spite of the tendency to design breakwaters for the fullest dissipation of wave energy, current diffraction diagrams in use are for full reflection. They have a very strict view of the breakwaters themselves; solely an infinitely thin, fully reflecting, vertical breakwater without any transmission or overtopping. This study focuses on the changes to the diffraction patterns once construction aspects related to breakwater reflection are considered. The alteration to diffraction coefficients due to wave spectra variations, both directional and frequency, is examined through the use of theory and advanced modeling systems. It is found that the directional spectrum causes large changes while frequency variation is negligible. This primary driving component is than related to breakwater layout and incident angles. A study of literature related to breakwater reflection is used in advanced computer models to determine where and how much this characteristic contributes to the diffraction pattern. Two separate types of models, phase-resolving and phase-averaging, are used to test breakwater reflection in diffraction modeling. This comparison confirms theoretical assumptions, finds possible faults in current diffraction diagrams, and creates new questions about breakwater tip physics. The full analysis shows that breakwater reflectivity causes little change to the diffraction pattern beyond the area closest to the breakwater. The areas of greatest influence are located at the breakwater tips, which can be associated with reflection off the breakwater’s front face, and along the leeside of the breakwater, which can be attributed to the absorption of the leeside’s reduced reflectivity. Therefore, the need for new, updated diffraction diagrams based on reflectivity is unnecessary. As a secondary objective of this study, the usefulness of using the SWAN modeling program for diffraction studies was examined. This topic is of importance as SWAN is an efficient alternative to more computationally intensive modeling programs. In addition, it is a free software which is universally available. Believed to be incapable of computing rapidly changing wave environments near breakwaters or in harbor situations, SWAN performed diffraction remarkably well for broad directional waves. More narrow directionally spread waves had good accuracy throughout with only minor scaling differences. SWAN, within this study, was unable to replicate the effects of reflection on both sides of a breakwater and therefore was not used for the primary objective.

Diffraction
Reflection
SWAN

http://resolver.tudelft.nl/uuid:bfaf4c51-1234-4eff-a5c0-12c982faa8f6

Student theses

master thesis

(c) 2009 Boshek, M.R.