The effect of artificial reef configuration on wave breaking intensity relating to recreational surfing conditions
Date
2009-03
Authors
Johnson, Craig Michael
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
Multi purpose reefs are a relatively new concept that incorporate functionalities of beach
stabilization, breakwater/seawall protection, biological enhancement and recreational amenity.
Economic benefits increase their attractiveness. There is, however, some degree of uncertainty
in design guidelines as to the predictability of each of these aspects. With regards to recreational
amenity enhancement, one such uncertainty exists in the ability to predict the reef configuration
required to give a certain degree of surfability of a reef, and more specifically, to predict the
shape of a plunging wave.
An extensive survey of the relevant literature has been conducted to provide a background
on multi purpose reefs and the uncertainties in predicting the success of multi purpose reefs
in achieving their design objectives. A study of wave breaking has been done, along with an
analysis of existing breaker height and breaker depth formulae. The effects of bottom friction, refraction,
shoaling, winds currents and varying water level on wave breaking has been addressed.
Surfability aspects were reviewed including a definition of breaking intensity which is defined
by the wave profile in terms of vortex shape parameters, and other surfability parameters that
influence the surfability of a reef. Background on numerical modelling methods has been given,
along with a description and some trial runs of a new and promising method, Smooth Particle
Hydrodynamics. Numerical models were run using the open source SPHysics package in order
to assess the applicability of the package in measuring vortex shape parameters. The SPHysics
package is, however, still in a stage of development, and is not yet suitable for reef studies with
very long domains and with high numbers of particles (required for sufficient resolution in the
plunging vortex).
A theoretical examination was done on the relevant literature in order to gain an insight into
the dynamics affecting the development of the plunging vortex shape. A case study of a natural
surf reef was carried out in order to give qualitative estimation of the wave dynamics and reef
structure required to give good quality surfing waves and high breaking intensity. The West-
Cowell surfing reef factor was used as a tool in predicting wave focusing effects of a naturally
occurring reef. Extensive two dimensional physical model laboratory studies were conducted in
order to quantify the effects of the reef configuration and wave parameters on breaking intensity.
Design guidelines were developed in order to assist in the prediction of breaking intensity for
reefs constructed with surfing amenity enhancement as one of their design objectives.
The results show that large underwater topographic features can significantly affect the
shape and size of incoming waves. Refraction, focusing and shoaling can transform ordinary
waves into waves deemed suitable for surfing. The West-Cowell surfing reef factor gives reasonable
results outside its applicable range. The 2D physical model laboratory tests show significant
variations in vortex shape parameters due to interactions between broken and unbroken waves
in a wave train and also to the reflections developed in the flume. Results show that the predicted
trends agree with the observations. The results also show that the junction between the
seaward reef slope and the horizontal crest may have an effect on the wave shape in the form
of a secondary crest between the primary crests. Design guidelines based on the results are
presented, and show that breaker height formulae for smooth planar slopes show good agreement
with the values of breaker heights measured in the physical model tests, and that existing
breaker depth formulae show average agreement. The design guidelines could assist with more
effective design of artificial reefs for surfing purposes.
Description
Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2009.
Keywords
Sunset Reef, Theses -- Civil engineering, Dissertations -- Civil engineering