Definitions for wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 42

the breaking does not generate whitecaps. Obviously, large breakers, which do produce
whitecapping, also break the skin layer ( Jessup et al. , 1997b ).
Lowen & Siddiqui ( 2006 ) compared three methods of detecting micro-breaking: the
wave slope, the areal extent of the thermal wake and the variance of the vorticity in the
micro-wave crest region, based on empirical threshold values. The comparison showed
that the vorticity method is the most accurate, although it is obviously a difficult method to
apply in field conditions. Infrared remote-sensing techniques intended to deal with micro-
breaking will be discussed in more detail in Section 3.6 .
Tulin & Landrini ( 2001 ) investigated micro-breaking by means of numerical simula-
tions of the breaking onset with account taken of capillary effects, and by means of radar
observations of micro-breakers in the laboratory. They provide interesting insights into
the hydrodynamics of formation and propagation of micro-breaking waves. According to
them, surface tension has a significant impact on the breaking of waves shorter than 2m, to
such an extent that for waves characterised by (2.45) no jet is produced at the crest. Instead,

“a forward facing bulge growing out of the wave crest is formed ... Unlike energetic breakers where
the shape continually evolves in transient fashion, ... the microbreaker can propagate for a consider-
able distance without significant change of shape”. Tulin & Landrini ( 2001 ) note that for very short
waves (sub-micro-breakers) the bulge separates from the main flow “forming a cup on top of the
wave, as noted by Ebuchi et al. ( 1987 )”.

2.9 Criteria for breaking onset

Wave breaking obviously happens when the water surface loses stability and collapses.
What is the physics leading to this collapse has been one of the main questions for wave-
breaking studies, and is one of the main topics of this topic. The understanding of this
physics has been elusive, but dealing with breaking is necessary across a very broad range
of oceanographic applications as described throughout the topic. Therefore, in the course of
wave-breaking investigations, various criteria which supposedly indicate a breaking onset
and therefore may help to detect it have been employed. They may or may not assist in
the understanding of the physics, and may not even rely on the physics as such, but they at
least need to be outlined at this stage in order to explain various experimental, analytical,
statistical, probabilistic and numerical methods described later in this topic.
Wave-breaking criteria are typically broadly classified into three categories: geometric,
kinematic and dynamic criteria (e.g. Wu & Nepf , 2002 ). Most commonly employed are
those that follow from the Stokes limiting steepness ( Stokes , 1880 ):

H /λ = 1 / 7 = 0 . 142 . (2.46)

If converted into the nowadays commonly used steepness parameter (1.1) , this criterion

reads:

limiting = ( ak ) limiting = 0 . 443 . (2.47)