EM 1110-2-1100 (Part II)
31 July 2003
ξo > 3.3
0.5 < ξo < 3.3
ξo < 0.5
(c) As expressed in Equation II-4-2, spilling breakers tend to occur for high-steepness waves on gently
sloping beaches. Plunging breakers occur on steeper beaches with intermediately steep waves, and surging
and collapsing breakers occur for low steepness waves on steep beaches. Extremely low steepness waves
may not break, but instead reflect from the beach, forming a standing wave (see Part II-3 for discussion of
reflection and Part II-5 for discussion of tsunamis).
(d) Spilling breakers differ little in fluid motion from unbroken waves (Divoky, Le Mhaut, and Lin
1970) and generate less turbulence near the bottom and thus tend to be less effective in suspending sediment
than plunging or collapsing breakers. The most intense local fluid motions are produced by a plunging
breaker. As it breaks, the crest of the plunging wave acts as a free-falling jet that may scour a trough into the
bottom. The transition from one breaker type to another is gradual and without distinct dividing lines.
Direction and magnitude of the local wind can affect breaker type. Douglass (1990) showed that onshore
winds cause waves to break in deeper depths and spill, whereas offshore winds cause waves to break in
shallower depths and plunge.
(2) Breaker criteria. Many studies have been performed to develop relationships to predict the wave
height at incipient breaking Hb. The term breaker index is used to describe nondimensional breaker height.
Two common indices are the breaker depth index
in which db is the depth at breaking, and the breaker height index
Incipient breaking can be defined several ways (Singamsetti and Wind 1980). The most common definition
is the point that wave height is maximum. Other definitions are the point where the front face of the wave
becomes vertical (plunging breakers) and the point just prior to appearance of foam on the wave crest (spilling
breakers). Commonly used expressions for calculating breaker indices follow.
(3) Regular waves.
(a) Early studies on breaker indices were conducted using solitary waves. McCowan (1891) theoretically
determined the breaker depth index as γb = 0.78 for a solitary wave traveling over a horizontal bottom. This
value is commonly used in engineering practice as a first estimate of the breaker index. Munk (1949) derived
the expression Ωb = 0.3(Ho / Lo)-1/3 for the breaker height index of a solitary wave. Subsequent studies, based
on periodic waves, by Iversen (1952), Goda (1970), Weggel (1972), Singamsetti and Wind (1980), Sunamura
(1980), Smith and Kraus (1991), and others have established that the breaker indices depend on beach slope
and incident wave steepness.
(b) From laboratory data on monochromatic waves breaking on smooth, plane slopes, Weggel (1972)
derived the following expression for the breaker depth index
Surf Zone Hydrodynamics