corresponding to the group celerity of the short waves and thus are refracted with the
short waves. In shallow water the height of the bound long waves will increase quite
dramatically due to shoaling.
In the surf zone the short waves lose height and energy and can no longer balance
the bound long waves which are therefore released as free long waves. The free long
waves are substantially reflected from the beach and either progress back out to sea
(for normally incident short waves), termed the leaky mode or refract and turn back
to the shore to be re-reflected, termed the trapped mode. The trapped free long waves
then form three-dimensional edge waves with a wave height which decreases with
distance from the shore.
Another mechanism for generating long waves in the surf zone is variation in set-up
caused by breaking wave groups. Surf beat is the variation of set-up on the shoreline
and may be caused by a combination of free long waves in the surf zone, generated at
sea as bound long waves and free long waves generated in the surf zone due to
variations in set-up.
Cell circulation is the term used to describe currents within the surf zone that are
not parallel to the shore. The existence of cell circulations is evidenced by rip currents
(a common hazard for swimmers!) Rip currents are a seaward return flow of water
concentrated at points along the beach. They are caused by a longshore variation of
wave height and hence set-up which provides the necessary hydraulic head to drive
them. The longshore variation of wave height can be caused either by refraction
effects or by the presence of edge waves. Under the latter circumstances a regular
pattern of cell circulations and rip currents will exist and beach cusps may be formed.
The interested reader is referred to Komar (1976) and Huntley et al. (1993) for further
details.