  Infra-gravity waves are generally known as small-amplitude waves of periods
between 25 seconds and 5 minutes. They originate from the presence of wave
groups in the open ocean waves and can move freely after being released near
the surf zone where they can be further fueled with energy from the spatially
varying break point of swell waves . As these waves approach the shore, the
relative importance of the infra-gravity wave signal increases, and its impact
on the shorter waves gets stronger. In addition, infra-gravity waves drive
strong cross-shore currents, which lead to significant back-and-forth motion of
the underlying sea water. This strong cross-shore motion has been made visible
by recent field studies, where significant cross-shore movement was detected
and found to be correlated with the infra-gravity wave signal. In the present
work, the connection between infra-gravity waves is explored further using
linear wave theory and an established numerical nearshore wave model (BOSZ). It
is shown that in all cases, the presence of infra-gravity waves leads to strong
cross-shore motion. This behavior can be understood by considering the
infra-gravity waves as separate free waves, and then following the fluid
particle trajectories excited by these waves. As it is shown, these
trajectories have a very large horizontal extent which -- if not separated from
the main gravity wave field -- appears as a large, but often not directly
visible back-and-forth motion, underlying the more readily observable gravity
ocean waves.
