oncoming waves is small, only finer sediments are moved onshore. The
sediment transported over a period of time will depend on the variations
in the wave climate, including the direction of wave attack. In addition to
the onrush and downrush of water on the beach, two important types of
currents are set up by the wave motion: rip currents and longshore
currents.
The rip current is the narrow and strong flow seaward as a result of
bed features and the formation of opposing along-shore currents. For
example, when water is trapped behind obstacles such as sandbars, it
forms a current along the coast which can drain with a high velocity
current towards the sea through an opening in the sandbars (Fig. 15.1).
Thus the transport of water over the sandbar (e.g. by a large breaker)
feeds the rip current to form a nearshore circulation.
When waves break at an angle to the coastline, a longshore current
parallel to the coastline is established (Fig. 15.2), responsible for the net
630 COASTAL ENGINEERING
Fig. 15.1 Rip currents
Fig. 15.2 Longshore currents
