NEARSHORE CIRCULATIONLearning Objective: | Recognize and explain the nearshore circulation system. |
 T and the nearshore current system. The COASTAL SYSTEM is a relatively uniform drift that flows roughly parallel to shore. It may be composed of tidal currents, wind-driven currents, or local, density-driven currents. The NEARSHORE SYSTEM is more complex and is composed of shoreward (verso riva) moving water in the form of waves at the surface, a return flow or drift along the bottom in the surf zone, nearshore currents that parallel the beach (longshore or littoral), and rip currents. See figure 1-1-2.
Longshore or littoral currents occur in the surf zone and are caused by waves approaching the beach at an angle. If you ever swim in the surf where ocean waves are approaching the beach at an angle, you will most likely become aware of the longshore current.Most people like to swim in the immediate proximity of the beach where they enter the water.However, what many swimmers find out after being in the water awhile is that they are transported quite a distance from where they entered the water. The transporting mechanism is the longshore current.At times the current is almost imperceptible, but at other times, it can be quite strong. Generally, longshore currents increase with (1) increasing breaker height, (2) increasing breaker-crest speed, (3) increasing angle between breaker crests and bottom contours, and (4) decreasing wave period. Also, under otherwise identical conditions, a steep beach will have a stronger longshore current than a more gently sloping beach. Another factor to consider is development of a longshore sandbar. These bars channel the current in the trough between the bar and the beach, and quite a strong current can result.Rip currents are quite often erroneously called "rip tides". A rip current is not associated with the tides, but is caused by the return flow of water from the beach. The current resembles a small jet or neck in the breaker zone, which fans out (si apre a ventaglio) behind the breakers and becomes quite diffuse. The current extends from the surface to the bottom and is quite strong. The strength or intensity of rips is not predictable, but is determined using the same factors that control longshore currents.The speed of feeder currents flowing into the jet increases as they near the jet.Rip currents may or may not occur along a stretch of beach. When they occur, they may be irregularly spaced, or regularly spaced at long or short intervals. They commonly form at the down-current end of a beach where a headland (a point where the land juts out into the water) deflects the longshore current seaward.
|
