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6 - Surf zone circulation

from Part II - Coastal Processes



Wave shoaling and breaking result in the transfer of momentum as well as a mass of water into the breaker and surf zones and leads to an increase in the water level against the shoreline, termed set-up. In turn, the set-up drives an offshore-directed flow that returns the water offshore beyond the breaker line. Mass transport of water onshore occurs relatively uniformly alongshore in the wave above the level of the wave trough. The return flow takes the form of either a two-dimensional undertow or a three-dimensional rip cell circulation. The resultant surf zone circulation is important for sediment transport and the development of nearshore and beach morphology and the flows are also important for marine organisms living in the bed and in the water column.

Undertow occurs as a seaward directed current in the zone between the bed and the wave troughs. The undertow occurs essentially uniformly alongshore and, because the cross-sectional area of discharge is large, the corresponding mean flows are relatively small – on the order of a few cm under low waves to a few tens of centimetres in intense storms. Because this flow takes place in the lower part of the water column where sediment concentrations are highest it is an important mechanism for the seaward transfer of sediment and for the development and migration of nearshore bars.

In rip cell circulation the seaward return flow is concentrated in narrow zones (rips) that periodically breach the breaker line alongshore.

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