Author(s): M. Kitou; I. Katopodi; B. A. O'Connor
Linked Author(s): Irene Katopodi
Keywords: Longshore current; Time scale; Adaptation time; Current response; Random waves
Abstract: The present study deals with the time scale of the development of the longshore current in the nearshore zone. The response of the longshore current to wave climate starting from rest is examined. The analysis of the governing equations for a simplified case with longshore uniformity identifies a characteristic time-scale parameter, the adaptation time. It is shown that the adaptation time increases rapidly with depth due to larger inertia, reduced bed friction and smaller driving forces (weak dissipation) in deep water. For the purposes of the analysis, use is made of a numerical steady state cross-shore model with spectral wave input. It is shown that the current response in time is not the same in all parts of the cross-shore domain. The zone close to the shore adapts to steady state fast (in minutes) whereas the offshore zone adapts slowly (in days), with an adaptation time that exceeds the time scales of weather changes and therefore of the induced driving forces themselves. The effect of bed friction enhancement due to waves is also examined. Finally, conclusions are drawn for the behaviour of steady state and time dependent computations in relation to the wave driving forces.
Year: 2003