Modelling nearshore currents driven by waves and set-up gradients

Abstract

During the fall of 2003, an extensive measurement campaign called the “Nearshore Canyon Experiment” (NCEX) was set up along the coast of La Jolla, California. The project site covered an area in which two submarine canyons are located, respectively La Jolla and Scripps Submarine Canyon. Over 20 institutions collaborated on recording the hydrodynamics and morphological changes. The collective objective of these institutions was to gain more insight into nearshore processes. The canyons are responsible for substantial changes in wave energy distribution over an alongshore distance of a few hundred meters. The irregular wavefield that arises in the direct vicinity of the canyons results in a complex nearshore circulation and in morphological changes in the surfzone. The following research objectives were proposed for this study: 1) “What is the relative contribution to the alongshore current of (i) alongshore water level gradients due to the alongshore variation of wave set-up and (ii) obliquely incident waves?” And 2) “Can the numerical model Delft3D reproduce the order of magnitude and direction of the nearhore currents?” This master thesis describes the hydrodynamic circulation in the nearshore during the measurement campaign and the results of the numerical model Delft3D for two specific cases, respectively a day with wind sea and a day with swell waves. The incident waves drive currents and create wave set-up. Owing to the influence of the submarine canyons, alongshore gradients develop in the wavefield. It is concluded that the influence of the submarine canyons becomes more prominent in case of swell waves and therefore a larger alongshore set-up gradient arises. In both the observations and the model predictions an alongshore current develops strong enough to counteract the wave-driven flow.