Storm-Induced Variability of Longshore Sediment Transport in Bou-Ismail Bay, Algeria

Abstract

This study investigates the longshore sediment transport (LST) dynamics under extreme wave conditions along Bou-Ismail Bay, Algeria. A hindcast dataset covering 29 years (1992-2020) was analysed using a POT (Peak Over Threshold) extreme value approach, combined with DHI MIKE 21
spectral wave modelling to simulate wave transformation from offshore to nearshore. LST rates were then quantified using the recalibrated formulation of Mil-Homens et al. (2013). The results indicate that LST responds non-linearly to storms. An increase of only 1 to 2 m in significant wave height (Hs)
was sufficient to multiply transport rates, highlighting the crucial role of extreme conditions relative to mild wave conditions. The Sidi Fredj headland proved to be a dominant geomorphological factor, protecting Khelloufi beach from northeast swells while exposing Colonel Abbas to the direct action of
northern and western waves. Morphodynamic contrasts further explain the differences observed in the LST. The steeper breaking zone at Colonel Abbas (slope of 0.09) increased wave height, while the finer sands (260 μm) were more mobile than those at Khelloufi, where the slope is gentler (0.03) and the
sediments coarser (330 μm). The integrated methodology proved to be robust and reliable revealing that extreme-event-driven LST must be incorporated into sediment budgets, risk assessments, and shoreline management for effective and sustainable coastal planning.