The Safi boulders (Morocco): Evidence of past extreme wave events

Abstract

Tsunamis and storm surges are among several hazards threatening the Atlantic coast of Morocco. During the last two decades, sedimentological deposits left by these events, in the form of fine-grained sediments (washover) and boulders, have been studied along the Moroccan shores to determine the return period of these events and how they fluctuate in terms of intensity. The present work investigates two new boulder fields located along the Safi coast. Field surveys were accomplished in the two areas to collect the position/elevation and dimensions of 164 boulders using a GPS/DGPS and a traditional meter, respectively. The field data was combined with hydrodynamic equations to estimate the minimum velocity and storm/tsunami wave height to move the measured boulders. In site 1, the boulders are situated on a rocky intertidal platform, while in the second, they are on the top of a cliff. For both sites, the size of the boulders ranges from 0.6 to 5.60 m, 0.54 to 3.6 m, and 0.27 to 1.5 m on the major (a), medium (b), and minor (c) axes, respectively. They reach a volume of up to 25.20 m3 and a weight of 55.44 t. The maximum distance and elevation recorded are around 41.77 m and 13 m, respectively. According to hydrodynamic equations, the coastal platform boulders (site 1) can be moved by tsunami waves with heights between 0.10 and 0.59 m, and storm waves from 0.42 to 2.37 m. To be displaced, the cliff-top boulders (site 2) need tsunami and storm waves with at least 5.91 and 8.64 m, respectively. The results from the field surveys and hydrodynamic equations suggest that both tsunami and storm waves can move the boulders in site 1. For the cliff top-boulders, the data favours tsunamis as a source for this deposit.