Mohamed-Ali Geawhari

The Loukkos is among the largest river in Morocco. To date, there has been no thorough investigation of the underwater bedforms in the Loukkos. To characterize variability in bedform geometry, and to provide a detailed description of the morphodynamics of dunes in the river channel, images of the sediment-water interface were collected using an Ocean Server Iver2-62 AUV and an Echosounder 320 BP. Grab samples were taken for understanding sedimentological distribution. Series of bedforms of different sizes are clearly observed in the study area. They are presented as fields developed at depths of 2–10 m. Medium dunes in terms of Ashley's (1990) classification system, are the most dominant. The wavelengths and the heights vary from 1.5 to 52.5 m and 0.3–3.23 m respectively. Overall, the dunes identified along the Oued Loukkos have an asymmetrical cross-section with variable bedforms configuration from 2 D straight crest dunes to sinuous 3D dunes. In addition, the dunes show no clear correlation between dune height and wavelength with depth with previous studies, but this suggest that other parameters should be involved.

ABSTRACT An episode of seiche, with peak-to-peak amplitudes of 6 to 11 cm, was observed and recorded in the Port of Larache, Morocco, at the mouth of the Oued Loukkos, using a Sea-Bird Electronics CTD. The CTD measurements, which exhibited characteristics of seiche, were processed to remove the much larger tidal signal and preserve the lower amplitude, higher frequency signal of the seiche. Based on spectral estimates, which were performed using the time series containing both well-defined and ill-defined seiche-like water level variations, the most energetic seiche period was approximately 17.5 minutes. A less energetic seiche period of approximately 70 minutes was also detected. The seiche is possibly sustained by reflections of a propagating gravity wave at points where there are sudden changes in the hydraulic cross-section (depth and/or width) of the Oued Loukkos. The genesis of the observed episode of seiche is unknown, but a likely prospect is that it was engendered by non-linear interaction of the tidal flow with the river bed. Un episode de seiche observe au port de Larache, côte atlantique du Maroc. Résumé. Un épisode de seiche, avec une amplitude variant entre 6 et 11 cm, a été observé et enregistré dans le port de Larache, au Maroc, à l'embouchure de l'oued Loukkos grâce à la bathysonde « Sea Beard Electronic ». Les mesures CTD, qui présentent les caractéristiques de seiche, ont été traitées pour soustraire le signal de la marée et préserver le signal de la seiche. D'après les estimations spectrales qui ont été appliquées sur une série temporelle contenant à la fois des données d'élévation de surface (seiche) bien et mal définie, la période de seiche la plus énergique était d'environ 17,5 minutes. Une période de seiche moins énergique d'environ 70 minutes a également été détectée. La seiche est peut-être soutenue par des réflexions d'une onde de gravité se propageant dans les endroits où il y a des changements soudains dans la section hydraulique (profondeur et / ou la largeur) d'Oued Loukkos. La genèse de l'épisode observé de seiche est inconnue, mais une perspective probable c'est qu'elle a été engendrée par interaction non linéaire de l'écoulement de marée avec le lit de la rivière.

Several different classifications to characterize estuarine systems have been proposed. In this present paper, one of the most important estuaries in North Africa, the Oued Loukkos (Morocco), forms a case-study for proposing a systematic classification of this particular tidal estuary according to the vertical salinity gradient. This study, conducted using a CTD, shows that the spatial-temporal distribution of salinity depends on the stage of the tide and the upstream distance from the mouth of the river. In this case, it is also evident that the morphology of the bottom was capable of impacting the distribution of salinity by locally changing the water circulation. Based on the vertical salinity gradient measurement, the Oued Loukkos represents an estuarine environment with one section near its mouth that can be characterized as a mixed mesotidal estuary and another section upstream which can be characterized as a stratified mesotidal estuary. Between, there is an intermediate zone...