Morphological Evolution and Sedimentological Study of the Essaouira Bay and the Mouth of Oued Ksob, Essaouira, Morocco

Abstract


Introduction
The morphological evolution is the result of the conjunction of many natural factors and particularly the variation of sea level change, wind, swell, currents, and importance of the sedimentary contributions of rivers (El Mimouni, 2009;Lahmadi, 2013).The winds are also at the origin of important transfers of sand in all dune areas.Thus, along Essaouira Bay, the removal of aggregates in 2016 for the reconstruction of the port has contributed to the depletion of coastal sediments.The evolution and dynamic balance of Essaouira Bay rely on sediment exchanges between the foreshore, the open beach, and the dunes.In this study, we'll concentrate on the level of geomorphological change brought on by the accumulation and the accretion of coastal spits (Taghya, 2014), processing of satellite images allows us to observe and describe this evolution.
The Essaouira coastline, which is a complex coastal area is characterized by aeolian and hydro-sedimentary processes that act at various spatial and temporal scales (El Mimouni, 2009).To make a sedimentological study and compare the sediments of different environments of the study area of Essaouira Beach, a sedimentological study was conducted on the sands of these environments.The sedimentological techniques used in this work are granulometry, morphoscopy, calcimetry, and X-ray diffraction.
The study area is located on the Atlantic coast of Morocco at 31°30' North latitude and 9°50' West longitude.Due to its dual position (oceanic border and proximity to the High Atlas), it belongs to the northwestern sector of the Atlantic Atlas, defined as the region stretching from east to west, from the western High Atlas to the Atlantic Ocean, and from south to north, from the plain of the Oued Souss to that of the Oued Tensift (Weisrock, 1980).

Materials and Methods
To carry out this work, two components were treated: a sedimentological study of the sands of the Essaouira coast and a study of the evolution of the Essaouira Bay and the mouth of Oued Ksob by processing satellite imagery .

Sedimentological Study of the Sands of the Essaouira Coastline
For the sedimentological study, samples are taken from the following morphological units: beach, intertidal, mouth, and dunes.The samples are then processed in the sedimentology laboratory and the analysis and characterization center (ACC).All the results are presented in Figs. 1, 2, 3, 4 and 5.
The field sampling is done directly at a depth of 20 cm in the case of a study on a dune, a beach, a sandbank, more generally on the intertidal domain.
The samples were taken with a small shovel and about 200g to 300g of sediment were taken and put in plastic bags that were then labeled and destined for granulometric and mineralogical analyses (Fig. 6).The four sedimentological techniques used are: • granulometry analysis, which aims to quantitatively determine the distribution of particles by diameter classes.The number and size of the diameter classes will depend on the objective and possibly on the size of the sample available (Founier, 2012) • Morphoscopy is defined as a means to help in the "statistical determination of the different types of quartz in sandy deposits".More broadly, morphoscopy is concerned with observing the various traces visible on the surface of detrital quartz grains to determine their origin and the environments through which they pass.(Le Ribault L., 1977) ; • Calcimetry allows measuring the volume of CO2 released by the action of hydrochloric acid (HCl) on the calcium carbonate (CaCO3), of a sample of soil or rock, using a calcimeter ; • The mineralogical analysis method by X-ray diffraction (XRD) is generally used to identify the different crystalline mineral phases contained in a sample (Fig. 7).

Study of the Evolution of Essaouira Bay and the Mouth of Oued Ksob by Processing Satellite Imagery (1985-2021)
The analysis of the coastline evolution of Essaouira Bay and the mouth of the oued ksob was determined through the comparison of Landsat satellite imagery of several years from 1985 to 2021.
Satellite image processing was performed using GIS software such as ENVI classic, ESA SNAP, and ArcGis.Numerous operations have been performed on these satellite images, they consist in visualizing and showing the morphology of the Essaouira Bay and the mouth of Oued Ksob.
Using ENVI software, the images are processed by RGB visualization as well as the band ratios to obtain a well-defined final result with a resolution of 15 m (Fig. 8).

Sedimentological Study
According to the calculations of several parameters and granulometric indices of the sample ''Dune'' (Sample 1), it is about sands of dune very well classified and an asymmetry with the presence of fine particles.The most abundant form of quartz grains is round matte which explains the long eolian transport.The chemical composition of the sands of this sample oscillates between a high percentage of silica and a low percentage of limestone, as well as the presence of chlorite and a significant percentage of illite.
The study of the sample ''Mouth'' (Sample 2) indicates that most of the sands of the mouth of Oued Ksob come from a river-type environment which explains the shining effect of quartz grains, the presence of round shining quartz grains highlights the role of turbulence.The chemical composition shows an important percentage of SiO2 and a weak percentage of CaCO3.The lowest levels generally occupy calm and sheltered areas (Flor, 1978;Aouiche, 2016), with the presence of considerable percentages of chlorite, illite, and kaolinite.
Morphoscopic examination of quartz grains in the last two samples (southern estuary and northern estuary) shows the abundance of blunt-shiny grains, which highlights the influence of aquatic transport corresponding here to littoral hydrodynamics conditioned by local hydrodynamic forces: waves and swells, tidal currents.The high rates of illite and calcite in these two samples can be explained by the origin of the latter which is mixed, it can be marine as it can be continental.
The CM diagram (Fig. 1) shows the mode of transport of the different samples.The sample ''Dune'', which interests us, is located at the level of the CM diagram in the domain of gradual suspension, that is to say, that the grains are transported by the graded suspension.On the other hand, the three samples, Embouchure, South estuary, and North estuary, are located in the saltation domain.An aquatic environment for the 3 samples having a transport by saltation and for the sample ''Dune'' is characterized by an eolian environment.

Morphological Evolution
From 1985 to 2021, it is noted that the Bay, which is a coastal indentation drawing a concave coastline open to the sea, has undergone an increase in the surface area reflected by an accretion that is likely related to sedimentary inputs from the Oued Ksob, which represents the main source of sedimentation for sector 1 which is the Essaouira Bay (Gentile, 1997;EL Mimouni et al., 2007;El Mimouni, 2009).A considerable amount of sand and sediment, as well as the solid load, were transported to Essaouira Bay during the flood periods (Fig. 9) Sector 2, which is the estuary area (Fig. 10), is a very complex area from a morphological and sedimentological point of view, given the interaction of many factors (river dynamics, marine dynamics, wind transport ...) (Gentile, 1997;Simone, 2000;El Mimouni, 2009) In 1985, the estuary had two passes to the beach.Then in 1990, there was the mouth represented by a single pass and the beginning of an accumulation of material, sands, by lateral accretion due to the littoral drift.Then in 1995, on both sides of the banks of the Oued, two littoral spits began to be shaped.Later in 2000, the estuary presented on the left bank an advancement of a littoral arrow in the opposite direction to the lateral accretion with the appearance of a point that advances towards the sea.In 2010, the concave bank of Oued Ksob, which is located outside the meander is eroded by the water current.On the other hand, at the level of the convex bank deposits of sandy sediments accumulate.
In addition, in 2016 and 2021, we observe the presence of a shoreline spit, shaped by the coastal drift of orientation of NE-SW and another shoreline spit oriented SW-NE likely due to the sinuosity of Oued Ksob, which makes its filling / draining by the tides, without forgetting the role of the action of the tides that contribute to the shaping of this shoreline spit.
Beyond the mouth of Oued Ksob to the south, there is an intense hydrodynamic action because the swell in this sector 3 is not influenced by the island of Mogador.It is an open beach, directly subjected to the action of the incidental swell characterizing the Atlantic Ocean and which generates a fairly strong littoral drift towards the south.

Conclusions
The use of satellite images has allowed us to restore the morphological evolution of the entire coastline of Essaouira Bay and the mouth of Oued Kosb in the long term, over a period from 1985 to 2021.This photo-interpretation analysis was carried out using GIS tools and software (ArcGis, SNAP, and ENVI).The results of this study confirm the spatial and temporal variability of shoreline change.We found a strong variability at the mouth of Oued Ksob, we also distinguished a spatial variability that is reflected in the existence of three very different sectors: • The first one is located north of the estuary of Oued Ksob: because of its closed nature (a Bay), this area receives most of the flood inflow of Oued Ksob, which helps to maintain its morpho-sedimentary balance.• The second sector is composed of the estuary and the mouth of Oued Ksob: it is characterized by a hydrodynamic both fluvial and maritime, this is explained by the presence of two coastal spits, one resulting from the influence of the coastal drift oriented NE-SW and the other resulting from the action of the tides of an SW-NE orientation.• The third sector is located south of the estuary of Oued Ksob: this sector is an open beach that is not affected by the effect of the presence of the obstacle which is the island of Mogador.The sedimentological analysis, according to the various studies conducted, granulometric, morphoscopic, calcimetric and the results obtained by X-ray diffraction as well as the presence of certain minerals explains and confirms the origin of the sediments, at the level of the four areas: the dunes, the mouth, the area south of the estuary and the area north of the latter, which is continental and marine.
In addition to the natural factors that condition the morphological evolution of the entire coastline of Essaouira, the island of Mogador presents an obstacle by playing an important role in the morphogenesis as well as sedimentation along the coastline.