Skip to navigation – Site map

HomeNumérosvol. 20 - n° 3Morphological change on a wadi-in...

Morphological change on a wadi-influenced beach: Essaouira, Morocco

Changements morphologiques d’une plage sous influence d’un oued : Essaouira, Maroc
Abdelhadi Elmimouni, Lahcen Daoudi and Edward J. Anthony
p. 243-250

Abstracts

Field topographic surveys were conducted at intervals of months from 2004 to 2006 in order to highlight the sediment circulation patterns of the embayed beach of Essaouira, on the Atlantic coast of Morocco, especially under the influence of a nearby wadi, the Ksob, which is the main sediment purveyor of the beach, notably via flash floods. The profile data highlight longshore and cross-shore variations in beach morphology. Constant winds from the north rework the upper beach to form barchans that migrate southward towards the mouth of the Ksob. During important river floods, two of which occurred late in 2005, these sand inputs by aeolian action, as well as fresh inputs brought in by the wadi, are flushed out into the nearshore zone of Essaouira bay, and are subsequently redistributed onshore, and along the beach by longshore currents generated by highly refracted swell. The sediment mobility pattern in Essaouira bay describes a rather original process of beach "rotation" involving not just wave redistribution of sand, but also significant aeolian activity, and high-energy flash floods.

Top of page

Editor's notes

Article soumis le 21 mai 2012, accepté le 30 janvier 2013.

Full text

We thank David Menier, Mouncef Sedrati and an anonymous reviewer for their constructive review.

Introduction

1In many embayed and pocket beaches, net sand inputs from adjacent stream mouths are an essential element of the sediment budget, especially on high-relief coasts (Inman and Jenkins, 1999), although documentation of this relationship has been sparse (Pranzini et al., 2013). The impact of river mouths on coastal dynamics is manifest in two ways: (1) tidal and non-tidal estuarine processes and interactions involving river flow, waves and tides, and (2) bedload exchanges between the mouth and the adjacent shorelines. The morphosedimentary processes implied in bedload exchanges between river mouths and adjacent shorelines are, however, rather poorly known (Anthony, 2009). These processes are controlled by the river flow regime on the one hand, and by waves and wave-induced currents on the other hand, notably the alongshore distribution of wave energy (e.g., Barnard and Warrick, 2010). The river flow regime, modulated by topography, generally varies with climate. The largest river bedload discharges to the sea occur during floods, which, in wadis in semi-arid to arid areas, are extremely irregular, often of high intensity and rather sudden. The example treated in this paper concerns the morpho-sedimentary dynamics of an embayed beach in Essaouira, on the Atlantic coast of Morocco. The study highlights an original sediment circulation system involving a wadi, the bay hydrodynamic system and aeolian activity.

Study site

2Essaouira beach is 7 km-long, oriented NNE-SSW, and flanks Essaouira bay which lies on the western maritime flank of the High Atlas. The beach is bound to the north by the rocky headland of Moulay Bozerktoun, behind which the port of Essaouira is sheltered, and to the south by the mouth of Wadi Ksob (fig. 1).

Fig. 1 – Location of Essaouira bay.
Fig. 1  Le site d’Essaouira.

Fig. 1 – Location of Essaouira bay. Fig. 1 – Le site d’Essaouira.

A: Essaouira bay on the coast of Morocco; B: the Wadi Ksob catchmen; C: map of Essaouira beach showing profile locations and local wind and wave roses.
A : la baie d’Essaouira sur la côte marocaine ; B : le bassin-versant de l’oued Ksob ; C : carte de la plage d’Essaouira montrant la localisation des profils ainsi que les roses de vents et de houles.

3The northern part of the bay bounds the urban front of the city of Essaouira, from which it is separated by a sea-front promenade, thus cutting off the beach from the massive aeolian dune complex behind the city. The beach connection with the dunes occurs near the mouth of the Ksob, which is characterized by a large spit platform recurved to the north. The river-mouth sector of this wadi comprises a rock outcrop on which the Portuguese constructed a fort in the 15th century, the remnants of which are still standing. Another morphological feature of interest to the dynamics of the study site is Mogador Island located just offshore of the bay. The extension of the city is occurring at the expense of the dunes, which are more or less fixed by fences and vegetation (fig. 1). The beach and dunes consists of fine, well-sorted sand.

4Wadi Ksob drains a basin of 1480 km2, with a peak elevation of 1694 m, and the river bed has a mean slope of 1.5% (Elmimouni et al., 2010). The wadi mouth is characterized by marked sandy infill from both wadi sediments and aeolian inputs from the dune massif in the northeast. Apart from bedload brought down by the Ksob, the bed of the wadi thus constitutes an important sink for dune sand transported to the south, especially during dry windy periods (Chahboun, 1988). However, the bed of the mouth of the wadi is swept clear of sediment during every important flood event. The turbidity plumes associated with these floods indicate that the sediment transported offshore is rapidly deposited and stored in the nearshore zone of Essaouira bay at depths of 0 to ‑5 m and distances of 100 to 200 m (Elmimouni, 2009). The rapid fanning out of the fluvial jet results in poorly sorted nearshore deposits of sand, gravel and some silt.

5The region of Essaouira is subject to a rather particular climate resulting from its geographic situation. It is located in a semi-arid to arid area that is strongly influenced by the Atlantic Ocean. It experiences a dry season of up to 7 months. Temperatures are relatively mild as a result of upwelling on this part of the Moroccan coast which is affected by the cool Canary current. The mean maximum temperature is 22.3°C in August and September, the hottest months, and 9.5°C in January, the coolest month. Mean precipitation in the Ksob catchment is 295 mm/a, and exhibits marked intra-annual and interannual irregularity. This translates into sudden, sometimes devastating, and especially unpredictable floods, with heavy solid discharge. The liquid discharge can increase from a few m3/s under normal conditions to several thousand m3/s during floods (2550 m3/s for the Novembre 2005 flood). Winds blow with remarkable intensity over 280 days a year. They are generally unidirectionnal (N to NNE) with variable speeds depending on the season. Mean wind speeds commonly exceed 12 m/s during storms (fig. 1C) from April to July, which are the windiest months.

6The wave regime is one of moderate fetch (6 to 10 s), generated by North Atlantic storms. Summer significant wave heights are 1 to 3 m, and the wave regime does not show a strong link with the strong summer wind regime. The highest waves can exceed 5 m in autumn. The dominant wave direction is from NNW, followed by NW (fig. 1C). The northerly wave approach, reinforced by winds from the same sector, generates strong and permanent longshore drift to the south, except within Essaouira bay where waves are refracted and diffracted by the port breakwater and Mogador Island (Elmimouni, 2009). The coast of Essaouira experiences a semi-diurnal tide with a spring tidal range of about 4 m. Essaouira beach is characterised by a relatively low-sloping (0.4 to 0.8%) nearshore zone. At low tide, the beach ranges in width from 300 to 600 m. The mouth of the Ksob experiences relatively weak tidal intrusion, generally not exceeding 3 km during spring tides.

Methods

7Eight topographic field surveys were conducted between June 2004 and June 2006 using a very high-resolution LIECA TC600 electronic total station. 12 cross-shore profiles were established between the northern extremity of the beach and the north bank of the Ksob, and three others on the open beach south of the Ksob (fig. 1C). Profiles went from the promenade wall or the dune to the lowest part of the beach at the time of the surveys, carried out at low tide. Aerial surveys aimed at establishing digital elevation models (DEMs) were also conducted. Profile and DEM data were treated using Surfer (Golden Software). The error margin of each profile and DEM was calculated on the basis of the length of each profile or area covered by the DEM. This error margin is 5% and represents the sum of instrument, operational and analytical uncertainties.

Results

8Six profiles have been selected to highlight the evolution of the beach during a year without flooding by the Ksob (June 2004 to June 2005, fig. 2).

Fig. 2 – Selected beach profiles over the period June 2004 to June 2005.
Fig. 2  Profils représentatifs de la plage sur la période de juin 2004 à juin 2005.

Fig. 2 – Selected beach profiles over the period June 2004 to June 2005. Fig. 2 – Profils représentatifs de la plage sur la période de juin 2004 à juin 2005.

MHWS = Mean high water springs; MHWN = Mean high water neaps; ML = Mean level; MLWN = Mean low water neaps. None of the profiles covered Mean Low Water Springs.
MHWS = Pleines mers de vive-eau ; MHWN = Pleines mers de morte-eau ; ML = Niveau moyen ; MLWN = Basses mers de morte-eau. Aucun des profils n’a été étendu au niveau des basses mers de vive-eau.

9The profiles summarize the seasonal and spatial trends. The year 2004-2005 was exceptionally dry, and the mean maximum discharge of the Ksob did not exceed 59 m3/s; this maximum discharge was recorded in February 2005. As a result, the sediment input of the Ksob to Essaouira bay was virtually nil. The year 2005-2006 was, in contrast to the preceding one, characterized by two exceptional floods, on 29th November 2005, with a peak discharge of 2550 m3/s, and on 25th December with a peak discharge of 1553 m3/s. Given the fact that wadi Ksob represents the main immediate source of sediments for Essaouira beach, floods of this magnitude must strongly influence the morphology of the beach because of the important amount of sand they mobilize. The evolution of the beach over these two contrasting years is thus synthesized in DEMs enabling comparison and insight into the potential influence of the wadi on the beach.

Beach profiles

10Situated at the northern end of the beach well behind the port breakwater, profile 1 (fig. 2) runs up to the promenade wall, covering a narrow beach that is completely submerged at high tide. The beach here is thus situated below the level of mean high water neap tides (MHWN). In this relatively sheltered part of the beach, topographic variations over the survey period were quite weak, and mainly concerned the lower beach below the mean level (ML). Profile 4 (fig. 2) also runs up to the sea wall. The most important morphologial changes concerned the upper beach between MHWN and MHWS (Mean High Water Spring) levels, a zone characterized by a well developed berm mildly reworked by aeolian action. As in profile 4, the most important variations in profile 6 (fig. 2) essentially concerned the upper beach, between MHWN and MHWS, and were related to active aeolian forms, notably small barchans. As in the previous profile, accretion occurred in autumn-winter and erosion in spring. Profile 10 was morphologically the most stable of all the profiles (fig. 2). The upper beach, which merges into a dune front stabilized by vegetation, showed no significant topographic variations, except for a single phase of significant accretion in spring, between April and June 2005. Profile 13 (fig. 2), on the south bank of the Ksob, extends 340 m from a stabilized dune front to the Portuguese fort. The upper beach is a 200 m wide complex dune field composed of barchans, star dunes and transverse dunes. It showed remarkable morphological changes under the action of these dunes, whereas the lower beach below MHWN showed more subdued changes throughout the survey period. The general morphology of this profile evolved constantly especially in the upper beach zone of barchans. Profile 15 (fig. 2), located 650 m south of profile 13, represents conditions facing the open ocean (fig. 1C). The beach morphology in this sector exhibited net accretion throughout the study period. The upper and mid-beach zones were characterised by small barchans 20 to 50 cm high and profile variations from one survey to the next were due essentially to the formation and mobility of these forms.

Digital elevation models

11Both the DEMs representing the years 2004-2005 and 2005-2006 showed relatively mild overall change, with the exception of the wadi mouth sector where change was more prominent (fig. 3).

Fig. 3 – Digital elevation models synthesizing the overall beach sediment budget.
Fig. 3  Modèles numériques de terrain synthétisant le bilan volumétrique global de la plage.

Fig. 3 – Digital elevation models synthesizing the overall beach sediment budget. Fig. 3 – Modèles numériques de terrain synthétisant le bilan volumétrique global de la plage.

A: between June 2004 and June 2005; B: between June 2005 and June 2006.
A : entre juin 2004 et juin 2005 ; B : entre juin 2005 et juin 2006.

12Over the year 2004 to 2005 (fig. 3A), the northern end of the beach, represented by profiles 1 et 2, showed mild accretion. South of this sector, the beach shows a linear band of mild erosion from profile 3 to profile 6. Accretion prevailed further south (from profile 6 to profile 9), whereas the lower beach showed mild erosion. The environs of the wadi mouth showed dominant erosion. In the field, this led to the exposure of gravel in the bed of the wadi. Overall, the beach underwent a net sediment gain (+637 m3). The year 2005-2006 (fig. 3B) was characterised by net mild accretion over much of the beach, with a net contrast, however, in the wadi mouth sector which underwent net erosion. The erosion in this sector explains the sediment deficit (-440 m3) over this year.

Discussion and conclusion

13The topographic variations exhibited by Essaouira beach highlight a dynamic pattern that is relatively well defined in space and time. Profile 1 represents the lowest energy sector of the bay where the upper beach directly adjoins the sea wall. Alternations of erosion and accretion in this sector are controlled by seasonal wave energy fluctuations. Change in this sheltered sector, where the upper beach is systematically submerged at high tide, and which is not affected by wind action, is thus forced by waves and wave-induced currents: accretion during relatively fair weather summer conditions (June to September) and erosion in winter (October to February). Profile 4 marks the zone of onset of aeolian influence on Essaouira beach, important throughout the rest of the beach further south. Beach change is forced by wave activity and sand movement between the mid- and lower beach, and is frequently manifested by swash bar migration, and by a dominant aeolian regime on the upper beach where much of the overall morphological and volume change is controlled by nascent barkhanes (fig. 4). This aeolian influence gains momentum towards the mouth of the Ksob as the wind fetch increases (Elmimouni, 2009). Profiles 10 and 13 flank the wadi mouth which is characterised by intense aeolian activity (fig. 5) that generates marked mobility of the wadi channel.

Fig. 4 – Ground photograph showing migrating barkhans being reworked by waves during the rising tide.
Fig. 4  Photographie montrant des barkhanes migrantes en voie de remaniement par la houle lors de la marée montante.

Fig. 4 – Ground photograph showing migrating barkhans being reworked by waves during the rising tide.Fig. 4 – Photographie montrant des barkhanes migrantes en voie de remaniement par la houle lors de la marée montante.

Fig. 5 – Ground photograph showing the important accumulation of dune sand at the mouth of wadi Ksob near the remnants of the Portuguese fort (in the left).
Fig. 5  Photographie montrant une accumulation dunaire importante au niveau de l’embouchure de l’oued Ksob près des ruines du fort portugais (visibles à gauche).

Fig. 5 – Ground photograph showing the important accumulation of dune sand at the mouth of wadi Ksob near the remnants of the Portuguese fort (in the left). Fig. 5 – Photographie montrant une accumulation dunaire importante au niveau de l’embouchure de l’oued Ksob près des ruines du fort portugais (visibles à gauche).

14Profile 15, representing the exposed beach south of the Ksob, is not directly affected by the wadi mouth but subject to important aeolian sand flux mobilised over the surface of the massive south bank sand accumulation. The beach south of the Ksob is therefore an area of important aeolian sand accumulation. It benefits from a long aeolian fetch wherein maximum sand mobilisation occurs at around profile 14, which showed a net loss of over 100 m3 over the period 2004-2005. Beach profile variations are much more important in the southern part of the bay, in the wadi mouth sector and the more exposed beach south of the wadi. The wadi mouth sector is extremely complex, affected by both intense aeolian activity and flash floods.

15The net aeolian sand mobility towards the south of the bay does not appear to affect the stability of the beach to the north of profile 4. In contrast, despite this southward sand mobility, the wadi mouth shows a net sediment loss over the two-year survey period (fig. 3). In 2005-2006, the wadi mouth sector also underwent more significant erosion than in 2004-2005, thus illustrating the intensity of the sand flushing effect caused by the two floods at the end of 2005. Beach erosion in the course of these events prevailed up to profile 8. Aeolian input from the beach north of this profile did not succeed in compensating for this erosion.

16The sand mobility on this beach implies a "rotation" process that is not just under the command of waves. This circulation pattern is schematically depicted in figure 6.

Fig. 6 – Schematic sediment circulation pattern in Essaouira bay.
Fig. 6  Schéma synthétique de la circulation sédimentaire au sein de la baie d’Essaouira.

Fig. 6 – Schematic sediment circulation pattern in Essaouira bay. Fig. 6 – Schéma synthétique de la circulation sédimentaire au sein de la baie d’Essaouira.

The pattern is under the joint command of waves, aeolian activity and flash floods affecting the mouth of Wadi Ksob.
La circulation sédimentaire est commandée conjointement par la houle, l’activité éolienne et des crues brusques affectant l’embouchure de l’oued Ksob.

17Beach rotation most commonly reflects changes in wave approach direction. These may occur on a seasonal basis (e.g., Norcross et al., 2002; Sedrati and Anthony, 2007; Jeanson et al., 2013) or generated by multi-annual meteorological ENSO (Ranasinghe et al., 2004; Short et al., 2004) or NAO-type (Thomas et al., 2010) oscillations, or even by bathymetric changes induced by large alongshore migrating mud banks (Anthony and Dolique, 2004). The rotation on Essaouira beach involves two extra sources of sand mobility: active aeolian mobility of sand towards the wadi mouth, and flash floods that redistribute both wind-transported and fluvial sand. The gradual accretion and build-up of the wadi-mouth storage zone, notably via spit platform growth, is periodically interrupted by high-energy flash floods that redistribute sand offshore. This is an important process, as it ensures redistribution of sand trapped southwards in the wadi mouth area as a result of the constant winds blowing from the north. Sand is further abstracted from the spit platform and transported by wind towards the open beach south of Essaouira bay. The recovery of Essaouira beach should, therefore, occur via wave-induced onshore transport, with redistribution towards the lower-energy northern sector of the beach via longshore drift induced by wave refraction and diffraction by Mogador Island, with subsequent wave energy gradients in the bay. This conforms to the classic process of wave-induced sand redistribution, but which is only a part of the picture on Essaouira beach. It is this drift direction that has determined the northward orientation of the massive sand spit platform on the south bank of wadi Ksob, whereas the regional longshore drift direction on the Atlantic beaches of Morocco is to the south (Elmimouni, 2009).

Top of page

Bibliography

Anthony E.J. (2009)Shore Processes and their Palaeoenvironmental Applications. Developments in Marine Geology Volume 4. Elsevier Science, Amsterdam, 519 p.

Anthony E.J., Dolique F. (2004) – The influence of Amazon-derived mud banks on the morphology of sandy, headland-bound beaches in Cayenne, French Guiana: A short- to long-term perspective. Marine Geology 208, 249-264.

Barnard P.L., Warrick J.A. (2010) – Dramatic beach and nearshore morphological changes due to extreme flooding at a wave-dominated river mouth. Marine Geology 271, 131-148.

Chahboun A. (1988)Les formations sableuses fluviales; littorales et éoliennes aux embouchures des oueds Tensift, Ksob et Sous (Maroc). Thèse de Doctorat, Université de Paris VI, 187 p.

Elmimouni A. (2009) - Approche expérimentale de suivi de la dynamique morphosédimentaire du littoral d’Essaouira, Maroc : implications pour un aménagement raisonné. Thèse de Doctorat, Université du Littoral Côte d'Opale, Dunkerque, 248 p.

Elmimouni A., Daoudi L., Saidi M.E., Baiddah A. (2010) – Comportement hydrologique et dynamique d’un bassin versant en milieu semi-aride : exemple du bassin versant du Ksob (haut atlas occidental, Maroc). Cuaternario y Geomorfologia 24, 99-112.

Inman D.L., Jenkins S.A. (1999) – Climate change and the episodicity of sediment flux of small California rivers. Journal of Geology 107, 251-270.

Jeanson M., Anthony E.J., Dolique F., Aubry A. (2013) – Wave characteristics and the morphology of pocket beaches fronted by a coral reef-lagoon system, Mayotte Island, Indian Ocean. Geomorphology 182, 190-209.

Norcross Z.M., Fletcher C.H., Merrifield M. (2002) – Annual and interannual changes on a reef-fringed pocket beach: Kailua Bay, Hawaii. Marine Geology 190, 553-580.

Pranzini E., Rosas V. (2009) – Pocket beach response to high magnitude-low frequency floods (Elba Island, Italy). Journal of Coastal Research Special Issue 50, 969-977.

Pranzini E., Rosas V., Jackson N., Nordstrom K.F. (2013) – Beach changes from sediment delivered by streams to pocket beaches during a major flood. Geomorphology 199, 36-47.

Ranasinghe R., McLoughlin R., Short A., Symonds G. (2004) – The Southern Oscillation Index, wave climate, and beach rotation. Marine Geology 204, 273-287.

Sedrati M., Anthony E.J. (2007) – A brief overview of plan-shape disequilibrium in embayed beaches: Tangier Bay, Morocco, revisited. Méditerranée 108, 125-130.

Short A.D., Trembanis A.C. (2004) – Decadal scale patterns in beach oscillation and rotation Narrabeen Beach, Australia - time series, PCA and wavelet Analysis. Journal of Coastal Research 20, 523-532.

Thomas T., Philips M.R., Williams A.T. (2010) – Mesoscale evolution of a headland bay: Beach rotation processes. Geomorphology 123, 129-149.

Top of page

Annex

Version française abrégée

Les interactions morphosédimentaires entre les embouchures des cours d’eau et les plages adjacentes restent encore mal connues. Elles sont contrôlées par le régime hydrologique du cours d’eau, par le régime de houle et de courants côtiers, ainsi que par le vent dans les zones à activité éolienne notable. Les apports les plus conséquents de sédiments en charge de fond sont associés aux crues des oueds en marge du domaine aride, dont une grande irrégularité, une brutalité et une intensité épisodique sont les marques caractéristiques. L’exemple présenté dans ce travail porte sur le fonctionnement morpho-sédimentaire de la plage de la baie d’Essaouira, sur le littoral atlantique marocain à fort développement dunaire (fig. 1). La plage est bordée sur une grande partie par le front urbain de la ville d’Essaouira, dont le port et ses installations constituent la fermeture nord de la baie, et l’embouchure de l’oued Ksob la fermeture sud. La connexion dunaire directe avec la plage se fait tout près de l’embouchure de l’oued Ksob. La baie est largement protégée des houles du large par l’île de Mogador. La plage est caractérisée par une morphologie de proche avant-côte assez plate. À basse mer, l’estran est dégagé sur 300 à 600 m et la pente intertidale est très faible (0,4 à 0,8 %). Le sable est fin et bien trié. L’oued Ksob draine un bassin versant de 1 480 km2, avec un pic d’altitude de 1 694 m, et son lit a une pente moyenne de 1,5 % (Elmimouni et al., 2010). L’embouchure de l’oued est caractérisée par une avancée sableuse très marquée dont l’origine est liée conjointement à la charge sableuse remaniée par l’oued et aux accumulations de sables éoliens provenant du massif dunaire au nord-est. À part des apports propres du Ksob en période de crue, le lit de l’oued constitue donc un piège important de sables dunaires en transit vers le sud, notamment en période de sécheresse (Chahboun, 1988). Toutefois, dès la première crue importante, le lit encombré est évacué. Les panaches de turbidité associés à ces crues montrent que les sédiments charriés par l’oued sont en grande majorité évacués et stockés au niveau de l’avant-côte dans la baie d’Essaouira dans des fonds allant de 0 à -5 m. L’amortissement très rapide du jet fluvial conduit à des dépôts mal triés par excès de charge sur des distances faibles de l’ordre de 100 à 200 m (Elmimouni, 2009). Les précipitations moyennes au niveau du bassin du Ksob ne dépassent guère 295 mm par an, et présentent une irrégularité intra-annuelle et interannuelle très marquée. Ceci se traduit par des crues violentes, parfois dévastatrices mais surtout imprévisibles, avec une charge solide très importante. En effet, le débit liquide peut passer de quelques m3/s en période normale à des milliers de m3/s lors d’une crue (2 550 m3/s pour la crue de novembre 2005). Les vents soufflent avec une intensité remarquable pendant 280 jours par an. Ils sont généralement unidirectionnels (N à NNE) avec des vitesses variables en fonction des saisons. Les vitesses peuvent dépasser le seuil de 12 m/s pendant des tempêtes d’avril à juillet, mois les plus venteux. Le littoral d’Essaouira est affecté par une houle avec des fréquences variables selon les périodes de l’année. Il s’agit d’une houle à fetch modéré (de 6 à 10 s) générée par des tempêtes dans l’Atlantique nord. En été, la hauteur est comprise entre 1 et 3 m, ce qui correspond à la saison la moins agitée qui, elle, est donc non corrélée aux vents locaux forts de cette saison. Les hauteurs les plus fortes peuvent dépasser 5 m en automne. La direction générale dominante de la houle incidente est NNW, et secondairement NW (fig. 1C). Cette houle alimente en permanence une dérive littorale de direction N-S, un sens de transit renforcé par les vents constants du secteur nord. Toutefois, dès son arrivée à l’entrée de la baie d’Essaouira, la houle est fortement réfractée et diffractée du fait de l’île de Mogador et de la digue du port d’Essaouira (Elmimouni, 2009). Le littoral d’Essaouira subit une marée semi-diurne avec deux pleines mers et deux basses mers d'importance sensiblement égale. Le marnage maximal observé sur ce littoral dépasse rarement le seuil de 4 m, ce qui en fait un littoral mésotidal. L’influence de la marée affecte l’embouchure de l’oued Ksob sur une distance relativement faible, de deux à trois kilomètres sur le cycle morte-eau/vive-eau.

Un suivi topographique a été effectué à des intervalles de plusieurs mois de 2004 à 2006 afin de décrire le système de circulation sédimentaire caractérisant la plage, notamment sous l’influence de l’embouchure de l’oued Ksob. Les données mettent en évidence des variations morphologiques longitudinales et transversales de la plage. De façon générale, on remarque une ampleur plus nette des variations de la plage dans la partie sud de la baie, au niveau de l’embouchure de l’oued et au sud de celle-ci, zone plus exposée au vent et à la houle, et surtout sous influence de l’oued. La transition entre le secteur nord et le reste de la plage, représentée par le profil 4, marque le début d’une activité éolienne qui semble dominer la dynamique de la partie supérieure de la plage, induisant une variabilité spatio-temporelle du profil (fig. 2). Des vents constants du nord remanient le haut de plage pour former des barkhanes (fig. 4) qui migrent vers l’embouchure du Ksob au sud. Plus au sud, la zone de l’embouchure est très complexe et la rive sud, avec sa flèche très développée, montre une érosion généralisée qui traduit d’une part le transport éolien vers la plage ouverte au sud de l’oued (profil 15), et d’autre part le remaniement estuarien. Ce remaniement atteint son pic lors des crues brutales. En ce qui concerne l’influence éolienne, une bonne partie du sable transporté par cette voie sur la plage au nord de l’oued Ksob est ainsi piégée et évacuée vers le large au niveau de l’embouchure. La flèche est fortement exposée aux actions éoliennes. Le sable repris de la plateforme de cette flèche s’accumule sous forme de dunes embryonnaires dont une partie est piégée près d’une palissade érigée en aménagement. Ces dunes embryonnaires, très mobiles, prennent de l’importance et se déplacent rapidement vers le profil 15. Les crues importantes rejettent vers le large les sables s'accumulant au niveau de l’embouchure. Ces sables sont ensuite progressivement remaniés et transportés vers la plage par une dérive littorale générée par une houle très réfractée dans la baie. En 2005-2006, l’embouchure a subi une érosion beaucoup plus importante qu’en 2004-2005 (fig. 3), témoignant ainsi de l’intensité du déblaiement qui a accompagné les deux crues de la fin de l’année 2005.

La mobilité de la plage d’Essaouira décrit donc un processus de balancement, dit aussi de « rotation », qui diffère toutefois des schémas de rotation classique dépendant uniquement de variations de la direction de la houle, qui peuvent être saisonnières (e.g., Norcross et al., 2002 ; Sedrati et Anthony, 2007 ; Jeanson et al., 2013) ou impulsés par des oscillations météorologiques pluri-annuelles de type ENSO (Ranasinghe et al., 2004 ; Short et al., 2004) ou NAO (Thomas et al., 2010), ou encore induits encore par d’énormes bancs de vase sur le littoral sous influence amazonienne d’Amérique du Sud (Anthony et Dolique, 2004). Dans la baie d’Essaouira, ce processus de rotation, synthétisé dans la figure 6, implique non seulement la redistribution opérée par la houle, mais aussi une activité éolienne significative et des crues d’oued subites de haute énergie. Ce schéma implique un recyclage éolien unidirectionnel de sables vers le sud, un stockage de ces sables dans le giron de l’embouchure de l’oued Ksob, suivi, lors des crues importantes, d’une exportation de ces sables, ainsi que des sables fluviatiles frais, vers l’avant-côte. Le ré-équilibrage, dans ce contexte de plage de baie, est alors opéré par la houle très réfractée qui induit des transferts vers la plage et le long de celle-ci du sud vers le nord. C’est cette dérive qui a imprimé l’orientation nord de la flèche de rive sud de l’embouchure du Ksob, dans un contexte où les plages ouvertes au nord et au sud de la baie d’Essaouira sont assujetties à une dérive littorale bien marquée vers le sud (Elmimouni, 2009).

Top of page

List of illustrations

Title Fig. 1 – Location of Essaouira bay. Fig. 1  Le site d’Essaouira.
Caption A: Essaouira bay on the coast of Morocco; B: the Wadi Ksob catchmen; C: map of Essaouira beach showing profile locations and local wind and wave roses. A : la baie d’Essaouira sur la côte marocaine ; B : le bassin-versant de l’oued Ksob ; C : carte de la plage d’Essaouira montrant la localisation des profils ainsi que les roses de vents et de houles.
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-1.png
File image/png, 179k
Title Fig. 2 – Selected beach profiles over the period June 2004 to June 2005. Fig. 2  Profils représentatifs de la plage sur la période de juin 2004 à juin 2005.
Caption MHWS = Mean high water springs; MHWN = Mean high water neaps; ML = Mean level; MLWN = Mean low water neaps. None of the profiles covered Mean Low Water Springs.MHWS = Pleines mers de vive-eau ; MHWN = Pleines mers de morte-eau ; ML = Niveau moyen ; MLWN = Basses mers de morte-eau. Aucun des profils n’a été étendu au niveau des basses mers de vive-eau.
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-2.png
File image/png, 256k
Title Fig. 3 – Digital elevation models synthesizing the overall beach sediment budget. Fig. 3  Modèles numériques de terrain synthétisant le bilan volumétrique global de la plage.
Caption A: between June 2004 and June 2005; B: between June 2005 and June 2006.A : entre juin 2004 et juin 2005 ; B : entre juin 2005 et juin 2006.
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-3.png
File image/png, 1.4M
Title Fig. 4 – Ground photograph showing migrating barkhans being reworked by waves during the rising tide.Fig. 4  Photographie montrant des barkhanes migrantes en voie de remaniement par la houle lors de la marée montante.
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-4.png
File image/png, 1.6M
Title Fig. 5 – Ground photograph showing the important accumulation of dune sand at the mouth of wadi Ksob near the remnants of the Portuguese fort (in the left). Fig. 5  Photographie montrant une accumulation dunaire importante au niveau de l’embouchure de l’oued Ksob près des ruines du fort portugais (visibles à gauche).
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-5.png
File image/png, 392k
Title Fig. 6 – Schematic sediment circulation pattern in Essaouira bay. Fig. 6  Schéma synthétique de la circulation sédimentaire au sein de la baie d’Essaouira.
Caption The pattern is under the joint command of waves, aeolian activity and flash floods affecting the mouth of Wadi Ksob. La circulation sédimentaire est commandée conjointement par la houle, l’activité éolienne et des crues brusques affectant l’embouchure de l’oued Ksob.
URL http://journals.openedition.org/geomorphologie/docannexe/image/10682/img-6.png
File image/png, 502k
Top of page

References

Bibliographical reference

Abdelhadi Elmimouni, Lahcen Daoudi and Edward J. Anthony, Morphological change on a wadi-influenced beach: Essaouira, Morocco Géomorphologie : relief, processus, environnement, vol. 20 - n° 3 | 2014, 243-250.

Electronic reference

Abdelhadi Elmimouni, Lahcen Daoudi and Edward J. Anthony, Morphological change on a wadi-influenced beach: Essaouira, Morocco Géomorphologie : relief, processus, environnement [Online], vol. 20 - n° 3 | 2014, Online since 01 January 2016, connection on 28 March 2024. URL: http://journals.openedition.org/geomorphologie/10682; DOI: https://doi.org/10.4000/geomorphologie.10682

Top of page

About the authors

Abdelhadi Elmimouni

Laboratoire de Géosciences et Environnement – Faculté des Sciences et Techniques de Marrakech – B.P.549 – Marrakech – Maroc (abdel_elmimouni@yahoo.fr).

Lahcen Daoudi

Laboratoire de Géosciences et Environnement – Faculté des Sciences et Techniques de Marrakech – B.P.549 – Marrakech – Maroc.

By this author

Edward J. Anthony

Aix-Marseille Univ. – Institut Universitaire de France – CEREGE, UMR 34 - Europôle Méditerranéen de l'Arbois – BP 80 – 13545 Aix-en- Provence Cedex 04 - France.

By this author

Top of page

Copyright

The text and other elements (illustrations, imported files) are “All rights reserved”, unless otherwise stated.

Top of page
Search OpenEdition Search

You will be redirected to OpenEdition Search