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