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Érosion, enjeux et aménagements

A brief overview of plan-shape disequilibrium in embayed beaches: Tangier bay (Morocco)

Mécanismes de déséquilibres longitudinaux au sein de plages de baie, baie de Tanger, Maroc
Mouncef Sedrati and Edward J. Anthony
p. 125-130

Abstracts

Embayed beaches commonly exhibit long-term equilibrium plan shape configurations that reflect either swash-alignment or drift-alignment (Davies, 1980) relative to the wave field, involving, respectively, virtually no longshore drift, and a balanced longshore drift over space and time. Within the context of an overall sediment budget in equilibrium, imbalances in longshore drift in embayed beaches are essentially generated by medium-term (order of several years) wave modifications in the incident wave field due to: (1) cyclic changes in deepwater wave approach directions caused by climatic oscillations; (2) human constructions, notably port breakwater structures. The latter type of engineered modification is perfectly exemplified by Tangier Bay. The beach in the eastern part of this bay has been steadily eroding over the past 50 years as a result of important port development in the western end of the bay that has perturbed waves from the Atlantic, generating, in particular, destabilization of a hitherto equilibrium drift-aligned system. The long-term resolution of problems generated by such perturbations requires a clear integrated management approach.

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Index terms

Geographical index :

Maroc, Tanger
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Author's notes

Constructive remarks by the anonymous reviewers have helped in improving the manuscript. Denis Marin is thanked for preparing the illustrations.

Full text

1 - Introduction

1Beaches are continuously mobile, undergoing accretion and erosion in response to natural oceanographic and sediment supply factors. However, open-coast beaches subject to longshore drift are generally much more sensitive to these changing conditions than embayed beaches which are generally bounded by cell boundaries that are commonly headlands, and very rarely river mouths. Embayed beaches are generally associated with a dominantly closed sediment circulation system, with cross-shore exchanges, but they may also experience sediment leakage around headlands or artificial structures (Short and Masselink, 1999). These beaches tend to develop equilibrium concave shapes that minimize longshore gradients in erosion or accretion (Silvester, 1960; Hsuet al., 1987, 1989). This can be attained in short beaches (hundreds of metres to a few kilometres long) through the long-term development of a swash-aligned configuration (Davies, 1980) wherein differences in refraction and wave incidence angles along the beach are minimal, so that net longshore drift gradients do not develop. Examples are common in both sandy (Short and Masselink, 1999) and gravel beach settings (Orfordet al., 2002). However, wave parameters and the way they are affected by headlands may be such that embayed beaches are subject to longshore drift gradients. These may occur either within the framework of seasonal beach ‘rotation’ (Short and Masselink, 1999), wherein the bay is subject to seasonal reversals in wave approach and therefore longshore drift, eventually with a balanced sediment budget, or may be neutralized by longshore gradients in beach clast sizes that minimize transport gradients (Carter, 1988). It is clear that in situations of a homogeneous longshore clast size, drift gradients must not persist in space and time since this would generate longshore imbalances in beach sediment budget and therefore entrain differential erosion and accretion, ultimately resulting in a disequilibrium plan shape.

2The beach in Tangier Bay, Morocco (Fig. 1), is a fine example of such a perturbation. Tangier Bay not only offered an ideal situation for the construction of Morocco’s second most important port at the junction between the Atlantic and the Mediterranean, but its embayed beach constitutes an important recreational and leisure outlet for both the citizens of the city of Tangier (population: 600,000) and tourists flocking to Morocco’s sunny coast. Although the bay is laterally bounded by headlands that result in a closed sediment cell, the beach in the eastern half of the embayment has suffered severe erosion due largely to construction of the port of Tangier under the strong pressures associated with the developing economy of Morocco. The beach morphology, sediment dynamics and rehabilitation schemes aimed at controlling the erosion of these beaches have generated several unpublished consultancy reports as well as published efforts (LCHF, 1974a, 1974b; CBC 1984; Long, 1998a, 1998b, 1999; Longet al., 1999; Sedrati and Raissouni, 2000; El Arrim, 2001; El Moumni et al., 2002; Snoussi and Long, 2002; Bouzidiet al., 2004; Sedratiet al., 2004), thus illustrating the economic importance of this bay and the necessity of finding adequate management solutions to beach erosion. The aim of this paper is not to reiterate the results of these various works but to briefly examine the dynamics underlying beach shoreline fluctuations in a classical embayed configuration using the example of Tangier Bay. In this paper, the focus is on interpretation of the longshore processes prevailing in the bay in terms of a fine example of human-induced perturbation of an embayed Mediterranean beach system.

Fig. 1 - Location (a) and close-up view (b) of Tangier Bay, Morocco.
Localisation (a) et vue approchée (b) de la Baie de Tanger, Maroc.

Fig. 1 - Location (a) and close-up view (b) of Tangier Bay, Morocco.Localisation (a) et vue approchée (b) de la Baie de Tanger, Maroc.

2 - From equilibrium to disequilibrium in Tangier Bay

3Tangier Bay is a 5 km-long embayment facing the Strait of Gibraltar (Fig. 1). The bay is limited by the sandstone headland of Jbel El Kébir in the west and by limestone marl formations that form Cape Malabata in the east. The bay comprises a sandy beach except at its eastern extremity which now consists of shingle. Present fluvial supply of beach-sized sediments into the bay comes from three wadis (Fig. 1), but only amounts to a total of 5000 to 10,000 m3 yr-1 (LCHF, 1974a, 1974b). The western extremity of the bay was chosen as a site for the construction of Morocco’s second most important commercial port in the 1950s, at the entrance to the Mediterranean. Tides affecting the bay are semi-diurnal and micro-tidal with a spring range of 1.4 m. As a result, the tidal influence is moderate. East to north-east winds are largely dominant on this coast during the summer season while north-west to south-west winds are frequent during the rest of the year. Because of its geographical location, Tangier Bay experiences a unique mixed wave regime consisting of a mixture of Atlantic swell and wind waves from west to north-west with peak periods of 5 to 15 s and significant heights up to 4 m, and shorter-fetch Mediterranean wind waves from east to north-east, generated by the synoptic winds, with periods of 4 to 7 s and significant heights that do not exceed 2 m.

4The Atlantic waves undergo strong refraction and diffraction in the western sector of the bay due to the port breakwater, while the shorter waves from the northeastern sector are less refracted. On the basis of incident wave statistics published by LCHF (1974a, 1974b) and Zakarya (1994), Bouzidiet al. (2004) calculated, using the NERC formula, the longshore drift potential in the bay, highlighting a disequilibrium bi-directional regime with an estimated annual east to west drift of about 430,000 m3 and a west to east drift of about 115,000 m3. This thus results in a residual annual westerly longshore drift of about 315,000 m3 (Bouzidiet al., 2004). The east to west aeolian sediment transport generated by the dominant east to north-east winds has been estimated at 36 m3 m yr-1 (Sedrati and Raissouni, 2000), and should be considered as an important source of sediment supply that further contributes to the surplus sediment budget in the western sector of the bay.

5The longshore imbalance is translated by marked variations in shoreline width that have been highlighted from ortho-rectified aerial photographs (Sedrati and Raissouni, 2000; El Arrim, 2001; El Moumniet al., 2002). These variations are summarized in Fig. 2. Shoreline retreat in the eastern part of the bay since 1958 has varied, on average, from 2 to 3 m yr-1. Over this same period, the beach in the western sector of the bay prograded by over 2 km, with rapid accretion in response to the important east to west longshore drift and aeolien sediment transport. Accretion of Municipal beach next to the port ranged from 3 to 6 m yr-1, this rate dropping to about 3 m yr-1 on Sanaa beach in the central sector of the bay. Although the rate of shoreline advance in the western sector has become less in the last few years, dropping from a high of 6 m yr-1 to a low of 3 m yr-1, the severe retreat of the beach in the eastern sector has led to the destruction of tourist and transport infrastructure and continues to pose a serious threat in this sector (Fig. 3). If this situation continues to go unchecked, it will irreversibly compromise the sea-front tourist vocation of the bay and of the city of Tangier, in the face of the massive demand for beach recreation that is today one of the strong drivers of the Moroccan economy.

Fig. 2 - Aerial photographs showing Tangier Bay beaches in (a) 1958, (b) 1963, (c) 1996; (d) shorelines identified following orth-rectification, and successive phases of the construction of Tangier harbour.
Photographies aériennes montrant les plages de la Baie de Tanger en (a) 1958, (b) 1963, (c) 1996 ; en (d) les traits de côtes identifiés après l’ortho-rectification des photographies aériennes et les phases consécutives d’aménagement du port de Tanger.

Fig. 2 - Aerial photographs showing Tangier Bay beaches in (a) 1958, (b) 1963, (c) 1996; (d) shorelines identified following orth-rectification, and successive phases of the construction of Tangier harbour.Photographies aériennes montrant les plages de la Baie de Tanger en (a) 1958, (b) 1963, (c) 1996 ; en (d) les traits de côtes identifiés après l’ortho-rectification des photographies aériennes et les phases consécutives d’aménagement du port de Tanger.

Fig. 3 - Ground photographs showing the erosion impact on Ghandouri beach (top) and on the Marbel residence (bottom). Photos by M. Sedrati, June 2000.
Deux photographies montrant l’impact de l’érosion sur la plage de Ghandouri (haut) et la résidence Marbel (bas). Photos de M. Sedrati, juin 2000.

Fig. 3 - Ground photographs showing the erosion impact on Ghandouri beach (top) and on the Marbel residence (bottom). Photos by M. Sedrati, June 2000.Deux photographies montrant l’impact de l’érosion sur la plage de Ghandouri (haut) et la résidence Marbel (bas). Photos de M. Sedrati, juin 2000.

3 - Discussion: disequilibrium in embayed beaches and the case of Tangier Bay

6The persistent imbalance in longshore drift has been clearly shown to be responsible for the shoreline variations in Tangier Bay (Sedrati and Raissouni, 2000; El Arrim, 2001; El Moumniet al., 2002; Bouzidiet al., 2004). However, the mechanisms underlying this imbalance have not been convincingly explained so far. The strongly embayed character of Tangier Bay suggests that there is insignificant longshore leakage of sand from the confines of the bay, while beach-offshore sand exchanges are expected to operate within the framework of a balanced long-term budget. Although there was already a tendency for a wider beach in the western sector, especially at the western extremity in the lee of the harbour breakwater, the bay shoreline exhibited a relatively balanced beach width in 1958 (Fig. 2). This suggests that the beach in Tangier Bay was largely a classical swash- to drift-aligned equilibrium embayment beach with a long-term adaptation to the bi-directional longshore drift. If it had existed in the past, the longshore drift imbalance prevailing today would have long since led to the disappearance of the eastern half of the beach.

7Within the context of a balanced overall embayment sediment budget (implying no permanent offshore leakage through submarine canyons, for instance (see Cohen and Anthony, this volume), and no lateral cell limit bypassing), imbalances in longshore drift in embayed beaches are due essentially to wave energy gradients alongshore and their eventual effects in terms of wave-generated currents. Longshore currents are by far the dominant, but by no means the sole manifestation, of these wave gradients, other potential manifestations being rip currents fed by such longshore currents (Short and Masselink, 1999). Longshore wave energy gradients may undergo medium-term (order of several years) modifications in one of at least three ways, rarely in combination: (1) commonly cyclic changes in deep-wave approach directions due to climatic oscillations; (2) cyclic changes in nearshore bathymetry due to large-scale intrusion of longshore migrating sediment bodies into the wave field affecting bays; and (3) perturbation of the wave field by human constructions, notably port breakwater structures. Type 1 modifications are common on beaches where seasonal changes in longshore drift occur, but the temporal scale of these variations and their regular cyclic nature are too short to durably imprint imbalances in longshore drift. They become much more important where medium-term changes in wave directions are caused by variations in climatic oscillations (e.g., Shyuer-Ming and Komar, 1994; Short and Masselink, 1999; Ranasingheet al., 2004). Type 2 modifications are much rarer and have only been reported from embayed beaches affected by large mud banks migrating along the Amazon-influenced coast of South America (Anthonyet al., 2002; Anthony and Dolique, 2004). Type 3 modifications are much more common, with disequilibrium and beach erosion engendered by development pressures (Anthony, 2005). Such beach erosion is not only an economic loss in its own right but may be a significant hazard along low densely populated coasts, especially in the face of rising sea level and climate change. Type 3 modifications are perfectly exemplified by Tangier Bay.

8Bouzidiet al. (2004) carried out a fine-scale analysis of drift directions in Tangier Bay and highlighted from 12 refraction diagrams a multiple cell system involving what they identified as ‘source’, ‘stable’ and ‘receiving’ cells, and related these cells to the wave parameters affecting the bay. These multiple cells are purported to explain longshore changes in beach morphology, and marked spatio-temporal variability in such morphology. This relationship, based solely on drift gradients identified from wave refraction maps, largely excludes, by and large, morphodynamic feedback and cross-shore beach morphodynamic processes which have been shown to be important in the sediment dynamics of embayed beaches (Short and Masselink, 1999). The multiplicity of such ‘cells’ must be expected to play a negative feedback role by disorganizing longshore drift within the bay, and annulling the significant accumulation that has characterized the western part of the bay to the detriment of the eastern part. This cell multiplicity is thus incompatible with a clear and regular pattern of progressive beach accumulation towards the west, with a maximum in the lee of Tangier harbour, the role of which is not clearly brought out in the refraction patterns observed by these authors.

9The east to west increase in shoreline accretion within the bay highlights the destabilizing influence of the port on the longshore distribution of the bay shoreline sediment budget through: (1) direct refraction and diffraction of Atlantic waves, and (2) longshore wave height gradients engendered by these effects. The destabilizing influence suggests a significant contribution of Atlantic waves to the disequilibrium in the bay budget and the ensuing strong residual drift from east to west to the benefit of the western port sector. Atlantic waves approaching from a west window would, in the past, have contributed to balancing longshore drift from the east generated by Mediterranean waves. In spite of being less frequent, the former are longer-fetch and higher-energy waves, compared to the latter. Since the construction of the harbour breakwater and its successive extensions, diffraction of Atlantic waves in the lee of this structure results in a relative ‘wave shadow’ zone that promotes counter-drift (relative to the wave approach direction from the west). This simple wave shadow effect has not been reported in previous studies, but it is a major mechanism in the modification of wave parameters and in engendering medium to long-term disequilibrium in embayed beaches, whatever the nature of the wave-field perturbation. There is therefore a simple case of drift divergence for Atlantic waves, somewhere to the east beyond the harbour breakwater, probably between Sanaa and Marbel beaches (Fig. 1). The refraction diagrams published by Bouzidiet al. (204) clearly indicate an east-west gradient in Atlantic wave breaker heights due to this wave shadow effect that contributes to drift to the west. This westerly drift complements sand drift generated by the more frequent but shorter and less refracted Mediterranean waves from the north-east.

10The diminished rates of shoreline accretion in the western sector over time may indicate the interplay of feedback mechanisms, one of which is a subtle change in shoreline orientation towards a weaker drift-alignment, although this would require a finer level of analysis of wave refraction and diffraction patterns, and the contribution of beach morphodynamic state changes. This implies that sediment release from the eastern sector is decreasing, as a result of depletion. This may also account for the shingle that now characterizes the eastern extremity of the bay, as sand has been evacuated from the system.

4 - Concluding remarks: the fate of Tangier Bay

11Types 1 and 2 perturbations of embayed beaches are generally reversible manifestations of longer-term, as opposed to seasonal, beach rotation (Short and Masselink, 1999). Whether wave modifications are caused by climatic oscillations or by massive mud banks on their way from the Amazon to the Orinoco, the embayed beaches generally undergo reversible longshore changes, as the source of perturbation of the wave field cyclically disappears. This is, unfortunately, not the case where such wave modifications are engendered by large-scale human constructions that are practically irreversible. Since the 1970s, several strategies to prevent further erosion of the beach in Tangier Bay and to restore acceptable beach widths have been proposed by LCHF (1974a, 1974b), CBC (1984), and INRS Canada (Longet al., 1999). These projects have only been partially implemented but without any success. Following hydrographic and other field surveys, LCHF (1974 a,b) proposed two rehabilitation projects that consisted in the construction of a series of groynes or breakwaters in the eastern part of the bay, but none of the two proposals was implemented. In 1987, the Moroccan authorities implemented the first part of the project proposed by CBC (1984). It consisted in building two groynes and one breakwater. Ten years later, the installed structures turned out to be inefficient in reducing erosion, let alone stopping it. The last project to date, proposed by INRS Canada (Longet al., 1999), consisted in nourishing the eastern sector with sand extracted from the western sector and in restructuring the existing defence works, notably by shortening the groynes and the breakwater in front of Tarik beach (Fig. 1). A critical analysis of this project shows that the nourishment and structural solutions proposed were not likely to overcome the erosion problem, since, on wider beaches with less reflective profiles and shortened groynes, longshore drift during storms is liable to induce strong surf zone sand transport to the west, resulting in rapid loss of the nourished material (Sedratiet al., 2004).

12It is very likely that at this advanced stage of erosion, soft solutions alone such as beach nourishment are not enough to solve the problem. The strong development pressures that generate much needed income for the burgeoning tourist economy of Morocco need to be balanced by a proper management strategy involving a good understanding and an integrated view of beach and coastal dynamics, as well as defence strategies covering not only the beaches but also the dunes and the nearshore zone. This can only be effectively done within the framework of the immediate implementation of Integrated Coastal Zone Management, not just for the beleaguered beach of Tangier Bay, but for the whole of the 5000 km-long Moroccan coastline.

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List of illustrations

Title Fig. 1 - Location (a) and close-up view (b) of Tangier Bay, Morocco.Localisation (a) et vue approchée (b) de la Baie de Tanger, Maroc.
URL http://mediterranee.revues.org/docannexe/image/190/img-1.jpg
File image/jpeg, 88k
Title Fig. 2 - Aerial photographs showing Tangier Bay beaches in (a) 1958, (b) 1963, (c) 1996; (d) shorelines identified following orth-rectification, and successive phases of the construction of Tangier harbour.Photographies aériennes montrant les plages de la Baie de Tanger en (a) 1958, (b) 1963, (c) 1996 ; en (d) les traits de côtes identifiés après l’ortho-rectification des photographies aériennes et les phases consécutives d’aménagement du port de Tanger.
URL http://mediterranee.revues.org/docannexe/image/190/img-2.jpg
File image/jpeg, 84k
Title Fig. 3 - Ground photographs showing the erosion impact on Ghandouri beach (top) and on the Marbel residence (bottom). Photos by M. Sedrati, June 2000.Deux photographies montrant l’impact de l’érosion sur la plage de Ghandouri (haut) et la résidence Marbel (bas). Photos de M. Sedrati, juin 2000.
URL http://mediterranee.revues.org/docannexe/image/190/img-3.jpg
File image/jpeg, 77k
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References

Electronic reference

Mouncef Sedrati and Edward J. Anthony, « A brief overview of plan-shape disequilibrium in embayed beaches: Tangier bay (Morocco) », Méditerranée [Online], 108 | 2007, Online since 01 January 2009, connection on 26 March 2017. URL : http://mediterranee.revues.org/190 ; DOI : 10.4000/mediterranee.190

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About the authors

Mouncef Sedrati

Coastal Geomorphology and Shoreline Management Unit - EA 3599, Université du Littoral Côte d’Opale, MREI 2 - 189a, avenue Maurice Schumann 59140 Dunkerque, France - mouncef.sedrati@univ-littoral.fr

Edward J. Anthony

Coastal Geomorphology and Shoreline Management Unit - EA 3599, Université du Littoral Côte d’Opale, MREI 2 - 189a, avenue Maurice Schumann 59140 Dunkerque, France - anthony@univ-littoral.fr

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