The Northwestern Mediterranean Coast of Tunisia: Wave Processes, Shoreline Stability and Management Implications Nabila Halouani, Moncef Gueddari & Omran Frihy
Arabian Journal for Science and Engineering ISSN 1319-8025 Volume 38 Number 7 Arab J Sci Eng (2013) 38:1851-1860 DOI 10.1007/s13369-012-0401-4
1 23
Your article is protected by copyright and all rights are held exclusively by King Fahd University of Petroleum and Minerals. This eoffprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.
1 23
Author's personal copy Arab J Sci Eng (2013) 38:1851–1860 DOI 10.1007/s13369-012-0401-4
RESEARCH ARTICLE - EARTH SCIENCES
The Northwestern Mediterranean Coast of Tunisia: Wave Processes, Shoreline Stability and Management Implications Nabila Halouani · Moncef Gueddari · Omran Frihy
Received: 27 January 2011 / Accepted: 27 December 2011 / Published online: 12 January 2013 © King Fahd University of Petroleum and Minerals 2013
Abstract The 10-km long coastline of the Tabarka–Berkoukech on the northwestern Mediterranean coast of Tunisian is characterized by marked morphological features, including embayments, long extensions, headlands, pocket beaches, sand dunes and sea-cliffs. The undulated nature of this coastline has permit sand, originated from three rivers, to confine between headlands, embayments, pocket beaches and the downdrift leeside of Tabarka Harbor breakwaters. Wave-related coastal processes simulated by STWAVE model together with the analysis of aerial photographs are jointly combined to evaluate degree of stability of this coastline. Analysis of aerial photographs dated 1963 and 2001 (38 years) using the Digital Shoreline Analysis System document significant beach changes in the sandy beaches. Maximum accretion occurs downdrift of Tabarak Port (3.8 meter/year), followed eastward by a minor erosion of −1.5 meter/year at El Morjene beach. The morphological characteristics of this coastline exhibit a wide range of beach dynamics resulted from interactions of waves, shoreline orientation and sediment supply. Pattern of longshore sediment transport direction has been deduced from the interplay between incident waves versus average shoreline orientation (angle of incidence). Results indicate that the W and NW waves are responsible for the generation of the sediment transport to the east throughout the year. This transport process is also accompanied with a net westerly reversals induced from the N and NE waves. Such wave reversibility,
in general, generates a long-term equilibrium between the opposing east and westward sand movement within the coastline, yielding to an effective zero net littoral drift with a minimum sand loss resulting in stable beaches. Collectively, results obtained are jointly discussed in the context of sustainable management of this coastline. Keywords Waves simulation · Longshore drift · Beach changes · Wave energy dissipation · Wave propagation · Wave refractive · Shoreline management
N. Halouani (B) · M. Gueddari Department of Geology, Faculty of Sciences, Tunis El Manar University, Campus Universitaire, 2092 El Manar, Tunis, Tunisia e-mail:
[email protected] O. Frihy Coastal Research Institute, 15 El Pharaana St., El Shallalat, Alexandria 21514, Egypt
123
Author's personal copy 1852
Arab J Sci Eng (2013) 38:1851–1860
1 Introduction In most of the Mediterranean shores, waves and currents generated are the most important factors in the transportation and deposition of littoral sediments which consequently induce morphodynamic changes in coastlines and the seabed configuration. Waves play an important role in the sedimentary transport when approaching the coast at an angle, particularly in the surf zone. As waves approach the coast, orthogonal feature generates are strongly modified by the combined effects of bathymetric configuration, refraction, shoaling and diffraction. These wave orthogonal changes result in energy dissipation or by an energy concentration. Understanding the various transport components of sediments and distinct areas of concentration/diminish of wave energy are useful for selecting the best sites, which are subjected to less wave energy, and, therefore, in planning and taking development decisions. Such knowledge is also essential in the establishment of setback distances for the safe construction of resort buildings and also in the design of shore-control structures including beach nourishment projects. Planners and designers are always interested in calm areas to execute coastal projects for safe anchorages, harbors and beaches suitable for swimming. In wave-dominated shores such as the Tunisian coastline, naturally sheltered areas such as leesides of headlands provide calm environments suitable for recreation beaches or harbors. In a similar way, numerous submerged bedrock exists parallel to the coastline act as a natural breakwater, resulting in a low-energy coastal area that may be appropriate for coastal development. This submerged bedrock is the seaward part of the Tyrrhenian sandstone ridges extend across the backshore. Such potentialities are discussed herein to explore them for future recreational development of the study coastline. The above-raised concepts are the main subject of this paper, with application to the western Mediterranean coast of Tunisia. Literature review has confirmed that previous investigations in this particular area are very limited. Field observations have reported significant coastal erosion along the Tabarka–Berkoukech coastline [1–3]. Main geomorphologic units of the coastal plain of Tunisia (aeolian, dunes, sea-cliffs, wetlands and lagoons) have been mapped by Oueslati [4,5]. Paskoff and Sanlaville [6] identified a series of sandstone ridges in the backshore of the study area. Grain size characteristics of the beach and dunes have been investigated by Boukaaba [7]. Seabed sediment texture in the nearshore zone has been mapped by Halouani et al. [8], in which coarse-tomedium sands are much dominated between 0 and 5 m water depth, followed seaward by medium-to-fine sediment. The present work, therefore, is the first attempt to integrate into coastal morphology, shoreline stability, driving forces and management implications along the Tabarka–Berkoukech
123
coastline of Tunisia. These, in turn, are discussed in the context of shoreline management guidelines and sustainability. Together, the mathematically simulated wave-related processes and the wave data consulted serve interpreting driving forces prevail in the region. Degree of shoreline stability is evaluated from the analysis of historical aerial photographs acquired in 1963 and 2001. 2 Study Area The Tunisian coast on the Mediterranean Sea is mainly composed of open rocky beaches, except to the east where relative sand is locally isolated between rocky headlands. The present study is primarily focused on the western coast of Tunisia from Tabarka Port to Berkoukech River (Fig. 1). The coastline, ∼10 km long, is oriented to the SW–NE and is backed by a wide range of landscape components. These include: discontinues shore-parallel calcareous arenite seacliffs (7–15 m height), carbonate-cemented siliciclastic sand dunes (4–12 m elevation), alluvial deposits and wadies [4]. Most of these components are partially green covered by indigenous plants (Fig. 2d, e). Dunes and sea-cliffs are jointly acting as a natural defense against wave processes. The seacliffs and dunes are yellowish–reddish in color and are composed of quartz with minor fragments of volcanic rocks in carbonate cement that ranges between 40 and 60 %. These rocks are entirely composed of Tyrrhenian sandstone formed approximately 125,000 years ago [4,5]. Part of these rocks extends underwater of the littoral zone forming a series of shore-parallel bedrock, as confirmed by field observations and from Google Earth satellite images. In addition to Tabarka Harbor (Fig. 2a), the coastline is interrupted by a pronounced series of rocky headlands separating embayments and bays covered by a veneer of medium sand ranges in mean grain size from 0.21 to 0.54 mm. Tabarka Island and Borj Arif cape are the most bulge rocky headlands in the region (Fig. 2a, b). The heavy urbanization of the western coast has resulted in the connection between Tabarka Island and the mainland (Figs. 1, 2a). The seafront side of Tabarka is considered as a principal resort beach in this region. The only source of sediment supplied to the sea in the study area is the sediment discharged from the three main rivers that flow seasonally from highlands in the backshore, namely El Kébir, Bouterfess and berkoukech. A total amount of 100.2 × 106 m3 of water and 596 × 103 tons of sediments are annually discharged from these rivers into the Mediterranean Sea [9]. The backshore flat between the beach and seacliffs is wide in places but absent in others. Sandy beaches ranges in width between 30 and 140 m. The coastline of this region has a unique aesthetic view and recreation potentials, including the white sand covers the sub-aerial beach, the green-covered dunes and sea-cliffs. Despite these recreation
Author's personal copy Arab J Sci Eng (2013) 38:1851–1860
1853
Fig. 1 Map of the study area in the northwestern Mediterranean coast of Tunisia showing major geomorphological units, seafloor bathymetry, sandy and rocky shores (modified from [5]). Position of the wave gauge used in this study is denoted as small star in the upper inset map
potentials, the coastline is relatively still under-developed and almost unpopulated with the exception of the Tabarka and El Morjene beaches (Figs. 1, 2a). The nearshore bathymetric contours are nearly parallel to the shoreline with a relatively steep sloping seabed. The depth contours at 5 and 10 m exist at a relatively closer distance from the coastline, being 350 and 700 m, respectively [8]. Seabed slope mostly has a narrow surf zone where waves break close to the shore and develop directly into an intense swash that runs up and down the beach surface. This condition combined with hazardous rip currents implies that the study beaches are unsafe for swimming. Alternatively, artificial swimming pools have been built in the beachfront zone of most of the recreation centers in the region.
3 Methodology Wave models have been developed to numerically solve the equations governing changes in wave height and direction (through processes such as refraction, shoaling and breaking). As offshore wave data sets become available, wave transformation models have become standard tools for evaluating changes in nearshore wave climate. Due to the complexities of surf zone hydrodynamics, sediment transport analysis techniques typically depend on information from wave models at the line of wave breaking. The well-known mathematical model, steady-state spectral wave (STWAVE) model, developed by the US Army Corps of Engineers
[10–15] is used in the present study to simulate wave refraction pattern. This model also calculates energy wave dissipation across the nearshore zone of the study area, in which wave breaking was calculated using the dissipation function developed by Battjes and Janssen [16], where D = 0.25Q b Fm (Hmax )2 ; D is the energy dissipation; Q b the percentage of waves breaking based on truncated Rayleigh distribution of wave height; and Fm is the mean frequency. The input data used to run this model incorporate the bathymetry grid with a spatial resolution of 50 m surveyed in 1881, shoreline position of 2001, and wave data measured west of Tabarka Harbor. Waves that arrive from the NW and NE sectors are chosen for running the model, with 3 and 7 m heights. The model was not calibrated or turned in any way for the Tabarka–Berkoukech application, but all numerical models are sensitive to the quality of the input data. In this study, wave climate is discussed in relation to its impact on beach morphology based on data measured between 1971 and 1980, using a wave gauge deployed at ∼100 m water depth to the west of Tabarka Island (Lat: 09◦ −10◦ N, Long: 37◦ −38◦ E) (Fig. 1). Wave data were provided by the Agency of Littoral Protection of Tunisia. To quantify beach stability and changes in the shoreline positions along the Tabarka–Berkoukech stretch, analysis was undertaken using digitized orthorectified aerial photographs acquired in 1963 and 2001, at scale 1:25,000 and 1:20,000. This analysis was carried out using Geographic Information System (GIS). The digitized orthorectified aerial
123
Author's personal copy 1854
Arab J Sci Eng (2013) 38:1851–1860
Fig. 2 Photographs showing prominent geomorphologic features in the study area (locations are shown in Fig. 1). a El Corniche beach formed downcoast of Tabarka Harbor experiencing low-energy environments because its sheltering effect from the NW waves. Submerged sand bars are predominated in the surfzone. b A group of rocky headlands separates pocket and embayment beaches. c Pocket beach formed
123
between headlands. d Embayment beach backed by rocky sea-cliffs. e Sand dunes partially fixed by endogenous plants. f Near symmetrical tombolo formation at Berkoukech beach indicates long-term equilibrium due to periodic reversals in sediment transport as a result of the effect of the NW and NE waves yielding to an effective zero net littoral drift. g Near symmetrical bulge beach, h Negro Cape
Author's personal copy Arab J Sci Eng (2013) 38:1851–1860
1855
photographs of 2001 were used as baseline image to correct the geometrical distortions of the 1963 photographs (Ministère de l’Agriculture Tunisien). The aerial photographs of 1963 were georeferenced, orthrectified and mosaicked by the ARCMAP GIS in projection UTM Zone N32, Datum Carthage. The average annual rates of shoreline change along the Tabarka–Berkoukech stretch have been consulted from the study of Halouani et al. [8]. In their study, rates of shoreline change were calculated using the ARCVIEW extension abbreviated as DSAS (Digital Shoreline Analysis System) developed by the USGS [17]. To map the variations in shoreline positions and calculate rate of changes, a hypothetical baseline was created more or less parallel to the present-day coastline geometry with a position of ∼30 m distance behind. A total of 180 equally spaced transects were extracted perpendicular to the baseline and spaced 50 m a part (Fig. 3c). The distance at each transects measured between the baseline and the shoreline positions of the 1963 and 2001 aerial photographs provides a reliable data for quantifying beach changes over the 38 years time frame. The largest shoreline position errors were found to be within a margin of ±9 m, equals to 0.24 meter/year. 3.1 Coastal Processes Waves are the main hydrodynamic process acting along the Tabarka–Bouterfess coast, compared with the effect of tide, which has a maximum of 0.6 m tidal range [18]. This section focuses on discussing the relationships between wave approach and shoreline orientation, which in turn is the most important tools for assessing sediment movement along the study coastline. 3.2 Wave-Induced Currents Wave data consulted in this study comprise monthly averages proportions of wave height measured at eight directional sectors (Table 1). The statistical analysis of this data indicates marked seasonality in wave climate, intervening with storm waves in winter (>2–8 m height) and lower waves in other months (
