3.2.6.1. Beach classification and transition
Wright et Short (1984) developed a single-bar beach model for wave-dominated coasts from Australian beaches with 6 beach states. This model was completed by Short et Aagaard (1993), extending it for 2- or 3-bar systems. Previous studies on Gulf of Lions beaches have enabled the description of complex bar typologies and transitions over short distances, increasing the knowledge of bar patterns (Ferrer et al., 2009; Aleman et al., 2011). On a regional scale, this present study shows a great diversity of beach states with many morphologically continuous areas over several kilometres. These bar-beach systems are classic with respect to the literature, either from a morphological (this study) or a dynamical point of view (Aleman et al., 2013). However, two unusual beach states can be highlighted.
First, the “subtidal rocky platform” class (19.5% of the coastline), presents a steep beach face following by a seaward bedrock outcrop that can nourish the berm with pebbles and cobbles.
This state evolves in a reduced sediment environment with a narrow sandy prism. Despite this, 1 or 2 straight bars are present in this confined nearshore zone. These bars are small and eventually a “Net Offshore Migration” can occur at a very slow migration rate (Aleman et al., 2013). The offshore rocky plateau is wide and shallow (-10 m) and could explain this relative slowness by attenuating strong waves further offshore due to the outcropping bedrock (Anfuso et al., 2003; Sabatier et al., 2004). Beaches with bedrock/rock flats/reef are common systems in the literature (Sanderson et Eliot, 1999; Short, 2006), but the presence of bars is not mentioned. This often-observed state in the Languedoc-Roussillon is morphologically original, as is the control exerted from the bedrock on bars and beachface characteristics.
Secondly, despite the fact that the “non-barred dissipative” class is represented in only a short part of the Languedoc-Roussillon, this state is unusual in a microtidal environment. This concave featureless profile is mainly identified in meso- and macro-tidal environments (Short,
Hegge et al. (1996), sediment size apparently determines the morphodynamic state of the beach under sheltered low-energy conditions where the wave fetch is restricted.
A key element to understanding the representation of bar-beach state and its longshore variability is to have data on a large spatial scale and 3D accuracy. Previous observations in the world vary from aerial photographs, having the advantage of good spatial extension (regional scale) but limited 3D pattern, to sporadic topo-bathymetric data (DGPS, video), with low spatial extension. These limiting factors encourage authors to use the Dean parameter ( ) in spite of the uncertainties they can induce. Moreover, many field sites have well-marked boundaries (headland, rocks, reef, islets or islands) inducing a short average length of the beach system with individual beach state compartments (Jackson et al., 2005; Short, 2006;
Gómez-Pujol et al., 2007; Klein et al., 2010; Scott et al., 2011). In this context, the spatial transition between bar-beach states has been poorly documented. The use of 3D topo-bathymetric LiDAR in the present study associated with a regional quasi-continuous sandy beach reaching nearly 200 km long, allows us to focus on these transitions. Some characteristics are highlighted: 1) The spatial amplitude of class cells varies from a few kilometres to several tens of kilometres. 2) Some transitions are sudden when the beaches are disturbed by large rocky headlands (Cap Leucate, Cap d’Agde, Mont Saint Clair). 3) On the other hand, transitions can also occur progressively over a few hundred metres on continuous beaches. In these sectors, bar patterns are complex (Aleman et al., 2011) and entangled. 4) Harbours do not seem to disturb the regional beach state pattern. Only one of twelve constitutes a boundary between the reflective and intermediate state. However, nothing suggests that this boundary is in relation with this modification. Harbour impacts are local with bars cut by jetties and slight longshore evolution of the sea slope. 5) Moreover, small coastal structures of sea defence do not seem to affect the regional/local beach state observed.
Their impact is low and noticeable at a small scale with rip configurations and complex bar patterns in shallow water (Aleman et al., 2011). However, this impact is not in the same magnitude as that observed in the Netherlands coast (Short, 1992).
3.2.6.2. Geological framework
Most beach classification models do not take into account the geological constraints on shoreface morphodynamics. Current studies emphasize the importance of these constraints (headlands/capes, rock outcrops, reef) to explain the observed morphologies (Jackson et al., 2005; Short, 2006; Jackson et Cooper, 2009; Scott et al., 2011).
First, the presence of capes plays a role in the sedimentary cell size and sediment transport (Sanderson et Eliot, 1999), rip formation (Short, 1999; Short, 2006; Short, 2010) or wave propagation (Gómez-Pujol et al., 2007). In this study, some headlands/capes are observed along the Gulf of Lions and each induces a particular behaviour of the shoreface characteristics: 1) The most original influence is observed at Cap Leucate, which is the boundary between intermediate (southward) and dissipative (northward) beach states. Its presence plays a significant role by interrupting some of the northward sediment transport.
Indeed, medium-coarse sediment from coastal rivers of the Roussillon does not bypass the obstacle formed by the cape. Fine sediment is trapped beyond the wave closure depth. Only a small part of this fine sediment can pass through the outer bar and supply the intermediate beaches. Sediment at the north of the cape mostly comes from ancient input of the Rhone which is located further north, outside the study area. This sediment is finer (fine-medium sand) and better distributed. Most studies have focused on beaches enclosed between two
capes (Sanderson et Eliot, 1999; Gómez-Pujol et al., 2007). However, the presence of a significant headland on a long linear sandy coast may constitute a natural barrier to sediment transport resulting in significant variability and a sorting of the sedimentary families. Thus, a differentiation of beach morphologies can be observed on both sides of the cape. 2) Headlands can have a high influence on wave propagation and dissipation. This is usually observed on sheltered open beaches (near rocky coasts) inducing a reflective state. This behaviour is classic in the literature. 3) Another headland observed in the field site corresponds to the boundary of rock outcrops north of Mont Saint Clair and the “subtidal rocky platform” state.
4) Finally, the influence of the cape of Agde is more complex to identify. However, it is the boundary between a dissipative (southward) and intermediate-dissipative (northward) state. In conclusion, rocky headlands seem to play a key role for beach state transitions by their impact on both sediment transport and hydrodynamics.
Secondly, the accommodation space has a significant impact on shoreface morphodynamics and observed beach state as mentioned in this study and in the literature.
Bedrock outcrops allow wave energy mitigation (Sanderson et Eliot, 1999; Short, 2006; Short et Woodroffe, 2009; Muñoz-Perez et Medina, 2010; Short, 2010), the emergence of topographic rips (Short, 1999; Short, 2006; Short, 2010), but also influence and constrain the accommodation space and sedimentary stock (Jackson et al., 2005; Scott et al., 2007; Jackson et Cooper, 2009; Scott et al., 2011). In the northern area, in the subtidal rocky platform compartment, the depth of closure is between -6 to -10 meters (Sabatier et al., 2004) and the substratum outcrops at a depth between -5 to -10 meters. The sedimentary stock is limited and close to the shore. The geophysical data record only a 4 m thickness of sand and sometimes the bedrock outcrops on the inner and outer trough of bars (Certain et al., 2005b). Thus, the full seaward development of the profile is not possible and its characteristics are modified.
Moreover, the bedrock modulates wave energy and limits the onshore transfer of sand between the upper and lower shoreface. It is only during high-energy events that sediment transfer can be observed, but the input consists of coarse sediment or pebbles. This geological context causes reduced and steep beaches.
3.2.6.3. Dean parameter
The Dean parameter is generally used to determine beach-bar states. Nevertheless, the field observations show a greater variety of beach typologies for a same value of (Fig. 3.11). The use of the Dean parameter (Dean, 1973) as a tool for the prediction of beach morphologies has been questioned (Anthony, 1998; Levoy et al., 2000; Masselink et Pattiaratchi, 2001; Jackson et al., 2005), particularly when it used in large tidal ranges and low-energy environments, but also in microtidal contexts. Moreover, there are no standardized rules for hydrodynamic period to consider and sediment samples locations. Such considerations have received little attention in the literature but they do have a bearing on the representativeness of sediment-wave parametric combinations (Anthony, 1998). Thus, in several field studies, the authors found important discrepancies between the predicted and observed beach morphologies (Wright et al., 1987; Sanderson et Eliot, 1999; Benaventes et al., 2000; Klein et Menezes, 2001; Thieler et al., 2001; Jackson et al., 2005). However, those authors concluded that these parameters are useful in discriminating between reflective and dissipative extreme beach states, but do not adequately characterize intermediate situations (Wright et al., 1987; Bauer
Dean parameter under particular conditions. This parameter is only based on the sediment characteristics (Ws), the breaker height (Hb), and the wave period (T). However, many other variables affect beach morphologies, such as dynamic factors (secondary wave motions, tidal currents, etc.), sedimentary factors (grain shape, porosity, bimodal nature, etc.) and geological controls (accommodation space, sediment volume, and bedrock/reef outcropping) (Cooper et Pilkey, 2004). Moreover, the waves are not monochromatic and it is not always easy or possible to assign unique values to the grain size, wave height and wave period at a given time (Jackson et al., 2005). Anthony (1998) noted that "the performance of wave-sediment parameters must surely depend on local environmental contexts" particularly within lower-energy beach systems with a long response time. In the Languedoc-Roussillon, the key parameters for the calculation of , such as hydrodynamics and particle size, were analysed to identify the potential source of error.
Different sediment samples used cause great heterogeneity of values. The choice of sediment sample location across the active profile can induce a high inaccuracy of the Dean parameter calculated (Benedet et al., 2004a). However, its influence has been little studied in the literature (Short, 1992; Medina et al., 1994; Muehe, 1998; Benedet et al., 2004a; Benedet et al., 2004b; Albino et Suguio, 2011). Indeed, the utilization of supratidal environments such as upper beach, berm and beach step gives poor results compared to field observations. The microtidal context is probably not unrelated to this result due to the very low tidal range and the upper beach influenced by hydrodynamic conditions only during storm surges (a few times per year). As reported by Jackson et al. (2002), the infrequent inundation of the upper beaches affected by surge results in pronounced differences in sediment characteristics between the active lower foreshore and nearshore, and the relict upper foreshore. Subtidal environments such as inner bar, outer bar and lower shoreface are more realistic.
The average annual wave heights are low with 80% of Hs lower than 0.80 m. However, climatic conditions are extremely variable with very low wave heights in summer and important storms in winter. These environmental settings differ from those described by Jackson et al. (2002) as a low-energy environment but also differ from the open ocean beaches with high energy (Gómez-Pujol et al., 2007). Hydrodynamic seasonality induces important variations of the Dean parameter, especially in the dissipative field. Gómez-Pujol et al. (2007) noticed that the Dean parameter could be useful to describe gross beach classification but tends to fail with seasonal variability. Jiménez et al. (2008) emphasized the importance of taking into account the duration and intensity of the wave forcing necessary for the morphological reaction of the beachface. In this case, we could consider the calm conditions in August 2009 (one month before the LiDAR survey) as the wave-forcing period responsible for the observed morphologies. Indeed, calculated for this period gives the best prediction of beach classes ( o/ p of 84.5%). However, annual average hydrodynamic conditions provides better results if we consider the bar typologies ( o/ p of 72.3%). If considering only the storm waves (Hs>2m) as the most morphogenic conditions, the
predicted/ observed ratio is very low.
In conclusion, it seems that the discrimination quality of is related to both the temporal and spatial disparity of the hydrodynamic conditions and sediment characteristics. It appears necessary to accurately identify the parameters responsible for the morphological reactions of beach systems. In the Languedoc-Roussillon, the best predicted/ observed ratio is obtained for beach classes (Reflective, Intermediate and Dissipative) using the mean particle size of subtidal environments (inner bar, outer bar and lower shoreface) coupled with the average hydrodynamic conditions prior to field observations. The best prediction for the bar states (R, LTT, TBR, RBB, LBT and D) are obtained using the mean particle size of subtidal
environments coupled with annual average hydrodynamic conditions. Nevertheless, as mentioned by Scott et al. (2011): “Beach classification models based on environmental parameters are, by necessity, simplifications and should be used as tools for understanding morphodynamic systems, rather than beach type prediction”.