Morphodynamics of nearshore rhythmic sandbars in a mixed-energy environment (SW France): 2. Physical forcing analysis

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Abstract

The morphology and migration of rhythmic intertidal ridge and runnel systems, and subtidal crescentic bars that border the southwest coast of France were characterized using in situ surveys and maps obtained by remote-sensing methods. The period from 1986 to 2000 was investigated. A total of 35 km of coast was mapped. This data set shows several specificities, the origin of which are examined in the present report using hydrodynamic data. A complete analysis of the influence of wave climate on both the shape and the movements of these rhythmic sedimentary patterns was performed. In addition, SWAN and MORPHODYN-coupled numerical models were used to provide quantification of both wave breaking and longshore currents for wave parameters that were representative of the mean values and of the energetic conditions. This study demonstrated the short time response of intertidal systems to the wave forcing. When the offshore significant wave height (Hs) was lower than 2.5 m, regular coastal ridge and runnel systems developed in the intertidal zone and migrated in the longshore-drift direction at a rate of 1.7–3.1 m day−1. By contrast, the ridge and runnel system morphology abruptly changed when the Hs exceeded 2.5 m, and after the storm, the typical ridge and runnel rhythmic topography was recovered within 5–9 days. The crescentic bars, which had a convex seaward shape, were affected by waves with Hs values greater than 3 m (slightly less for short waves). Depending on the wave orientation, the crescentic bars moved in the longshore-drift direction at a rate that reached 1 m day−1. The data suggested a slight negative correlation between the mean alongshore length of the crescentic bar and the mean Hs. Finally, it seemed that increasing the wave obliquity with respect to the coast resulted in the flattening of the crescentic bars. Thus, coupling Spot and in situ mapping to hydrodynamic records allow the characterization of coastal morphology and dynamics, with time and space samplings that are particularly well adapted to the little studied alongshore morphodynamics. This approach should improve the difficult parameterization of morphodynamic models in high-energy environments.

Introduction

Increasing our morphodynamic knowledge of the coastal zone is essential for the development of beach-change models. These are required by decision makers to plan the development of the littoral zone, which is fragile but of high economic potential. This paper provides an innovative method for analyzing the alongshore morphology and dynamics of nearshore sandbars, which are common features along mesotidal and macrotidal sandy coasts that are exposed to oceanic waves. Nearshore bars frequently include regular or quasi-regular alongshore non-uniformities, which are collectively referred to as rhythmic topography. Beach cups (Coco et al., 2003), rip-current channels (Brander and Cowell, 2003) and crescentic bars (Van Enckevort and Ruessink, 2003b) are examples of nearshore rhythmic patterns that are repeated along the coast. A combination of several rhythmic systems in the cross-shore direction is commonly observed (Aagaard, 1991, Short and Aagaard, 1993). The morphology of nearshore sandbars evolves over time, and the changes in shape comprise alongshore uniform and non-uniform components. The former include onshore and offshore migrations (reviews by Shand et al., 1999 and Van Enckevort and Ruessink, 2003a). The latter encompass changes in quasi-rhythmic non-uniformities (Konicki and Holman, 2000, Van Enckevort and Ruessink, 2003b). These consist of alterations in length scale, cross-shore amplitude and alongshore migration. The results of the latest investigations, based on extensive video or field surveys coupled to wave-forcing analysis, have revealed the diversity in alongshore length (from a few meters to a few kilometers) and the behavior of these non-uniform features. They have also shown that the alongshore migration of nearshore bars is of the same order of magnitude (1–10 m day−1) as the cross-shore migration of rhythmic patterns.

The SW sandy coast of France (about 200 km) exhibits two distinct rhythmic sedimentary systems that are repeated along the shore. Ridges that are regularly interrupted by runnels are found in the intertidal zone, whilst crescentic bars appear at about 500 m offshore the ridges and runnels in the subtidal zone. The morphology and dynamics of these bars have been extensively studied using both in situ surveys (Michel and Howa, 1999, De Melo Apoluceno et al., 2002, Desmazes et al., 2002) and remotely sensed maps (Froidefond et al., 1990, Lafon et al., 2002a, Lafon et al., 2004). On a weekly scale, field investigations demonstrate that the shape of these rhythmic patterns changes over time, and that they are susceptible to movements both in the cross-shore and alongshore directions (the data and analytical results are summarized in Section 2). The yearly average alongshore lengths of the nearshore crescentic bars and ridge and runnel systems suggest that they are not under morphological control. However, satellite images and photographs often show two ridge and runnel systems coupled with one nearshore crescentic bar (Castelle and Bonneton, 2004). Although quantitative, these observations need to be explained and, particularly, to be precisely linked to the hydrodynamics, which is principally driven by the wave dynamics in this case. As wave estimates are obtained in deep water, numerical wave and hydrodynamic models describing the wave propagation towards the bars and the wave forcing at the bars were used here. An analysis of the meteorological and hydrological forcing is provided in Section 3. This paper aims to analyze the influence of wave climate on: (1) ridge and runnel shape; (2) ridge and runnel migration rate; (3) crescentic bar shape and (4) length scale; and (5) crescent-shaped bar alongshore migration rate. The results of this study are detailed in Sections 4–8. In the discussion (Section 9), the results are compared with the response of bars to wave forcing observed at other locations.

Section snippets

Physical settings

The Gironde coast forms the northern part of the SW coast of France (Fig. 1). It is about 120 km in length. In this area, the beaches are composed of homogeneous quartz sands, with mean grain sizes ranging from 200 to 400 μm (Pedreros et al., 1996). Rocks are uncommon, except in the vicinity of the inlet of the Gironde estuary. The semidiurnal tide generates clockwise rotating currents with speeds on the continental shelf that do not exceed 0.25 m s−1 (Lorin et al., 1979). Mean tidal and mean

Wave database

The Hs and Ts have been recorded since 1980 by the non-directional Datawell buoy of Biscarosse (Météo-France). This data set is discontinuous. To supplement the wave in situ measurements, wave climate (including wave direction) was used according to the Vag-Atla numerical model developed by Météo-France (AVISO database). Vag-Atla is a deep-water wave-prediction model that has produced wave forecasts four times per day over the Atlantic since 1996 (Guillaume, 1987). The output at the nearest

Method

To explain the influence of the wave climate on ridge and runnel shape, the available wave parameters preceding each map were investigated. The period between the mapping and the first preceding storm was considered, going back in time a maximum of 2 months. Storm conditions (Hs > 3 m) were defined on the basis of simulation results giving the percentage of wave breaking as a function of wave height (Section 3.2). The temporal variation of wave height, period and direction were examined to

Ridge and runnel system alongshore migration rate

Based on the Spot and in situ mapping, it was shown that the ridge and runnel migration rate varied significantly over time. A rate of about 1.2 m day−1 was obtained in 1999 (31 May–13 September), whilst it was about twice as high in 1989 (29 July–4 October). Fig. 4 displays the occurrences of Hs (a) and Ts (b) obtained from the wave gauge of Biscarosse for the 1989 investigation period (gray area) and those provided by Vag-Atla modified to reconstruct the Biscarosse buoy for the 1999 period

Method

Non-symmetrical bar shapes are logically linked to oblique waves that occur perpendicular to the coast (Blondeaux, 2001). To explain the occurrence of flattened crescentic bars along the SW coast of France in July 1989 (map C), wave statistics computed over the 12 months preceding map C were compared with 12-month wave statistics prior to cases F, G and H, which show convex bars. For this purpose, the Hs and Ts values obtained from the wave-rider buoy were investigated. Wave direction was not

Length scale of the crescentic bars

The alongshore length of the crescentic bars changed over time (cf. Table 1). From 1986 to 1992, they were generally long with mean length scales varying from 706 to 818 m. Six years later, in 1998, their mean length scale was as small as 579 m. In fact, between 1992 and 1998, the number of crescentic bars longer than 800 m decreased by three-quarters. Finally, from 1998 to 2000, their spacing increased up to 692 m. Wave-climate modifications were investigated to explain the shortening and the

Crescentic bar alongshore migration rate

Crescentic bar alongshore movements did not occur systematically. Based on the Spot maps, deformations and motions were clearly in evidence from May to September 1991, whilst, on the contrary, crescentic bars were unchanged from April to August 2000 (cf. Table 1), at least at the scale of the observation (20 × 20 m pixels).

Between May and September 1991, wave-gauge records from 7 June to 8 July were missing. The remainder of the data set was used and compared with wave conditions corresponding to

Discussion

The investigation of various sources of hydrodynamic data was necessary in order to obtain a substantial and useful data set for explaining the morphology and dynamics of rhythmic sandbars that border the Gironde coast. Indeed, wave observations in the area were sparse and wave direction was not provided, except by numerical models and the recent data set. Comparing, for instance, the numerical simulation results (or the empirical results for wind-wave generation) with the in situ data required

Conclusions

In this paper, a characterization of the alongshore morphodynamics of both the ridge and runnel systems and the crescentic bars that border the southwest coast of France was initiated by combining large-scale coastal mapping derived from high-resolution satellite measurements with a complete set of hydrodynamic data. About 35 km of coast was investigated, and, on average, 75 and 41 ridge and runnel systems and crescentic bars were studied, respectively. This work provides sound, although

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