Sand spit rhythmic development: A potential record of wave climate variations? Arçay Spit, western coast of France
Introduction
Interest in coastal studies has grown worldwide over the last decade. Sea level rise (IPCC, 2001) and a worsening wind and wave climate in the Northeast Atlantic during the second half of the twentieth century (WASA Group, 1998) have had a strong effect over coastal sedimentary system evolutions. Among the different types of coastal systems, sand spits can be defined as a type of barrier forming a narrow and elongated sandbody, attached to a land mass at one end and terminating in open water at the other (Evans, 1942, Zenkovitch, 1967, Davis and Fitzgerald, 2004). Sand spits protect the mainland coast from the forces of the ocean, particularly during storms. Wave-induced processes are the main factors controlling sand spit development (Evans, 1942, King, 1970, Schwartz, 1982). Other factors such as the geological framework (Riggs et al., 1995, Nielsen and Johannessen, 2001, Honeycutt and Krantz, 2003); sea-level rise (Héquette and Ruz, 1991, Ollerhead and Davidson-Arnott, 1995); the sediment availability (Aubrey and Gaines, 1982, Héquette and Ruz, 1991, Park and Wells, 2007); winds and tides (Tricart, 1967) and human impacts (Bouaziz et al., 2005, Lorenzo et al., 2007, Simeoni et al., 2007) can also have key roles in sand spit morphological evolution and illustrate the complexity of studying these systems. Most of these studies have dealt with the formation and the morphological evolutions of sand spits but few have discussed the link existing between wave climate and sand spit morphological evolutions (Héquette and Ruz, 1991, Ollerhead and Davidson-Arnott, 1995, Park and Wells, 2007). The purpose of this paper is to investigate relationships between variations in sand spit morphological evolution and variations in wave climate, in order to show the potential of sand spits in recording past wave climates. To address these questions, the present study focuses on the Arçay Spit (Western coast of France), because: (1) it is rapidly growing (Galichon, 1984, Galichon, 1985), thus its variations in growth rate should be easily observed; (2) it seems to be dominated by wave-induced longshore transport mainly and results from a downdrift accumulation of sand (Bertin et al., 2007); and (3) it seems to result from episodic growth due to variations in longshore transport rate as it displays a hook-like shape morphology (Schwartz, 1982, Galichon, 1984, Galichon, 1985).
Section snippets
Geographical and geomorphological settings
The Arçay Spit is located in the middle part of the French Atlantic coast, belonging to the passive continental margin of the Bay of Biscay (Fig. 1a). The continental shelf extends 130 km offshore and displays a very low gradient (1/1000 to 2/1000). The Arçay Spit is located in the landward part of the wide macrotidal Pertuis Breton estuary (Chaumillon and Weber, 2006). It borders the river mouth of the Lay River characterised by a very low freshwater discharge (9 m 3 s− 1; Viguier, 1986).
Methods and data
Considering the main objective of this study was to investigate the relationships between variation in sand spit development and wave climate, a combination of various data and methods was used and includes: (1) historical maps, aerial photographs and SPOT images to demonstrate morphological changes; (2) long-term offshore wave data archives and wave numerical modelling to get time series of wave conditions at the study site and; (3) numerical modelling of sediment transport to get time series
Geomorphological results at secular, decadal and annual time scales
Sand spit morphological evolution results are successively described from secular to decadal and annual time scales in terms of elongation, area gain and hook ridges accretion. Elongation and area gain are calculated using the boundary between the upper beach part made of coarse sands (i.e. the active portion of the spit, according to Meistrell, 1972) and the lower beach part made of fine sands (i.e. spit platform, Fig. 2b) and is referred to as “sand spit boundary” in the following. The high
Hydro-sedimentary results: wave climate and longshore transport rate
Monthly-averaged significant wave heights (Hs), peak wave periods (Tp), wave angle at breaking (α) and longshore transport (Qs) were computed from 1979 to 2005 (Fig. 9a, b, c, d) from the 3-hourly time series described in Section 3.2.
Discussion
In the following section, relationships between variations of modelled wave conditions, longshore transport and annual spit sand gain will first be discussed, after which the origin and long-term (decades to centuries) morphological evolution of Arçay Spit will be tentatively correlated with past wave climate.
Conclusions
The purpose of this paper was to investigate the relationships between variations in wave climate, longshore transport and sand spit development in order to show the sedimentary record of wave climate by sand spit systems. Arçay Spit has been chosen for that purpose because: (1) since the beginning of the nineteen century it is a rapidly growing sand spit; (2) it is the result of wave-induced longshore transport and (3) it displays several hook ridges suggesting a rhythmic character of sand
Acknowledgments
The estuarine and coastal zone team of LNEC is thanked for allowing us to use its numerical resources permitting to simulate wave propagation over the period 1979–2005. Xavier Bertin is funded by the European Commission through a Marie Curie Intra-European Fellowship (contract IMMATIE MEIF-CT-2006-041171).
The authors are grateful to Michel Benoit and Florence Lafon from the Electricité de France-Laboratoire National d’Hydraulique et Environnement (EDF/R&D-LNHE) for their cooperation in making
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