The role of the inter-bar depressions in hydrodynamic and sediment transport processes of sandy macrotidal beaches

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1 The role of the inter-bar depressions in hydrodynamic and sediment transport processes of sandy macrotidal beaches Adrien CARTIER 1,2, Arnaud HÉQUETTE 1,2 and Philippe LARROUDE 3 1. Laboratoire d Océanologie et de Géosciences, UMR CNRS 8187 LOG, Université du Littoral Côte d Opale, 32 Ave Foch, Wimereux, France. 2. Univ. Lille Nord de France, F Lille, France. Adrien.cartier@univ-littoral.fr, Arnaud.hequette@univ-littoral.fr 3. LEGI-UMR 5519 UJF, Grenoble Université, larroude@legi.grenoble-inp.fr

2 Context Sandy Macrotidal beaches are less studied in the world than other beaches Characterized by a bar-trough topography (King and Williams, 1949) Lots of studies have focused their attention on Hydrodynamic and morphological processes. Bar migrations, stability of the bar-trough topography (E.G. Sedrati and Anthony, 2007; Masselink, 2006; Anthony et al., 2004; Kroon and Masselink, 2002) Davies (1980) Cartier and Héquette, 2013

3 Context Sandy Macrotidal beaches are less studied in the world than other beaches Characterized by a bar-trough topography (King and Williams, 1949) Lots of studies have focused their attention on Hydrodynamic and morphological processes. Bar migrations, stability of the bar-trough topography (E.G. Sedrati and Anthony, 2007; Masselink, 2006; Anthony et al., 2004; Kroon and Masselink, 2002) Davies (1980) High variations of longshore and cross-shore sediment transport rates across the beach (Cartier and Héquette, 2013, Cartier et al., 2012) Cartier and Héquette, 2013

4 Cartier and Héquette, 2013 Highest longshore sediment transport rates are located over the intertidal bars due to the combination of wave breaking, wave-driven currents and tidal currents (Cartier and Héquette, 2013) Sediment transport in troughs is reduced as they are characterized by lower wave energy conditions

5 Cartier and Héquette, 2013 Onshore sediment transport higher than longshore sediment transport

6 Anthony et al (2004) Few morphological changes and low migration rates of intertidal bars (Wright et al, 1982; Anthony et al, 2004, 2005 )

7 The morphodynamic role of currents in the trough region is limited, but may be significant (Masselink, 2006) Longshore sediment transport in the troughs may be substantial and significantly contribute to the overall sediment budget (Anthony et al., 2004, 2005) Few measurements have been undertaken in these depressions How does beach morphology influence hydrodynamics? What is the role of troughs in the morphodynamic of macrotidal beaches?

8 Study Area Northern France Sandy beaches of Wissant Bay Facing the Dover Strait Macrotidal environment Tidal Range > 7m

9 The system studied Runnel mouth Bar Bar Trough Bar Trough

10 The system studied A B C 4 Hydrographical Instruments (Wave and current meters) 2 Hz Longshore Sediment Transport (LST) calculated using Van Rijn formula (1989) (Camenen and Larroudé, 2003)

11 Results: Hydrodynamic Significant wave height (m) Longshore current velocity (m.s -1 ) West East Low wave energy and current conditions H av. = 0.17 m and Vl av. = 0.19 m.s -1

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13 Rising Tide: -Opposite direction of mean flow over the bar and in the trough = topography driven -Beyond 1m depth, flow in the trough is tidally driven -Longshore sediment transport follows longshore current velocity variations -Sediment transport rates are higher in the trough during inition stage of flooding

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15 Mid Tide: -Nearshore currents are tidally driven and eastward directed = flood -Decrease of current velocity - Sediment transport rates are similar over the beach and eastward directed

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17 Falling Tide: -Current in the trough keeps on flowing eastward while intertidal bar experiences ebb that is directed westward -Under 1.5m depth in the trough, flow is channelized Drainage (15 min) Strong acceleration of current velocity (0 m.s -1 to 0.26 m.s -1 )

18 Falling Tide: -Local increase of current velocities at the runnel mouth due to the runnel drainage -Sediment transport higher in the trough and in the drainage channel than over the intertidal bar

19 Rising Tide Bar m 3.s -1.m -1 Onshore sediment transport Flooding m 3.s -1.m -1 West Origin of sediment in the trough: from flooding and onshore sediment transport wave-driven East

20 Mid-Tide Tide to 10-7 m 3.s -1.m -1 Longshore sediment transport tidally driven is dominated West East

21 Falling Tide Bar m 3.s -1.m -1 Onshore sediment transport Drainage m 3.s -1.m -1 West East

22 Conclusion Sediment transport in the troughs can be significant and higher than over the intertidal bars especially under low wave energy conditions Strong runnel drainage/flooding is significant when bar-trough topography is well developped. Efficient mechanism but only under very shallow water conditions when the bar is slightly emerged This mechanism contributes to the stability of the barred morphology of these macrotidal beaches by inducing a residual circulation of wave-driven onshore transported sands from the upper to the lower beach during the falling tide

23 Thank you for your attention