Tidal and fluvial controls on the morphological evolution of a lobate estuarine tidal bar: The Plassac Tidal Bar in the Gironde Estuary (France)

Evaluation of the recent morphological evolution of the Gironde estuary through the use of some preliminary synthetic indicators

“…A recent study localised around estuarine bars in the upper estuary (Billy et al, 2012) also demonstrated the long term control of the tide and the river flow on such sedimentary bodies. Our study is based on four situations covering 32 years, therefore it is not possible to investigate any effect at higher frequency.…”

“…As expected, the largest intertidal areas are in the lower estuary, at the level of the widest sections (between km 70 and 90), where they represent an area comprised between 15 and 25 km 2 . Upstream, intertidal areas are very narrow, with a relative increase between km 40 and 50 because of the presence of many islands and sandbanks in this zone (Billy et al, 2012). Time variations of intertidal areas are weak and do not seem to show significant trend.…”

“…It forms a funnel-shape estuary, with a length of 75 km between the junction and the mouth, and a total surface of 635 km 2 . A recent work from Billy et al (2012) described the morphology as a tripartite zonation (Darlymple et al, 1992), comprising from upstream to downstream: (1) a fluvially sourced bay-head delta that consists of estuarine meandering channels and tidal bars deposited in the estuary funnel (actually the riverine portion formed by the Garonne and Dordogne rivers), (2) a wide central basin filled with muddy sediments, centered at around Pauillac, and (3) an estuary mouth with a deeply scoured tidal inlet terminating landward and seaward in sandy flood and ebb deltas in front of Royan. In the estuary, tides are semi-diurnal; because of the variation of the tidal range, the estuary is macrotidal and hypersynchronous, with tidal amplitudes ranging from 1.3 to 5 m at Royan and from 3 to 6 m at Bordeaux (Figure 1).…”

“…The last available bathymetry (from year 2000) is not presented here because of uncertainties of the data. In this study, the region covered corresponds to zones (2) and (3) decribed by Billy et al (2012). In the riverine portion, data are available only in a short sector of the Garonne comprised between Bordeaux and the confluence of Ambes (25 km length).…”

Recognition and significance of bayhead delta deposits in the rock record: A comparison of modern and ancient systems

“…Drowned river valleys along the northern Gulf of Mexico, a wave‐dominated microtidal coast (Rodriguez et al ., ), reveal multiple bayhead deltas developing in estuaries (Mobile, Galveston, Corpus Christi, Nueces and Matagorda bays) behind sand spits and barriers (Figs and ). In estuaries dominated by strong tidal currents (such as the Atlantic coast of the USA and France), the bays will have an open or semi‐open mouth with a bayhead delta that is intensely reworked by tidal currents (Allen & Posamentier, , Féniès & Tastet, ; Billy et al ., ).…”