Morphodynamics of intertidal sand bars: field studies in the northern Adriatic, NE Italy

Swash bars onshore migration have been measured on intertidal area of two microtidal beaches of Northern Adriatic during very low wave energy conditions. Bed microtopography was performed at every low tides, and migration of features followed by rods in a cross-shore array. Very small swash bars (A<0.07m, 2<L<10 m) were identified all over the intertidal zone, and their formation was always associated with a slack water tidal level. Despite the virtual absence of waves (Hs of 0.04-0.08 m at the Volano site), a important landward migration was measured (0.5-3 m/day). This migration is associated with an onshore sediment transport. The very low wave energy permits to assess the action of swash processes alone on the bar morphodynamics. A direct relationship was found between the bar migration rate and the duration of swash action over the bar crests. A similar study was performed on a bigger intertidal bar at Lido di Dante, while it was exposed to swash processes. Despite differences in bar nature and evolution, the landward bar migration also showed a strong relationship with the duration of swash action, excepted when longshore currents were evidenced, decreasing the deposition on the slip landward face, and thus the migration rate.