The formation and morphodynamics of complex multi-hooked spits and the contribution of swash bars

Numerous studies have been devoted to recurved and hooked spits in micro- and meso-tidal environments, but none have really focused on the peculiar right-angle plan shape exhibited by some spit hooks. The morphodynamics of a complex multi-hooked spit in a macrotidal inlet in Normandy, France, have been investigated using aerial photographs and 12 topographic LiDAR surveys from 2000 to 2017, complemented by hydrodynamic data. Merville Spit is a 2.5 km-long hooked spit, composed of four 'nested' ridges each with a right-angle hook at its tip, that has developed in the vicinity of the sand-rich outlet of the Orne River. Monitoring of the two last ridges shows that ridge formation starts with the attachment of the tip of a swash bar to the fulcrum point of the previous ridge, and ends following its westward elongation and counter-clockwise rotation when its hook impinges on the Orne outlet. The main factor controlling ridge plan-view shape is swash bar dynamics in the vicinity of the spit. The offshore wave direction has no significant impact on ridge and hook behaviour due to the large number of swash bars on the ebb-tidal delta. Over the study period, between 5 and 10 swash bars massively assured sand supply to the upper foreshore, favouring the growth of existing ridges, but also enabling the building of a new ridge. The bars contribute to imparting hook right angles at the tips of the ridges by creating local wave refraction or by providing a sheltering effect. Strong tidal currents in the Orne channel act as a hydrodynamic barrier that stops hook elongation at the edge of the river's outlet. The similar behaviour of the two studied ridges has been synthesized into a new and original conceptual model of complex multi-hooked spit formation and evolution under the influence of swash bars.