Study of beach cusps via high resolution TLS acquisitions on the pocket beach of Porsmilin (Brittany)

Beach cusps are rhythmic shoreline features made up of series of horns and embayments. Their build-up occurs in specific conditions (steep beachface, low-energy wave conditions...). These features can notably be characterized by the cusp spacing λ and their prominence ɛ (difference in beachface gradient between embayment and horn). At present, two main theories confront to explain the formation of such features on natural beaches : standing edge waves (special class of waves propagating longshore) and self-organisation hypothesis. - Standing edge wave theory proposes that the superimposition of incident waves and standing edge waves generates longshore variations of swash height linked with the position of edge wave nodes and anti-nodes. These variations of swash height result in regular zones of erosion. Depending on the context, different types of edge-waves may occur. The predicted beach cusp spacing is : λ = (g T^2 tanβ) / π for a sub-harmonic edge wave model λ = (g T^2 tanβ) / 2π for a synchronous edge wave model with : λ : beach cusp spacing (m) g : gravitational acceleration (9.81 m/s) T : incident wave period (s) tanβ : beach gradient - Self-organisation theory suggests that a combination of interactions and feedbacks between swash flow and beach topography leads to the growth of morphologic irregularities of a given wavelength (because of flow divergence or convergence), resulting in beach cusp formation and maintaining. The predicted beach cusp spacing is then : λ = f S with : λ : beach cusp spacing (m) S : horizontal extent of the swash flow (m) f : empirical constant (~1.5) Three multitemporal Terrestrial Laser Scan acquisitions have been carried out for three consecutive days on the sandy beach of Porsmilin (Brittany, France) with a spatial resolution varying from few centimetres to few metres. Moreover the hydrodynamic conditions have been obtained thanks to the Previmer project website (http://www.previmer.org/), notably based on WaveWatch3 and MARS-2D models. This study proposes to profit from the high resolution and accuracy of Terrestrial Laser data to measure the geometry and the spacing of beach cusps, to compare the measured parameters to the predicted ones (with both theories) and thus to attempt to identify the hydrodynamic process which sparks off their formation.