A new high frequency remote sensing based method: application to the swash zone of a very high reflective beach under high energetic conditions (Grand Popo, Benin)

Abstract

Abstract:A new remote sensing method based on video imaging is tested on a steep reflective beach (Grand Popo, Benin) to characterize the variation of the bed level and water level in the swash. The set up used enables the measurement of the wave by wave impact of swash on the beach morphology. Large variations up to 10 cm of the bed level are observed at periods longer than group periods, suggesting a substantial morphological control on the swash dynamics. The effect of individual swash events on high frequency change of bed level is taken into account relating hydrodynamics and morphodynamics variables to the bed evolution. Preliminary results show a strong correlation of the high frequency bed change with swash hydrodynamics and group influence. Keywords: Nearshore, Swash, Sediment transport, Beach Cusp, Video, Remote sensing, Pixel intensity, Individual swash, Swash interaction, Benin. 583Thème 3 -Instrumentation, mesures, imagerie et télédétection IntroductionThe swash zone is a highly dynamic zone of the nearshore experiencing large levels of sediment transport, turbulence and large rates of morphological changes in the short term (swash to swash events) to long term (tidal cycle, days). In particular, the processes associated to the energy transfer between short and long waves and beach reflection remains a research challenge, but the last is crucial to understand how the swash zone affects the long wave behavior, the standing wave patterns and the interaction with the morphology (MASSELINK & PULEO, 2006). For example, recent studies using laboratory and numerical data (ALMAR et al., 2012) have shown the major control of swash slope on the outgoing reflected wave as well as on the standing long wave pattern, being the swash a low frequency filter in which the flatter the beach the lower frequency of the outgoing component. Moreover, the impact of individual swash events is still poorly addressed. In fact, recent observations have shown complexity of individual swash impact event on erosion/accretion and suggested the role played by the interaction between the uprush and backwash on net sediment transport as well as the swash interaction of individual events due to swash saturation (BLENKINSOPP et al., 2010). Regardless the importance of this zone, the wave transformation on this area is insufficiently understood due to lack of field data. For a sequence of waves approaching the beach, the degree of swash interactions depends on the ratio between the natural period of individual swash events Ts and the incident wave period T (BROCCHINI & BALDOCK, 2008). For T > Ts little or no interaction occurs, where swash interaction occur between incident waves and the uprush or backwash of preceding waves for Ts > T. This interaction may generate a range of scales of new motion such as hydraulic jumps, low frequency waves, turbulence and a high degree of energy dissipation. A new method based on remote sensing video imaging processing was developed to measure swash zone variations from short scales (swash to swash ev...