MORPHSWASH · Morphodynamic of the Swash zone: Experimental and numerical modelling

Periodic Report Summary - MORPHSWASH (Morphodynamic of the Swash zone: Experimental and numerical modelling)

Objectives The main goal of the MORPHSWASH project was to increase the understanding of the nearshore sediment dynamics, by developing new concept models which implement recent advances on the Swash Zone (SwZ) dynamics. The study encompassed numerical and experimental work. Work performed The project has encompassed numerical and laboratory experiments. The numerical analysis had the objective of developing a short-term numerical model based on a Lagrangian description of the flow. A Lagrangian analysis has been developed obtaining results valid for some specific flow conditions. Two sets of equations describing the flow in terms of depth-averaged Lagrangian coordinates have been derived. Such sets of equations correspond to either non-linear shallow water or a Boussinesq set of equations in depth-averaged Lagrangian coordinates. The range of validity and functioning has been defined by both analytical and numerical means. The experimental investigation has been supported by several laboratory experiments, performed at the medium-size wave flume of the Università Politecnica delle Marche (UNIVPM) and at the large-scale facility of the Universitat Politecnica de Catalunya (CIEM-UPC). Large-scale data have been collected by using different experimental setups. Some experiments performed at UNIVPM aimed to develop Particle tracking velocimetre techniques (PTV) focused to the SwZ dynamics. PTV techniques have been developed at UNIVPM and applied to study the flow-sediment dynamics around a submerged pipeline (Mattioli et al., 2012). Results and expected impact Several results have been obtained within MORPHSWASH. An important result is the analysis of the shoreline influence on the beach dynamics (Alsina et al., 2012). We have provided experimental evidences of a link between the SwZ dynamics and the surf zone morphodynamics. It was shown that more dissipative SwZ conditions significantly reduce the rate of seaward sandbar migration with respect to a corresponding case characterised by more reflective SwZ conditions. This was explained in terms of the SwZ dynamics. Important results were also obtained on the dynamics of suspended sediment at the SwZ and the influence of long waves and wave groups. The importance of a wave-by-wave analysis of the SwZ flows has been highlighted (Caceres and Alsina, 2012). A limited number of hydrodynamic types of events, inducing large suspended sediment concentrations were described. A clear predominance of incident wave / bore as a forcing term in the surf zone was also highlighted, whilst the SwZ was dominated by wave-swash interactions. The study of the influence of wave groups (Vicinanza et al., 2011) has revealed that random waves have an influence on the beach dynamics similar to that of bichromatic wave groups, promoting offshore transport, in comparison to monochromatic conditions. It has also been shown that free long waves promote onshore transport. Finally, our study suggested that the inclusion of long wave and wave group sediment transport is important for improved nearshore morphological modelling of the cross-shore beach profile evolution. These studies will significantly impact on all the engineering applications concerned with the management of the near-shoreline region (e.g. forecast of beach erosion, design of beach protection structures, etc.). Experimental PTV investigations (Mattioli et al., 2012) have highlighted the importance of studying the flow-pipe interactions and the turbulent structure for the understanding of the sediment dynamics around a submerged pipeline. It was shown that the vortical structures which dominate the flow around the pipeline generate sweeps and ejections associated to clockwise/anticlockwise rotating vortices. These structures are seen to largely influence the sediment dynamics near the bed, in particular their entrainment from the bed with particles captured by ejections and, settling to the bed with particles captured by sweeps. This work will mainly impact on the assessment of the pipeline-soil-flow interactions, which are fundamental for all stability analyses of underwater pipelines placed on erodible seabeds. Finally, the numerical analysis has brought to the implementation of a novel hydrodynamic model written in terms of depth-averaged Lagrangian coordinates. Such a code works well for large unbroken waves but displays some instabilities when predicting the dynamics of steep broken waves. This activity will impact on improving the state-of-the-art capabilities of accurately predicting nearshore flows.

Data: CORDIS, © European Union