Journal of Coastal Research, Special Issue 50, 2007 Journal of Coastal Research SI 50 379 - 383 ICS2007 (Proceedings) Australia ISSN 0749.0208 Design of scaled movable bed experiments using numerical models J.M. Alsina†, A. Sánchez-Arcilla‡, X. Gironella‡ and T.E. Baldock† † Dept. of Civil Engineering, University of Queensland, Brisbane QLD 4072, Australia. josealsina@uq.edu.au t.baldock@uq.edu.au ‡ Lab. of Maritime Engineering Universidad Politécnica de Cataluña, Barcelona 08034, Spain. agustin.arcilla@upc.edu xavi.gironella@upc.edu ABSTRACT ALSINA, J.M., SÁNCHEZ-ARCILLA, A., GIRONELLA, X. and BALDOCK, T.E., 2007. Design of scaled movable bed experiments using numerical models. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 379 – 383. Gold Coast, Australia, ISSN 0749.0208 Scaling designs for mobile bed short wave experiments have been analysed using a suite of numerical models to test different sediment scaling configurations. The numerical simulations have revealed that geometrical sediment scaling gives similarity in the boundary layer flow but the sediment transport mode switches from bed- load in the prototype to suspended-load in the model. This is due to the high relation of prototype to model sediment sizes. However, scaling to maintain the relative fall speed allows smaller sediment size relations and closer similarity in the dominant transport processes. A set of corrections have been proposed to minimise the scaling effects in sediment transport rates and bottom evolution. ADDITIONAL INDEX WORDS: Bed-load sediment transport scaling, Suspended-load sediment transport scaling, Morphodynamic modeling, Mixing length boundary layer models. INTRODUCTION Scaled movable models are commonly used to study sediment transport, beach evolution and coastal problems. The basic philosophy is to gain a better understanding of the dominant hydraulic/sediment processes, by ensuring that the relative magnitudes of all dominant processes are the same in the model and prototype. However, this is usually an impossible task in scaled models (HUDSON et al., 1979). The design of scaled movable experiments relies therefore, on empiricism. No rigorous methodology to design scaled sediment transport models has been developed (HUGHES, 1993) and consequently results obtained from a scaled physical model may not compare well with reality. Nevertheless, physical modelling remains a useful qualitative tool in understanding the dominant forces and response mechanisms of sediment. Numerical models provide a low-cost complement to physical modelling, since many design conditions and parameters may be tested in numerical simulations. This is supported by the idea that an experimental model is a simplified version of reality (as are numerical models) where the forcing conditions are controlled within a certain range. The aim of this study is to introduce numerical techniques to 1) enable improved design of a scaled short wave sediment transport model and 2) to upscale or interpret scaled physical model results. The present study has been limited to the design of movable bed experiments aimed to study cross-shore morphodynamic beach processes in which incident waves play an important role. The results are therefore applicable for short-wave hydrodynamic models. The numerical techniques are tested using Barceloneta Beach, Spain, as a case study. REVIEW OF SCALING LAWS IN MOVABLE BED EXPERIMENTS Short wave hydrodynamic scaling must obey (HUGHES, 1993): 1) The model is geometrically undistorted to correctly simulate wave transformation across the beach profile. Consequently, all lengths are scaled with the geometrical length-scale (n): p X Y Z m L n n n n L = = = = (1) where n indicates scaling ratio and the sub-indices express the different spatial coordinates. L p and L m indicate a length scale in the prototype and in the model, respectively. 2) The Froude number must be similar in the model and prototype, and is given by r U F gL = (2) where U is velocity, g gravitational acceleration and L the wave length. Froude number similarity implies that the Froude scale must be n Fr = 1 and from (2) the time and velocity scales are obtained as, T n n = (3) The time and velocity is then scaled in according to the Froude number. Sediment transport scaling is a more complex question. It is assumed that perfect similitude in bed-load and suspended load sediment transport may not be achieved. Therefore a compromise between a bed-load or suspended load dominated model must be chosen.