Sediment trend models fail to reproduce small-scale sediment transport patterns on an intertidal beach GERHARD MASSELINK*, DANIEL BUSCOMBE*, MARTIN AUSTIN  , TIM O’HARE   and PAUL RUSSELL   *School of Geography, University of Plymouth, Plymouth, UK (E-mail: daniel.buscombe@plymouth.ac.uk)  School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth, UK ABSTRACT A rigorous test is presented of the application of sediment trend models to an intertidal beach environment characterized by bar morphology. Sediment samples were collected during low tide from a regular grid and their sediment fall velocity distributions, obtained using a settling tube, were analysed using moment analysis. The net sediment transport direction determined from beach surveys, hydrodynamic measurements, wave ripple observations and sediment transport modelling was compared with predictions by sediment trend models based on the spatial distribution of sediment parameters. It was found that the sediment transport pathways and patterns of sedimentation predicted using sediment trend models were at odds with field observations, and varied significantly depending on whether surface or sub-surface sediment samples were used. The sediment trend models are thought to fail because, in energetic and morphologically variable beach environments, spatial patterns in sediment characteristics are mainly attributed to the presence of different hydrodynamic regions and associated morphology, rather than sediment pathways. The use of sediment trend models cannot replace the collection of morphological, hydrodynamic and sediment transport data in the field to define relationships between flows, forms and sedimentation patterns on a dynamic intertidal beach. Keywords Beach morphology, hyperbolic triangle, log-hyperbolic distribu- tion, nearshore sediment transport, particle analysis, sediment trend models. INTRODUCTION The textural characteristics of beach sediments are not constant, but change substantially over space and in time (e.g. Masselink et al., 2006). This is an important observation, because sedi- ment size plays a crucial role in sediment trans- port processes and, hence, morphological change. The sediment transport rate is linked directly to the bed shear stress through the quadratic stress law (or a variation thereof), which takes account of both the flow velocity and the roughness of the sea bed controlled by the sediment size (Van Rijn, 1993). Once a sediment particle is suspended, its fate strongly depends on the sediment fall veloc- ity. Because the spatial and temporal variability of beach sediment characteristics are generally poorly understood, a constant sediment size often is assumed. This assumption represents a major limiting factor in models of beach change (e.g. Soulsby, 1997), because the errors introduced into morphodynamic models of beaches due to uncertainties in grain parameters and/or the use of a single time/space-averaged sediment size value may be significant. A consideration of changes in grain size may, in fact, improve the output of models of nearshore change, such as demonstrated by Gallagher et al. (1998) in their modelling of sand bar migration. Early studies emphasized that textural variabil- ity in beach environments may be due to different hydrodynamic processes acting on different por- tions of the profile (Bascom, 1951; Inman, 1953; Miller & Ziegler, 1958), resulting in textural Sedimentology (2008) 55, 667–687 doi: 10.1111/j.1365-3091.2007.00917.x Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists 667