Marine and River Dune Dynamics - 1-3 April 2008 - Leeds, United Kingdom 337 1 INTRODUCTION Bed forms as a product of the interaction of a de- formable bottom and forcing currents have been a phenomenon fascinating to the casual observer as well as to the scientific community for more than a century (e.g Darwin, 1883). Despite the simplicity in form, their ubiquitous nature and a multitude of ap- plied methods and approaches, a satisfactory formu- lation of the underlying physical principles probably still needs several more years of dedicated research (cf. Kennedy, 1963). However, in the recent past new sensors and positioning technology in the field and laboratories on the one hand as well as enhanced mathematical schemes on the other hand have led to new opportunities in exploring the natural system and modelling its behaviour. This contribution pre- sents preliminary findings from two different model approaches and compares these with high-resolution morphology of compound bedforms in a tidal chan- nel. As mistakable nomenclature might obscure the scientific progress, we avoid the terms “sandwave” and “dune” - which both have been understood purely descriptive by many (cf. Ashley, 1990) but also process related by others - in favour of the inno- cent term “bedform” for all morphological features observed in the study area of the Grådyb tidal chan- nel in the Danish Wadden Sea (Fig. 2). For decades echo soundings have shown the su- perimposition of bedforms of different sizes in tidal environments (Allen & Collinson, 1974; Rubin & Mcculloch, 1980; Ten Brinke et al., 1999), but only recently high-resolution multibeam sonars provide the opportunity to show the complex three- dimensional patterns and to assess temporal varia- tions in morphology along one direction and the same channel reach with high precision. In combina- tion with high-accuracy positioning, horizontal and vertical changes at sub-decimetre scale can be quan- tified (Ernstsen et al., 2006). The Grådyb tidal channel displays well developed compound bedforms that have been thoroughly in- vestigated in the last years (Bartholdy et al., 2002; Bartholdy et al., 2005; Bartholomä et al., 2004; Ernstsen et al., 2006; Ernstsen et al., 2005; Winter & Ernstsen, 2007)Ernstsen et al., 2007. Successive in- ter-tidal as well as repeated annual measurements have formed a sound data-base on the characteristics of this domain. Generally the compound bedforms of the central Grådyb tidal channel can be characterised by large asymmetric ebb-directed features with lengths of about one hundred meters and heights of about three meters, which migrate slowly in the or- der of ten meters per year towards the open sea. Smaller superimposed bodies with lengths of a few meters and heights in the order of several decimetres are much more dynamic. These very active bedforms are asymmetric and reverse direction within each semi-tidal cycle, the crests moving in the order of two meters. These super-imposed bedforms tend to grow in size from the trough towards the crest of the underlying large feature. Although the smaller bed- forms are expected to have an ebb directed net mi- gration, and thus contribute to the dynamics of the large ones in the course of time (Bartholdy et al., 2002), some observed tidal cycles also reveal a flood oriented net migration (Bartholomä et al., 2004; Ernstsen et al., 2006). On the superimposition of bedforms in a tidal channel C. Winter MARUM, University of Bremen, Germany G. Vittori DICAT, Genova, Italy V.B. Ernstsen MARUM, University of Bremen, Germany J. Bartholdy Department of Geography and Geology, University of Copenhagen, Denmark ABSTRACT: High resolution bathymetric measurements reveal the super-imposition of bedforms in the Grådyb tidal inlet in the Danish Wadden Sea. Preliminary results of numerical model simulations are dis- cussed: A linear stability model was tested to explain the large bedforms as being caused by tidal system in- stabilities. Results show comparable dimensions and migration rates. A three-dimensional morphodynamic model is shown to reproduce small scale transport rates but lacks realistic trends of morphodynamic evolu- tion.