River, Coastal and Estuarine Morphodynamics: RCEM 2007 – Dohmen-Janssen & Hulscher (eds) © 2008Taylor & Francis Group, London, ISBN 978-0-415-45363-9 Meandering channel dynamics in highly cohesive sediment on an intertidal mud flat Wiecher Bakx, Filip Schuurman, Maarten G. Kleinhans & Henk Markies Universiteit Utrecht, Fac. Geosciences, Dept. Physical Geography, Utrecht,The Netherlands ABSTRACT: On an intertidal mud flat in the Western Scheldt estuary (the Netherlands), small meandering channels (about 1 m wide) were studied with the aim to improve understanding of the effect of highly cohesive sediment on channel and meander geometry and dynamics. The morphology of several channels was mapped repeatedly using stereo photography. During a neap-spring tidal cycle the flow velocity and sediment concentra- tion was measured in detail in one meandering channel. Grain size analysis showed that the mud consists of about equal portions of clay and silt plus fine sand. Laboratory flume experiments were done on a carefully installed bed of sediment from the Western Scheldt intertidal flat. Three processes dominate erosion and morphodynam- ics of the meandering channels: 1) splash-erosion by rain, 2) erosive steps under hydraulic jumps and 3) bank erosion in very sharp bends in addition to the usual common bank erosion by undercutting. The flow velocity during flooding is negligible compared to the ebb flow. Rain storms at low tide cause much larger morphological change and sediment concentrations than the diurnal tide-related change and concentrations, even though the tidal discharge volume is larger than the rain runoff. Also storms cause larger morphological change, probably because of sediment stirring by gravity waves. Laboratory experiments with rainfall simulations verified that splash erosion by rain-drops breaks down the cohesive structure of the consolidated clay, resulting in an easy erodible surface layer of fluid mud.We hypothesise that the smaller, upstream channels in the channel network on the intertidal flat are generated by high-concentration, channelising rainfall runoff. Vertical downward steps ranging 0.01–0.1 m in height were observed particularly in the sections with steeper slopes near the low-tide base level of the Western Scheldt main channel adjacent to the intertidal flat.At these times the flow discharge sources were rainfall runoff and base flow, causing critical flow conditions with discharges far below bankfull. The steps occur under highly erosive hydraulic jumps and migrate upstream, resulting in the excavation of the thalweg and undercutting of the banks as confirmed in preliminary laboratory flume experiments. Very sharp meander bends with R/W < 2 are common while meander cutoffs are rare. Dye tracer experiments in several meander bends at bankfull discharge revealed recirculating flows in two places: just downstream of the apex in the inner bend and just upstream of the apex in the outer bend. Thus, the main flow impinges on the bank of the outer bend, causing the bank erosion pattern that leads to the sharp meander in agreement with literature. We discuss a simple extension of a meander simulation model to include very sharp bends. 1 INTRODUCTION 1.1 Scope and aim Intertidal mud flats commonly have small meandering streams (e.g Fagherazzi et al. 2004). Since mean- dering dynamics strongly depend on the strength of river banks, these meandering channels are inter- esting exemplars in the more cohesive part of the spectrum from noncohesive to very cohesive banks. The channels are so small that they could poten- tially be recreated in a laboratory flume. However, earlier attempts to do so proved that the meanders arrest after some time and become fixed in planform (e.g. Smithm, 1998). Dynamic meandering in cohesive sediment has not been reproduced in the laboratory because the unscaled cohesion is too large compared to the downscaled laboratory channels. However, the similar-sized meandering channels on intertidal flats are dynamic (pers. comm. with locals at our field site, and Fagherazzi et al. (2004)). The aim of this paper is to identify the main mech- anisms for meander dynamics on a cohesive intertidal mud flat. We study natural meander geometry and dynamics during a neap-spring tide cycle, and study erosive processes in more detail on natural mud in a laboratory flume. We found three processes that dom- inate erosion and morphodynamics of the meandering channels: splash erosion, erosive backward migrating 763