The migration of large-scale bed forms in an estuarine environment: observation and parameterisation for coastal area modelling

CONTEXT. The topic of this proposal relates within Oceans 2025 to: Shelf and coastal processes: 13 'Morphological modelling in Liverpool Bay and adjacent estuaries'. It fits mainly under WP 3.5 'Coastal morphodynamics and bathymetric evolution', while contributing also to WP 9.1 'Assessing and improving the accuracy of shelf sea models'. In addition, this studentship will add significant value to the NERC-funded FORMOST project (NE/E015123/1, 2008-2010, involving Bangor University and POL) which aims to (i) extend POL's field observational capabilities in sediment transport research, and (ii) develop new parameterizations of local sediment transport, based on Bangor's Sand Transport Model, that will here be implemented in two coastal area models (Telemac and POLCOMS). The Telemac system has already been upgraded to include a parameterised version of the Bangor model, and this will be available to the student via FORMOST as a working tool for data interpretation. In addition, new field data will be obtained during FORMOST in the outer channels of the Dee Estuary, within the area of POL's Coastal Observatory, providing background information for the studentship. BACKGROUND. The migration of large-scale sedimentary features (sand waves, banks and bars) contributes significantly to the morphological evolution of the seabed. Such features are not resolved within coastal sediment transport models, giving rise to uncertainty in morphological outcomes. This project will focus on the mobility and migration of large-scale bed features in the Dee Estuary, examining the key physical processes largely from an observational standpoint, but including a modelling element that will allow an informed interpretation of the experimental outcomes. An understanding of the role of estuaries like the Dee in acting as sources or sinks for mobile sediment is essential from a management viewpoint against a background of climate change. The results of a pilot study carried out using POL's X-band radar during summer 2007 (MSc project, Bangor University, jointly-supervised by POL) indicate that large-scale sand features are advancing into the Dee Estuary in the intertidal area between Hilbre Island and the Wirral mainland. More generally, POL's radar, which is capable of monitoring the movement of bed forms over a 4 km radius, has revealed that large sand features are also advancing into the estuary in deeper water. However, what is unknown is the relative contribution of the respective large bed forms to the sediment budget of the Dee Estuary. The DETAILED AIMS of the studentship are to: (i) track sand dunes in the inter-tidal area near Hilbre Island, thus elaborating the results of the 2007 pilot study. This will highlight the role of waves and currents on net sediment transport. Interpretation of the results will include Telemac simulations for the conditions measured on site. (ii) track using X-band radar the movement of sand banks and dunes in deeper water in mid-estuary. Interpretation will be provided both by available/new ADCP data and modelling. Telemac (and possibly POLCOMS) will be used to establish the 'drivers' responsible for morphological change. (iii) establish a methodology for the inclusion of large-scale bed features in coastal area models, taking as a starting point Van Rijn's (2005) roughness formulation, introducing this into Telemac, and critically testing its effect with regard to the bed forms in the Dee. End users. The PhD programme will be of relevance to end users concerned with management issues in the Dee. For example, the Countryside Commission for Wales (CCW) as well as the Environment Agency (EA) will be in attendance at FORMOST project meetings to which the student will contribute. The study will also contribute directly to the Centre for Catchment and Coastal Research, part of the Partnership Project between Aberystwyth and Bangor Universities.