The dynamics of Cobble Dunes, Severn Estuary UK

Abstract

Dunes are one bedform type which can be found underwater and which have an impact on flow resistance and play an important role in engineering structure design, navigation as well as dredging strategies. Existing literature focuses on the formation and processes associated with dunes developed by sandy sediment and has paid little attention to those developed in coarse sediment, i.e. cobble dunes.

This research will focus on the dynamics of a set of coarse cobble dunes located on Hills Flats, in the Severn Estuary, UK. The features are composed of a range of sediment sizes; from small boulders, coarse cobbles, pebbles, to finer components. The dunes are exposed during low water period, especially during Spring tides, when direct measurements of the dunes could occur. Specifically, data related to dunes dynamics were collected by a number of techniques including measuring the dune shape by using dGPS, recording near-bed flow velocity data with current meter, sampling bedload transportation to imply the movements of pebbles or cobbles, and measuring the basic morphology of dunes located on the site and to obtain first interpretations of dune dynamics. Data from these techniques were processed to demonstrate the roughness effects on tidal flows and the subsequent influence on the bulk flow field. The results of this study show that the dunes, D ₅₀~ 16.7 mm, move only on the highest Spring tides and reverse direction of migration with each flood and ebb tide while little movement on Neap tides is recorded. The migration is low with the maximum rate only 1.1 cm per tide. The dune scale varies but usually less than 1 m high and 4 – 10m long. Water depth, flow velocity and bed shear stress over the dunes could exceed 10 m, 2 m/s and 80 N/m2 respectively. The initial motion of pebbles could start in a range of 0.3 – 50 N/m2. The dynamics of the bedforms are explained by the tidal asymmetry which mediates the temporal distribution of bed shear stress, understanding of which provide benefits to several aspects such as engineering of structures and environmental management planning in this area and other similar locations.

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