Introduction

Ebb-tidal deltas are accumulations of sediment (mostly sand) at the seaward side of a tidal inlet. Previous research on sediment dynamics at ebb-tidal deltas has been integrated into general conceptual models by for instance [1]. The conceptual model is based on the hypothesis that sediment is transported seaward through the main tidal channel and deposited as a distal lobe at the position where the current diminishes below the sediment transport threshold. Waves will modify the distal lobe and the sediment is redistributed as swash bars on either side of the main channel. These swash bars generally migrate onshore and will eventually attach to the adjacent shoreline. Wave generated longshore currents, enhanced by the tide dominated currents that occur at the tidal delta margins, transport the major part of the swash bar's sediment back into the inlet.

Teignmouth study area[2] and analysis of the ARGUS images show that the morphodynamics at the Teign estuary ebb-tidal delta seem to correspond quite well to the general model. Although the southern side of the Teign inlet system faces a cliff, the northern side resembles the general ebb-tidal delta morphology model of [3] very well. Spratt Sand can be considered as the channel margin linear bar. On the distal lobe lies the intertidal bar Outer Pole that cyclicly migrates onshore. A marginal flood channel is also present between Spratt Sand and the beach. The Teign estuary ebb-tidal delta seems to be an excellent location for a process study on the conceptual sediment dynamics model as proposed by [1]. Focus of the study should therefore be on the northern part of the ebb-tidal delta where the intertidal bar Outer Pole migrates onshore.

Based on the above considerations, the main objective of the study is to identify the main sediment transport processes on the intertidal area in the northern part of the ebb-tidal delta and to determine the relative influence of wave, wave driven current and tidal current induced sediment transport on the onshore movement of the intertidal sandbar.

Teignmouth is situated on the southern coast of Devon, UK, and was one of the two sites of the COAST3D project. A large amount of data was gathered during this project which finished in March 2001.

Teignmouth study area

The coastal environment of Teignmouth consists of a tidal inlet which connects the Teign estuary with Lyme Bay, part of The English Channel. A typical ebb-tidal delta exists on the northern part of the seaward side of the inlet. Compare the idealized ebb- tidal delta morphology proposed by [3] and the Teign ebb-tidal delta (photo copyright South West Water).

The idealized ebb-tidal delta morphology proposed by Hayes
Teignmouth ebb-tidel delta (photo copyright South West Water)
A major difference between the two ebb-tidal deltas is the cliff that borders the southern side of the inlet against which the main channel is fixed.

Project approach

The tidal and wave field characteristics at the ebb-tidal delta will be analysed using COAST3D measurements, consisting of several tripod measurements, ARGUS images and X-band RADAR, and with DELFT3D model calculations. The models were fully calibrated during the COAST3D project and performed fairly well[4]. The analysis will lead to more insight in the general conditions that force the build-up and the onshore movement of the intertidal bar.

Detailed sediment transport and hydrodynamics measurements will be conducted at the onshore moving intertidal bar to gain more insight in the relative contribution of waves and currents and the effect of the tide on the onshore movement of the intertidal bar.

Please contact Pim van Santen for more info on this project.

References