Patterns and drivers of daily bed-level dynamics on two tidal flats with contrasting wave exposure

Zhan Hu,Tjeerd J Bouma,Daphne Van Der Wal,Peng Yao

doi:10.1038/s41598-017-07515-y

Abstract

Short-term bed-level dynamics has been identified as one of the main factors affecting biota establishment or retreat on tidal flats. However, due to a lack of proper instruments and intensive labour involved, the pattern and drivers of daily bed-level dynamics are largely unexplored in a spatiotemporal context. In this study, 12 newly-developed automatic bed-level sensors were deployed for nearly 15 months on two tidal flats with contrasting wave exposure, proving an unique dataset of daily bed-level changes and hydrodynamic forcing. By analysing the data, we show that (1) a general steepening trend exists on both tidal flats, even with contrasting wave exposure and different bed sediment grain size; (2) daily morphodynamics level increases towards the sea; (3) tidal forcing sets the general morphological evolution pattern at both sites; (4) wave forcing induces short-term bed-level fluctuations at the wave-exposed site, but similar effect is not seen at the sheltered site with smaller waves; (5) storms provoke aggravated erosion, but the impact is conditioned by tidal levels. This study provides insights in the pattern and drivers of daily intertidal bed-level dynamics, thereby setting a template for future high-resolution field monitoring programmes and inviting in-depth morphodynamic modelling for improved understanding and predictive capability.

Highlights

Tidal flats are highly important ecosystems, given their ecosystem services including providing habitats for unique plants and benthic invertebrates[1,2,3], contributing to coastal defence[4,5,6], as well as hosting migratory birds[7,8,9]
Based on high spatiotemporal resolution data, we have extended on previous studies and reveal detailed pattern and hydrodynamic drivers of bed-level dynamics on two tidal flats with contrasting wave exposure
We show that the magnitude of short-term bed-level variations is high comparing to the net bed-level changes at both study sites

Summary

Introduction

Tidal flats are highly important ecosystems, given their ecosystem services including providing habitats for unique plants and benthic invertebrates[1,2,3], contributing to coastal defence[4,5,6], as well as hosting migratory birds[7,8,9]. Recent studies have shown that such short-term (daily) bed-level fluctuation on tidal flats form a major disturbance to the newly settled benthic invertebrates and vegetation seedlings[23,24,25,26,27,28,29]. The short-term bed-level dynamics can induce the onset of cliff erosion in saltmarsh systems, forming one of the critical tipping points for cyclic saltmarsh dynamics[29,30] As these short-term (daily) bed-level fluctuations are key to controlling the ecological development and thereby biogeomorphic functioning of these tidal flats, we elucidate the processes controlling these crucial but understudied dynamics. Strong erosion during storm events was highlighted[32,34] In these studies, bed shear stress (BSS) induced by wave and tidal current as the chief hydrodynamic forcing was not analysed in concert with the daily bed-level changes. The relation between hydrodynamic forcing and the spatiotemporal pattern of short-term bed-level dynamics remains unknown

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Conclusion