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

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.

Quantifying Bed Level Change at the Transition of Tidal Flat and Salt Marsh: Can We Understand the Lateral Location of the Marsh Edge?

“…Zuidgors did not result in sufficient bed level heights after conversion. However, the obtained bed level changes were comparable with studies wherein similar measurements were conducted (e.g., Hu et al, ; Lawler, ).…”

“…This confirms the findings of earlier studies having spatial data on short-term bed level dynamics (e.g., Bouma et al, 2016;. It furthermore agrees with long-term studies (multiple years) that also showed increasing bed level dynamics over the tidal flat toward the sea (e.g., Andersen et al, 2006;Hu et al, 2017;O'Brien et al, 2000). The previously mentioned studies were extended by the present study with measurements at the transition from tidal flat to marsh, resulting in observations both on the vegetated marsh and tidal flat.…”

“…Bed level change is an interesting parameter in that it integrates a suite of physical and biological processes on the tidal flat and salt marsh that potentially influence lateral dynamics: wave and/or tidal forcing exerting bed shear stresses (Friedrichs, ; Green & Coco, ; Hu et al, ; Hunt et al, ), grain size properties determining the critical erosion threshold (Deegan et al, ; Grabowski et al, ), bioturbation and bioaggregation (seasonally) modifying the critical erosion threshold of the bed (Andersen et al, ; Corenblit et al, ; Murray et al, ), and external sediment supply controlling sedimentation rates (Hu, Wang, et al, ; Mariotti & Fagherazzi, ; Willemsen et al, ). By affecting both salt marsh expansion and retreat, short‐term bed level dynamics (i.e., seasonal and shorter changes in sediment elevation or bed level) at the tidal flat in front of a marsh have been indicated as the driving mechanism connecting long‐term cyclic behavior of the marsh to large‐scale physical forcing (Bouma et al, ).…”

Wave climate and sediment dynamics in the tidal flats and marshes of Galgeschoor: Synthesis report

“…Deloffre et al (2005) concluded that tidal forcing was also the dominant driving force responsible for the erosion of a mudflat located in the upper macrotidal Seine estuary, France. This characteristic time-scale is also present in the variation of τc as well as τ tot during storm events, which was significantly correlated to bed-level dynamics at Zuidgors and Baarland (Hu et al 2017). Yet, contrary to Hu et al (2017) tidal forcing cannot explain variation in long-term bed level dynamics along the Galgeschoor.…”

“…This characteristic time-scale is also present in the variation of τc as well as τ tot during storm events, which was significantly correlated to bed-level dynamics at Zuidgors and Baarland (Hu et al 2017). Yet, contrary to Hu et al (2017) tidal forcing cannot explain variation in long-term bed level dynamics along the Galgeschoor. Spatial variations in sediment characteristics (mud content, particle size, Figure 3.16) interact with tidal forcing in shaping elevation changes at the high plot of GSc2 over the long-term, and possibly other factors may be at work.…”