The Importance of Combined Tidal and Meteorological Forces for the Flow and Sediment Transport on Intertidal Shoals

Abstract

Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind‐driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.

Variations in storm-induced bed level dynamics across intertidal flats

“…The magnitude of the flow velocity on the flat at the peak of the storm is in line with the estimate of wind-driven velocities of order 1/40 of the wind speed 28 , with smaller contributions for larger tidal forcing (tidal forcing is limited in these hours around low water) and larger water depths (where inertia is significant). When the flow was wind-dominated (e.g., at the peak of the storm), the flow was aligned with the wind direction while geometrical constraints by the bathymetry still had to be followed 28 , 32 (i.e., predominantly alongshore directed flow). Secondly, the limited flow and wave effects in the channel imply large variations in sediment transport between the channel and the intertidal flat (i.e., implying bed level changes).…”

“…At the low elevations of the intertidal flat the tidal flow velocities are small (almost stagnant) for the critical water depths (Fig. 7 h), but can be amplified by the wind especially for small tidal velocities 28 . At higher elevations the tidal velocities may peak at the critical water depths (as these velocity peaks occur at higher water levels), although the magnitude of these velocities is there less (Figs.…”

Effects of Shoal Margin Collapses on the Morphodynamics of a Sandy Estuary

“…The rate of infilling of erosion scars was less in the model compared to field observations (Figure a), whereas the field observation of erosion of the associated deposit compares more closely to the modeled results. The difference indicates that the model has difficulties with the simulation of the shallower regions, maybe because no extreme water level conditions are included (only one repeated month of real time series) nor combined astronomical and meteorological forces (De Vet et al, ). Additionally, filling of the scars increases when a mud fraction is added to the model (Van Schaick, ).…”

Morphodynamic Resilience of Intertidal Mudflats on a Seasonal Time Scale

“…Tidal forcing and wave action have been long recognized as the main drivers for the mudflat's shape and morphodynamic evolution (e.g., Friedrichs & Aubrey, 1996, Bearman et al, 2010Hunt et al, 2015;Mariotti & Fagherazzi, 2013a). Wind-driven flows (e.g., de Vet et al, 2018), human interventions (de Vet et al, 2017), and the amount of sediment transported into the system with either marine or fluvial origin play an important role as well.…”

“…Variations occur on time scales ranging from hourly storms, to intertidal dynamics, century varying sediment supply, and beyond (Friedrichs, 2011). Examples are wave action during storm conditions (e.g., Kohsiek et al, 1988;Zhu et al, 2017), changes in tidal difference due to damming (e.g., Louters et al, 1998, de Vet et al, 2017, neap-spring tidal oscillations (e.g., Kohsiek et al, 1988, Zhu et al, 2017, seasonal variations in presence of biofilms (e.g., Paarlberg et al, 2005), seasonally varying wave climate (Fan et al, 2006) or wind climate (e.g., De Vet et al, 2018), or yearly and decadal changes in sediment supply (e.g., Allison et al, 1995;Jaffe & Foxgrover, 2006). Belliard et al (2019) further suggest that the relative importance of wave forcing compared to tidal forcing varies depending on the location on the flat.…”