Observed and modelled tidal bar sedimentology reveals preservation bias against mud in estuarine stratigraphy

Abstract

Mud plays a pivotal role in estuarine ecology and morphology. However, field data on the lateral and vertical depositional record of mud are rare. Furthermore, numerical morphodynamic models often ignore mud due to long computational times and simplifications of mixed depositional processes. This study aims to understand the spatial distribution, formative conditions and preservation of mud deposits in the intertidal zone of bars in high‐energy sand‐dominated estuaries, and to elucidate the effects of mud on morphology, ecology and stratigraphic architecture. To meet these objectives, field data (historic bathymetry, bio‐morphological maps and sediment cores of the shoal of Walsoorden, Western Scheldt estuary, the Netherlands) were combined with complementary hydro‐morphodynamic numerical modelling (Delft3D). Based on the field observations, two types of mud deposits were distinguished: (1) mudflat deposits, which are thick (>10 cm) mud beds at the surface associated with high elevations and low accumulation rates; and (2) mud drapes, which are thin (millimetre to centimetre) buried laminae that form and preserve at a wide range of elevations and energy conditions. Model results show that deposition on mudflats occurs just after high‐tide slack water in areas shielded from high flood velocities, suggesting that mud accumulation is mostly controlled by elevation, flow velocity and flow direction. Mud accumulation increases shoal elevation, sometimes to supratidal levels. This reduces flow over the shoal, which in turn reduces chute channel formation, stabilises bar morphology and decreases local tidal prism. These effects further promote mud deposition and vegetation settling. Although observations show that mud cover at the surface is relatively high (20%–40% of the intertidal area), mud constitutes only a small percentage of the total estuary volume (ca 5%) revealing that only a small fraction is preserved in the stratigraphy. Due to this mismatch between surface and subsurface expression of mud, interpretations of estuarine stratigraphy risk underestimating the influence of mud at the surface on morphodynamics and habitats.

Reservoir Mud Releasing May Suboptimize Fluvial Sand Supply to Coastal Sediment Budget: Modeling the Impact of Shihmen Reservoir Case on Tamsui River Estuary

“…These results coincide with the observations from the world's mouth bar deposits, which remain sandy in spite of the mud‐dominated sediment supply (see Braat et al. (2023), van der Vegt et al. (2020), and references therein).…”

“…Despite a vast amount of mud released from the reservoir, muds transported to the mouth zone were almost entirely (>97%) exported to offshore by flood advection and wave dispersion. These results coincide with the observations from the world's mouth bar deposits, which remain sandy in spite of the mud-dominated sediment supply (see Braat et al (2023), van der Vegt et al (2020), and references therein). In the tide-dominated estuary zone, sand also contributed >90% of the sediment deposit.…”

“…For example, field observations from the Mekong Delta revealed that mobilization of bottom sands and suspended and bedload sand transports were restricted by estuarine mud mantling (Allison, Dallon Weathers, & Meselhe, 2017; Stephens et al., 2017). Field studies of the Scheldt estuary showed that up to 40% of the intertidal area was mantled by muds, limiting the transport of sands that made up 95% of the estuary sediment volume (Braat et al., 2023). A numerical study applying the Delft3D model on an idealized estuary revealed that as the mud supply increased, larger proportions of the bed were mantled by muds whose contents would rise to a mud‐dominated regime, as a result sands became even less erodible (Braat et al., 2017).…”

Mixed depositional processes in coastal to shelf environments: Towards acknowledging their complexity

“…Braat et al (2023) studied the relationship between the surface (horizontal) and subsurface (vertical) expression of mud in tidal bars and its morphodynamic and stratigraphic implications. Due to the limited availability of field data on the lateral and vertical depositional record of mud, new field data was gathered from the Shoal of Walsoorden (Western Scheldt estuary, the Netherlands) and analysed for this study.…”