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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. [ABSTRACT FROM AUTHOR]

Channels in rivers and estuaries are the main paths of fluvial and tidal currents that transport sediment through the system. While network representations of multi‐channel systems and their connectivity are quite useful for characterisation of braiding patterns and dynamics, the recognition of channels and their properties is complicated because of the large bed elevation variations, such as shallow shoals and bed steps that render channels visually disconnected. We present and analyse two mathematically rigorous methods to identify channel networks from a terrain model of the river bed. Both methods construct a dense network of locally steepest‐descent channels from saddle points on the terrain, and select a subset of channels with a certain minimum sediment volume between them. This is closely linked to the main mechanism of channel formation and change by displacement of sediment volume. The two methods differ in how they compute these sediment volumes: either globally through the entire length of the river, or locally. We compare the methods for the measured bathymetry of the Western Scheldt estuary, The Netherlands, over the past decades. The global method is overly sensitive to small changes elsewhere in the network compared to the local method. We conclude that the local method works best conceptually and for stability reasons. The associated concept of alluvial connectivity between channels in a network is thus the inverse of the volume of sediment that must be displaced to merge the channels. Our method opens up possibilities for new analyses as shown in two examples. First, it shows a clear pattern of scale dependence on volume of the total network length and of the number of nodes by a power law relation, showing that the smaller channels are relatively much shorter. Second, channel bifurcations were found to be predominantly mildly asymmetrical, which is unexpected from fluvial bifurcation theory. [ABSTRACT FROM AUTHOR]

Unsustainable exploitation of groundwater in northwestern India has led to extreme but spatially variable depletion of the alluvial aquifer system in the region. Mitigation and management of groundwater resources require an understanding of the drivers behind the pattern and magnitude of groundwater depletion, but a regional perspective on these drivers has been lacking. The objectives of this study are to (1) understand the extent to which the observed pattern of groundwater level change can be explained by the drivers of precipitation, potential evapotranspiration, abstraction, and canal irrigation, and (2) understand how the impacts of these drivers may vary depending on the underlying geological heterogeneity of the system. We used a transfer function-noise (TFN) time series approach to quantify the effect of the various driver components in the period 1974–2010, based on predefined impulse response functions (θ). The dynamic response to abstraction, summarized by the zeroth moment of the response M 0, is spatially variable but is generally large across the proximal and middle parts of the study area, particularly where abstraction is high but alluvial aquifer bodies are less abundant. In contrast, the precipitation response is rapid and fairly uniform across the study area. At larger distances from the Himalayan front, observed groundwater level rise can be explained predominantly by canal irrigation. We conclude that the geological heterogeneity of the aquifer system, which is imposed by the geomorphic setting, affects the response of the aquifer system to the imposed drivers. This heterogeneity thus provides a useful framework that can guide mitigation efforts; for example, efforts to decrease abstraction rates should be focused on areas with thinner and less abundant aquifer bodies. [ABSTRACT FROM AUTHOR]

Estuaries are characterized by intertidal bars, i.e. shoals, that are encompassed by ebb and flood channels. Division of the flow at channel junctions into the ebb and flood channels indicates that the estuary consists of bifurcations that become confluences when flow reverts. While disturbances, such as shoal margin collapses (Van Dijk et al., 2019) only have a near-field effect on erosion and deposition at individual channel scale, the disturbance has a far-field effect on elevation jumps and flow division at the channel junctions. Because tidal asymmetry, produced by the distortion of the propagating tidal wave, is observed, we expect that a disturbance will migrate differently through the flood- and ebb-dominated channels. Our aim is to determine if we can approximate tidal bifurcations as river bifurcations, and quantify how dredging and disposal affect the channel network and bifurcation asymmetry at channel junctions. Therefore, we analysed bathymetry of the Western Scheldt (The Netherlands) since 1955 and used a Delft3D schematization of the Western Scheldt to isolate the effect of dredging and disposal strategies. We applied a novel channel network extraction tool, to determine changes in the bifurcation asymmetry and tidal asymmetry between the main, side and connecting channel scales.The tidal asymmetry in the Western Scheldt, represented by peak velocity ratio and period of flood-ebb ratio, shows that generally, the duration of the ebb flow is longer, whereas the flow current is stronger. Model results indicate that the bifurcation asymmetry is hardly affected by dredging. Tidal asymmetry changes depending on the disposal locations. The ebb period becomes longer and stronger at bifurcations in case dredged sediment is disposed of in the side channels. Dredging of the main channel leads to an increased elevation jump between the high-order channel and the bifurcating channel. We expected that the bifurcating channels would close off because the increase in elevation jump, however, both channels remains open because the increase in gradient is opposed by an increase in sinuosity, indicated by the bifurcation angle between the ebb and flood channels. Bifurcations become less stable in the case of dredging, which is indicated by the decrease in the number of bifurcations and the decrease in the number of ebb and flood channels in the channel networks.We conclude that tidal bifurcations, described by a bifurcation angle and elevation jump, can be approximated as river bifurcations. While dredging and disposal affect the stability of bifurcations by increasing the ebb period and decreasing the peak velocity ratio. The stability of bifurcations is important as it determines the number of connecting channels and the continuation of the multi-channel system, which affects the area of tidal flat and biodiversity in the Western Scheldt.Van Dijk, W.M., Hiatt, M.R. Van der Werf, J.J. and Kleinhans, M.G. (2019), Effects of shoal margin collapses on the morphodynamics of a sandy estuary. Journal of Geophysical Research – Earth Surface. [ABSTRACT FROM AUTHOR]

Hydromorphological data for many estuaries worldwide is scarce and usually limited to offshore tidal amplitude and remotely-sensed imagery. In many projects, information about morphology and intertidal area is needed to assess the effects of human interventions and rising sea-level on the natural depth distribution and on changing habitats. Habitat area depends on the spatial pattern of intertidal area, inundation time, peak flow velocities and salinity. While numerical models can reproduce these spatial patterns fairly well, their data need and computational costs are high and for each case a new model must be developed. Here, we present a Python tool that includes a comprehensive set of relations that predicts the hydrodynamics, bed elevation and the patterns of channels and bars in mere seconds. Predictions are based on a combination of empirical relations derived from natural estuaries, including a novel predictor for cross-sectional depth distributions, which is dependent on the along-channel width profile. Flow velocity, an important habitat characteristic, is calculated with a new correlation between depth below high water level and peak tidal flow velocity, which was based on spatial numerical modelling. Salinity is calculated from estuarine geometry and flow conditions. The tool only requires an along-channel width profile and tidal amplitude, making it useful for quick assessments, for example of potential habitat in ecology, when only remotely-sensed imagery is available. [ABSTRACT FROM AUTHOR]