Alluvial connectivity in multi‐channel networks in rivers and estuaries

Abstract

Channels in rivers and estuaries are the main paths of fluvial and tidal currents that transport sediment through the system. While network representations of multichannel 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.

Building and raising land: meandering rivers and filling estuaries

“…This would be in contrast to the current theory of macro cells in tidal systems (e.g. Van Dijk et al, 2019;Sonke et al, 2022), which suggests that perturbations do not propagate to other downstream or upstream macro cells. This question requires further work.…”

Effects of Sea‐Level Rise on Dredging and Dredged Estuary Morphology

“…The experiments represent idealized estuaries with uninterrupted flow and sediment transport connectivity (Sonke et al., 2022), but many urban systems have upstream dams and downstream sluices which determine the flow of both sediment and water (Cox, Huismans, et al., 2021; Nienhuis & van de Wal, 2021; van Wesenbeeck et al., 2014). These structures can divide and separate the effects of SLR unevenly in different parts of the system.…”

“…The channel centerline was identified as the lowest path from river to mouth by an algorithm for channel detection (Sonke et al., 2022). The migration of meander bends and the overall change in river planform was quantified as sinuosity, which was computed as the length of the main channel divided by the valley length (i.e., 18 m).…”

Connectivity and directionality in estuarine channel networks

“…However, this approach is currently limited by a lack of network‐scale flux measurements in estuaries due to logistical or instrumentation constraints. Validated numerical modeling results combined with networks extracted from high‐resolution topography (e.g., Sonke et al, 2021, this issue) are good candidates to test the efficacy graph theory to produce reasonable estimates of tidal flux asymmetry in estuaries.…”

“…Additionally, physics‐based numerical models that qualitatively reproduce channel and shoal patterns in estuaries (Lanzoni & Seminara, 2002; Townend, 2012; Van der Wegen & Roelvink, 2012) have improved our understanding of processes and emergent patterns in estuarine morphodynamics. Recent studies (e.g., Hiatt et al, 2020; Sonke et al, 2021, this issue; van Dijk et al, 2019) have used algorithmic channel detection to develop networks for geomorphological analysis in estuaries using networks, but using formal graph theory on estuarine networks remains unexplored. Accordingly, we still lack quantitative measures to properly compare and quantify patterns in real‐world estuarine channel networks that could be used to validate numerical and physical experiments of estuarine morphodynamics.…”