Abstract
River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake.In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works.
Highlights
Scour holes are common features in rivers and estuaries
The causes that trigger scour hole formation include turbulent flows induced by river bends (e.g., Engelund, 1974; Zimmermann and Kennedy, 1978; Odgaard, 1981; Andrle, 1994; Gharabaghi et al, 2007; Blanckaert, 2010; Beltaos et al, 2011; Ottevanger et al, 2012; Vermeulen et al, 2015), confluences (e.g., Mosley, 1976; Kjerfve et al, 1979; Best, 1986; Ginsberg and Perillo, 1999; Pierini et al, 2005; Best and Rhoads, 2008; Ginsberg et al, 2009; Ferrarin et al, 2018), local channel narrowings and structures, like bridge piers, groynes and bed protection (e.g., Wang et al, 2017; Pandey et al, 2018; Liang et al, 2020)
The third reason is the lithological influence on scour hole formation, which in current analyses proves to be a major influence on scour hole initiation, growth rate, and shape, and which in certain cases even overrules the above causes and controls
Summary
Scour holes are common features in rivers and estuaries. With their steep slopes and large depths, these scour holes can threaten the stability of nearby infrastructure like embankments, bridge piers, tunnels and pipelines (e.g. Gharabaghi et al, 2007; Beltaos et al, 2011; Wang et al, 2017; Pandey et al, 2018; Liang et al, 2020). In an exploratory study by Huismans et al (2016), the link between scour hole occurrence and the composition of the subsurface lithology was made directly by combining multibeam surveys with detailed geological maps constructed based on lithological data from corings. The Rhine-Meuse Estuary in the Netherlands forms an ideal study area, as more than a hundred scour holes are identified in this area, of which many are expected to be influenced or triggered by heterogeneities in the subsurface lithology (Huismans et al, 2016). Scour hole characteristics in two subreaches with distinct lithological composition are analysed, highlighting the differences in lithological control on the size, growth rate and direction of scour holes
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