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.