Abstract
River bifurcations have become recognised over the last decade as being
critical but poorly understood elements in many channel systems,
including braided and anastomosing rivers, fluvial lowland plains and
deltas. They control the partitioning of both water and sediment with
consequences for the downstream evolution and for river and coastal
management. Avulsion studies and bifurcation modelling suggest that
symmetrical bifurcations are inherently unstable. However, the
simultaneous activity of channels in deltas, anastomosing rivers and
large braided rivers such as the Jamuna suggest that symmetrical
bifurcations are stable in agreement with sediment transport
optimisation theories. These theories are still a matter of debate. Our
objective is to understand the stability and evolution of the braid
pattern through studying the dynamics of the bifurcations under natural
discharge conditions: both during floods and low flow. Using a series of
Landsat TM images taken at irregular intervals showing inter-annual
variation, we studied the evolution of a large number of bifurcations in
the Jamuna river between 1999 and 2004. The images were first classified
into water, bare sediment and vegetation. The contiguous water body of
the river was then selected and translated into a network description
with bifurcations and confluences at the nodes and interconnecting
channels. Channel width is a crucial attribute of the network channels
as this allows the calculation of bifurcation asymmetry. The key step
here is to describe river network evolution by identifying the same node
in multiple subsequent images as well as new and abandoned nodes, in
order to distinguish migration of bifurcations from avulsion processes.
Nodes in two subsequent images were linked through distance and angle of
the downstream connected channels. Once identified through time, the
changes in node position and the changes in the connected channels can
be quantified Along the entire river the well-known total and active
braiding indices were derived from the network. The development of
bifurcation asymmetry is assessed by comparing the width asymmetry of
each bifurcation in two different, linked scenes. The formation of a
dominant channel is observed during persistent, constant low flow
condition. A vast majority of the bifurcations in the dataset showed an
increase in bifurcation asymmetry during persistent flow conditions,
causing a decrease of the number of parallel channels and the formation
of one or only a few dominant channels. This is in agreement with
bifurcation instability found in bifurcation studies (de Haas and
Kleinhans, this conference). The braided character of the river is
maintained due to the reactivation of abandoned channels and a reset of
bifurcation asymmetry during high discharges in the rain season, causing
an increase in the number of channels. We conclude that, in order to
counteract the inherent instability of bifurcations, the prevalence of
parallel channels in a braided river is the result of bifurcation
re-opening forced by discharge peaks.
Original language | English |
---|---|
Pages | EP31C-0749 |
Publication status | Published - 1 Dec 2010 |
Event | American Geophysical Union Fall Meeting 2010 - San Francisco, USA Duration: 1 Jan 2010 → … |
Conference
Conference | American Geophysical Union Fall Meeting 2010 |
---|---|
City | San Francisco, USA |
Period | 1/01/10 → … |
Bibliographical note
American Geophysical Union 2010 Fall meeting.[EP] Earth and Planetary Surface Processes, [EP31C] The Morphodynamics of Big Rivers: What Do and Don't We Know? II Posters. EP31C-0749Keywords
- [1821] HYDROLOGY / Floods
- [1825] HYDROLOGY / Geomorphology: fluvial
- [1855] HYDROLOGY / Remote sensing
- [1856] HYDROLOGY / River channels