Hydraulic modelling and flood inundation mapping in a bedrock-confined anabranching network: the Mekong River in the Siphandone Wetlands, Laos

Abstract

Anabranching fluvial networks recently have become the focus of attention from
environmental specialists, especially in the hydraulic field. Anabranching networks can be
found in different physical environments; however, the hydraulic and geomorphological
natures of such river networks are still not well known leading to on-going discussions on the
definition and nature of the networks. Even though, alluvial anabranching networks generally
have common features like vegetated islands, low water surface slope and stable channel
planform, bedrock-confined anabranching networks also have their own characteristics
inherited from the geological and structural controls imposed on the single channels that
compose the network complex.

This thesis focuses on the provision of a benchmark describing the bulk hydraulic
characteristics of a large bedrock-confined, anabranching river network, located within
southern Laos. The network can be separated into: (i) the upper river network constituted by
two bifurcations and one confluence with an interpolated bathymetry based on soundings of
cross-sections along the navigation channels; and, (ii) the downstream river network
characterised by a complex anabranching network with five bifurcations and five confluences
for which there is no bathymetric survey.

The river network as whole is a ‘composite’ – partly bedrock (especially the channel-bed)
and partly alluvial-filled and as such it does not accord fully with any prior description or
classification of anabranching channel networks (e.g. Huang and Nanson, 1996). To
understand the hydraulic nature of the river network, the energy approach in a onedimensional
(1D) steady-flow hydraulic model (HEC-RAS) was applied to the network.
Significant challenges arose due to the lack of boundary conditions throughout the model,
namely: (i) unknown splitting discharge ratios at each bifurcation; (ii) partly non-survey
bathymetry; and, (iii) ungauged downstream boundary condition of one of the channel
outlets. To determine the discharge entering each channel, the splitting discharge ratio at each
bifurcation was defined originally by the ratio of the cross-sectional area of the first crosssection
of each downstream channel and then adjusted based on the Flow Optimization
function in HEC-RAS to minimize any rise or drop of the modelled water surface around a
junction. For the channels with non-surveyed bathymetry, a SPOT satellite image was
processed to construct a pseudo-bathymetry showing a range of elevations, including shallow
and deep portions of channels, rather than detailed bed elevations as would be obtained from
a measured bathymetry. To define the boundary condition of the ungauged channel outlet, the
water surface elevation was interpolated and validated according to predefined assumptions
(i.e. the water surface slope along the ungauged channel was interpolated according to the
available DEM and cross-sectional width extracted from a SPOT image for low discharge
conditions was assumed to be similar to the gauged channels for flooding discharges).
In general, the study has helped to develop methods to model the complex river network with
data constraints (i.e. the boundary conditions). The findings include: (i) the developed
pseudo-bathymetry based on a SPOT image is useful to model a large river network using the
energy approach in a 1D hydraulic model in which the cross-sectional area is important in
modelling the bulk hydraulic parameters but the influence of the cross-sectional shape is
subordinate; (ii) the in-channel hydraulic roughness coefficient at each cross-section may be
significantly different from neighbouring values due to the variation in the local bedrock
roughness and the roughness of intervening alluvial reaches; and, (iii) the hydraulic
roughness of the riparian land cover along the floodplains does not contribute noticeably to
the modelled stage along the river network nor to the planform extent of flooding for
overbank flooding discharges. Rather, changes in land-cover, and hence the riparian
roughness, are registered as small, but measureable, changes in the local velocity over the
riparian floodplain and in the average in-channel velocity.
Citations:
Van, P.D.T., 2009. Hydraulic modelling and flood inundation mapping in a bedrockconfined
anabranching network: The Mekong River in the Siphandone wetlands, Laos.
Unpublished PhD thesis submitted to the Faculty of Engineering, Science and Mathematics,
University of Southampton, England.

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ORCID for P.M. Atkinson: orcid.org/0000-0002-5489-6880

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