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

Anabranching fluvial networks recently have become the focus of attention from<br/>environmental specialists, especially in the hydraulic field. Anabranching networks can be<br/>found in different physical environments; however, the hydraulic and geomorphological<br/>natures of such river networks are still not well known leading to on-going discussions on the<br/>definition and nature of the networks. Even though, alluvial anabranching networks generally<br/>have common features like vegetated islands, low water surface slope and stable channel<br/>planform, bedrock-confined anabranching networks also have their own characteristics<br/>inherited from the geological and structural controls imposed on the single channels that<br/>compose the network complex.<br/><br/>This thesis focuses on the provision of a benchmark describing the bulk hydraulic<br/>characteristics of a large bedrock-confined, anabranching river network, located within<br/>southern Laos. The network can be separated into: (i) the upper river network constituted by<br/>two bifurcations and one confluence with an interpolated bathymetry based on soundings of<br/>cross-sections along the navigation channels; and, (ii) the downstream river network<br/>characterised by a complex anabranching network with five bifurcations and five confluences<br/>for which there is no bathymetric survey.<br/><br/>The river network as whole is a ‘composite’ – partly bedrock (especially the channel-bed)<br/>and partly alluvial-filled and as such it does not accord fully with any prior description or<br/>classification of anabranching channel networks (e.g. Huang and Nanson, 1996). To<br/>understand the hydraulic nature of the river network, the energy approach in a onedimensional<br/>(1D) steady-flow hydraulic model (HEC-RAS) was applied to the network.<br/>Significant challenges arose due to the lack of boundary conditions throughout the model,<br/>namely: (i) unknown splitting discharge ratios at each bifurcation; (ii) partly non-survey<br/>bathymetry; and, (iii) ungauged downstream boundary condition of one of the channel<br/>outlets. To determine the discharge entering each channel, the splitting discharge ratio at each<br/>bifurcation was defined originally by the ratio of the cross-sectional area of the first crosssection<br/>of each downstream channel and then adjusted based on the Flow Optimization<br/>function in HEC-RAS to minimize any rise or drop of the modelled water surface around a<br/>junction. For the channels with non-surveyed bathymetry, a SPOT satellite image was<br/>processed to construct a pseudo-bathymetry showing a range of elevations, including shallow<br/>and deep portions of channels, rather than detailed bed elevations as would be obtained from<br/>a measured bathymetry. To define the boundary condition of the ungauged channel outlet, the<br/>water surface elevation was interpolated and validated according to predefined assumptions<br/>(i.e. the water surface slope along the ungauged channel was interpolated according to the<br/>available DEM and cross-sectional width extracted from a SPOT image for low discharge<br/>conditions was assumed to be similar to the gauged channels for flooding discharges).<br/>In general, the study has helped to develop methods to model the complex river network with<br/>data constraints (i.e. the boundary conditions). The findings include: (i) the developed<br/>pseudo-bathymetry based on a SPOT image is useful to model a large river network using the<br/>energy approach in a 1D hydraulic model in which the cross-sectional area is important in<br/>modelling the bulk hydraulic parameters but the influence of the cross-sectional shape is<br/>subordinate; (ii) the in-channel hydraulic roughness coefficient at each cross-section may be<br/>significantly different from neighbouring values due to the variation in the local bedrock<br/>roughness and the roughness of intervening alluvial reaches; and, (iii) the hydraulic<br/>roughness of the riparian land cover along the floodplains does not contribute noticeably to<br/>the modelled stage along the river network nor to the planform extent of flooding for<br/>overbank flooding discharges. Rather, changes in land-cover, and hence the riparian<br/>roughness, are registered as small, but measureable, changes in the local velocity over the<br/>riparian floodplain and in the average in-channel velocity.<br/>Citations:<br/>Van, P.D.T., 2009. Hydraulic modelling and flood inundation mapping in a bedrockconfined<br/>anabranching network: The Mekong River in the Siphandone wetlands, Laos.<br/>Unpublished PhD thesis submitted to the Faculty of Engineering, Science and Mathematics,<br/>University of Southampton, England.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Van, Tri Pham Dang

Advisors dc:contributor.advisor

• Carling, Paul
• Atkinson, P.M.