Modelling the bulk flow of a bedrock-constrained, multi-channel reach of the Mekong River, Siphandone, southern Laos Tri P. D. Van, 1 Paul A. Carling 2 * and Peter M. Atkinson 2 1 School of Environment and Natural Resources, Can Tho University, Ho Chi Minh City, Vietnam 2 Geography & Environment, University of Southampton, Southampton, UK Received 12 January 2011; Revised 16 November 2011; Accepted 17 November 2011 *Correspondence to: Paul A. Carling, Geography & Environment, University of Southampton, Southampton, SO17 1BJ, UK. E-mail: P.A.Carling@soton.ac.uk ABSTRACT: The general nature of bulk flow within bedrock single-channel reaches has been considered by several studies recently. However, the flow structure of a bedrock-constrained, large river with a multiple channel network has not been investigated previously. The multiple channel network of the Siphandone wetlands in Laos, a section of the Mekong River, was modelled using a steady one-dimensional hydraulic model. The river network is characterized by a spatially-varying channel-form leading to signifi- cant changes in the bulk flow properties between and along the channels. The challenge to model the bulk flow in such a remote region was the lack of ideal boundary conditions. The flow models considered both low flow, high inbank and overbank flows and were calibrated using SPOT satellite sensor imagery and limited field data concerning water levels. The application of the model highlighted flow characteristics of a large multi-channel network and also further indicated the field data that would be required to properly characterize the flow field empirically. Important results included the observation that adja- cent channels within the network had different water surface slopes for the same moments in time; thus calibration data for modelling similar systems needs to account for these significant local differences. Further, the in-channel hydraulic roughness coefficient strongly varied from one cross-section to the next (Manning’s ‘n’ range: 001 to 010). These differences were amplified during low flow but persisted in muted form during high discharges. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: (1D) hydraulic modelling; bedrock-constrained; anastomosed; anabranch; Mekong River Introduction One-dimensional (1D) hydraulic models have a long tradi- tion of application, especially being applied to elucidate the bulk hydraulic characteristics of large single-thread rivers (Pappenberger et al., 2005; Remo and Pinter, 2007; Roberts et al., 2007; Shahrokhnia and Javan, 2007; Thompson et al., 2007; Patro et al., 2009). However, to understand how a river behaves when there is a network of interconnecting channels, often with more than one outlet section, remains a challenge. One-dimensional modelling of complex river systems may be an appropriate approach to identify specific flow conditions and locations which require detailed consideration [i.e. two- dimensional (2D) and three-dimensional (3D) modelling]. In ad- dition, such 1D models help identify where additional hydraulic field data are needed, and thus, help focus field research effort effectively. In fact, focusing field effort is an important concern in large complex river networks in remote regions where there are inadequate baseline data and wherein intensive fieldwork is neither practical nor economic. However, a challenge is that limited baseline data may make specifying the boundary condi- tions even for 1D modelling difficult, if not impossible. In remote areas, even if the land surface terrain is well described by a digital elevation model (DEM), there may be limited data describing the wetted channel section (Flener et al., 2010). In addition, other boundary conditions may be quantified only partially, including the inflow characteristics; i.e. measured discharge and stage for only some cross-sections within a selection of the network channels. Moreover, where structurally-constrained bedrock channels are concerned (as in this study), it is most likely that there will not be a smooth down- stream adjustment in channel geometry. Bedrock-constrained channels are defined herein as having a bedrock floor and allu- vial banks. Structural features may constrain bedrock-channel geometries in contrast to alluvial channels which adjust their form readily to the imposed flow and sediment loads. Rather, in bedrock rivers the channel geometry adjusts in the short term only locally due to sediment deposition with erosion of the bed- rock boundary being precluded at the timescales of individual floods. Thus, one might expect the bulk flow parameters of the bedrock-constrained river network to vary complexly due to structurally-imposed channel junction angles, rapidly varying planform, sectional channel geometries and boundary roughness values as structural and lithological controls dictate. Values of the hydraulic roughness of bedrock channels are known to range widely and to include high values (due to struc- turally-controlled sharp steps and joints), including values in excess of those recorded from alluvial systems [e.g. Manning’s EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 37, 533–545 (2012) Copyright © 2012 John Wiley & Sons, Ltd. Published online 3 January 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.2270