Modelling the bulk flow of a bedrock‐constrained, multi‐channel reach of the Mekong River, Siphandone, southern Laos

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 significant 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 adjacent 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.

Discussion of ‘Field evidence and hydraulic modeling of a large Holocene jökulhlaup at Jökulsá á Fjöllum channel, Iceland’ by Douglas Howard, Sheryl Luzzadder-Beach and Timothy Beach, 2012

“…More importantly, whilst Howard et al (2012) seemed to acknowledge that they should have varied Manning's n in Section 3.4 they failed to do this. This omission is a problem because the stage-dependant nature of Manning's n is well known; by way of example, the paper by Van et al (2012) highlights the importance of varying Manning's n as stage changes. Whilst Alho et al (2005) used Manning's n values for a flood of up to 40 m flow depth, Howard et al (2012) suggested floods of 160 m and 210 m deep.…”

Modeling of water flow in multi-channel river system in the Narew National Park

“…Tabata and Hickin (2003) concluded their study, concerning hydraulic effi ciency of anastomosing Columbia River (Canada), that there is a great need for additional studies investigating inter-channel hydraulic properties and fl ow conditions in anastomosing rivers. Similar to ours, but more simplifi ed study was presented by Van et al (2012), who developed a one-dimensional, steady-state HEC-RAS model for anastomosing section of Mekong river, considering two different hydrological conditions (high and low water stages). Since they declared no fi eld measure- Yang et al (2014) stated that selection of an appropriate value for Manning's coeffi cient signifi cantly impacts the accuracy of a hydraulic model.…”

“…Less popular, because of the limited number of research areas, is the use of models for multi-channel rivers. The specifi city of such systems requires detailed recognition of fl ow distribution conditions between existing anabranches and accurate measurement of their geometry ( Van et al 2012). An important factor infl uencing the fl ow resistance in channels of the anastomosing river system is vegetation (Kiczko et al 2011).…”

Model-Based Analysis of Macrophytes Role in the Flow Distribution in the Anastomosing River System

“…In both studies, Manning's coefficient was used to describe channel roughness. Moreover, Van et al [32] used Manning's coefficient to estimate the impact of different land cover change scenarios on water flow. Similar to this study, they used fixed Manning's values for different types …”

“…In cases where 1D models were used and the energy conservation equation was solved, the modeling was limited to only one bifurcation, as for proper model validation discharge measurements in all anabranches are required [32]. In experiments investigating vegetation-flow interactions and more complex geomorphological responses [10][11][12][13], upstream boundary conditions of flow were fixed either as a hydrograph or a constant flow.…”