Modelling the River Mekong using HEC-RAS

Challenge

The Issues of flooding, development, environment and climate change in the Mekong are significant issues for many within the six countries sharing the water of it. Change is occurring at a rapid pace and it is not easy to show how changes in one sector or country will affect another. Experience in the UK and elsewhere shows that the very graphic representation of 2D modelling can help.

The new 5.0 series of HEC-RAS offers many of the features needed so a number of tests have been carried out. Currently 1D/Quasi 2D models of the Cambodia Flood Plain and Mekong Delta are used, in either Mike or ISIS packages.

The models are linked to a decision support system used for planning or flood forecasting and may be run for single yearly events or up to 100-year continuous simulations that are subsequently statistically analysed. The same models are used for water quality and salinity intrusion simulations.

The distance along the mainstream to the sea is 550km but the 1D model has over 10,000km of channel. This, however is a significant simplification as in the Vietnam delta alone there are over 50,000km of primary and secondary channels with flood banks. Previous attempts at 2D modelling of this situation either use large grid sizes or significant simplifications. The more natural condition in Cambodia is easier to model and a HEC 2D model based on the ISIS information has been developed.

Requirements for 2D Hydrodynamics:

  • Sufficient detail
  • Whole Floodplain
  • Tidal
  • Structures
  • Local Rainfall/Inflows
  • Long term events
  • Calibration and Verification
  • Attractive outputs.

Other parameters:

  • Fine Sediments /Channel Morphology
  • Salinity intrusion.

Solution

As a first step, an overtopping model for Phnom Penh was successfully tested. Versions with the urban part only and with the main river in 2D were encouraging.

The preparation of a fully 2D model in HEC seems to be easier to set up than linking 1D sections and has a small-time penalty. However, calibrating the 2D channel requires a robust version of the software including the cell interactions and diffusion used, and thus 1D calibration presently seems more robust.

Based on the available Digital Elevation Model (DEM) the 2D area was defined with break lines for the main rivers at a basic grid of around 200m giving 2.2m cells. This model ran successfully as an initial trial, with one wet season completing in around 24 hours – representing a natural condition on top of which further infrastructure is being added. The main roads and banks showed the expected flow constrictions and HEC-RAS showed the known pattern of flood paths and nicely illustrated patterns using particle tracking and animations.

The next challenge of adding in the numerous channels and banks of the Mekong delta results of this study will start once the model is running under 5.05 and some further calibration and speed comparisons are completed.

Benefit

The HEC 2D software shows promise but there is still a long way to go before ‘proving’ the model can replace the quasi 1D/2D. Efficiently representing the bank and channels of the Mekong delta in a model that can run quickly enough is a major one. The changes in versions 5.03-5.05 should help in the long term but with limited resources it has not yet been possible to compare and benefit fully from the improvements. This is therefore a priority next step for testing.

Want to know more?

Email Tony Green for more information on the River Mekong and Mekong Modelling Associates, our consultancy company for the ASEAN region. You can also find out more about our work in this area on our flood modelling web pages.



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