Hydrology and Hydraulics

Can Tho city is the largest city and the economic heart of the Mekong Delta, Vietnam. Due to its economic importance and envisaged development goals the city has witnessed a large growth in both population size and extend over the last two decades. Although city development plans exist, the city is growing uncontrolled in many areas, among which some are quite flood prone.

This study focus on the suspended sediment dynamics and sediment-nutrient deposition in the whole Mekong Delta (MD), by apply quasi-2D hydrodynamic model combined with a measurement campaign on sediment-nutrient deposition in floodplains in Vietnam.

The hydrological model SWIM (Krysanova et al., 1998) was set up to assess the discharge and sediment dynamics of the  Mekong  basin down to the gauging station in Kratie/Cambodia. The model is driven by ERA-INTERIM reanalysis data (Dee et al., 2011) and APHRODITE (Yatagai et al., 2009) precipitation data. The model calibration and validation is, at a first step, based on the discharge and sediment data provided by the Mekong River Commission (MRC) and a multi-objective calibration aiming at optimal model performance in the time and frequency domain. With respect to the heterogeneous (temporal and spatial) data availability, different parameter regions are defined (see Fig.  1).

Floodplain sedimentation plays an important role for the agro-ecosystem in the Mekong Delta. In phase 1 of WISDOM the sediment dynamics and processes were studies in detail on a selected part of the floodplain. In this second phase, the spatial variability of floodplain sediment deposition is both monitored and simulated. For this purpose an extensive monitoring scheme was implemented for the flood season 2011. It consists of approximately 450 sediment traps distributed over the main inundation areas of the Mekong Delta (Plain of Reeds and Long Xuyen Quadrangle).

Flood hazard analysis is an indispensable input for flood risk assessment.  An essential part is the determination of probabilities of occurrence of floods of different magnitudes. However, the underlying assumption of stationarity does not hold for most of the observed discharge time series in general, and in particular not for future climate conditions. This is of particular importance for low lying coastal areas and estuaries like the Mekong Delta, which is one of the most vulnerable areas for climate change impacts world-wide.

This short report shows some results of the floodplain monitoring in the Dong Thap province of the Mekong Delta. Figure 1 compares water level time series for two stations, representative for main river (T1) and secondary channel in floodplains (H11), respectively, for the years 2008 and 2009. It can be seen that the hydrograph characteristics in the channels and the floodplains are generally identical, except of the tidal influence. The tidal influence is dampened in the floodplains due to lower inundation depths and the low momentum of the huge inundation area in the floodplains.

A 1D hydrodynamic model of the Mekong Delta developed at SIWRR was further improved by collecting more data on dike lines and elevations and an improved representation of the floodplains in the one-dimensional model domain. With the new data it was calibrated against the record flood in 2000 and additionally validated by simulating the flood in 2001. Both calibration and validation runs show a very good agreement with the recorded discharges.

Part I – the instrumentation scheme

The different inundation survey systems were deployed to the field as illustrated in Figure 1.

Part I – the instrumentation scheme

The different inundation survey systems were deployed to the field as illustrated in Figure 1.

The investigation area in Tam Nong has been equipped with a large number of sensors in order to monitor the inundation and sediment transport dynamics. The whole instrumentation scheme encompasses: