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Floodplain sediment dynamics

Suspended sediment dynamics are the primary source for a sustainable agro-ecosystem in the Mekong Delta by providing nutrient input for the subsequent farming season. However, little is known about the dynamics of suspended sediment in the floodplains of the Delta. In particular, quantitative data about the inundation processes in the floodplains are non-existent. In order to investigate the suspended sediment transport in both channels and floodplains in the Plain of Reeds in the Mekong Delta, an extensive monitoring network was established in a representative study area of 165 km2.

A network of continuously operating water quality monitoring stations recorded SSC in floodplains and channels over the three year period 2008-2010. The network was complemented by short-term campaigns to monitor the longitudinal and vertical distribution of SSC in the channel network. It could be shown that the suspended sediment dynamics in the floodplains differ significantly from the channels (cf. Fig. 1), and that the floodplain sedimentation is predominantly controlled by the initial inundation and the last stages of receding water levels. During both phases the human interference with the inundation process is largest, in particular in compartments protected by high dikes.

 

 

 

Fig. 1: a) different sediment concentration in channel (right) and floodplain compartments (left), b) sediment concentration at T2 (channel) and T4 (floodplain) in 2009. SSC in the floodplain compartment (T4) decreases with time, while SSC in the channel (T2) shows a strong variation related to the flow regime. SSC inside the compartment is also less influenced by tidal effects. The floodplain data show that not all SSC will be deposited in the compartment, as the concentration stabilizes at about 20 mg/l.

The analysis of the measurements allows us to conclude:

  • The suspended sediment dynamics in the floodplain of the Mekong Delta rely on two mechanisms: one being the flood wave, the other being the tidal backwater effects having a much higher frequency.
  • The anthropogenic influence on SSC can be qualitatively detected. This influence increases SSC during two periods: sluice gate opening in August and pumping water out of the paddy field between November to December.
  • SSC decreases with in the channels with distance to the main river. An exponential decrease occurs within 10km from the Mekong River; beyond this distance, the concentration tends to stabilize at about 42 mg/l.
  • Not all suspended sediment entering the paddy fields is deposited, because particles are very fine. Therefore, the observed background concentration of SSC in the floodplain was about 20 mg/l.
  • To maximize sediment trapping, i.e. also nutrient input on the floodplains, some important factors should be taken into account:

a)      the suspended sediment concentration in the channel (magnitude and time of occurrence)

b)      the channel flood stage in comparison to the paddy field topography levels

c)       sluice gates located further from the Mekong River should be opened earlier.

 

A manuscript covering this topic is currently under peer review.

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