Has dyke development in the Vietnamese Mekong Delta shifted flood hazard downstream?

Abstract

Abstract. In the Vietnamese part of the Mekong Delta (VMD) the areas with three rice crops per year have been expanded rapidly during the last 15 years. Paddy-rice cultivation during the flood season has been made possible by implementing high-dyke flood defenses and flood control structures. However, there are widespread claims that the highdyke system has increased water levels in downstream areas. Our study aims at resolving this issue by attributing observed changes in flood characteristics to high-dyke construction and other possible causes. Maximum water levels and duration above the flood alarm level are analysed for gradual trends and step changes at different discharge gauges. Strong and robust increasing trends of peak water levels and duration downstream of the high-dyke areas are found with a step change in 2000/2001, i.e. immediately after the disastrous flood which initiated the high-dyke development. These changes are in contrast to the negative trends detected at stations upstream of the high-dyke areas. This spatially different behaviour of changes in flood characteristics seems to support the public claims. To separate the impact of the highdyke development from the impact of the other drivers -i.e. changes in the flood hydrograph entering the Mekong Delta, and changes in the tidal dynamics -hydraulic model simulations of the two recent large flood events in 2000 and 2011 are performed. The hydraulic model is run for a set of scenarios whereas the different drivers are interchanged. The simulations reveal that for the central VMD an increase of 9-13 cm in flood peak and 15 days in duration can be attributed to high-dyke development. However, for this area the tidal dynamics have an even larger effect in the range of 19-32 cm. However, the relative contributions of the three drivers of change vary in space across the delta. In summary, our study confirms the claims that the high-dyke development has raised the flood hazard downstream. However, it is not the only and not the most important driver of the observed changes. It has to be noted that changes in tidal levels caused by sea level rise in combination with the widely observed land subsidence and the temporal coincidence of high water levels and spring tides have even larger impacts. It is recommended to develop flood risk management strategies using the high-dyke areas as retention zones to mitigate the flood hazard downstream.

Floodplains and Complex Adaptive Systems—Perspectives on Connecting the Dots in Flood Risk Assessment with Coupled Component Models

“…Last but not least, anthropogenic interventions are more or less the most relevant driver of flood risk in a floodplain; that is, the construction of flood defenses such as levees and dams [61][62][63] or river restoration projects [64][65][66]. Furthermore, the construction of levees as flood protection measures in one floodplain can have adverse effects in downstream floodplains [67][68][69][70][71], and thus result in trade-offs between upstream and downstream floodplains [72,73]. Reviews on the impacts of land use changes and regulations on floods are given by Rogger et al [74], Burby et al [75], and O'Connell et al [76].…”

Towards risk-based flood management in highly productive paddy rice cultivation – concept development and application to the Mekong Delta

“…In the final step, we investigated how flood risk will change by two land-use scenarios. Triet et al (2017) proved that construction of high-dyke areas in the northern delta provinces An Giang and Dong Thap increased the flood hazard in the centre of the delta. Thus, the first scenario considers the opening of the sluice gates in the high-dyke areas in these two provinces to introduce floodwater to the paddy fields during the main flood period September-October.…”

“…A large share of the overestimated exposed area of rice crops can be attributed to the simulated inundation extents, although huge efforts have been made in updating, refining, and calibrating the model (Dung et al, 2011;Manh et al, 2014;Triet et al, 2017). The main source of uncertainty stems from the interpolation of 1D model results to a 2D raster, 20 which could not be reduced even by the high resolution LiDAR DEM.…”

“…hydropower dams), local flood control (e.g. dyke systems), climate change impacts and sea level rise by hydrodynamic modelling (Le et al, 2007;Van, 2009;Dinh et al, 2012;Van et al, 2012;Toan, 2014;Triet et al, 2017;Dang et al, 2018). A comprehensive flood hazard analysis for the whole MD can only be found in the study of Dung et al (2015).…”

The Cattle Grooming Behavior and Some Problems with Technological Grooming Instruments for Cow WelfareFlood Hazard Mapping at Long Xuyen Quadrangle in 2015 Using Geographic Information System and Remote Sensing Technologies

“…In contrast, local residents living in downstream section of the delta are not used to tidal-induced floods leading to negative impacts (e.g., damages of the agriculture and aquaculture) [12], leading to negative impacts on the local livelihood [15,16]. Thus, a dense canal system has been created in flood-prone areas to efficiently drain flood waters from the LXQ and the PoR (Plains of Reed) to the West Sea (Gulf of Thailand) and to the Vam Co Rivers, respectively and the high dyke system is intended to reduce local natural flood hazards [17]. The high dykes system in the LXQ can reduce the discharge of the Song Tien, diverting around 7% of the total volume to Song Hau and the inundation periods are slightly shorter due to flood withdraw to the West Sea [18].…”