Future projections of flood dynamics in the Vietnamese Mekong Delta

Abstract

The annual flood pulse of the Mekong River is crucial to sustain agriculture production, nutrition, and the livelihood of millions of people living in the Vietnamese part of the Mekong Delta (VMD). However, climate change impacts on precipitation, temperature and sea-level combined with land subsidence, upstream hydropower development, and water infrastructures (i.e. high-dykes construction) are altering the hydrological regime of the VMD.This study investigates future changes in flood hazard and agricultural production caused by these different scales of human-induced stresses. A quasi-two-dimensional (quasi-2D) hydrodynamic model was used to simulate eight scenarios representing the individual and compound impacts of these drivers for a baseline and future (2036-2065) period. The scenarios map the most likely future pathway of climate change (RCP 4.5) combined with the best available Mekong upstream hydropower development, and land subsidence scenarios as well as the current delta development plan. We found that sea-level rise and land subsidence would cause the highest changes in flood hazard and damage to rice crop, followed by hydropower and climate change impacts. Expansion of high-dyke areas in two northernmost delta provinces (An Giang and Dong Thap) would have the smallest impact. The combination of all modelled drivers is projected to increase delta inundation extent by 20%, accompanied with prolonging submergence of 1-2 months, and 2-3 times increase in annual flood damage to rice crops in the flood-prone areas of the VMD. These findings of likely increasing risk of tidal induced flood hazard and damage call for well-planned adaptation and mitigation measures, both structural and non-structural. Highlights Climate change, hydropower, dyke development, sea-level rise and delta land subsidence strongly alter the delta future flood hazard and damage.  Higher flood extent (22-26%) and prolong inundation (1-2 months) are mainly driven by effective sea-level 5 rise and climate change.  Hydropower development might counteract the increase flood hazard and damage driven by climate change.  A reliable seasonal flood forecast (by August) would effective help to minimize agriculture flood damage.  Emerging call to develop flood management plan with focus on tidal induce inundation hazard.

Accounting for Multisectoral Dynamics in Supporting Equitable Adaptation Planning: A Case Study on the Rice Agriculture in the Vietnam Mekong Delta

“…Previous studies support this, as they show almost the same change in precipitation and evaporation, which are the two main drivers of river discharge, up to 2050 under both RCP 4.5 and 8.5 in Cambodia and the VMD (Lee & Dang, 2018; van Oldenborgh et al., 2013). This also aligns with a recent study that finds that in the short to medium term, climate‐induced discharge changes do not substantially increase flood risks in the delta (Triet, Dung, Hoang, et al., 2020).…”

Evaluating the Impacts of Rice-Based Protection Dykes on Floodwater Dynamics in the Vietnamese Mekong Delta Using Geographical Impact Factor (GIF)

“…It applies a dynamic wave description to solve the vertically integrated equations of the conservation of continuity and momentum referred to as Saint Venant equations [36]. This has been described in previous studies to serve modelling of water levels and floods for the whole VMD and its floodplains [9,10,13,20,35,37,38].…”

“…However, the delta has been facing several natural challenges due to river-bed mining and dyke construction [3], coastal erosion [4], land subsidence [5], salinity intrusion [6,7], hydropower dam developments [8], and especially flooding [3,9,10]. In addition, the VMD is one of the deltas most vulnerable to increased flooding with climate change [8,11] and sea level rise [12][13][14].…”

“…They evaluated the impacts of high dykes in the LXQ and PoR with the boundary conditions of high floods in 1981, 1991, and 2000. Last but not least, Triet et al [13] examined the projections in flooding hazard and agricultural production due to different scenarios of human activities between two periods of baseline and future (2036-2065). The study found that sea level rise and land subsidence are the major factors for the changes in flood hazard and influenced on rice cropping, the secondary factors relating to the development of hydropower in upstream countries as well as the impact of climate change.…”

Phytoplankton of the delta of the Mekong River during the dry season

“…In the territory of Vietnam, the Mekong divides into 9 large distributaries and a considerable number of minor anabranches, forming a large delta. The area of the delta is about 40 000 km², equaling 12% of the total territory of the country (Triet et al, 2020). The river is characterized by extremely high species diversity of aquatic inhabitants.…”