WATER LEVEL CHANGES UNDER INCREASED REGULATED FLOWS AND DEGRADED RIVER IN VIETNAMESE MEKONG DELTA

Abstract

 Maintaining stable water levels during dry seasons is crucial for the irrigation of rice crops in the Vietnamese Mekong Delta (VMD) as agriculture, i.e. rice crops, is the main economic sector in the delta. Dry season water levels in the VMD are impacted by regulated flow discharges and river bathymetry. Hydropower dams, especially six mega mainstream dams in the upper Mekong River have caused tremendous reduction in the sediment budget of the VMD which, in turn, cause severe river bed degradation. Using in-situ bathymetric measurements and 2-D hydrodynamic modelling, this study investigates the alterations of water levels in the upper reach of the VMD as a combined impact of modified flow discharges and river bed degradation scenarios. Three thresholds of increased discharges associated with different degrees of river bed degradations were distinguished. Under low river bed degradation (< 1.5 m), water levels of dry seasons can be improved when the low-flow discharges increase by 10%. Under moderate degradation (1.5 m ≤ degradation ≤ 3 m), an increase of 20% of the low-flow discharge would help conserve the initial water levels. However, under high degradation scale (3 m < degradation ≤ 6 m), an increase of 50% of the low-flow discharge from upstream dams is expected.