Floodwater impacts on residential areas in floodplain areas along Day River system in emergency situation

Abstract

Hanoi, the capital of Vietnam, is protected from floods by dike systems for the Red and Day Rivers. In emergency situations resulting from catastrophic floods, floodwater from the Red River is discharged into the Van Coc Lake, which is a regulating reservoir that drains into the Day River system through the Day Weir at the downstream end of the Van Coc Lake. The authors investigated the flood mitigation function of the Van Coc Lake and identified an optimal operation scenario for the lake for the largest recorded flood, which occurred between the 16th and 27th of August 1971. However, a comprehensive evaluation of the flood risk level in residential areas within the floodplain areas located along the Day River system to determine an optimal operation scenario has not been conducted. In this study, a two-dimensional depth-integrated hydrodynamic model was employed to investigate flood risk levels in these residential areas when the flood-water level reached 11.0 m, 12.0 m, and 12.72 m (the flood peak) at the Day Weir and at 10 and 30 h after the flood peak, under the optimal operation scenario. The time at which floodwater from the Van Coc Lake reached the Day Weir was referred to as Time-0 in this study and was used to estimate the movement of floodwater. The study area, from the Day Weir to the downstream end, was more than 100 km in length and was divided into three zones: the northern part, designated Zone 1; the central part, designated Zone 2; and the downstream end part, designated Zone 3. The simulation results were used to produce reliable flood susceptibility maps that identify the areas at greatest risk of being impacted by floodwater. At the flood peak time, residential areas in the study area were inundated at levels of 84.00% (9.50 km(2)) in Zone 1, 71.84% (29.18 km(2)) in Zone 2, and 98.59% (13.31 km(2)) in Zone 3. Floodwater needed 75, 87, and 135 h from Time-0 to reach 11.0 m, 12.0 m, and the flood peak at 12.72 m at the Day Weir. The results of this study form a basis for future land planning and the design of risk reduction strategies for the Day River system and serve as a strong warning about the dangers of floodwater throughout the study area.