The Da River Basin is an international basin where available access to hydrological data is limited; it has a total basin area of 52,900 km 2 , about 50% of the area in which it is located, Vietnam. The Da River is the primary source of water for agriculture in 25 provinces and cities, and the primary source of drinking water for more than 30 million people in both urban and rural areas. It has huge economic and historical value. However, flood forecasting for the Da River basin has not been adequately addressed yet because of the challenge of the inconsistency, scarcity, poor spatial representation, as well as difficult access and incompleteness of the availability of ground observed rainfall data. In this research, the IFAS model has been utilized to assess the benefits of using satellite-based precipitation products to create flood forecasting for the whole research area. The results showed that the Integrated Flood Analysis System (IFAS) model was able to integrate the satellite-based precipitation products for simulating the flood event in the Da River basin. Also, the 3B42RT algorithm showed a definite improvement in reproducing the flood peak and low flow very well in the research area. These results could be used to enhance the effectiveness of flood management strategy in the basin.

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 16 th and 27 th 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 floodwater 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.

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Van Coc Lake located in the Dan Phuong and Phuc Tho, Hanoi, Vietnam, has an area of 30.83 km 2. It is usually dry and includes arable land and residential areas. However, to protect Hanoi City from catastrophic floods, in emergency situations, it receives the floodwater from the Red River as a regulating reservoir and drains the floodwater to the downstream Day River through the Day Weir. We performed numerical simulations to assess the movement of floodwater in this lake and evaluated the impact of floodwater on its residential areas. A two-dimensional depth-integrated hydrodynamic model was constructed to simulate inundation in this lake. The model was validated and the effects of Manning's coefficient of roughness were examined. In case of emergency, it has to receive an inflow of 2,500 m 3 /s into the area. Inflow rates of 1,200 and 600 m 3 /s were also examined to compare the results. The simulation results indicate that this lake was entirely inundated in 13, 28, and 56 h by inflow rates of 2,500, 1,200, and 600 m 3 /s, respectively. Between 66% and 99% of residential areas were inundated. The areas with highest velocity were primarily focused behind the gate with a velocity of 1.5-1.8 m/s.