Apply MIKE models to study the impacts of climate change on flooding and design adaptation measures in Can Tho City in the Vietnamese Mekong Delta.

Can Tho City in the Mekong Delta usually has pluvial and fluvial floods every year. The situation will get worst in the future under climate change. This study aims (i) to apply the MIKE 11, MIKE 21, and MIKE FLOOD model to simulate the impact of climate change scenarios on flooding in Can Tho city (ii) to suggest climate change adaptation measures for sustainable social-economic activities of the city. We considered the combined effect of sea-level rise including RCP 2.6, RCP 4.5, and RCP 8.5, and changes in climate factor (i.e., precipitation) for the years 2025, 2030, and 2050. The results show that the flood model is appropriate to simulate the flood. The flooded areas of the city under the scenarios of RCP 2.6 and RCP 8.5 will be increased from 2025 to 2050, and RCP 4.5 will be increased progressively from the year 2025 to 2030 and reduced to 2050. In the scenario of RCP 8.5, the flooded area of the city will be gradually increased in 2025, 2030, and 2050 at 3,559 ha, 3,719 ha, and 4,476 ha, respectively. Fifteen adaptation measures to climate change from 2021 to 2050 are suggested.

We simulated the flooding under climate change scenarios in Can Tho City by MIKE 11 and MIKE FLOOD models.Then, we gave some climate change adaptation measures for sustainable social-economic activities of the city.

Material and Methods
We used MIKE FLOOD, MIKE 11 HD, and MIKE 21 FM to simulate the floods with three scenarios: RCP 2.6, RCP 4.5, and RCP 8.5 due to the effect of sea-level rise and change of climate factors (i.e., precipitation).This study only focused on precipitation and SLR scenarios, not including the impacts of tidal gates.

2.1.
Urban flooding modeling The river network in the study area, the location of the boundaries, and the stations for calibration and validation of water levels.Hydraulic simulation (MIKE 11 HD) includes 1 upstream water level boundary, 9 downstream water level boundaries, and 2 water level stations used for calibrationvalidation (Figure 1).The collected data include DEM in the study area, river cross-section data collected from the Southern Institute of Irrigation Science, and water level data at measuring stations in the study area collected from the National Hydrometeorological Center in 2017 [16].

2.2.
Climate change scenarios Sea Level Rise Scenarios.In this study, sea-level rise (SLR) scenarios were RCP 2.6, RCP 4.5, and RCP 8.5 in three periods: 2025, 2030, and 2050.The data were referenced from the Climate Change and Sea Level Rise scenarios for Vietnam.The East and West Seas are defined by the Ministry of Natural Resources and Environment (MONRE) [17].The East Sea includes Cape Ke Ga to Cape Ca Mau.The West Sea includes Coastal Area from Cape Ca Mau to Kien Giang.The SLR scenarios for 2016 are slightly lower than the SLR scenarios for 2020 ranging from 1cm to 2cm for 2030 and 2050 of RCP 2.6 and RCP 4.5 at both east and west sea regions [18].For RCP 8.5, the minor differences are 2cm and 3cm for the years 2030 and 2050, respectively.The differences between the 2020 and 2016 scenarios released by MONRE of Vietnam are not significant enough to warrant major changes in planning and mitigation measures for Can Tho City.Thus, this study used the values of SLR released in year 2016 (

3.2.
Results of flooding simulation The sections below present the inundation scenarios in Can Tho city under climate change (i.e., precipitation and temperature) and SLR for 2025, 2030, ad 2050 due to RCP 2.6, RCP 4.5, and RCP 8.5.The areas near Hau River have the most significant impact compared to the other locations.The flooding caused lost land use, such as agriculture, and inundated streets during flooding time.In this study, we mainly analyzed the inundated areas (ha) and inundation rate (% flooded area compared to the total area of the district) of the districts.Besides, the floodwater level is remarkable in some streets near Hau River with 0.3 -0.5m.
In 2025, the risk of flooding in Can Tho is 2.7%, with a flooded area of 3,814ha.Three districts with the highest flooded area are O Mon, Thot Not, and Cai Rang with 501.5ha, 846.8ha, and 1,241.3ha,respectively.In 2030, the risk of flooding is 3.2%, with a flooded area of 4,484 ha.The three districts with the highest flooded area are Thoi Lai, Thot Not, and Cai Rang, with 816.2ha, 880.2ha, and 1,290.2ha,respectively.In 2050, the risk of flooding is 3.4%, with a flooded area of 4,793ha.Three districts with the highest flooded area are Thoi Lai, Thot Not, and Cai Rang, with an inundated area of 888.3ha, 908.8ha, and 1,387.5ha,respectively.2, Figure 3, and Figure 4, respectively.
In 2025, the flood risk of Can Tho city is 3.82%, with a flooded area of 5,381ha.The three districts with the highest flooded area are Thot Not, Cai Rang, and Thoi Lai, with 894.08 ha, 1,278.94ha, and 1,395.01ha, respectively.
In 2030, the flood risk of Can Tho city is 6.23%, with a flooded area of 8,784 ha.Three districts with the highest flooded area are Cai Rang, Co Do, and Thoi Lai, with 1,334.39ha, 1,515.43ha, and 3,127.84ha, respectively.In Thot Not, Tan Loc ward with an area of 32.68 km², most of the area surrounding the ward adjacent to the Hau River, 150m from the Hau River, is flooded with a depth of 0.5.The most flooded area in O Mon District in the southeast area of Phuoc Thoi Commune, close to the Hau River, with a 0.5m.
Although Cai Rang District has a small inundated area in 2030, it is one of the most affected areas due to sea-level rise combined with saline intrusion in Tan Phu Ward and Phu Thu Ward.This district is in the city's southernmost district and closest to the Hau River and the sea compared to other districts.Phu Thuthe suburban ward -is the most affected ward of Cai Rang district, especially for agricultural land.Currently, agricultural land is 818.45 hectares, accounting for 40.56% of the whole ward's land area.Land for rice and crops, perennial crops, and aquaculture land has an area of 511.04 ha; 305.99 ha; 1.42 ha, respectively.
Besides, the floodwater level is remarkable in some districts.Figure 3 shows inundated streets in RCP 4.5 for 2030.The flooded areas are mainly alongside the Hau River.The Floodwater level near the Hau riverbank can be up to 1 m and decrease to 0.3-0.5 m to the inner land.Flooded streets are mainly in Ninh Kieu and Cai Rang districts since they are urban areas in Can Tho city.Some roads with heavy inundation are Vo Nguyen Giap, Nguyen Trai, and Cach Mang Thang Tam.Especially, Con Khuong and Cai Khe islands in Ninh Kieu district can face up to 1 m floodwater depth.When a flood happens, the airport in Binh Thuy district can be flooded up to 0.3m if no embankment is built.Besides, Le Hong Phong Street -one of the main roads to the airport and surrounding areas can be inundated by 0.5 -1 m.Other peri-urban and rural districts have a negligible effect on streets and infrastructures than urban areas with 0.3 m floodwater levels.
In 2050, the flood risk of Can Tho City is 5.58%, with a flooded area of 7,861ha.3 districts with the highest flooded area are Co Do, Cai Rang, and Thoi Lai with the flooded area of 1,119.15ha,1,430.09ha,and 2,596.91ha,respectively.

RCP 8.5 scenarios
For the RCP 8.5 scenario, the flooded area of Can Tho city will gradually increase in 2025, 2030, and 2050 at 3,559 ha, 3,719 ha, and 4,476 ha, respectively (Table 5).The flooded area of Thoi Lai district increased the highest in 2025, 2030, and 2050, respectively, at 286.2ha, 356.8ha, and 671.1ha.The inundation areas in Can Tho increase gradually by 2025, 2030, and 2050.Specifically, in 2025, the flood risk of Can Tho city is 2.5%, with a flooded area of 3,559ha.The three districts with the highest flooded area are O Mon, Thot Not, and Cai Rang, with 489.4ha, 829.7ha, and 1,226.8ha.In 2030, the flood risk of Can Tho city is 2.6%, with a flooded area of 3,719ha.The three districts with the highest flooded area are O Mon, Thot Not, and Cai Rang, with 503.2ha, 844.4ha, and 1,261.5ha,respectively.In 2050, the flood risk of Can Tho city is 3.2%, with a flooded area of 4,476ha.The three districts with the highest flooded area are Thoi Lai, Thot Not, and Cai Rang, respectively, with 671.1ha, 903.3ha, and 1,416.3ha.According to the results, both precipitation and sea level rise are dominant factors that impact flooding in Can Tho City.The other studies also indicated the important contribution of rainfall and river water level to the regular flooding in Can Tho City [20,21].Thus, management strategies need to address both issues together to mitigate the risks and impacts of floods in the area.

Climate change adaptation measures to 2030 and vision to 2050 for Can Tho city
The climate change adaptation plan for Can Tho City related to water flow and flooding is based on this study on flooding simulation, climate change scenarios, and the social-economic development master plan of Can Tho City until 2030 and the vision of 2050 (Table 6).The implemented mitigation strategies targeted the regions that were identified to have sustained the greatest impact from flooding, as determined by the aforementioned simulation, specifically encompassing Thoi Lai, Ninh Kieu, and Cai Rang.Besides, the suggestion also highlighted the current projects in Can Tho City for some other districts such as O Mon and Binh Thuy which were suitable for the adaptation measures. Improve the perviousness areas on the roads in the city (Department of Construction).

2030-2050
 Strengthening and protecting the area of Can Tho thermally power plant against the risk of flooding (Department of Industry and Trade).

Figure 1 .
Figure 1.River network for MIKE11 modeling and the stations in Can Tho City.

Figure 2 .Figure 3 .
Figure 2. Flood risk map of Can Tho City year 2025 under RCP 4.5 scenario due to climate change and SLR

Figure 4 .
Figure 4. Flood risk map of Can Tho City year 2050 under RCP 4.5 scenario due to climate change and SLR

 2030 
Build a system of flood prevention for the inner city of Can Tho (Can Tho City ODA Project Management Unit). The project "Protecting the environment of Can Tho Green and Clean," prevention and control of pollution and flooding in the city from 2021 to 2025 (Department of Construction, Department of Natural Resources and Environment). Prevent flooding for the schools in Can Tho City (Department of Education and Training)  Research and apply technical solutions to adapt to floods caused by heavy rains and high tides for urban areas, and residential areas, and prevent flash floods and landslides in areas susceptible to natural disasters.(Department of Construction).The project of planning and building multi-purpose flood prevention combined with improving and upgrading the city's waterway transportation system in Can Tho (Department of Agriculture and Rural Development, Department of Construction).2025-Build an embankment of Hau River in the area near Can Tho airport before the risk of flooding (Department of Agriculture and Rural Development). Study the feasibility of flood control at traffic routes through the areas likely to be flooded in the future (Con Cai Khe and Con Khuong, part of Nguyen Trai and Cach Mang Thang Tam Streets (NH91); part of Vo Nguyen Giap Street, Le Hong Phong traffic route north of Binh Thuy district extending to Le Hong Phong section near Can Tho Airport) (Department of Construction).

Table 1 .
). SLR scenarios for Can Tho City for RCP 2.6, RCP 4.5, and RCP 8.5 (Unit: centimeter) Table2shows the change in precipitation used to predict the risk of flooding in the study area.The scenarios are low emission scenario RCP 2.6, medium emission scenario RCP 4.5, and high emission scenario RCP 8. 5.The data were collected from the Ministry of Natural Resources and Environment in 2016, for climate change scenarios of Vietnam in the Southern region.

Table 3 .
Flooded areas for the districts in Can Tho City in 2025, 2030, and 2050 under climate change and SLR for RCP 2.6 3.2.2.RCP 4.5 scenariosFor the RCP 4.5 scenario, the flooded area of Can Tho city in 2025, 2030, and 2050 was estimated at 5,381ha, 8,784ha, and 7,861ha, respectively (Table4).Which, the flooded area of Co Do district increases the highest in 2025, 2030, and 2050 at 454.27ha, 1,515.43ha,and 1,119.15ha,respectively.The maps of flood risk in 2025, 2030, and 2050 due to the combined impacts of climate change under the RCP 4.5 scenario are shown in Figure

Table 4 .
Flooded areas for the districts in Can Tho City in 2025, 2030, and 2050 under climate change and SLR for RCP 4.5

Table 5 .
Flooded areas for the districts in Can Tho City in 2025, 2030, and 2050 under climate change and SLR for RCP 8.5

Table 6 .
The fifteen measures for three periods 2025 -2050 corresponding to the short-term, middleterm, and long-term periods in Can Tho City to adapt to flooding under climate change and SLR Project on embankment erosion control in My Khanh market area" (Department of Agriculture and Rural Development). "Emergency embankment erosion control at a hazardous location on Tra Noc river, Tra An ward (from Tra Noc bridge to Xeo May bridge, Binh Thuy district, Can Tho city)" (Department of Agricultural and Rural Development). "Embankment erosion control of O Mon River (from Ba Rich dam to Tam Vu canal), Thoi Hoa, and Thoi An Wards, O Mon district (on the left bank of O Mon river, from O Mon bridge to Hau River)" (Department of Agriculture and Rural Development). Dredging the O Mon -Xa No canal system (Department of Agriculture and Rural Development). Complete technical guidance on technical solutions to the construction of infrastructure works to cope with urban flooding (ground leveling, drainage, regulating reservoir, embankment) (Department of Construction).