Hydraulic modelling for flood vulnerability assessment, case study in river basins in North Central Vietnam

Transcription

1 Hydraulic modelling for flood vulnerability assessment, case study in river basins in North Central Vietnam Tran Ngoc Anh 1*,2, Dang Dinh Kha 1,2, Dang Dinh Duc 2, Nguyen Thanh Son 1 1 Faculty of Hydrology Meteorology and Oceanography, Hanoi University of Science, VNU, 334 Nguyen Trai, Ha Noi, Viet Nam 2 Center for Environmental Fluid Dynamics, Hanoi University of Science, VNU, 334 Nguyen Trai, Ha Noi, Viet Nam 1. Introduction Vietnam is one of the most affected countries by natural disasters such as storm, typhoon, hot spell, flash flood, among those typhoon associated with heavy rainfall causing seriously flood is considered as the most serious threat in Central Vietnam. Due to its own geographical characteristics: very steep in surface, rivers are mostly short, the flood plains in downstream are narrow,... the flood in Central Vietnam has caused a lot of damages in life and property of the local communities. In order to mitigate these damages, there are many approaches ranging from non-structural measures to structural measures as reforestation, strengthening the early warning system, reasonable land-use planning, construction of dyke system, building dam and regulation reservoirs etc. Among them, Vulnerability assessment has been addressed intensively over past decades with a lot of achievements contributing to mitigation of damages due to natural disaster. The vulnerability assessment results provide the useful information for the local authorities in planning and communities to strengthen their capacity in response to natural disaster. The vulnerability assessment was started with the qualitatively approach that mostly based on experience of community together with other social-economic information (Son and Van, 2012). These approach give the very useful information of the historical events happen in the region and how to deal with the former problems if the event comes, but it does not provide good enough information for the changes in natural conditions (e.g climate change) or human activities (e.g new construction of dam, reservoir etc). Therefore, the integrated approach that use the advance of quantitative modelling with socio-economic assessment might bridge these gaps and reach the practical demand of disaster risk management activities. This paper aims to present the assessment results of flood and inundation risk in downstream of some main river basins in North Central Vietnam and an illustration of using this result in conducting vulnerability index of downstream area of Lam river basin in Nghe An and Ha Tinh provinces. This semi-quantitative approach also show the role of hydraulic modelling in vulnerability assessment. 2. Methods and Materials 2.1 Vulnerability concept Figure 1. Study area * tnanh2000@yahoo.com

2 Vulnerability is the extent to which a system is susceptible to floods due to exposure, a perturbation, in conjunction with its ability (or inability) to cope, recover, or basically adapt (UNESCO-IHE). There several formulas calculating the vulnerability (Conner, 2007; Sheuer et al., 2010; Villagran et al., 2006; Trinh et al 2010; etc) and the guideline by UNESCO-IHE (Son and Van, 2012) was applied in this study which vulnerability is a summation of Exposure, Susceptibility and Resilience as: where Exposure is the predisposition of a system to be disrupted by a flooding event due to its location in the same area of influence Susceptibility is the element exposed within the system, which influence the probabilities of being harmed at times of hazardous floods Resilience is the capacity of a system to endure any perturbation, like floods, maintaining significant levels of efficiency in its social, economical, environmental and physical components 2.2 Hydraulic modelling In this study, the river systems are modelled using MIKE FLOOD model developed by Denmark Hydraulic Institute (DHI) with rainfall-runoff modul (NAM) to simulate discharge at upstream and inflow boundaries, MIKE 11 for one dimensional river network, MIKE 21 for the flood plain and MIKE FLOOD to couple 1D and 2D model in the downstream region. The Digital Elevation Model (DEM) was developed for the downstream region of 3 basins with the resolution of 30x30m based on the 1:10,000 maps enhanced with surveyed elevation points collected from different validated sources Lam river basin Lam river is one of the 9 largest river systems in Vietnam, originates from Laos runs toward Vietnamese East Sea through Nghe An and Ha Tinh province (Figure 1). It has catchment are of km 2, which km 2 is in Laos. The total main river length is 258,2 km with 3 main tributaries: Hieu river (220 km), Giang river (25,7 km) and La river (12,5 km). These system was modeled with 461km and 292 cross-sections (Figure 2) in MIKE 11 model. The upstream boundaries are simulated discharges at Cua Rao in the main river, Tan Ky in Hieu river, Thac Muoi in Giang river, Son Diem and Hoa Duyet in La river, and downstream boundary is tidal water level at Cua Hoi (river mouth). The flood plain region in downstream was modelled in MIKE 21 then linked with MIKE 11 using the procedures in MIKE Flood. The model system was calibrated with catastrophic flood event in 2010 (Figure 3) and verified with the flood event in 2013 (Figure 4). Figure 2. 1D hydraulic network of Lam river system

3 a) Linh Cam station b) Yen Thuong station c) Dua Station d) Nam Dan station Figure 3. Calibration results Comparison of simulated and observed water level at hydrological stations in Lam river basin in flood event 13 th - 28 th October 2010 a) Linh Cam station b) Yen Thuong station c) Dua Station d) Nam Dan station Figure 4. Verification results Comparison of simulated and observed water level at hydrological stations in Lam river basin in flood event 13 th 29 th October 2013

4 2.2.2 Nhat Le river basin Nhat Le river basin is in Quang Binh province ranging from border between Vietnam and Laos to the Vietnamese East Sea (Figure 1). The total area of the basin is km 2, with the main river is Kien Giang in upstream and after the conjunction with the largest tributary - Long Dai, the river is called as Nhat Le and flow to the sea through Nhat Le river mouth at Dong Hoi city. The river network was modeled with 189km in length and 54 cross-sections (Figure 5) in MIKE 11 and 332,9 km 2 of flood plain in MIKE 21. In this modelling system, the upstream boundaries are simulated discharges from rainfall-runoff model (NAM) at Kien Giang and Tam Lu stations while the downstream boundary is tidal level at Nhat Le river mouth. The couple 1D-2D was calibrated with historical flood event in 1999 and calibrated with flood event in 2010.! Figure 5. 1D hydraulic network of Nhat Le river system! Figure 6. 1D hydraulic network of Thach Han Ben Hai river system!! Thach Han Ben Hai river basin Quang Tri province has two main rivers are Thach Han and Ben Hai. These two rivers connect with each other through Canh Hom river and also connect with O Lau river in Thua Thien Hue province through Vinh Dinh river, therefore these 3 rivers are modeled together in a complicated system (Figure 6). Thach Han, Ben Hai and O Lau rivers all originate from the mountainous area near border with Laos, running toward Vietnamese Eastern Sea with the length of 156 km, 64,5 km and 65 km respectively through a steep area with narrow flood plain. Each river has only one main tributary as Sa Lung in Ben Hai basin, Hieu (Cam Lo) river in Thach Han basin and Thac Ma river in O Lau basin. The modelled system includes total 198 km in length with 140 cross-sections. The upstream boundaries are simulated discharges (using NAM model) at Sa Lung and Gia Vong in Ben Hai river system, Cam Tuyen and Dakrong in Thach Han river system and at Hai Son and Pho Trach in O Lau river system. The flood plain with about 717 km 2 was modeled in MIKE 21 then the coupled system was calibrated with flood event in October 2005 and verified with flood in November a) Flooding Depth b) Inundation duration c) Flood Peak Velocity d) Flood Risk Figure 7. Flood risk map in downstream of Lam river basin Using the above developed system model, three main inundation variables of the downstream region (maximum flooding depth, inundation duration and flood peak velocity) were simulated at every cells in the domain, then the flood risk variable was calculated from three main variables using

5 weighting parameters collected from the surveys (Kha et al, 2013; Van et al, 2013). Figure 7-9 show the flood risk maps in those three river basins in the historical floods. a) Flooding Depth b) Inundation duration c) Flood Peak Velocity d) Flood Risk Figure 8. Flood risk map in downstream of Nhat Le river basin a) Flooding Depth b) Inundation duration c) Flood Peak Velocity d) Flood Risk Figure 9. Flood risk map in downstream of Thach Han Ben Hai river basins As definition, exposure is depends on the flood risk and the property are expose to those risks, therefore the exposure maps were developed by overlapping the flood risk maps (developed above) with landuse maps. The results of exposure in Lam river basin are presented in Figures 10a. In order to determine the susceptibility and resilience of the local communities in Lam river basin, series of surveys were conducted in the flood plain regions with questionnaires to collect socioeconomic information. Total of 5120 questionnaires for different key-informants (local authorities and head of households) were collected from surveys for about 20 questionnaires for each commune. The susceptibility and resilience are determined by 32 indices (Kha et al, 2013; Van et al, 2013) which 20 indices calculated from questionnaires and 12 indices from yearly statistical books at commune level. Then the susceptibility and resilience indices were averaged for each commune in the region and depicted in Figure 10b and 10c. These index maps were then overlapped with exposure maps by GIS tools to establish the vulnerability maps in Figure 10d. a) Exposure map b) Susceptibility map c) Resilience map Figure 10. Flood vulnerability of Lam river basin d) Vulnerability map

6 3. Results and Discussions The vulnerability map for the Lam rivers (Figure 10d) shows the most vulnerable areas in red color and was developed based on the information from a certain design flood and local planners might extract their needed information in proposal of solutions for flood prevention. The modelling system was well developed for main rivers in North Central Vietnam with the series of thematic maps, therefore the vulnerability maps in Nhat Le, Thach Han and Ben Hai rivers could be developed in the same approach. And furthermore, these systems could be applied for some future scenarios such as: climate change and sea level rise by replacing the increased rainfall for upstream boundaries and raising sea level at downstream boundaries; construction of dyke system by updating the surface elevation database (DEM) in 2D modelling; or construction of reservoir by changing discharge at upstream boundaries, etc. The hydraulic modeling apparently plays an important role in vulnerability assessment since it could provide the flood information at very fine resolution (upto 30mx30m in this case) that would help the local planners in land-use planning as well as local authorities in proposal of prevention measures for communities in flood plain in details. Furthermore, this is only approach could also provide the information of future changes in flood variables and consequently vulnerability due to changes in natural conditions and human activities. 4. Acknowledgment The results of this paper are from the results of studies sponsored by projects 11-P04-VIE (CPIS) and BĐKH-19, the presentation at the workshop was sponsored by The Agence Universitaire pour la Francophonie, the authors would like to thank sponsors for their very valuable continuous supports. 5. Reference 1. Conner F. R. (2007). Flood vulnerability index IPCC (2001). Climate change 2001: Impacts, Adaptation and Vulnerability, Summary for Policymakers, WMO 3. Đặng Đình Khá, Trần Ngọc Anh, Nguyễn Thanh Sơn, Nguyễn Tiền Giang, Cấn Thu Văn (2013) Xây dựng bộ mẫu phiếu điều tra khả năng chống chịu với lũ lụt của người dân phục vụ đánh giá khả năng dễ bị tổn thương do lũ. Tạp chí khoa học Đại học Quốc gia Hà Nội. Khoa học Tự nhiên và Công nghệ Tập 29, số 2S tr Nguyễn Thanh Sơn, Cấn Thu Văn (2012) Các phương pháp đánh giá tính dễ bị tổn thương - Lý luận và thực tiễn. Phần 1. Khả năng ứng dụng trong đánh giá tính dễ bị tổn thương lũ lụt ở Miền Trung Việt Nam, Tạp chí khoa học Đại học Quốc gia Hà Nội. Khoa học Tự nhiên và Công nghệ Tập 28, số 3S tr Cấn Thu Văn, Nguyễn Thanh Sơn, Trần Ngọc Anh, Đặng Đình Khá, (2013) Các phương pháp đánh giá tính dễ bị tổn thương - Lý luận và thực tiễn. Phần 2: Áp dụng thử nghiệm tính toán chỉ số dễ bị tổn thương do lũ thuộc lưu vực sông Lam-tỉnh Nghệ An. Tạp chí khoa học Đại học Quốc gia Hà Nội. Khoa học Tự nhiên và Công nghệ Tập 29, số 2S tr Villagran de Leon JC (2006). Vulnerability conceptual and methodological review. Studies of the university: research, counsel, education, publication series of UNU-EHS4/2006. Bonn. 7. Viet Trinh, Lars Ribbe, Jackson Roehrig & Phong Nguyen (2010). Flood risk assessment for the Thach Han River Basin, Quang Tri Province, Vietnam. Proc. of the Sixth World FRIEND Conference: Global Change: Facing Risks and Threats to Water Resources in Fez, Morocco, October IAHS Publ. 8. Sebastian Scheuer, Dagmar Haase, Volker Meyer (2010). Exploring multi-criteria flood vulnerability by integrating economic, social and ecological dimension of flood risk and coping capacity: from a starting point view towards an end point view of vulnerability, Natural Hazards and Earth System Sciences, Springer, Accepted: 3 November DOI /s