THE INTEGRATED STRATEGY FOR URBAN FLOOD MANAGEMENT TO ADAPT WITH CLIMATE CHANGES IN HO CHI MINH CITY

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1 THE INTEGRATED STRATEGY FOR URBAN FLOOD MANAGEMENT TO ADAPT WITH CLIMATE CHANGES IN HO CHI MINH CITY Ho Long Phi Vietnam National University Ho Chi Minh City Project manager at the Steering Center of Flood Control Ho Chi Minh City, Viet Nam 1

2 Content Urbanisation Climate changes Urban flood and flood management projects Results and discussions Integrated Strategy for urban flood management Conclusions 2

3 URBANISATION 3

4 Urbanisation Mn. Inhab Mn. Inhab Mn. Inhab Mn. Inhab. Urbanising areas have been developed on lowland under pressure of economic growth. 4

5 Population growth Population grows in all recently urbanised districts, where locate in low-land areas. About 3 Mn people have resettled in new urbanised districts since the last ten years 5

6 Development Plan to

7 CLIMATE CHANGES 7

8 Rainfall Volume of yearly-max rainfall events increased about 50mm upon the past 50 years. Counts of 3-hour rainfall events with volume >100mm Period * Counts *The last record consists of just 7 years data. 8

9 Decreasing return frequency R>30mm R>50mm R>100mm R>115mm R>130mm Moving averaged data show that within the last 20 years the return frequencies of heavy rainfall events have reduced as much as 2-3 folds. What should be the design criteria for storm sewer systems?. 9

10 Water level at lower boundaries Yearly max water level at surrounding stations Z(cm) 150 Phu An 140 Vung Tau 130 Thu Dau Mot 120 Nha Be Ben Luc Year Stationary Increasing T-Student tests confirm the statistically difference between means of two sub-series and

11 Local trends in water level rise Yearly max water level Mực nước cao nhất hàng năm Water level (cm) 170 NB: y = x M ự c n ư ớc (cm ) PA VT 130 NB VT: y = x PA: y = 1.452x Year Năm Sea level at Vung Tau (red line) has been hardly changed, while all water levels at river stations has shown an average increasing of 1.5cm per year since

12 Statistical analysis of trend Table 1: Trend analysis of yearly max water level at main stations Zmax (cm) n Mann-Kendall Z Signific. Sen's Q Sen's B COV Remark Phu An *** Strong rise Vung Tàu No change Thu Dau Mot *** Strong rise Nha Be ** Strong rise Bien Hoa Weak rise Ben Luc *** Strong rise Tan An ** Strong rise 12

13 URBAN FLOOD and FLOOD MANAGEMENT PROJECTS 13

14 Urban Flood in Ho Chi Minh City The flooding situation in central districts has been improved since 2007, thank to flood management projects. Note that count of heavy rainfall events keeps on increasing. New flooded sites are occurring in new urbanised districts. 14

15 Flooded sites Flooded sites are shifting outwards to new urbanised districts. 15

16 Flooded area at water level km2 of HCM City would be flooded at water level +2.0, apart from Can Gio area. 16

17 Urban Drainage Projects Master plan JICA (6/2001) Tham Luong-Ben Cat (USD 400 Mn., under construction ) Nhieu Loc-Thi Nghe (USD 300 Mn., under construction) Tan Hoa-Lo Gom (USD 200 Mn., under construction) Tau Hu Ben Nghe (USD 350 Mn., under construction) Tide control project 1547 (USD 2 Bn., under preparation) Hundreds of small projects since 90. $6 Bn. is required for the next decades. 17

18 Tide Control Project (MARD) Hình 9 QUY HOẠCH HỆ THỐNG CÔNG TRÌNH KIỂM SOÁT MỰC NƯỚC CHỐNG NGẬP ÚNG KHU VỰC BỜ HỮU SÔNG SÀI GÒN NHÀ BÈ HCMC will become a part of a large polder system 18

19 Impacts of Climate Changes on Urban Flooding CASE STUDY Nhieu Loc -Thi Nghe Project

20 Project outline Catchment area: 3300 ha. Cost: USD 300 Mn. Situation: phase Under construction 20

21 Impact of water level rise Zmax = 154cm Flooded area: 223 ha Without supplementary tide gate, some areas will remain flooded just after finishing the construction 21

22 Flooding areas under extreme rainfall events (with tide gate) Rainfall volume (mm) Flooded area (ha) Rainfall volume (mm) Flooded area (ha) As heavy rainfall event keeps on increasing as resulted by Urban Heat Island effect, the project performance would be reduced remarkably. 22

23 Results and discussion Climate changes may result in non-stationary time series of rainfall and water level and require new approaches. Climate changes would reduce performance of storm sewer systems remarkably. An integrated strategy is required. Tide control polder alone could not solve the flood problems. 23

24 Integrated Strategy for Urban Flood Management 24

25 Integrated strategy of urban flood management to adapt with climate changes Adaptation Protection Retreat A sustainable urban flood management strategy should include Adaptation and Retreat components. The optimal balance among the 3 components may vary with location and/or time and should be decided by a robust DSS. 25

26 S-Principles for HCMC OFT: An important principle of Sustainable Urban Flood Management is to introduce SOFT components, which can be adjusted to adapt with UNCERTAINTIES of urban development and climate changes. ELF REGULATION: Urban development projects should not overload neighborhoods with excess runoff or reduction of original discharge capacity. CATTERED: Distributed systems for flexibility, adaptability and risk probability. YSTEMATIC: Operation should be centralized and early-warning based. 26

27 Adaption The settlement within vulnerable areas, if not avoidable, should be supported with Adaptation policies, Awareness and appropriate Facility. There is no 100% guarantee protection, risk minimizing should be considered for extreme cases. Detention spaces could be expandable to adapt with climate changes. 27

28 Retreat (giving back) To adapt with uncertainties of climate changes, the policy of giving back rooms for water by introduction of regulation spaces (3% - 5% of sub-catchment area) is essential. For urbanizing areas, the land use policy should be applied to investor/proprietor as an approval criterion. For existing urbanized districts, the policy could be introduced during urban upgrading process and city compaction. In a robust DSS, urban land use planning should be treated as one of climate changes adaptation alternatives, not a precedence. 28

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30 Conclusions While sea level rise is still a potential danger, negative impacts of humaninduced activities has been direct causes of the urban flood in HCMC. To adapt with climate changes, Protection measures such as polder, pumping station and storm sewer upgrade are also needed, but may not be adequate. A balance among Protection, Adaptation and Retreat is requirement for the flood sustainable management strategy and can be determined by a robust DSS. 30

31 Thank you for your attention 31