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
Content Urbanisation Climate changes Urban flood and flood management projects Results and discussions Integrated Strategy for urban flood management Conclusions 2
URBANISATION 3
Urbanisation 75-2005 6 Mn. Inhab. 50-75 2 Mn. Inhab. 1850-50 0.5 Mn. Inhab. 2020 12 Mn. Inhab. Urbanising areas have been developed on lowland under pressure of economic growth. 4
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
Development Plan to 2025 6
CLIMATE CHANGES 7
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 52-61 62-71 72-81 82-91 92-2002 2003-2009* Counts 0 1 2 2 4 8 *The last record consists of just 7 years data. 8
Decreasing return frequency 80-89 90-99 2000-2009 R>30mm R>50mm 0.03 0.20 0.05 0.13 0.04 0.09 R>100mm R>115mm R>130mm 3.33 5.00 10.00 1.67 5.00 5.00 0.91 1.67 3.33 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
Water level at lower boundaries Yearly max water level at surrounding stations 170 160 Z(cm) 150 Phu An 140 Vung Tau 130 Thu Dau Mot 120 Nha Be Ben Luc 110 100 90 80 82 84 86 88 90 92 94 96 98 00 20 02 20 04 20 06 20 Year Stationary Increasing T-Student tests confirm the statistically difference between means of two sub-series 60-89 and 90-2009. 10
Local trends in water level rise Yearly max water level Mực nước cao nhất hàng năm 90-2007 Water level (cm) 170 NB: y = 1.1744x - 2208.1 160 M ự c n ư ớc (cm ) 150 140 PA VT 130 NB VT: y = -0.0774x + 289.4 120 PA: y = 1.452x - 2766 110 100 89 90 91 92 93 94 95 96 97 98 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 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 90. 11
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 18 4.03 *** 1.455 108.82 0.06 Strong rise Vung Tàu 18-0.04 0.000 134.00 0.06 No change Thu Dau Mot 18 4.46 *** 0.900 100.70 0.05 Strong rise Nha Be 18 3.27 ** 1.167 116.58 0.06 Strong rise Bien Hoa 18 1.86 + 1.800 138.30 0.11 Weak rise Ben Luc 18 3.88 *** 1.917 92.00 0.09 Strong rise Tan An 18 2.88 ** 1.941 97.76 0.12 Strong rise 12
URBAN FLOOD and FLOOD MANAGEMENT PROJECTS 13
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
Flooded sites Flooded sites are shifting outwards to new urbanised districts. 15
Flooded area at water level +2.0 300 km2 of HCM City would be flooded at water level +2.0, apart from Can Gio area. 16
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
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
Impacts of Climate Changes on Urban Flooding CASE STUDY Nhieu Loc -Thi Nghe Project
Project outline Catchment area: 3300 ha. Cost: USD 300 Mn. Situation: phase Under construction 20
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
Flooding areas under extreme rainfall events (with tide gate) Rainfall volume (mm) Flooded area (ha) Rainfall volume (mm) Flooded area (ha) 100 40.1 200 926 As heavy rainfall event keeps on increasing as resulted by Urban Heat Island effect, the project performance would be reduced remarkably. 22
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
Integrated Strategy for Urban Flood Management 24
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
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
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
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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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
Thank you for your attention hlphi@hcmut.edu.vn 31