Climate risks and adaptation in Asian coastal megacities: A synthesis

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severe flooding caused by Typhoon “Ondoy” inSeptember 2009 has raised public awareness of theunderlying causes of flooding. Recent analysis pointsto mostly anthropogenic causes behind the extremeflood event, including (a) a decrease in river channelcapacity through encroachment of houses, siltationfrom deforestation, and garbage; (b) disappearance of21 km of small river channels; (c) urbanization acceleratingrunoff concentration and reducing infiltrationlosses; (d) loss of natural retention areas; and (e) landsubsidence. Among these, land subsidence is the leastunderstood but important cause. It is being drivenby groundwater pumping and possibly geologicprocesses associated with the West Marikina ValleyFault (Siringan 2009). Land subsidence continuesdecades after the groundwater pumping stops, asillustrated by the Bangkok city case study.Flood control and mitigationprojects—1990 Master PlanFlood control and mitigation projects have beenplanned and implemented in Metro Manila overthe years. 34 A key feature of flood control in Manilais the Mangahan Floodway, which was constructedin 1985 and diverts flood flows from the MarikinaRiver into Lake Laguna as a method of attenuatingpeak discharges and protecting Manila’s urban areasfrom excessive inundation. JICA supported thedevelopment of a Flood Protection Master Plan in1990, which still serves as a basis for current and futureflood protection planning. The flood protectionmeasures undertaken as part of these projects areprimarily infrastructure projects involving attenuatingpeak flood flows in Laguna de bay, improvingconveyance of flood waters through Manila to ManilaBay, and preventing overbank spillage where theflood profile is higher than the adjacent river bank.Climate change flood simulation parametersTo simulate the future climate-change-induced flooding,the city study applied the statistical downscaledresults from the University of Tokyo’s IntegratedResearch System for Sustainability Science (IRS3)for temperature and extreme precipitation increases.For SLR in 2050, the study applied approximately50 percent of the increase in sea level rise in 2100projected in the IPCC AR4. 35 Historic storm surge inManila Bay was calculated at 0.91 m during majortyphoons; for 2050, a 10 percent factor increase wasapplied based on Ibaraki University methodology, 36bringing the storm surge estimate for 2050 to 1.0 m.34See Manila study for a list of planned and ongoing floodcontrol projects and studies.35In terms of boundary conditions for the hydrologicalmodel, the SLR values were added to high tidal level; thatis, mean spring high water level in Manila Bay was set asthe base line water level for the flood simulation, to whichwas added an SLR increase in accordance with the twoIPCC scenarios.36The university provided a detailed technical methodologyfor calculating the increase in storm surge based on ananalysis of historic surges and forecasted increased typhoonintensity.Figure 3.8 ■ Major Watershed and Drainage Areas of ManilaPasig-SanJuanMarikinaWest MangahanSource: Muto et al. (2010)34 | Climate Risks and Adaptation in Asian Coastal Megacities: A Synthesis Report
Table 3.6 ■ Manila Climate Change ParametersSimulation CaseTemperature Rise (°C)(downscaled)Sea Level Rise(cm) (global)Increased Rate of Rainfall24-hr event (%)Storm Surge Height(m) at Manila Bay1 Status quo climate (no change in climate0.0 0.0 0.0 0.91dependant variables)2 B1 with no change in storm surge 1.17 19 9.4 0.913 B1 with strengthened storm surge level 1.17 19 9.4 1.004 A1FI with no change in storm surge 1.80 29 14.4 0.915 A1FI with strengthened storm surge level 1.80 29 14.4 1.00Source: Muto et al. (2010).The factors applied are summarized in Table 3.7. The24-hr precipitation factor increase is reasonable as itis close to the time of concentration of flood flows forthe Manila watershed. Similarly, SLR is conservative,since it is taken from the IPCC estimates and doesnot include polar ice cap melt.Infrastructure scenarios assumed in baseyear and in 2050Box 3.2 ■ What does Metro ManilaLook Like in the Future?For the hydrological mapping, the team used 2003 topography datafor the structure of the city. It was assumed that in terms of numberof buildings and road network, Metro Manila in 2050 would lookvery similar to what it is today. Further, while the team intendedto use future land use in the hydrological and GIS mapping, thiswas not possible due to logistical reasons. Regarding populationgrowth, Manila uses a linear extrapolation of current trends to projectpopulation to the future. The city is expected to be a megacity witha population greater than 19 million by 2050. Commercially growingsections of Manila are expected to see a decline in population,with growth occurring in other parts of the city. Areas such as thePasig-Marikina river basin are seeing a decline in population, whichis expected to continue; growth will likely continue to occur in WestMangahan and Kamanava. Future regional GDP is not projected, buta real growth rate of 5 percent is applied to a variety of economicvariables and damage costs.Source: Muto et al. (2010).For the simulation runs, the study examined twoflood infrastructure alternatives based on the 1990Master Plan. The first was to assume the implementationof the Master Plan would be halted in 2008,the base year—referred to as “existing” infrastructureand abbreviated as “ex.” The other alternativewas that the full plan would be implemented by2050—referred to as “business as usual” and abbreviatedas “BAU.” To do this and include the climatechange factors noted above, the team developed 22simulation runs (Muto et al. 2010).Assumptions made regarding populationincrease, land use, urbanization in 2050Numerous assumptions regarding what Metro Manilamight look like in 2050 compared to the baseyear were made in the study (Box 3.2).Findings from theHydrological Analysisand GIS Mapping for MetroManilaSignificant increase in area exposed toflooding by extreme events (1-in-100-yearevent) in 2050 under B1 and A1FI scenariosThe flood pattern in Manila for 2050 is likely to besignificantly affected by the extent to which the 1990Master Plan is implemented. Table 3.8 shows that thearea exposed to flooding from a 1-in-100-year eventunder current conditions will increase from 82.62km 2 to 97.63 km 2 under the 2050 A1FI 1-in-100-yearevent, an increase of 18.2 percent. This represents asignificant increase in the inundated area. With theEstimating Flood Impacts and Vulnerabilities in Coastal Cities | 35
Page 4 and 5: © 2010 The International Bank for
Page 7 and 8: Figure 4.6 Damage Costs Associated
Page 9: AcknowledgmentsThis synthesis repor
Page 13 and 14: Executive SummaryIntroduction and R
Page 15 and 16: For each city, complex hydrometeoro
Page 17 and 18: infrastructure but a high emission
Page 19: these include strategies that focus
Page 23 and 24: policy implications have broader re
Page 25: 2Methodologies forDownscaling, Hydr
Page 28 and 29: Table 2.1 ■ Climate Change Foreca
Page 30 and 31: Return periods considered 18In this
Page 32 and 33: Figure 2.2 ■ Manila Rainfall-Runo
Page 34 and 35: Figure 2.3 ■ Estimation of Damage
Page 36 and 37: sion and distribution lines, and en
Page 38 and 39: Figure 2.5 ■ Possible Relationshi
Page 40 and 41: prices and the same prices are appl
Page 43 and 44: 3Estimating Flood Impactsand Vulner
Page 45 and 46: Table 3.2 ■ Bangkok Monthly Avera
Page 47 and 48: Estimation of storm surge in the ca
Page 49 and 50: Climate Change Impact and Adaptatio
Page 51 and 52: Figure 3.5a ■ Affected Condensed
Page 53: Manila’s current climate—Tropic
Page 57 and 58: Figure 3.10 ■ Areas of High Popul
Page 59 and 60: is also a key economic and financia
Page 61 and 62: Box 3.3 ■ HCMC in 2050Future land
Page 63 and 64: original swamp land on which the ci
Page 65 and 66: from 68 percent to 71 percent; that
Page 67 and 68: Table 3.15 ■ Districts Affected b
Page 69 and 70: Table 3.16 ■ Effects of Flooding
Page 71 and 72: 4Assessing Damage Costsand Prioriti
Page 73 and 74: Table 4.2 ■ Summary of Flood and
Page 75 and 76: uilt outside the cities and are not
Page 77 and 78: ■■■■■■For the eastern p
Page 79 and 80: Table 4.6 ■ Flood Damage Costs Wi
Page 81 and 82: Table 4.8 ■ Flood Damage Costs in
Page 83 and 84: Figure 4.5 ■ Loss Exceedance Curv
Page 85 and 86: Box 4.3 ■ Increased Time Costs an
Page 87 and 88: Table 4.11 ■ Adaptation Investmen
Page 89 and 90: Marikina River areas through a dam,
Page 91 and 92: Table 4.14 ■ Present Value of the
Page 93 and 94: of reducing vulnerability of the po
Page 95 and 96: 5Conclusions andPolicy Implications
Page 97 and 98: open land use is expected to be flo
Page 99 and 100: hydrometeorological data, yielding
Page 101 and 102: BibliographyAllen, M. R., and W. J.
Page 103: Reduction: Inventory and Prospect.
Page 106 and 107:
a paper commissioned by JICA (Sugiy
Page 108 and 109:
mercial and industrial establishmen
Page 110 and 111:
ADB. The selection and prioritizati
Page 113 and 114:
AnnexCAdaptation to IncreasedFloodi
Page 115:
of people and institutions to manag
Page 120:
THE WORLD BANK1818 H Street, NWWash
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Climate risks and adaptation in Asian coastal megacities: A synthesis

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