Climate risks and adaptation in Asian coastal megacities: A synthesis

AnnexCAdaptation to IncreasedFlooding: Brief OverviewIn response to the hazard of floods increasingfrom climate change, the Asian coastal cities inthe study have a range of possible adaptationapproaches drawn from existing flood managementtechniques. Each adaptation approach attempts toreduce risk by either reducing exposure or vulnerability,or by increasing capacity. The adaptationmeasures can be broadly grouped as: 59■■■■■■■■ProtectAccommodateRetreatImproved managementProtection—reducing exposure withstructural interventionsFlood protection measures seek to minimize theexposure of urban areas to inundation from floods.Common engineering interventions include floodembankments, polders, and sea walls, frequentlycombined with pumped drainage. To allow thetransfer of a flood wave past a city without damage,other protection techniques include (a) increasingthe hydraulic efficiency 60 of the flood channel withdredging, widening, and removal of obstructions;(b) diverting the flood flows around the city throughdiversion channels; and (c) attenuating 61 the floodflows upstream with reservoirs or through themanaged flooding of the agricultural and wetlandareas. With the exception of planned flooding ofagricultural and wetlands to attenuate flood peaks,the measures outlined above would be classifiedprimarily as structural adaptation measures.Within a city, storm drainage systems are designedto remove storm runoff without flooding.The design discharge for the conveyance systemsare normally based on storms with a return periodbetween 1-in-5-years to 1-in-10-years, as comparedto design standards of 1-in-30 to 1-in-100-year returnperiods commonly used for flood protectionembankments. In an urban setting, roads, buildings,and paved surfaces offer little opportunity forinfiltration of storm rainfall, which means that mostof the precipitation appears as storm runoff. In thisregard, the predicted increase in storm precipitation(ranging from 9 to 15 percent depending on the cityand the scenario) will generate a similar increase inthe design discharge for the storm drainage systems.To provide the same level of protection againststorm-induced urban flooding, the cities will needto adapt by either increasing the discharge capacityof their storm drainage systems or identifyingways to delay or reduce precipitation runoff. Forexample, adaptation measures might include stormattenuate ponds (using inner city parks) or makingstreets, alleys, and other paved areas more porousto rainfall to facilitate infiltration.Flood protection measures always carry a residualrisk of failure, which can be catastrophic as inthe case of New Orleans. An important outcome ofthe study is that even for cities with well-designedflood protection plans (e.g., Bangkok and Manila),the flood protection measures as currently plannedwill not offer the expected level of protection.59Drawn from IPCC, AR4, Chapter 6, Coastal systems andlow-lying areas, Figure 6.11 Evolution of planned coastaladaptation measures, pg. 342, 2007.60Improving the hydraulic efficiency lowers the flood profile.61Attenuation upstream lowers the flood profile downstream.93