Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.3.5.7 Flood Management ====

<div id="h3-34-siblings" class="h3-siblings"></div>

Cities are deploying a broad range of strategies to adapt infrastructure to flooding, with hard engineering approaches (e.g., dikes and seawalls) increasingly complementing soft approaches, including planning and use of nature-based solutions, that emphasise natural and social capital ( [[#Jongman--2018|Jongman, 2018]] ; [[#Sovacool--2011|Sovacool, 2011]] ). The infrastructure can alter downstream risks and lead to increased residual risk by encouraging more floodplain construction (Miller, Gabe and Sklarz, 2019; [[#Ludy--2012|Ludy and Kondolf, 2012]] ). Physical infrastructure is highly cost effective for large settlements, but not always for small settlements (Tiggeloven et al., 2020) and can be inaccessible to poorer communities (Sayers, Penning-Rowsell and Horritt, 2018; Van Bavel, Curtis and Soens, 2018). It is often inflexible once installed but new designs and adaptive pathways are emerging (Anvarifar et al., 2016; [[#Kapetas--2020|Kapetas and Fenner, 2020]] ).

As urban areas have expanded, so too have the number of vulnerable assets, and efforts may now emphasise reducing construction in high-risk regions (Paprotny et al., 2018a). The National Flood and Coastal Erosion Risk Management Strategy for England, for example, calls for reductions in inappropriate developments in floodplains ( [[#Kuklicke--2016|Kuklicke and Demeritt, 2016]] ; [[#UK%20Environment%20Agency--2020|UK Environment Agency, 2020]] ). Because climate change increases the flood risk profile of certain regions, reconsideration of design criteria has become more common ( [[#Ayyub--2018|Ayyub, 2018]] ). New York City now requires the sewer system currently designed for hydraulic capacity in 5-year design life should be designed for 50-year design life, taking into account climate changes over that period ( [[#NYC--2019|NYC, 2019]] ).

Adaptation strategies are diverse and often involve hybrid physical and NBS, and increasingly integrated management plans that consider both flood prevention and designing infrastructure and supporting people to cope with floods when they occur. Adaptation typically focuses on (i) increasing the standard of protection to compensate for the increased magnitude of extreme events; (ii) increased maintenance to cope with increased frequency of extremes and changes in ambient conditions; (iii) changed maintenance regimes from narrower maintenance windows, for example as assets are used more frequently (Sayers, Walsh and [[#Dawson--2015|Dawson, 2015]] ); (iv) land use planning and management to reduce exposure and manage hydrological flows; and (v) raising awareness, preparedness and incident management. In high population areas, hard interventions such as dikes and levees are generally cost effective ( [[#Jongman--2018|Jongman, 2018]] ; Ward et al., 2017).

Prevention or attenuation solutions include: rooftop detention, reservoirs, bioretention, permeable paving, infiltration techniques, open drainage, floating structures, wet-proofing, raised structures, coastal defences, barriers and levees, and have been deployed in diverse configurations and environments around the world ( [[#Matos%20Silva--2016|Matos Silva and Costa, 2016]] ). Barcelona (Spain) reached 90% impermeable surface cover by the 1980s, and has recently begun implementing artificial detention, underground reservoirs and permeable pavement technologies ( [[#Favaro--2018|Favaro and Chelleri, 2018]] ; [[#Matos%20Silva--2016|Matos Silva and Costa, 2016]] ). Florida Power and Light (USA), which provides service to approximately 10 million people, is investing USD 3 billion in flood protection and the hardening of assets (for example, upgrading wooden poles to steel and concrete) (Brody, Rogers and Siccardo, 2019). The City of Seattle recommends increasing preventative maintenance activities, the regular review of appropriate pavement technologies and modifications to subgrades and drainage facilities for high-risk areas ( [[#City%20of%20Seattle--2017|City of Seattle, 2017]] ), whilst also providing benefits to transport disruption (Arrighi et al., 2019). Adaptation in African cities is often dominated by informal responses ( [[#Owusu-Daaku--2018|Owusu-Daaku and Diko, 2018]] ). In the absence of centralised responses, low-income residents in Nairobi (Kenya) dig trenches and construct temporary dikes to protect homes, and in Accra (Ghana) the community has developed a range of social responses, including communal drains and local evacuation teams, to help protect people and critical valuables, although these innovations require connection to city-wide infrastructure to effectively reduce widespread risk ( [[#Amoako--2018|Amoako, 2018]] ).

More recent developments include sensor arrays to catalogue a river’s reach and how changing hydraulics interact with roadways (Forbes et al., 2019). Kuala Lumpur’s (Malaysia) stormwater management and road tunnel (SMART) during extreme rain events transitions the motorway to a stormwater conduit, an example of multifunctionality enabling agility ( [[#Isah--2016|Isah, 2016]] ; Markolf et al., 2019). Smart stormwater control systems are starting to use real-time control to dynamically manage the retention and movement of water during storms, though uptake at large scales which provide the greatest improvements in performance have been limited (Xu et al., 2020b).

In contrast to a ‘fail-safe’ approach to design which emphasises strengthening infrastructure against more intense environmental conditions, ‘safe-to-fail’ flood strategies allow infrastructure to fail in its ability to carry out its primary function but control the consequences of the failure. Examples include the use of a bioretention basin in Scottsdale (Arizona, USA) to accommodate excess runoff and help drain the city; a subsidy for affected farmers for lost crop production as part of the Netherlands’ Room for the River programme; targeted destruction of a levee to control flooding in the Mississippi River Valley in 2011 (Kim et al., 2019). Water-sensitive urban design, low-impact development, sponge cities, sustainable urban drainage and natural flood management involve deployment of systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration or use of stormwater to protect water quality and associated aquatic habitat. These are being designed and implemented at increasingly ambitious scales. For example, China’s Sponge City initiative sets a goal of 80% of urban land able to absorb or reuse 70% of stormwater through underground storage tanks and tunnels, and use of pervious pavements, in addition to NBS (Chan et al., 2018; [[#Muggah--2019|Muggah, 2019]] ). Similarly, several thousand water-sensitive urban design interventions have been implemented across the city of Melbourne (Kuller et al., 2018).

<div id="6.3.5.8" class="h3-container"></div>

<span id="coastal-management"></span>