Editing IPCC:AR6/SROCC/Chapter-4 (section)

== Box 4.1 Case Studies of Coastal Hazard and Response ==

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This box illustrates current coastal flood risk management and adaptation practices through four case studies from around the world, showing how current approaches could be refined using the new seal level rise (SLR) projections of this report, as well as findings on adaptation options, decision making approaches and governance (called Practice Consistent with SROCC Assessment in Tables 1–3). In an effort to illustrate some of the diverse social-ecological settings in this report, the locations are Nadi in Fiji, the Nile delta in Egypt, New York and Shanghai. The latter two studies are framed as a comparison. For each case, Current Practice reflects understanding, policy planning, and implementation that existed prior to SROCC. Recent improvements in understanding documented in this chapter suggest that significant, beneficial changes in the basis for design and planning are feasible in each case for addressing future risk.

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'''Box 4.1, Figure 1'''

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'''Box 4.1, Figure 1 | Historical and projected extreme sea level (ESL) events at four stations discussed in this box. The heights of ESL events are shown as a function of their return period. Observations (crosses) are derived from tide-gauge records. The historical return height (grey) is the best fit through these observations, and the […]'''
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[[File:710b9de8a61c92b9ab212ac9450c748f IPCC-SROCC-CH_4_Box_4_1_figure_1-3000x2599.jpg]]

Box 4.1, Figure 1 | Historical and projected extreme sea level (ESL) events at four stations discussed in this box. The heights of ESL events are shown as a function of their return period. Observations (crosses) are derived from tide-gauge records. The historical return height (grey) is the best fit through these observations, and the 5–95% confidence intervals (grey band) are shown. Note that the confidence interval for Lautoka is too narrow to be visible. Future ESL events represent the effect of regional sea level change for the period 2081–2100 for scenarios RCP2.6 (blue) and RCP8.5 (red). The increased height of the 100-year event for scenario RCP2.6 and RCP8.5 is 0.43 m and 0.80 m respectively for Burullus; 0.55 m and 1.03 m for Lautoka; 0.63 m and 1.04 m for New York; and 0.44 m and 0.79 m for Lusi. The increased frequency of the historical 100-year event for scenario RCP2.6 and RCP8.5 is a factor of 15 and 777 for Burullus; &gt;1200 and &gt;1200 for Lautoka; 67 and 541 for New York; and 6 and 26 for Lusi. The notation &gt;1200 indicates that the methodology allows for estimation of only a lower bound on the increased frequency.

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'''''Responding to Coastal Flooding and Inundation, Nadi, Fiji '''''

Hazards that contribute to riverine flooding and coastal inundation for Nadi Town and the wider Nadi Basin are heavy rainfall, elevated sea levels and subsidence of the delta. People and built assets in the Nadi River floodplain are already being affected by climate change. Observed sea level shows an increase of 4 mm yr –1 over the period 1992 – 2018. Over the past 75 years, extreme rainfall events have become more frequent. Of the 84 floods which occurred in the Nadi River Basin since 1870, 54 were post 1980, with 26 major floods since 1991 (Hay, 2017). In January 2009, large areas of Fiji were inundated by devastating floods which claimed at least 11 lives, left 12,000 people temporarily homeless and caused 54 million USD in damage. Worst hit was the Nadi area, with total damage estimated at 39 million USD (Hay, 2017). The increased frequency of flooding is not all attributable to increases in sea level and extreme rainfall events. River channels have become filled with sediment over time, largely owing to deforestation of the hinterland. Much of the mangrove fringe has been sacrificed for development of various kinds. The Nadi River Delta is subsiding, exacerbating the effects of SLR (Chandra and Gaganis, 2016 <sup>[[#fn:r837|837]]</sup> ). Various initiatives to help alleviate flooding and inundation in the Nadi Basin have been proposed. These include both hard protection and engineering (e.g., ring dikes, river widening, bridge rebuilding, retarding basins, shortcutting tributaries, dams and diversion channels) and accommodation (e.g., early flood warnings and improved land management practices in upper basin) measures (Box 4.1, Table 1).

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'''Box 4.1, Table 1'''
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Current coastal flood risk management and adaptation practices in the Nadi Basin and possible refinements using the new SLR and ESL projections of this report, as well as findings on adaptation options, decision making approaches and governance (Practice Consistent with SROCC Assessment). See Hay (2017) for background on current practice and practice consistent with SROCC assessment; see Box 4.1, Figure 1 for ESL event values. VLM is vertical land motion.

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'''''A Comparison of New York City and Shanghai Coastal Flood Adaptation Measures '''''

Hurricane Sandy (2012) and Typhoon Winnie (1997) are considered the largest recorded historical flood events for New York City and Shanghai, respectively (Xian et al., 2018 <sup>[[#fn:r838|838]]</sup> ). Hurricane Sandy killed 55 people in New York City and neighbouring states and caused over 19 billion USD losses to New York City (Rosenzweig and Solecki, 2014 <sup>[[#fn:r839|839]]</sup> ). Typhoon Winnie killed more than 310 people and caused damage exceeding 3.2 billion USD in China. Many dikes and floodwalls along coastal Shanghai and Zhejiang were breached by surge-driven floodwaters (Gu, 2005).

Past updates of the flood defences in Shanghai occurred after extreme flood events (i.e., typhoons in 1962, 1974 and 1981; Xian et al., 2018 <sup>[[#fn:r840|840]]</sup> ). In contrast, the flood tide of Hurricane Sandy stands out in the record at the Battery tide gauge. Unlike the numerous episodes of severe inundation experienced by Shanghai, NYC suffered relatively moderate consequences from individual events before Hurricane Sandy. This and other factors led to higher-standard flood protection measures in Shanghai, such as sea walls designed to protect its coastlines and critical infrastructure in developed areas against a 200-year (0.005 annual chance) flood level, and flood walls with 1000-year (0.001 annual chance) riverine flood return level along the Huangpu River. New York City, on the other hand, has relatively low protection, consisting of sandy dunes (e.g., on Staten Island), vegetation (e.g., in Queens) and low-rise sea walls or bulkheads in lower Manhattan. Another reason why Shanghai has repeatedly updated its flood protection while New York City failed to do so lies in their differing governance structures (Wei and Leung, 2005 <sup>[[#fn:r841|841]]</sup> ; Yin et al., 2011 <sup>[[#fn:r842|842]]</sup> ; Rosenzweig and Solecki, 2014 <sup>[[#fn:r843|843]]</sup> ), as well as rapid economic growth in China, providing funding for large-scale infrastructure (Zhang, 2003 <sup>[[#fn:r844|844]]</sup> ).

Since Hurricane Sandy in 2012, implementation of the ‘Big U’ project, a coastal protection system for lower Manhattan, has begun, and a variety of measures are planned, and some undertaken, to protect the subway system to a flood level of 4.3 m above street level (Jacob Balter, 2017 <sup>[[#fn:r845|845]]</sup> ; MTA, 2017). Newly built critical facilities will be located outside the flood zone and siting guidelines for publicly financed projects in the current and future flood zones have been tightened (New York City Mayor’s Office of Recovery and Resiliency, 2019). The degree to which these protection projects will be completed and the guidelines enforced remains uncertain. Home buyouts have enabled some permanent relocation away from hazardous areas, but relocation impacts on social networks and place-based ties hamper long-term recovery (Binder et al., 2019 <sup>[[#fn:r846|846]]</sup> ; Buchanan et al., 2019 <sup>[[#fn:r847|847]]</sup> ).

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'''Box 4.1, Table 2'''
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Current coastal flood risk management and adaptation practices in New York City and Shanghai and possible refinements using the new sea level rise (SLR) and extreme sea level (ESL) projections of this report, as well as findings on adaptation options, decision making approaches and governance (Practice Consistent with SROCC Assessment). See Xian et al. (2018) for background on current practice and practice consistent with SROCC assessment. See Box 4.1, Figure 1 for ESL event values. EbA is ecosystem-based adaptation.

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'''''Climate Change Adaptation in Nile Delta Regions of Egypt'''''

Coastal hazard for the Nile delta arises because large portions lie only ±1.5 m above sea level (Shaltout et al., 2015 <sup>[[#fn:r848|848]]</sup> ) and while the Delta includes only 2% of Egypt’s total area, it held 41% of its population as of 2006 (Hereher, 2010 <sup>[[#fn:r849|849]]</sup> ; World Bank, 2017 <sup>[[#fn:r850|850]]</sup> ) and is key to Egypt’s economy (Bucx et al., 2010 <sup>[[#fn:r851|851]]</sup> ). The delta is an important resource for Egypt’s ﬁsh farms (Hereher, 2009 <sup>[[#fn:r852|852]]</sup> ; El-Sayed, 2016 <sup>[[#fn:r853|853]]</sup> ) and contains more than 63% of Egypt’s cultivated lands (Hereher, 2010 <sup>[[#fn:r854|854]]</sup> ). The Nile Delta’s coastal lagoons are internationally renowned for abundant bird life, account for one fourth of Mediterranean wetlands and 60% of Egypt’s fish catch (Government of Egypt, 2016 <sup>[[#fn:r855|855]]</sup> ). Coastal flooding and salinisation of freshwater lagoons would negatively affect fisheries and biodiversity (UNDP, 2017 <sup>[[#fn:r856|856]]</sup> ).

The Nile Delta’s low elevation translates into high exposure to SLR (Shaltout et al., 2015 <sup>[[#fn:r857|857]]</sup> ), and the level of protection varies greatly from place to place (Frihy et al., 2010 <sup>[[#fn:r858|858]]</sup> ). Box 4.1, Figure 1 indicates that episodic flooding will increase substantially without effective adaptation measures. An estimated 2660 km 2 in the northern delta will be inundated by 2100 for GMSL of 0.44 m (Gebremichael et al., 2018 <sup>[[#fn:r859|859]]</sup> ; ''low confidence'' ), which is comparable to the RCP2.6 emission scenario. In addition, subsidence due to sediment diversion by the Aswan High Dam, water and natural gas extraction (Gebremichael et al., 2018 <sup>[[#fn:r860|860]]</sup> ) and some other critical natural aspects (Frihy et al., 2010 <sup>[[#fn:r861|861]]</sup> ) heightens vulnerability to coastal flooding (Box 4.1, Table 3) and reduces fresh water supply to the delta. Subsidence rates range from 0.4 mm yr –1 in the west delta to 1.1 mm yr –1 in the mid-delta and 3.4 mm yr –1 in the east delta (Elshinnawy et al., 2010 <sup>[[#fn:r862|862]]</sup> ), although rates as high as               10 mm yr –1 near natural gas extraction operations are also reported (Gebremichael et al., 2018 <sup>[[#fn:r863|863]]</sup> ). While there is ''low confidence'' in reported values, these indicate that subsidence makes locally a substantial contribution to RSL. Future construction of Ethiopia’s Grand Renaissance Dam (Stanley and Clemente, 2017 <sup>[[#fn:r864|864]]</sup> ) may heighten problems of fresh water availability and reduce hydropower production.

The low-lying northern coast and Nile Delta region are a high priority for adaptation to climate change (UNDP, 2017 <sup>[[#fn:r865|865]]</sup> ). The Egyptian government has committed 200 million USD to hard coastal protection at Alexandria and adopted integrated coastal zone management for the northern coast. Recent activities include integrating SLR risks within adaptation planning for social-ecological systems, with special focus on coastal urban areas, agriculture, migration and other human security dimensions (Government of Egypt, 2016 <sup>[[#fn:r866|866]]</sup> ; UNDP, 2017 <sup>[[#fn:r867|867]]</sup> ).

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'''Box 4.1, Table 3'''
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Current coastal flood risk management and adaptation practices in the Nile Delta and possible refinements using the new sea level rise (SLR) and extreme sea level (ESL) projections of this report, as well as findings on adaptation options, decision making approaches and governance (Practice Consistent with SROCC Assessment). See Elshinnawy et al. (2010) and text above for data sources for current practice. See Box 4.1, Figure 1 for ESL event values.

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