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

==== 4.3.2.3 Terrestrial Processes Shaping Coastal Exposure and Vulnerability ====

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Coastal areas, including deltas, are highly dynamic as they are affected by natural and/or human-induced processes locally or originating from both the land and the sea. Changes within the catchment can therefore have severe consequences for coastal areas in terms of sediment supply, pollution, and/or land subsidence. Sediment supply reaching the coast is a critical factor for delta sustainability (Tessler et al., 2018 <sup>[[#fn:r1037|1037]]</sup> ) and has declined drastically in the last few decades due to dam construction, land use changes and sand mining (Ouillon, 2018 <sup>[[#fn:r1038|1038]]</sup> ; ''high confidence'' ). For instance, Anthony et al. (2015) reported large-scale erosion affecting over 50% of the delta shoreline in the Mekong delta between 2003 and 2012, which was attributed in part to a reduction in surface-suspended sediments in the Mekong river potentially linked to dam construction within the river basin, sand mining in the river channels, and land subsidence linked to groundwater over-abstraction locally. Schmitt et al. (2017) <sup>[[#fn:r1039|1039]]</sup> demonstrated that these and other drivers in sediment budget changes can have severe effects on the very physical existence of the Mekong delta by the end of this century, with the most important single driver leading to inundation of large portions of the delta being ground-water pumping induced land subsidence. Thi Ha et al. (2018) estimated the decline in sediment supply to the Mekong delta to be around 75% between the 1970s and the period 2009–2016. In the Red River, the construction of the Hoa Binh Dam in the 1980s led to a 65% drop in sediment supply to the sea (Vinh et al., 2014 <sup>[[#fn:r1040|1040]]</sup> ). Based on projections of historical and 21st century sediment delivery to the Ganges-Brahmaputra-Meghna, Mahanadi and Volta deltas, Dunn et al. (2018) showed that these deltas fall short in sediment and may not be able to maintain their current elevation relative to sea level, suggesting increasing salinisation, erosion, flood hazards and adaptation demands.

Another rarely considered factor is the shift in TC climatology which also plays a critical role in explaining changes in fluvial suspended sediment loads to deltas as demonstrated by Darby et al. (2016) <sup>[[#fn:r1041|1041]]</sup> , again for the Mekong delta. More generally, most conventional engineering strategies that are commonly employed to reduce flood risk (including levees, sea walls, and dams) disrupt a delta’s natural mechanisms for building land. These approaches are rather short-term solutions which overall reduce the long-term resilience of deltas (Tessler et al., 2015 <sup>[[#fn:r1042|1042]]</sup> ; Welch et al., 2017 <sup>[[#fn:r1043|1043]]</sup> ). Systems particularly prone to flood risk due to anthropogenic activities include North America’s Mississippi River delta, Europe’s Rhine River delta, and deltas in East Asia (Renaud et al., 2013 <sup>[[#fn:r1044|1044]]</sup> ; Day et al., 2016 <sup>[[#fn:r1045|1045]]</sup> ). In regions where suspended sediments are still available in relatively large quantities, rates of sedimentation can vary depending on multiple factors, including the type of infrastructure present locally, as was shown by Rogers and Overeem (2017) <sup>[[#fn:r1046|1046]]</sup> for the Ganges-Brahmaputra-Meghna (Bengal) delta in Bangladesh as well as seasonal differences in sediment supply and place of deposition. For example, in meso-tidal and macro-tidal estuaries, during floods most of the sediments are depositing in the coastal zones and a large part of these sediments are brought back to the estuary during the low flow season by tidal pumping. This can lead to significantly higher deposition rates in the dry season as shown by Lefebvre et al. (2012) in the lower Red River estuary and by Gugliotta et al. (2018) <sup>[[#fn:r1048|1048]]</sup> in the Mekong delta. Enhanced sedimentation further upstream in estuaries and a silting-up of estuarine navigation channels can have high economic consequences for cities with a large estuarine harbour. In Haiphong city, in North Vietnam, the authorities decided to build a new harbour further downstream, for a cost estimated at 2 billion USD (Duy Vinh et al., 2018).

Overall, reduced freshwater and sediment inputs from the river basins are critical factors determining delta sustainability (Renaud et al., 2013 <sup>[[#fn:r1049|1049]]</sup> ; Day et al., 2016 <sup>[[#fn:r1050|1050]]</sup> ). In some contexts, this can be addressed through basin-scale management which allow more natural flows of water and sediments through the system, including methods for long-term flood mitigation such as improved river-floodplain connectivity, the controlled redirection of a river (i.e., avulsions) during times of elevated sediment loads, the removal of levees, and the redirection of future development to lands less prone to extreme flooding (Renaud et al., 2013 <sup>[[#fn:r1051|1051]]</sup> ; Day et al., 2016 <sup>[[#fn:r1052|1052]]</sup> ; Brakenridge et al., 2017 <sup>[[#fn:r1053|1053]]</sup> ). These actions could potentially increase the persistence of coastal landforms in the context of SLR. Next to decreasing sediment inputs to the coast, river bed and beach sand mining has been shown to contribute to shoreline erosion, for example, for shorelines of Crete (Foteinis and Synolakis, 2015 <sup>[[#fn:r1054|1054]]</sup> ), and several sub-Saharan countries such Kenya, Madagascar, Mozambique, South Africa and Tanzania (UNEP, 2015 <sup>[[#fn:r1055|1055]]</sup> ). At the global scale, 24% of the world’s sandy beaches are eroding at rates exceeding 0.5 m yr <sup>–1</sup> , while 28% are accreting for the period 1984–2016. The largest and longest eroding sandy coastal stretches are in North America (Texas; Luijendijk et al., 2018 <sup>[[#fn:r1056|1056]]</sup> ).

Shoreline erosion leads to coastal squeeze if the eroding coastline approaches fixed and hard built or natural structures as noted in AR5 (Pontee, 2013 <sup>[[#fn:r1057|1057]]</sup> ; Wong et al., 2014 <sup>[[#fn:r1058|1058]]</sup> ), a process to which SLR also contributes (Doody, 2013 <sup>[[#fn:r1059|1059]]</sup> ; Pontee, 2013 <sup>[[#fn:r1060|1060]]</sup> ). The AR5 further noted that coastal squeeze is expected to accelerate due to rising sea levels (Wong et al., 2014 <sup>[[#fn:r1061|1061]]</sup> ). Doody (2013) characterised coastal squeeze as coastal habitats being pushed landward through the effects of SLR and other coastal processes on the one hand and, on the other hand, the presence of static natural or artificial barriers effectively blocking this migration, thereby squeezing habitats into an ever narrowing space. Distinctions are made between coastal squeeze being limited to (1) the consequences of SLR vs. other environmental changes on the coastline and (2) the presence of only coastal defence structures vs. natural sloping land or other artificial infrastructure (Pontee, 2013 <sup>[[#fn:r1062|1062]]</sup> ). Recent publications have emphasised coastal squeeze related to SLR, although inland infrastructure blocking habitat migration is not necessarily limited to defence structures (Torio and Chmura, 2015 <sup>[[#fn:r1063|1063]]</sup> ; McDougall, 2017 <sup>[[#fn:r1064|1064]]</sup> ). Coastal ecosystem degradation by human activities leading to coastal erosion is also an important consideration (McDougall, 2017 <sup>[[#fn:r1065|1065]]</sup> ). Taking into consideration the current challenges to attribute coastal impacts to SLR (Section 4.3.3.1), it can be hypothesised here that as long as SLR impacts remain moderate, the dominant driving factor of coastal squeeze will be anthropogenic land-based development (e.g., Section 4.3.2.2). With higher SLR scenarios and in the case of no further development at the coast, SLR may become the dominant driver before the end of this century.

Preserved coastal habitats can play important roles in reducing risks related to some coastal hazards and initiatives are being put in place to reduce coastal squeeze, such as managed realignment (Sections 4.1, 4.4.3.1) which includes removing inland barriers (Doody, 2013 <sup>[[#fn:r1066|1066]]</sup> ). Coastal squeeze can lead to degradation of coastal ecosystems and species (Martínez et al., 2014 <sup>[[#fn:r1067|1067]]</sup> ), but if inland migration is unencumbered, observation data and modelling have shown that the net area of coastal ecosystems could increase under various scenarios of SLR, depending on the ecosystems considered (Torio and Chmura, 2015 <sup>[[#fn:r1068|1068]]</sup> ; Kirwan et al., 2016 <sup>[[#fn:r1069|1069]]</sup> ; Mills et al., 2016 <sup>[[#fn:r1070|1070]]</sup> ). However, recent modelling research has shown that rapid SLR in a context of coastal squeeze could be detrimental to the areal extent and functionality of coastal ecosystems (Mills et al., 2016 <sup>[[#fn:r1071|1071]]</sup> ) and, for marshes, could lead to a reduction of habitat complexity and loss of connectivity, thus affecting both aquatic and terrestrial organisms (Torio and Chmura, 2015 <sup>[[#fn:r1072|1072]]</sup> ). Contraction of marsh extent is also identified by Kirwan et al. (2016) <sup>[[#fn:r1073|1073]]</sup> when artificial barriers to landward migration are in place. Adaptation to SLR therefore needs to account for both development and conservation objectives so that trade-offs between protection and realignment that satisfy both objectives can be identified (Mills et al., 2016 <sup>[[#fn:r1074|1074]]</sup> ).

In summary, catchment-scale changes have very direct impacts on the coastline, particularly in terms of water and sediment budgets ( ''high confidence'' ). The changes can be rapid and modify coastlines over short periods of time, outpacing the effects of SLR and leading to increased exposure and vulnerability of social-ecological systems ( ''high confidence'' ). Without losing sight of this fact, management of catchment-level processes contribute to limiting rapid increases in exposure and vulnerability. Further to hinterland influences, coastal squeeze increases coastal exposure as well as vulnerability by the loss of a buffer zone between the sea and infrastructure behind the habitat undergoing coastal squeeze. The clear implication is that coastal ecosystems progressively lose their ability to provide regulating services with respect to coastal hazards, including as a defence against SLR driven inundation and salinisation ( ''high confidence'' ). Vulnerability is also increased if freshwater resources become salinised, particularly if these resources are already scarce. The exposure and vulnerability of human communities is exacerbated by the loss of other provisioning, supporting and cultural services generated by coastal ecosystems, which is especially problematic for coast-dependent communities ( ''high confidence'' ).

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