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

==== 6.3.4.5 Riverine Flood Impact Reduction ====

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NBS reduce both the volume of floodwater and the impact of floods ( ''medium evidence'' , ''medium agreement'' ). NBS reduce the volume of runoff by increasing infiltration and water storage (Shuster et al., 2005; Salvadore, Bronders and Batelaan, 2015), and affect the production and impact of flood waters through reducing river energy and flow speed through physical blockage, stabilising riverbanks during flood events, creating space for floodwaters to expand and combating land subsidence (Palmer, Filoso and Fanelli, 2014; Ahilan et al., 2018). Installing NBS to increase infiltration on low slopes and high-permeability soils can reduce the impacts of potential increases in urban flooding driven by climate change, especially for small- to medium-scale flood events (lower than 20% mean annual flood) (Moftakhari et al., 2018).

Source reduction strategies include creating permeable areas such as parks and open spaces, as well as engineered devices such as raingardens, bioswales and retention ponds that help retain stormwater runoff from impervious areas. River restoration can reduce flood peak flow and provide space for floodwaters to expand. Planting and maintaining vegetation along riverbanks, often in the form of parks or river restoration, maintains structural integrity during flood events. Wetland construction and improved connectivity to floodplains also reduces flood peaks. Efforts to restore floodplains are important to create space for floodwaters and reduce exposure by moving people out of the hazard zone. Floodplain restoration also provides access to the river that has multiple benefits including recreation, access to water for domestic use and other cultural ecosystem services. A key adaptation strategy is to reduce streambank erosion (a result of high peak flow) using riparian vegetation to stabilise riverbanks during flood events.

Cities manage flood risk using different types of adaptation and regulatory mechanisms ( [[#Naturally%20Resilient%20Communities--2017|Naturally Resilient Communities, 2017]] ). Built flood-control infrastructure, such as levees and stream channelisation, reduces the demand for nature-based flood impact reduction. Cities facing flood risk that do not currently have extensive grey flood-mitigation infrastructure may find NBS to be an appealing, lower cost solution (Keeler et al., 2019). In cities where flood-control grey infrastructure already exists, there is less demand for NBS of flood protection, but NBS may provide important back up, especially in a changing climate that may increase flood hazards ( [[#City%20of%20Los%20Angeles--2017|City of Los Angeles, 2017]] ; Elmqvist et al., 2019). Overall, city and basin-wide NBS for riverine flood impact reduction can reduce the generation of new hazards by making space for water which can reduce the potential for a false sense of security provided by traditional flood management approaches (Ruangpan et al., 2020; Turkelboom et al., 2021).

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