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Mangrove Vegetation Density and Channel Drainage Density Have Trade‐Off Effects on Nature‐Based Flood Risk Mitigation in Estuaries.

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Title: Mangrove Vegetation Density and Channel Drainage Density Have Trade‐Off Effects on Nature‐Based Flood Risk Mitigation in Estuaries.
Authors: Pelckmans, Ignace1 (AUTHOR) ignace.pelckmans@uantwerpen.be, Belliard, Jean‐Philippe1,2 (AUTHOR), Gourgue, Olivier2 (AUTHOR), Dominguez‐Granda, Luis E.3 (AUTHOR), Temmerman, Stijn1 (AUTHOR)
Source: Journal of Geophysical Research. Oceans. Jul2025, Vol. 130 Issue 7, p1-20. 20p.
Subject Terms: *Mangrove plants, *Flood damage prevention, *Floods, *Estuaries, *Green infrastructure, *Hydrodynamics, *Climate change, Stream channelization
Abstract: Mangrove conservation and restoration have been increasingly recognized as cost‐effective and sustainable strategies to mitigate the increasing coastal flood risks. As surge or tidal waves propagate through river deltas and estuaries, mangroves can lower peak water levels by exerting friction on the water flow, which results in within‐wetland attenuation of high water levels, and by providing flood storage, resulting in along‐channel attenuation. While the impact of channels and vegetation on the friction effect has been studied before, the impact on the storage effect is not. Here we present a hydrodynamic model in a tropical subestuary, calibrated and verified with field observations. Through a scenario analysis, we show that with a denser network of secondary subchannels and less dense vegetation, the storage effect becomes stronger, leading to higher along‐channel attenuation. The opposite can be observed for the friction effect: we simulate lower within‐wetland attenuation rates in case of higher channel drainage density and lower vegetation density. If 2 km wide mangroves fringe the channels, we found the strongest along‐channel attenuation in case of a dense network of secondary subchannels with low vegetation density. In contrast, when the mangrove extent is limited, for instance due the presence of aquaculture, a dense network of channels or sparse vegetation can result in the amplification of peak water levels. Future conservation and restoration efforts should consider this trade‐off between within‐wetland and along‐channel attenuation in order for nature‐based flood protection to safeguard both human settlements behind unchanneled wetlands and along deltaic channels fringed by mangroves. Plain Language Summary: Many coastal cities such as Guayaquil in Ecuador, Khulna in Bangladesh, Palembang in Indonesia and Ho Chi Minh in Vietnam house millions of people but are exposed to coastal floods, which will likely intensify with climate change and sea level rise. Mangroves can mitigate these flood risks. Communities which are located right behind mangroves are protected as the peak of a flood wave is lowered due to friction with the above‐ground roots and stems of mangrove trees. Communities which are located upstream along estuarine channels are protected as a part of the flood wave is temporarily stored in the mangroves fringing the channel downstream of the communities. We show that the denser vegetation and less channels dissecting the mangroves is beneficial for communities right behind the mangroves but disadvantageous for upstream settlements located along estuarine channels and vice versa. Future conservation and restoration efforts should consider these effects for nature‐based flood protection to safeguard both human settlements behind unchanneled mangroves and along deltaic channels fringed by mangroves. Key Points: Mangroves can attenuate extreme sea levels both within mangroves and along a channel fringed by mangrovesDenser vegetation improves within‐wetland attenuation but has negative effects on along‐channel attenuationA denser channel network improves along‐channel attenuation but has negative effects on within‐wetland attenuation [ABSTRACT FROM AUTHOR]
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Abstract:Mangrove conservation and restoration have been increasingly recognized as cost‐effective and sustainable strategies to mitigate the increasing coastal flood risks. As surge or tidal waves propagate through river deltas and estuaries, mangroves can lower peak water levels by exerting friction on the water flow, which results in within‐wetland attenuation of high water levels, and by providing flood storage, resulting in along‐channel attenuation. While the impact of channels and vegetation on the friction effect has been studied before, the impact on the storage effect is not. Here we present a hydrodynamic model in a tropical subestuary, calibrated and verified with field observations. Through a scenario analysis, we show that with a denser network of secondary subchannels and less dense vegetation, the storage effect becomes stronger, leading to higher along‐channel attenuation. The opposite can be observed for the friction effect: we simulate lower within‐wetland attenuation rates in case of higher channel drainage density and lower vegetation density. If 2 km wide mangroves fringe the channels, we found the strongest along‐channel attenuation in case of a dense network of secondary subchannels with low vegetation density. In contrast, when the mangrove extent is limited, for instance due the presence of aquaculture, a dense network of channels or sparse vegetation can result in the amplification of peak water levels. Future conservation and restoration efforts should consider this trade‐off between within‐wetland and along‐channel attenuation in order for nature‐based flood protection to safeguard both human settlements behind unchanneled wetlands and along deltaic channels fringed by mangroves. Plain Language Summary: Many coastal cities such as Guayaquil in Ecuador, Khulna in Bangladesh, Palembang in Indonesia and Ho Chi Minh in Vietnam house millions of people but are exposed to coastal floods, which will likely intensify with climate change and sea level rise. Mangroves can mitigate these flood risks. Communities which are located right behind mangroves are protected as the peak of a flood wave is lowered due to friction with the above‐ground roots and stems of mangrove trees. Communities which are located upstream along estuarine channels are protected as a part of the flood wave is temporarily stored in the mangroves fringing the channel downstream of the communities. We show that the denser vegetation and less channels dissecting the mangroves is beneficial for communities right behind the mangroves but disadvantageous for upstream settlements located along estuarine channels and vice versa. Future conservation and restoration efforts should consider these effects for nature‐based flood protection to safeguard both human settlements behind unchanneled mangroves and along deltaic channels fringed by mangroves. Key Points: Mangroves can attenuate extreme sea levels both within mangroves and along a channel fringed by mangrovesDenser vegetation improves within‐wetland attenuation but has negative effects on along‐channel attenuationA denser channel network improves along‐channel attenuation but has negative effects on within‐wetland attenuation [ABSTRACT FROM AUTHOR]
ISSN:21699275
DOI:10.1029/2025JC022398