Mangrove ecosystem properties regulate high water levels in a river delta

Abstract

Abstract. Intertidal wetlands, such as mangroves in the tropics, are increasingly recognized for their role in nature-based mitigation of coastal flood risks. Yet it is still poorly understood how effective they are at attenuating the propagation of extreme sea levels through large (order of 100 km2) estuarine or deltaic systems, with complex geometry formed by networks of branching channels intertwined with mangrove and intertidal flat areas. Here, we present a delta-scale hydrodynamic modelling study, aiming to explicitly account for these complex landforms, for the case of the Guayas delta (Ecuador), the largest estuarine system on the Pacific coast of Latin America. Despite coping with data scarcity, our model accurately reproduces the observed propagation of high water levels during a spring tide. Further, based on a model sensitivity analysis, we show that high water levels are most sensitive to the mangrove platform elevation and degree of channelization but to a much lesser extent to vegetation-induced friction. Mangroves with a lower surface elevation, lower vegetation density, and higher degree of channelization all favour a more efficient flooding of the mangroves and therefore more effectively attenuate the high water levels in the deltaic channels. Our findings indicate that vast areas of channelized mangrove forests, rather than densely vegetated forests, are most effective for nature-based flood risk mitigation in a river delta.

Mangroves as nature-based mitigation for ENSO-driven compound flood risks in a large river delta

“…The present modelling study builds further on the work by Pelckmans et al (2023), which presents the hydrodynamic model setup, calibration, and validation against observed tidal water levels for 11 tide gauge stations throughout the delta. The present paper focuses on the modelling of extreme high water levels under El Niño conditions, whereas Pelckmans et al (2023) only accounted for ENSO-neutral conditions (i.e. without additional El Niño-driven forcing).…”

“…While the Guayas delta is predominantly covered by Rhizophora mangle, other tropical deltas might be covered by a wider variety of mangrove species with different morphologies and, consequently, different mangrove-induced drag. Nevertheless, previous studies have pointed out a low sensitivity of mangrove-induced drag on water levels in the channels (Pelckmans et al, 2023;Horstman et al, 2015;Hu et al, 2015). Instead, a developed channel network is essential to distribute the HWLs from the main estuarine channel into the wetlands, and as such, upstream HWL reduction is more sensitive to wetland topography and degree of channelization than the vegetation-induced friction (Pelck- , 2023).…”

“…These data only cover 73 % of the watershed area which drains into the Guayas delta. Hence, we applied a precipitation-weighted linear extrapolation based on monthly precipitation data collected from OpenLandMap (Hengl and Parente, 2022), as similarly presented by Pelckmans et al (2023). The El Niñodriven landward boundary conditions, referred to as the El Niño riverine forcing, were defined using the daily extrapolated discharge series from 1-3 March 1998.…”