Mangroves are more and more recognized for nature-based mitigation of flood risks in low-lying coastal zones, which host a disproportionately large part of the global population. Aerial roots, branches, and canopy of mangrove trees exert friction on the water flow, thereby reducing the propagation of high water levels through the forest. Field measurements of high water level attenuation rates are limited so far to mangrove forests situated at higher-latitude subtropical settings, where aerial roots are much sparser and lower than in low-latitude tropical mangroves. Here, for the first time, we measured high water level attenuation in a tropical Rhizophora forest, where aerial roots are several meters high and water levels never exceeded the aerial root height. Our measurements reveal attenuation rates between 42 ± 9.8 and 46 ± 9.8 cm km−1, which are the highest attenuation rates ever recorded in a mangrove forest, but an exponential rate is more suited to quantify high water level attenuation. In contrast to observations inside the mangrove forest, our observations showed that the propagation of high water levels through a 20 km long tidal channel fringed by wide mangrove areas was amplified, but that high water level amplification was reduced for higher tides with deeper flooding of the fringing mangroves. Our results provide the first empirical assessment of flood protection by tropical Rhizophora mangroves. As Rhizophora is globally the most common genus among mangroves, we propose that our reported high attenuation rates should be incorporated in future assessments of nature-based flood risk mitigation by mangroves.

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.

Conservation and restoration of salt marsh ecosystems are becoming increasingly important because of the many ecosystem services they provide. However, the processes controlling salt marsh establishment and persistence, especially on bare tidal flats in muddy areas, remain unclear. As muddy sediments typically experience a restriction of soil drainage, we expect that a surface drainage relief due to a heterogeneity topography, as might occur on the edge of tidal channels, can facilitate the establishment of salt marsh vegetation on muddy tidal flats. By means of a manipulative field experiment, using “Mega-Marsh Organ” mesocosms, we investigated the impact of surface drainage and elevation relative to mean sea level on (1) the survival of Spartina anglica seedlings from three different age classes: 1-yr, 3-month, and 1-week; and (2) the growth performance of mature S. anglica marsh tussocks. S. anglica seedling survival, especially in the establishment phase, was positively affected by better surface drainage, increases of seedling age, and higher elevation relative to mean sea level. That is, the survival rate of S. anglica seedlings at the end of 6th week increased from 0% (at surface water undrained, 1-week, 0 cm elevation) to 94.44% (at surface water drained, 1-yr, 90 cm elevation). In contrast, surface drainage did not affect the performance of large S. anglica marsh tussocks, as only increased elevation relative to mean sea level was shown to affect S. anglica tussock growth in terms of plant height, shoot numbers, and dry biomass. Based on our findings, we proposed a conceptual model to understand how surface drainage-driven feedbacks in a heterogeneous topography may be reinforced to induce salt marsh establishment in muddy systems. Further testing of present hypothesized model would be beneficial for insights into salt marsh establishment on tidal mudflats.

SLIM is an unstructured mesh, finite element model of environmental and geophysical fluid flows, which is being improved to simulate fine-grained sediment transport in riverine and marine water systems. A 2D depth-averaged version of the model is applied to the Mahakam Delta (Borneo, Indonesia), the adjacent ocean, and three lakes in the central part of the Mahakam River catchment. The 2D code is coupled to a 1D section-averaged model for the Mahakam River and four tributaries. The coupled 2D/1D model is mainly aimed at simulating fine-grained sediment transport in the riverine and marine water continuum of the Mahakam River system. Using the observations of suspended sediment concentration (SSC) at five locations in the computational domain, the modelling parameters are first determined in a calibration step, for a given
period of time. A validation step is then performed using data related to another period of time. It is concluded that the coupled 2D/1D model
reproduces very well the observed suspended sediment distribution within the delta. The spatial distribution of sediment concentration in the delta
and its temporal variation are also discussed.