Small island developing states are among the most vulnerable areas to the impact of natural hazards and climate change. Flooding due to storm surges and extreme waves, coastal erosion, and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades. Reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures, but concerns remain that partial removal of the protective reef could increase wave loading on the islands. However, the available data and knowledge on the effects of these mining pits are currently very limited. This study provides new insights on the effects that pits may have on nearshore hydrodynamics and wave runup. Results are based on a large numerical data set of fringing reefs, derived using the validated XBeach nonhydrostatic+ process-based model. Model results indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef, which is partly caused by reduced wave transmission. Additionally, changes in sea and swell wave energy are attributed to reduced transmission, “a decrease” in wave dissipation, and (triad) wave–wave interaction. Furthermore, in 13% of all modeled cases, an increase in wave runup is observed, mainly due to more sea and swell wave energy reaching the shoreline. This probability is lowest for narrow pits relative to the reef flat width or pits located further from shore.

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The West African coastal barrier is maintained by significant wave-driven longshore sand transport. This sand originates from rivers and large coastal sand deposits. Today, however, much of the fluvial sand is trapped behind river dams and/or interrupted at several locations by port jetties. As a result, the sandy coastal barrier is eroding almost everywhere along its length.The aim of this study is to derive a large-scale sediment budget analysis, following a consistent approach, for the following countries: Republic of Côte d'Ivoire, Ghana, Togo and Benin, and pointing out the effects of major human interventions and climate change in this large common sediment system. The results are used as a basis to raise awareness among local governments and organizations on the effects and interdependency that major anthropogenic interventions (i.e. port jetties and river dams) and climate change (i.e. sea level rise, changes in wave climate, precipitation and temperature) may have on this shared sediment system. These detrimental effects can even occur in neighboring countries, as shown by some of the results. This estimation was carried out using a quantitative approach, based on one consistent numerical modelling system and validated with regional and local data.Based on the outcomes of the study, and with the support of a number of validation workshops in the different countries, suggestions are also provided for the setting up of a regional sediment management plan for the entire region.

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The Equatorian Daule and Babahoyo rivers combine into the tidal Guayas River with the largest estuary on the Pacific coast of South America. The city of Guayaquil, located along the Guayas, is the main port of Ecuador but at the same time the planet’s fourth most vulnerable city for future flooding due to climate change, after Guangzhou, Mumbai and Kolkata. Fluvial sedimentation is seen as one of the factors contributing to the risk of flooding. It increases water levels, it blocks the drainage from urban flooding and water treatment plants, it deteriorates navigation conditions, and it produces a mid-channel bar called “Islota del Palmar”. This bar attracts birds that hinder air traffic around the nearby airport. The growth of this bar pushes currents against the erodible bank along the Samborondón residential area. Sedimentation rates have increased in recent years, which is locally perceived as a result of deforestation in the upper catchment. The planning and design of effective mitigation measures, however, require a more precise understanding of what causes sedimentation. Therefore we carried out a systematic analysis using the new flexible-mesh version of the Delft3D modelling system. We found that sedimentation in the Guayas has no relation with upstream deforestation. Rather, the sediment originates from the downstream estuary instead of the upstream catchment. This complies with the presence of marine sands in the river as reported by a dredging company. We could explain the import of sediment from the asymmetry of the tide, and the recent increase in sedimentation rates to land reclamation and to a decrease of episodic flushing by river floods due to upstream dam construction. Only the deposition of silt and clay in connected stagnant water bodies could perhaps be ascribed to upstream deforestation. These insights will guide the planning and design of effective solutions.@en

The Equatorian Daule and Babahoyo rivers combine into the tidal Guayas River with the largest estuary on the Pacific coast of South America. The city of Guayaquil, located along the Guayas, is the main port of Ecuador but at the same time the planet’s fourth most vulnerable city for future flooding due to climate change, after Guangzhou, Mumbai and Kolkata. Fluvial sedimentation is seen as one of the factors contributing to the risk of flooding. It increases water levels, it blocks the drainage from urban flooding and water treatment plants, it deteriorates navigation conditions, and it produces a mid-channel bar called “Islota del Palmar”. This bar attracts birds that hinder air traffic around the nearby airport. The growth of this bar pushes currents against the erodible bank along the Samborondón residential area. Sedimentation rates have increased in recent years, which is locally perceived as a result of deforestation in the upper catchment. The planning and design of effective mitigation measures, however, require a more precise understanding of what causes sedimentation. Therefore we carried out a systematic analysis using the new flexible-mesh version of the Delft3D modelling system. We found that sedimentation in the Guayas has no relation with upstream deforestation. Rather, the sediment originates from the downstream estuary instead of the upstream catchment. This complies with the presence of marine sands in the river as reported by a dredging company. We could explain the import of sediment from the asymmetry of the tide, and the recent increase in sedimentation rates to land reclamation and to a decrease of episodic flushing by river floods due to upstream dam construction. Only the deposition of silt and clay in connected stagnant water bodies could perhaps be ascribed to upstream deforestation. These insights will guide the planning and design of effective solutions.@en

The Equatorian Daule and Babahoyo rivers combine into the tidal Guayas River with the largest estuary on the Pacific coast of South America. The city of Guayaquil, located along the Guayas, is the main port of Ecuador but at the same time the planet’s fourth most vulnerable city for future flooding due to climate change, after Guangzhou, Mumbai and Kolkata. Fluvial sedimentation is seen as one of the factors contributing to the risk of flooding. It increases water levels, it blocks the drainage from urban flooding and water treatment plants, it deteriorates navigation conditions, and it produces a mid-channel bar called “Islota del Palmar”. This bar attracts birds that hinder air traffic around the nearby airport. The growth of this bar pushes currents against the erodible bank along the Samborondón residential area. Sedimentation rates have increased in recent years, which is locally perceived as a result of deforestation in the upper catchment. The planning and design of effective mitigation measures, however, require a more precise understanding of what causes sedimentation. Therefore we carried out a systematic analysis using the new flexible-mesh version of the Delft3D modelling system. We found that sedimentation in the Guayas has no relation with upstream deforestation. Rather, the sediment originates from the downstream estuary instead of the upstream catchment. This complies with the presence of marine sands in the river as reported by a dredging company. We could explain the import of sediment from the asymmetry of the tide, and the recent increase in sedimentation rates to land reclamation and to a decrease of episodic flushing by river floods due to upstream dam construction. Only the deposition of silt and clay in connected stagnant water bodies could perhaps be ascribed to upstream deforestation. These insights will guide the planning and design of effective solutions.@en