Mangrove forests reduce wave attack along tropical and sub-tropical coastlines, decreasing the wave loads acting on coastal protection structures. Mangrove belts seaward of embankments can therefore lower their required height and decrease their slope protection thickness. Wave reduction by mangroves depends on tree frontal surface area and stability against storms, but both aspects are often oversimplified or neglected in coastal protection designs. Here we present a framework to evaluate how mangrove belts influence embankment designs, including mangrove growth over time and failure by overturning and trunk breakage. This methodology is applied to Sonneratia apetala mangroves seaward of embankments in Bangladesh, considering forest widths between 10 and 1000 m (cross-shore). For water depths of 5 m, wave reduction by mangrove forests narrower than 1 km mostly affects the slope protection and the bank erodibility, whereas the required embankment height is less influenced by mangroves. Sonneratia apetala trees experience a relative maximum in wave attenuation capacity at 10 years age, due to their large submerged canopy area. Once trees are more than 20 years old, their canopy is emergent, and most wave attenuation is caused by trunk and roots. Canopy emergence exposes mangroves to wind loads, which are much larger than wave loads, and can cause tree failure during cyclones. These results stress the importance of including tree surface area and stability models when predicting coastal protection by mangroves.

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This report investigates how embankments in Bangladesh could be upgraded with no or limited footprint increase. Many Bangladesh embankments need to be upgraded to improve flood protection. Traditionally, embankments are upgraded by adding more soil, heightening the crests while simultaneously widening the embankment maintain slope stability. However, in many cases there is insufficient space for embankment widening. Case studies show that in Bangladesh, private assets often directly border embankments at both the landward and seaward side. Also land is scarce in general and the land directly adjacent to embankments is often in use by the communities. For traditional embankment heightening and widening, these buildings need to be (re)moved. So owners of private assets and land need to be identified and compensated and the land must be acquired leading additional project costs and delays. This problem could be mitigated by using embankment upgrade techniques with a limited footprint increase. This requires knowledge of the available techniques and their suitability in the Bangladesh situation. Therefore, this study aims to provide an inventory of low-footprint embankment up-grade techniques used around the world, their (dis)advantages in terms of land use, risks, costs and O&M aspects, and their suit-ability for the situation in Bangladesh. @en