The Hondsbossche Dunes is a seven-kilometre-long dune, beach and shoreface nourishment that serves as a primary flood defence since 2015. In addition to its protective function, the Hondsbossche Dunes stimulate ecological habitat development in the coastal area, in line with Ecoshape's 'Building with Nature'-approach. Its morphologic development is monitored to simulate the morphological response to marine processes and aeolian sediment transport, and to determine the optimal profile for dune growth and nature development.The monitoring program of Ecoshape studied the volume change of the beach and the dunes. The decrease of sediment volume on the beach is not equal to the increase of sediment volume on the dunes. The hypothesis is that the remaining sediment must stem from the intertidal zone and the shoreface. The research objective of this study is to investigate the contribution of sediment supply and marine processes in the intertidal zone to changes in morphology and bed composition in the beach and dune. The results of this data analysis show that sediment is transported from the intertidal zone to the shoreface in the north and south of the Hondsbossche Dunes. This behaviour is different from the middle of the domain, where sediment is transported from the shoreface to the intertidal zone. The direction of cross-shore sediment transport between the shoreface and the intertidal zone may depend on the crest level of sandbars in the shoreface: The sandbars in the middle of the Hondsbossche Dunes break more short waves than those in the north and south, which causes 'freeing' of long waves and net landward sediment transport. The modelling study suggests that wave runup and the presence of soil moisture reduce aeolian sediment transport from the intertidal zone to the beach, because grains in the intertidal zone are immobile when they are moist. This reduction results in increased erosion at the dry part of the beach, because the wind picks up sediment from the beach instead of the intertidal zone. The soil moisture content also significantly coarsens the bed composition in the intertidal zone, because the difference in threshold for aeolian transport between fine and coarse grains becomes larger. Hydraulic mixing does not contribute to significant morphological changes in the intertidal zone, but redistributes grains on the bed of the intertidal zone toward its initial distribution by stirring up the bed of the intertidal zone. These marine processes only affect dune growth if aeolian sediment transport is supply limited, which is mainly the case for Profile 2 north and Profile 4. Aeolian sediment transport in Profile 3 south is mainly transport limited, because the dune growth volume in most simulations is equal to the dune growth capacity following the formulation of Bagnold (1941). It seems that dune growth in this profile is less affected by marine processes. These results are likely to be related to the finer bed composition and wider beach at Profile 3 south. ... Read more

The city of Guayaquil suffers from regular floods. During the wet season, typically from late December until late April or early May, multiple floods per week can occur. Mainly the excessive rainfall in combination with high tide penetrating into the city results in a high flood risk, but some flood-prone areas can also flood in case of spring tide only.
The main objective of this research is to investigate the possibility of reducing pluvial and coastal flooding in urban areas by constructing a (semi-permanent) barrier in a sea branch, which retains the incoming tide and creates storage for excessive rainfall. In addition, local storage areas spread over the city are considered to delay stormwater runoff into the sea branches. Based on a system analysis and by numerical modelling, several closure locations and their effects are assessed.
Temporary storage of stormwater behind a barrier in a sea branch is a suitable solution to prevent both coastal and pluvial flooding. Based on the results of this research and possible locations of the barriers, a combination of three selected barriers is most opportune, because all catchment areas adjacent to a sea branch can drain their stormwater in a closed-off part behind one of these barriers. In order for these barriers to be effective, they must be closed during low tide prior to heavy rainfall. All three barriers are able to withhold the stormwater volume from their corresponding catchment areas during a 10-year design rainfall event. Even in the event of the highest possible water level during low tide, being neap tide in combination with the storm surge of El Niño, the storage capacities are sufficiently large. Besides the large-scale and small-scale solutions that are currently considered by the local authorities, they are advised to also consider the intermediate-scale solution presented in this study.
Local stormwater storage in the form of water squares in parks and playgrounds is a small-scale solution to reduce pluvial flooding. The storage capacity of these areas is much smaller than the storage capacity behind a barrier, but it is a solution for low-lying urban areas that are not adjacent to a sea branch or river. When the storage capacity of parks and playgrounds in some catchment areas is not sufficient, underground storage basins can also be considered as local storage areas.
The local authorities are advised to set up regulations on return periods for designing flood risk-reducing structures and to assess the economic losses of floods in urban areas, in order to be able to estimate the acceptable cost of these structures.
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