Rijkswaterstaat and the water boards are undertaking the largest dyke reinforcement operation ever. The wave tests will be carried out in the Delta Flume at the end of this year. The bins are being transported to the Delta Flume of Deltares in Delft by truck on working days (1 to 2 shipments per day). That amounts to 68 bins each measuring 2 by 2 metres and 40 to 70 centimetres deep. A total of approximately 270 square metres of substrate has been removed. Substrate extraction is currently in full swing in the saltmarshes at Peazens-Moddergat, with a possible extension into October. Wetterskip Fryslân and other water boards will incorporate the research findings into their designs and dyke reinforcement work in saltmarsh areas. If the research shows that the saltmarsh grass does indeed have a dampening effect on the waves, then dykes could be redesigned to make them take up less space and work more effectively in the long term. The grass is expected to reduce the force of the waves that reach the dykes. As well as this dampening effect, saltmarsh grass also retains silt during high tides, so that it keeps pace with rising sea levels. UT Researcher Bas Borsje predicts that in addition to the height of the saltmarsh, the vegetation also contributes to the reduction of wave stress on the dyke. The Delta Flume simulates conditions in a superstorm, recreating extreme waves for instance, and this enables the wave-neutralizing effect of the saltmarsh vegetation to be measured. Saltmarshes that are overgrown with sea couch, a type of blue-grey grass, make an important contribution to the ability of the dyke to hold back the water. A saltmarsh is a strip of land beyond the dyke which borders the sea directly. The area covered by the Wetterskip Fryslân water authority includes many dykes with foreland in the form of saltmarshes. This innovation research is being funded by the National Flood Protection Programme (known as the HWBP). The substrate is being taken to the Delta Flume in Delft for research. Last month, the water authority has been removing quantities of substrate from the saltmarsh beyond the dyke to the east of the village of Peazens-Moddergat. This may make it possible to reduce the space required by dykes. Future comprehensive experiments will feature multi-frequency multi-angle measurements on a variety of sediment types, shell densities, interface roughnessess, and plumes of various sediment grain sizes, shapes and concentrations.The University of Twente, in partnership with Wetterskip Fryslân and the knowledge institute Deltares, is researching the effect of saltmarsh grass and foreland on the energy of the waves that reach the dyke. The results revealed that flume reverberation did not interfere significantly with bottom targets and that fine-sand plumes in the water column were clearly detected. Multi-frequency single- and multibeam echosounder data of the flume floor and of fine-grained, in-water sand plumes were recorded. In order to prepare for such comprehensive and ground-breaking experiments, we conducted a feasibility experiment in the Delta Flume, Deltares, Netherlands. Reference data derived from measurements in a controlled environment should optimize the use of empirical backscatter data for underwater observation and management. However, the current scarcity of sediment reference datasets of both bottom backscatter angular response and water column scattering, hampers the generation of applicable information. Multifrequency single- and multibeam echosounders are mature technologies for underwater mapping.
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