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- An important difference between North Sea Coast and the IJsselmeer experiments is that the North Sea sand engine contributes to beach and dune formation and the Lake IJsselmeer sand engines are not capable of transporting sand above the water line. The difference is that combinations of North Sea tides and winds allow sand to be moved towards the beaches. So far, in the current design, the Lake IJsselmeer sand engines do not create new fore-lands above water levels.
- The Workumerbuitenwaard sand engine was located in the dynamic zone, where waves break on the transition between deep and shallow water. The dynamic zone is about 600m off the coast. The wave energy was sufficient to erode the nourishment, but the sand transport direction proved to be north, parallel to the coast, instead of north-east towards the coast. The Oudemirdum sand nourishment was located close to the coast and here we see sand movement in the desired direction. In both cases wave energy is sufficient to erode and transport the sand. But the location choice in terms of: distance to the coast, water depth and wave dynamics, prove to be important in relation to the transport direction. Also the sand engine design seems to have played an important role. The Oudemirdum engine was designed as a hook perpendicular on the coast.
- The different outcomes may also be explained by differences in wave dynamics between Frisian’s west and south coast. The coastal morphology is similar, but storm wind directions are more west than southwest, creating more severe wave attacks on the western coast. Attribution of these differences to the experiment outcomes is difficult and more experiments and studies are needed to analyze this.
- Governance monitoring showed a sharp increase in interest for Building with Nature with local authorities. This interest was facilitated by the experiment’s real life character, which could be visited and discussed in the field. Many policy makers and authorities did visit the sand engine sites, which attracted media attention.
- Initially local stakeholders were not involved in planning and design. But it soon became clear that without local support it was difficult to implement the sand nourishments. Local entrepreneurs in the city of Hindeloopen prevented a sand engine construction along their beaches and in Oudemirdum the final design was the outcome of a lengthy negotiation process. Involvement of local stakeholders in planning and design proved essential. Also locals could contribute with detailed local knowledge on historic and current morphological behavior of the coast. An important lesson learned is to involve all important stakeholders from the beginning.
- An unexpected finding, resulting from the LIDAR coast-line measurements, was that coastal erosion was more severe than thought. This finding is unrelated to sand engine performance, but it showed the coast is vulnerable to wave attacks and measures are needed.
- The constructed sand engines are small in relation to average yearly sand transport volumes along the coast. Larger scale sand engines designed like the Oudemirdum experiment are expected to be effective to counter coastal erosion. A large scale sand engine should be carefully located in relation to recreational beaches and harbor entrances. Local entrepreneurs and authorities have shown their concern about the impact of unwanted sedimentation on their beaches or harbor entrance channels. In Hindeloopen these concerns have resulted in successful resistance against a sand engine along its recreational beaches. For that reason the potential of large scale sand engines along the Western Frisian IJsselmeer coast will remain limited, as this coast has more recreation and is dissected by several harbour entrances.
- Monitoring shows that the Oudemirdum sand engine survives for a long period of time and may function as a source of sand for coastal sedimentation. This sedimentation, together with the sand engine itself attenuates waves and reduce wave attacks on the coast. This may lead to lower height-design norms for the dike and therefore to cost reduction. Uncertainties in the extent of wave attenuation are yet considered too high to draw final conclusions on impacts of sand engines on flood defense efficiency. Research on the impacts of semi-natural forelands on wave attenuation as addition to dike reinforcements is being conducted (ref. Janssen, van Loon…etc.). The Frisian Water Authority responsible for flood control invested in the Oudemirdum sand engine as part of its innovation program, and is part of the monitoring project group. But integration of sand engines in its dike construction program is considered too risky, due to the extensive set of National Dutch norms and regulations dike designs need to fulfill to.
- New coastal policies do consider upscaling of sand nourishments. Therefore, the implementation of only sand engines along the Western coast is not a viable option because it is interrupted by shipping lanes. Building with Nature strategies along this coast must combine ‘soft’ engineering and the use of vegetation, like reeds or macrophytes, with hard infrastructures, like wave protections, underwater geotubes along beaches .The southern coast is not interrupted by shipping lanes and here large scale sand nourishments, in the form of sand engines, is a promising coastal protection and development strategy.
- The Building with Nature experiment was successful as a learning experience. It has yielded a wealth of knowledge on sand engine in specific and on the morphology ecology and governance of coasts in a fresh water lake environment in general. The experiment’s initiation took place in a time of heated policy debate on the future of IJsselmeer water level management and this together with its visibility associated media attention facilitated a shift in thinking with Frisian water authorities about coastal management. Building with Nature strategies are now part of a portfolio of coastal management possibilities to cope with the impacts of water level rises.