The main question of the project is whether sand nourishments (‘sand engine’) are a sustainable, cost-effective and ecologically resilient coastal reinforcement measure along the Frisian IJsselmeer coast. The function of a sand engine is based on the use of natural processes in the defense of the coast related to future water level rise and water level fluctuations.

A monitoring program in the period 2011 – 2017 was carried out to follow the effect of the sand engine on the coast, how and where the sand moves, and the effects on the vegetation under water and above water. The knowledge gained in this project will be used to develop a strategy for sustainable protection and enhance the spatial quality of the entire Frisian IJsselmeer coast.

The IJsselmeer is a former sea arm in the Netherlands, separated from the sea in 1932 by a 32 km long dam, the Afsluitdijk. Along the northeast coast of this lake, the former saltmarsh has developed into valuable freshwater wetlands which help protecting the shore of the province of Friesland. In 2009, planning started of three so-called ‘sand engine experiments’ along this shore, meant to investigate whether a gradual supply of sand would enable the wetlands to follow a rise in lake level along with sea level rise. The implementation is governed by a coalition of regional and national actors, led by It Fryske Gea, a regional NGO for nature protection.

The flexible water-level management that is needed to increase the buffer of freshwater requires additional protection of these areas outside the dikes. An increase in water level of ten centimetres results in more than 100 hectares of land that will be flooded. Without additional measures, the areas outside the dikes will be eroded. These areas are important for nature conservation, recreation and water safety and therefore the stakeholders along the Frisian IJsselmeer coast develop a strategy for sustainable coastal protection. The challenge is to make the Frisian IJsselmeer coast resilient enough to be able to withstand the negative effects of a higher water level. Also when the water level in the future increases even further.

The project ‘Zachte Zandmotor’ along the Frisian IJsselmeer coast aims to develop knowledge about how a sand engine can lead to a more sustainable, less expensive and environmentally robust way of coastal defences. The function of a sand engine is based on the use of natural processes in the defence of the coast related to future water level rise and water level fluctuations. With this knowledge, the Ecoshape partners want to provide a sound basis on the operation and effectiveness of a sand engine in a lake environment, in order to make a wider application of the concept possible. The knowledge gained in this project will be used to develop a strategy for sustainable protection and enhancing the spatial quality of the entire Frisian IJsselmeer coast.
 

The experiment was supported by national, provincial and water management authorities and by NGO’s, and each party framed the importance and the objectives according to their own interest. The Nature conservation NGO, for instance, stressed the importance of creating conditions for pioneer vegetation, because the salt water tidal estuarine habitats were replaced, after damming in 1933, by a stable water level, not allowing for ecological succession. The water authorities wanted innovation in flood prevention as an alternative to reinforcement of existing flood protection. The provincial authority aimed at improving the spatial quality of the Frisian coast. Lastly, the national government wanted to support adaptation of the Frisian coast after its decision to raise the IJsselmeer water level.

Selection of experiment locations had to consider processes of sand transport and sedimentation, ecosystem functioning, flood protection infrastructure, local recreational entrepreneurs and a governance system with multiple parties and interests.

To determine the best position of the nourishment, an advanced Unibest model (CROSSMOR) (van Rijn, 2006) was used to calculate sand transport and morphological changes perpendicular at the Workumerbuitenwaard (Folmer, 2010).  Using this model resulted in the placement of the nourishment in the dynamic zone at the edge of the shallow foreshore. For the design in Workum a long sand bar was chosen as it would create a long lee-side.

In Oudemirdum, the shape of a hook was chosen that stretched from close to the coast to the dynamic wave breaker zone to experiment with sand on the more shallow and less dynamical part of the foreshore.
 

The morphological changes around the sand nourishment are followed in detail by measuring land elevations with LIDAR, and underwater bathymetry with DGPS and single beam sonar readings. These measurements show how and where the sand is moving. In Workumerwaard, there is also an innovative experimental set-up used with 4 kilometre of fibre-optic Distributed Temperature Sensing (DTS), which allows us to follow the morphological changes near real-time. The results up to 2014 in the Workumerwaard demonstrate that the sand moves parallel along the shore, and the nourishment takes the shape of the sand ridges that were there before. So far, no effect of the sand nourishment was observed on the coast. The fibre-optic DTS measurements show that these have great potential to follow morphological changes in high temporal detail. The results in Oudemirdum show that the sand there is moving in line with expectations. In the picture you can see the spatial interpolation of fibre-optic DTS temperature data of 6 separate temperature measurements. The upper panels are from 2011, the lower panels are from 2012. The temperature anomaly on the lower left reflects the position of the nourishment.

At both locations, no major differences in underwater vegetation between control sites and nourishment sites are observed so far. Both pilot locations provide important data sets that can be used to calibrate morphological models.

Research on the governance aspects is focused on the lessons learned to scale up the application of such a sand engine, and simultaneously to create realistic expectations.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.

Literature

• Folmer, Wilms, Steijn & Lereninga 2010. Pilot eco-dynamiek Fryske kust. Building with Nature Deliverables. Dordrecht: Ecoshape
• Wiersma, A. P., Hattum, T. V., Lange, H. J. D. & Slobbe, E. J. J. V. 2017. Eindrapport Building with Nature pilot Zandmotor Friese IJsselmeerkust, Hoe effectief is de zandmotor als ecodynamische strategie voor het versterken van de Friese IJsselmeerkust?. Wageningen Environmntal Research rapport 2856. DOI: https://doi.org/10.18174/429949, ISBN: 978-94-6343-241-2.
• Smits, M. & Lulofs, K. 2012. Monitor building with nature IJsselmeer, phase 2: cases IJsseldelta Zuid-Bypass Kampen & BwN experiments Mij.

• It Fryske Gea 2011. Building with Nature Pilot Workumerwaard. MIJ 3.2 - Ecodynamic Design Deliverable. Building with Nature Deliverables. Dordrecht: Ecoshape.

• Arcadis 2011. Ontwerp Landtong Oudemirdum. In: Ecoshape (ed.). Dordrecht.

• Groot, A. E., Lenselink, G., Slobbe, E. J. J. V., Meurs, G. A., Noordhuis, R., Wiersma, A., Bos, M., Pasmans, I., Dankers, P. & Wilms, T. 2012. Natuurlijk IJsselmeer: ecodynamisch visie IJsselmeer 2100. Wageningen: Building with Nature - Ecoshape.

• Van Slobbe, E., Klimkowska, A., Van Dobben, H. & Wiersma, A. 2013a. De zachte zandmotor van de Workumer Buitenwaarden. Landschap: tijdschrift voor Landschapsecologie en Milieukunde, 2013, 219-227.