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Salt Marsh Development with a Mud Motor
Koehoal, Tzummarum, the Netherlands
The project Salt Marsh Development with a Mud Motor focused on the potential for furthering the development of salt marshes in the Wadden Sea by making optimal use of the sediment transportation capacity of ambient flows. Approximately 1.3 million cubic meters of mostly fine sediment is dredged annually from the harbor basins in the Port of Harlingen to maintain navigability. The dredged sediment is usually deposited in a designated disposal area in the Wadden Sea near the harbor. During typical operations, an unknown but possibly large proportion of the dredged sediment flows back into the port relatively quickly. This Building with Nature study, which is scheduled to be completed by EcoShape in 2018, suggests an alternative approach to sediment management: beneficially using the dredged sediment by depositing it further north of Harlingen and allowing natural processes to spread the sediment to nearby salt marshes.
Article cover: The Mud Motor serves as a semi-continuous source of sediment for the salt marsh. (Photo by Martin Baptist)
Producing Efficiencies
During the winters of 2016 and 2017, part of the sediment dredged from the Port of Harlingen (470,000 cubic meters in total) was deposited upstream from the targeted tidal flat and salt marsh study site. It is expected that this mud will be transported by the flood-dominated tidal currents to the study site. Within the project, detailed measurements of suspended sediment transport processes and sedimentation of the salt marshes and tidal flats were conducted. Studies on the influence of biota on salt marsh extension were also carried out. To determine how much of the disposed sediment was transported towards the target area, a tracer experiment was conducted. A fluorescent tracer was applied; the tracer had a particle-size distribution and behavior similar to sediment dredged from the Port of Harlingen. After mixing each sediment tracer with sediments in the hopper, the tracer particles were expected to be encapsulated in flocs formed by the natural sediments; therefore, they would all behave similarly. The results indicated that after five weeks, 80 percent of the mud disposed reached the intertidal area where salt marsh enhancement is desired, compared to only 20 percent from the original disposal location.
Using Natural Processes
Sediment is becoming an increasingly valuable resource, especially in view of accelerating sea level rise. The Mud Motor will serve as a semi-continuous source of sediment for salt marshes. Higher mud concentrations in the currents that feed a salt marsh will probably accelerate marsh-development processes, while maintaining the desired gradients that are associated with natural salt-marsh development.
Broadening Benefits
The Mud Motor is expected to generate three beneficial effects: (1) less recirculation towards the harbor—and consequently less maintenance dredging; (2) promotion of salt marsh growth and stability, thereby improving the Wadden Sea ecosystem and providing defense towards sea level rise; and (3) stabilization of the dike foreshores, and therefore less maintenance work on the dike.
Promoting Collaboration
The project was carried out by EcoShape consortium partners (Wageningen University & Research, Deltares, Arcadis, Royal HaskoningDHV and Van Oord) together with the Port of Harlingen and the local nature management nongovernmental organization It Fryske Gea. The applied research project by EcoShape is coupled with a fundamental research program financed by the Netherlands Organisation for Scientific Research (NWO); the program involves two PhD researchers and a postdoctoral researcher.
