The Mud Motor Koehoal experiment started in 2016. This trial, to test the beneficial use of dredged sediment for the development of salt marshes, runs until 2019. First results are now available. Project manager Martin Baptist: “Although we do not see the salt marsh grow, we do observe that the concept of the Mud Motor works: it is possible to influence the location where the dredged sediment deposits, by spreading it at a strategic location. Through the experiment, we gathered a lot of knowledge about the mud flat and salt marsh system.”
For the Mud Motor experiment in the Wadden Sea, 300,000 m3 of dredged sediment was spread upstream of the Koehoal salt marsh in the winter of 2016. In the following winter of 2017, another 170,000 m3 was spread. The Mud Motor was intensively studied by both the EcoShape team and the STW-financed fundamental research project team. Both teams benefited from each others’ research.
“We have seen that the dredged sediment reaches the salt marsh in large volumes. But we also observed that most of it washes away during storms. A net sedimentation of 5 to 10 centimeters per year remains, which is significantly more than we expected. However, we noticed that the salt marsh we chose to study the growth is actually at an intermediate stop for the sediment. A new location for a mud motor should be more at the end of the sediment transport chain to increase effectiveness”, says Martin. “We have also discovered that the salinity gradient in the tidal channel influences the sediment transportation. The sediment transport is less favorable at the Koehoal location because sediment travels in the direction of fresh water. Because the seawater near Harlingen is fresher than near Koehoal, and the fresher water pulls the sediment back from where we would like it to land.”
A salt marsh with a large sediment supply like Koehoal has the potential to grow apace with the sea level. Martin adds: “However, besides vertical accretion horizontal accretion is important as well. If the salt marsh grows, but the mud flat does not grow along with it, a cliff edge could form and cause the salt marsh to erode. Different processes affect the horizontal growth. For instance, samphire vegetation in the transition zone between salt marsh and mud flat is essential for the horizontal growth of a salt marsh.” An interesting discovery from the STW-research is that minuscule worms – called oligochaeta – live in the salt marsh soil, more than 100,000 per square metre. The worms stir up the soil that contains the samphire seeds. This slows down the germination of samphire. This is valuable knowledge for implementing measures to stimulate salt marsh growth.
In anticipation of the final report in 2019 the Building with Nature Guideline has been updated with the newly gained knowledge. Martin: “We have added technical design guidelines, criteria for sediment placement, practical and technical feasibility of mud motors and considerations for suitable locations. Also added to the Guidelines are lessons learned about permit requirements, contractual aspects and governance.”
The Mud Motor project is not finished yet. Martin: “In the coming winter the project team will monitor the salt marsh growth under the conditions that no sediment is placed, so to say when the mud motor is ‘switched off’. In the summer of 2019 we will deliver the final report on the Mud Motor.” The mud motor research is co-financed by the Wadden Fund.