Final report Mud Motor available

Near the harbour of Harlingen in the Netherlands an innovative method was tested between 2016 and 2019, where dredged sediment was reused to stimulate natural growth of the Koehoal-Westhoek salt marsh, titled: the Mud Motor. The final report describes the effect of the Mud Motor on the development of mud flats and salt marshes.

Important findings from the monitoring programme are the strong influence of wind strength and wind direction on sediment transport, and that sediment only stays on mud flats and salt marshes temporarily, depending on specific wind conditions. The importance of supplying extra sediment relative to the redistribution of local sediment is not as great as was expected. An increase of sediment dynamics resulting from the Mud Motor could not be established with certainty. During evaluation of the Mud Motor results, the team concluded that the hydrodynamic influence of waves and current on erosion is greater than previously anticipated.

Biotic processes also play an important role in salt marsh growth. Experiments where samphire seeds were sown in front of the rim of the salt marsh showed that the height as well as the duration of flooding of the salt marsh were favourable to the development of vegetation, and therefore also for growing the salt marsh. The most important limitation in the natural establishment of vegetation in the transition zone between mud flat and salt marsh, turned out to be availability of viable samphire seeds. It is likely that a shortage of viable samphire seeds limited the salt marsh growth in the Mud Motor project, and not a shortage of sediment.