Sediment for salt marshes: the ecological aspects of a Mud Motor

To safeguard navigation, about 1.3 million m3 of mainly fine sediments are dredged yearly in the harbour channels of Harlingen, The Netherlands, and are disposed in the vicinity of the harbour. The Mud Motor consists of disposing part of the dredged sediment further away, so that return flow to the harbour is minimised and the sediment will be transported by natural processes to the foreland of the dykes further north of Harlingen. This project aims to develop the scientific knowledge needed to assess the impact and cost-effectiveness on maintenance dredging volumes, salt marsh development and stability and maintenance of dykes of a multi-year semi-continuous mud nourishment, i.e. a Mud Motor.

We will quantify the effect of sedimentation rates on vegetation composition and lateral salt marsh expansion. This will be related to salt marsh development in general and to the Mud Motor pilot at Westhoek. Potential growth of the local salt marsh and development of the vegetation will be monitored throughout (and before) the nourishment pilot. Interaction of vegetation with the additional available sediment and possible engineering capacities of vegetation will be assessed. Macrozoobenthic community changes are also monitored and possible impact, such as consolidating or destabilizing the sediment, of changing communities will be studied in laboratory experiments. Mechanical limitations of macrozoobenthos regarding survival in unstable, muddy environments will be quantified by experiments with sediment in variable fractions of silt and sand. Overall, gains and losses of relevant ecosystem services in the affected habitat will be assessed. The results will provide a scientific basis to design guidelines for the application a Mud Motor elsewhere.