Re-use of dredged sediment – Mud Motor

Lessons Learned

  • Computer modelling can be used to predict the tidal flow conditions and to determine the best disposal location relative to tidal flows.
  • The tracer experiment showed that sediment disposal at a well-chosen location increases the chance that disposed sediment reaches the salt marshes significantly.
  • Mud disposal is often regulated and, because of environmental restrictions, only allowed in particular seasons and time slots. This strongly influences the strategy for mud disposal.
  • Capacity provided by dredging contractors must be adequate to extend the scope of maintenance dredging works. Longer sailing distance takes more time and results in higher costs. This is balanced by reduced maintenance dredging in the harbour and reduced dike maintenance cost as a result of salt marsh wave reduction.
  • Feasibility depends on assessment of extra travel time of dredger (extra costs), effectiveness on salt marsh growth (disposal close to salt marsh), reduced dredging harbour (reduced costs), practical issues (disposal location  far enough from salt marsh to have enough depth for dredger)
  • Disposal has, in many cases, to be done close to an existing salt marsh in order for enough sediment to reach the salt marsh.
  • More accurate monitoring of the dredger and the volumes dredged and disposed is required to assess efficiency.
  • Important findings from the project are 1) that the Mud Motor brings a significant volume of mud into the tidal gully, 2) that the transport of mud into the study area is highly affected by wind force and direction, as well as freshwater-induced circulation, and 3) that the mud remains only partially in the mudflats and salt marshes, mainly affected by specific storm conditions that induce sedimentary and/or erosive events.
  • The Mud Motor was intended to stimulate salt marsh development in the short term, that is to say, a period of weeks to a few months. The results so far indicate that the mud is only temporarily stored in our study area and probably is transported to the tidal divide further east. A possible mechanism is that the growth of the salt marsh is determined by long-term processes governed from the tidal divide. We consider it possible that an acceleration in salt marsh development due to the Mud Motor experiment will happen, however with a considerable delay.