Approximately 1.2 million m³ of mainly fine sediment is dredged annually from the harbour basins in the Port of Harlingen to maintain navigability. The dredged sediment is deposited in a designated disposal area in the Wadden Sea near the harbour. In current operations an unknown but possibly large proportion of the dredged sediment flows back into the port relatively quickly. This Building with Nature study suggests an alternative approach to sediment management: deposit the dredged sediment further north of Harlingen and let natural processes spread the sediment to nearby salt marshes (Mud Motor).

The Mud Motor is expected to generate three beneficial effects:

1. Promotion of the growth and stability of salt marshes, improving the Wadden Sea ecosystem;
2. Less recirculation towards the harbour, and therefore less maintenance dredging;
3. Stabilization of the foreshore of the dikes, and therefore less maintenance work on the dike.
   
   

The project is carried out by EcoShape consortium partners together with the Port of Harlingen and the local nature management NGO It Fryske Gea. The applied research project by EcoShape is coupled to a fundamental research programme financed by the Netherlands Organisation for Scientific Research (NWO) and involving two PhDs and a post-doc.

Salt marshes

Young salt marshes are important for the Wadden Sea ecosystem. Salt marshes dampen waves before they reach the shore and thereby contribute to the safety against flooding. Salt marshes are present north of Harlingen but their size is limited. This may either be due to limited mud supply or hydrodynamic conditions being too energetic. Experience with creating quiet hydrodynamic conditions to facilitate salt marsh growth has already been gained in the past. A logical follow-up is to increase the sediment supply by using the sediment available from maintenance dredging in the harbour.

Prefeasibility and Feasibility

Growth of the salt marshes is important to keep pace with sea level rise. Important natural processes that need understanding are deposition of fines on the tidal flats and salt marshes, erosion and compaction of the salt marshes, and vegetation growth. In the project Mud Motor Koehoal, the salt marshes at Koehoal along the northern coast of the Netherlands are studied. The salt marshes are valuable for the ecological functioning of the Wadden Sea, among others for birds (Veldrapportage vogelwaarnemingen bij drone-opnamen Koehoal-Westhoek), and also for protection of the dikes. Important is to understand the dynamics of the salt marshes and which processes are involved with erosion and expansion of the salt marshes. In addition, it is important to understand the transport of sediment from the disposal site through the tidal channels to the tidal flat and salt marsh system. The different processes that transport the sediment and keep the sediment in suspension are important to understand, in particular under different wave conditions and tidal currents.

The efficiency of the Mud Motor depends on a couple of aspects. One aspect is the disposal location offshore of the port. A preliminary study was conducted to assess the most efficient location for disposal of the dredged mud from the Port of Harlingen, with as much as possible deposition on the salt marshes and at the same time a disposal location close to the port (Vergunningaanvraag Natuurbeschermingswet 1998; Van Eekelen et al., 2016).

Another aspect is the timing of disposal, in particular in case of large fluctuations in flow conditions, such as in the Wadden Sea. In the Mud Motor Koehoal, disposal at the selected location is only executed during flood tide, with flow from the disposal location towards the salt marsh.

Location

Harlingen is a city situated in the northern part of The Netherlands, in the province of Friesland. This harbour city borders the Wadden Sea. Harlingen is an old town with a long history of fishing and shipping. The tide is semi diurnal with a mean tidal range of 1.90 m. Flow velocities are higher during flood than during ebb. Maximum flood velocity of about 0.9 m/s occurs about 2 hours before high tide and flood flow velocities are still considerable at high tide. Maximum ebb velocity of about 0.5 m/s occurs about 4 hours before low tide.The coastal area at Harlingen is typified by extensive tidal mud flats and deeper tidal channels (tidal creeks). Near Harlingen, the mean suspended sediment concentration is about 60 mg/l with a seasonal variation showing larger values in winter than in summer.The availability of mud facilitates the creation of salt marshes along the coast. Consequently, salt marshes are present at many locations along the Wadden Sea coast of Groningen and Friesland. These are mainly the result of human interventions in the 20th century, with brushwood dams and ditches built to create quiet hydrodynamic conditions and promote dewatering. In this way mud could be captured and vegetation could settle.The mud supply also results in relatively large mud maintenance dredging requirements. About 1.2 million m³ of mainly fine sediments are dredged in the Port of Harlingen. The dredged sediment is disposed in the Wadden Sea at the designated placement site, in the vicinity of the harbour, from where part of the placed sediment is experienced to return to the port.

Follow flow chart

Harlingen is situated in a tidal environment with no strong soil subsidence. The salt marshes north of Harlingen are sheltered by large areas of intertidal flats. Fine sediments are available from these intertidal flats and an initial gently sloping substrate is present consisting of sandy mud, just above high tide. A local seed source is present as pioneer vegetation and low and high marshes are already present in the neighbourhood. Consequently, the site had potential to develop a robust and sustainable marsh. This can be maintained and improved by adopting a modified and improved dredging strategy of the port of Harlingen using the fine sediments from this source to facilitate salt marsh growth.

Model computations have been made to check feasibility / efficiency of the proposed mud motor and to gain insight in the behaviour of mud disposed during flood. As a first step a numerical hydrodynamic model was run to determine a suitable new disposal location based on the predicted tidal currents. Based on the numerical flow simulations, a preliminary disposal location was chosen in shallow water on the right bank of the tidal gully (the landside of the Kimstergat, west of the Koehoal salt marsh).

The figure shows an example of the plume of disposed mud. The white dot in this plot indicates the disposal location. The simulations show that a large part of the disposed mud is transported in northeast direction and settles in the small water depths on the intertidal flats near the salt marshes, increasing the mud supply to the salt marshes.

The surface level of the salt marshes at the beginning of the project, before the first Mud Motor disposal, was measured using aerial photos and ground measurements (Veldrapportage Slibmotor T0). In following years, the deposition of mud on the salt marshes is measured at multiple locations. Sedimentation-Erosion Bars for measuring bed height were installed at 22 locations in September 2015, thus before the first Mud Motor disposal. In addition, 19 devices were installed in August 2016 on the mudflats in front of the salt marsh, and 15 devices were installed at a reference location. These Sedimentation-Erosion bars are checked five times per year.

Alongside this project, a fundamental research project 'Sediment for salt marshes: physical and ecological aspects of a Mud Motor' is investigating processes relating to the use of a Mud Motor. The aim is to develop the fundamental knowledge needed to understand and quantify the physical and ecological aspects of large-scale mud nourishment operations of this kind for further upscaling and exporting.

The ship-based measurements have shown that the natural background transport equals 800 tonnes per day. The extra amount of mud brought by the Mud Motor into the tidal gully equals 540-800 tonnes per day. The transport rate in the tidal gully is thus increased by a factor 1.6 – 2 due to the Mud Motor. Field measurements showed that wind direction and strength are a key factor in the transport of fine sediments onto the mudflat. This implies a large temporal variability, not only between seasons but also between years. We observed that the tidal flood flow direction (and thus the sediment fluxes toward the study area) can be reversed by a wind with opposite direction when the wind speed is about 10-12 m/s. Results of the measurements with Sedimentation Erosion Bars show relatively large changes in surface elevation. Layers of fluidised mud with a thickness of up to 10 cm were deposited in some locations in the salt marsh over a 2-months period, but also disappeared as quickly. In the permanent quadrats for vegetation composition we did not yet measure an increase in pioneer vegetation cover on the edges of the marsh. Neither did we measure increased succession in the vegetated plots within the short time period of the first two years.

• 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.

Uitvoeringsplan

Van Eekelen E, Baptist MJ, Dankers PJT, Grasmeijer B, van Kessel T & van Maren DS (2016) Muddy waters and the Wadden Sea harbours   

Veldrapportage Slibmotor T0

Veldrapportage vogelwaarnemingen bij drone-opnamen Koehoal-Westhoek

Vergunningaanvraag Natuurbeschermingswet 1998

Vergunningverlening Natuurbeschermingswet 1998

Vroom, J. (2015). Modelresultaten slibverspreiding t.b.v. slibmotor Koehool. Deltares memo 1209751-000-ZKS-0001.

Vroom, J, van Maren DS, Marsh J, van der Lelij AC (2017) ‘Effectiveness of the mud motor near Koehoal. Deltares rapport 1209751-004