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 1.3 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.
Why promote salt marsh development
Young salt marshes are important for the Wadden Sea ecosystem. Moreover, 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. This can be done by disposing the dredged sediments at the alternative placement site, further north of Harlingen as a semi-continuous source of sediment: the mud motor. The disposed sediment is expected to be transported by natural processes further into the area. The extra sediment supply is expected to lead to the formation and extension of salt marshes. The mud motor basically comes down to a modified and improved dredging strategy of the port of Harlingen. This would yield four favourable effects:
- less recirculation towards the harbour, hence less maintenance dredging;
- promotion of the growth and stability of salt marshes, improving the Wadden Sea ecosystem;
- stabilizing the foreshore of the dykes, and therefore less maintenance of the dyke.
- capturing mud on the salt marshes may result locally in a lower turbidity.
Higher mud concentrations in the tidal currents that feed a saltmarsh will likely speed up marsh development processes, while maintaining the desired gradients that are associated with natural saltmarsh development.
How: 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. The figure below shows an example of the plume of disposed mud. The white dot in this plot indicates the disposal location. The simulations show that 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.
Project implementation started in 2016, in combination with an extensive monitoring program. First results are expected in 2017.
- 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 extended scope of maintenance dredging works. (Longer sailing distance takes more time and results in higher costs. This is balanced by reduced maintenance dredging in harbour and reduced dike maintenance cost as a result of salt marsh wave reduction.
- Disposal has, in many cases, to be done close to an existing salt marsh in order for enough sediment to reach the salt marsh.
- 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)
- More accurate monitoring of the dredger and the volumes dredged and disposed is required to assess efficiency