Location and boundary conditions
The city of Delfzijl is located on the western side of the Ems-Dollard estuary. This city borders the estuary but seaviews are blocked by a large dike that protects the city from floods. Delfzijl has a middle size harbour that is protected from waves by an extra dike (the Schermdijk). The harbour is dredged regularly.
The estuary serves as an important area for primary production and has a function as habitat for many fish, (migratory) birds and seals. The estuary has large areas of intertidal flats (sandy and muddy). Salt marshes are present along the fringes of the estuary but they are not abundant. Most land water boundaries are hard (rock protected dikes).
Delfzijl has a mean semi-diurnal tidal range of 2,99 m. The sediment in the channels and on the large tidal flats consists mainly of sand. The Dollard area south of Delfzijl is more silty. The suspended sediment concentration (mud/fines <63 µm) in the whole estuary is rather high and increases from the estuary mouth toward the Ems river. Around Delfzijl the mean annual suspended sediment concentration is 90 mg/l. This concentration has been slowly increasing since many decades.
More specific information on the project location is presented on the worksheet Case Salt Marsh development, Marconi Delfzijl.(follows). Information on the complete Marconi project can be found on http://www.delfzijl.nl/waterfront-delfzijl-marconi/
Why a salt marsh
The ecological state of the Ems-Dollard estuary has been declining the last few decades. Suspended sediment concentrations are high and land-water boundaries are abrupt (no gradients). This, combined with a rising sea level and land subsidence due to gas extraction, gives a reason to rethink the coastal area. Provide more coastal safety while at the same time increase the ecological value. Specifically for the Delfzijl area the harbour is included in the formulation of a solution as this harbour has large amounts of sediment dredged every year that may be, beneficially, used for other purposes.
Based on the issues given above, a salt marsh area was proposed as a possible part of the solution for the coast of Delfzijl. A salt marsh at the outer side of the harbour dike (Schermdike) in this area would have several benefits:
- It creates a gradual water-land boundary
- provides space for flora and fauna
- decreases the wave attack on the present coastal defence
- may be able to decrease the suspended sediment concentration locally (above and around the salt marsh location)
- location for placement (beneficial use) of dredged sediment.
Another goal of this specific project was to learn from the process of creating a salt marsh at a location where no salt marsh is present. Therefore, the design of the salt marsh was such, that a maximum knowledge gaining level was obtained.
Design of salt marsh
There is no salt marsh present in front of Delfzijl. To find out whether the area is suitable for salt marsh development and how to initiate the development we followed the Salt Marsh Building Block Flow Chart.
There is no salt marsh present at the location. Probably as a result of the high wave energy in this area. Brushwood groynes are used to block wave energy.
To speed up the development of a full grown salt marsh the pioneer zone is constructed. This is done by using a sand body as base material. The top 1 meter layer is mixed with silt in various percentages. The height of 1 meter is chosen in order to create enough depth (space) for drainage channels to naturally develop.
The main research question is whether a sustainable salt marsh will develop. This is monitored closely. Based on the monitoring results, adjustments or management measures can be taken.
During the course of 2017 the sandy base layer is brought into place. The pioneer salt marsh is expected to be in place by the mid of 2018. From then till the end of 2019 an extensive monitoring campaign will be performed, focusing on morphological development of the salt marsh and vegetation development.
Lessons from the initiation phase
- Good governance seems to be a key element in initiating Building with Nature examples. Especially as benefits and risks are not always clear at the start of such a project. In this specific case there was a strong bond between a large group of stakeholders that were in favour of a natural solution.
- A lot of time is involved in getting stakeholders together and initiating such a project. Also because legislation is not always fit for purpose.