Marconi salt marsh development

Lessons learned

Below the lessons learned are summarized for the different phases of the project.

Lessons learned – Initiation, Planning and Design

The importance of the stakeholder process

The exchange of wishes, ideas and experiences leads to an integrated design that contains elements that are relevant and interesting for all parties involved. A well-thought-out stakeholder process facilitates this (Leuven et al., 2021). In the specific case of the EcoShape activities, the stakeholder process was only conducted with the Marconi parties. These were organized in a steering group of great common interest. This steering group, and especially the great common interest, has proved essential for the continuation and success of this project.

Laws and regulation

The implementation of a project within a Natura2000 area requires testing against the Nature Conservation Act, which often means that a permit must be requested based on a ‘Passende Beoordeling’. By considering the objectives of the Natura2000 area in the design, obtaining the Nature Conservation Permit Act is facilitated. By involving EcoShape in the preparation of the ‘Passende Beoordeling’, no separate assessment had to be drawn up for the possible influence of the research activities. Reuse of sediment / soil means that the (complex) legislation and regulations regarding quality requirements must be met. This has resulted in limitations, because only sludge that met the high-quality standard could be used in the project. Therefore dredged sludge from the harbour could not be used and mud had to be brought in from a nearby depot.

Cost-benefit analysis

The added value offered by a Building with Nature solution created enough interest from involved parties and stakeholders: the cheapest solution is not necessarily the best solution, because it offers less added value. The city salt marsh and pilot salt marsh are good examples of this. The construction of the salt marshes or stimulation of salt marsh growth makes a positive contribution to the landscape value, recreational opportunities, coastal defence and natural values. In addition, net CO2 and nitrogen are stored in salt marshes. Although the city salt marsh and the pilot salt marsh were not the cheapest solution, this solution did provide the most added value.

Lessons learned – Construction

Constructing the sediment base

The bandwidth for a suitable construction height is very narrow for a salt marsh landscape. The construction height affects the flood frequency which affects vegetation growth, succession, formation of creeks, sedimentation etc. Settlement and consolidation cause great uncertainty in the ultimate height of the salt marsh after realization. As it is difficult to calculate these factors with enough accuracy, you must be adaptive during realization and be prepared to make adjustments. It is important to proceed with caution: rather make the sediment base too low, so you must come back later, than too high. An alternative is to look for locations that are already close to the right height for salt marsh development.

Mixing 5, 20 and 50 percent mud in the soil

It turned out to be difficult to achieve precise percentages of mud in the soil. The work requires an experienced contractor and special equipment. For research purposes it was necessary to achieve the percentages of mud as accurately as possible, but for future projects, realizing the mud percentage within a certain bandwidth is enough. This makes the construction easier. High percentages of mud were more difficult to achieve and resulted in more (tire) tracks. This can be avoided by not working with high percentages of sludge.

Sowing glasswort

Based on experiments and old knowledge, a practically applicable method has been developed for harvesting, storing, processing and sowing glasswort. This showed that sowing glasswort is in principle feasible.

Lessons learned – Monitoring results

The main lessons learned based on the monitoring results are:

  • The design and construction of the salt marsh successfully enabled the rapid establishment and growth of pioneer vegetation.
  • Fine sediment is key for vegetation development. The vegetation cover was higher in the compartments with 20 or 50% mud in the upper meter of the subsurface and relatively low in the compartments with only 5% mud. Between 20 and 50% mud there is not much difference in vegetation cover. As mixing high percentages of mud during construction proved to be difficult a percentage around 20 percent seems the best.
  • Seeding Salicornia had no longer a significant effect on the vegetation development after the first growing season.
  • The contractor successfully realized compartments with different mud percentages in upper 1.0 m, although some local variation seems inevitable. However, applying fine sediment on top without mixing, as was done in the salt marsh park, was less challenging.
  • The initial elevation of the salt marsh is important for vegetation and tidal creek development. To construct the marsh at the desired elevation, accurate estimation of (initial) subsidence rates is important. This requires detailed geotechnical information about the original and new substrate.
  • Brushwood groynes alone are not enough for steering tidal creek formation at specific locations. In case tidal creeks are desired or unwanted at specific locations it may be needed to: (1) dig an initial tidal creek, (2) construct brushwood groynes immediately after the sediment base is installed and/or (3) limit the permeability of groynes at locations where channel formation is unwanted.
  • Construction of a new salt marsh including the foreshore can result in unforeseen – but not necessarily negative – morphological developments, such as the formation of sandbars. These sandbars have a sheltering effect in the case of Marconi (see also De Vries et al., 2021).

Lessons learned – Management and maintenance

Management and maintenance are different for a Building-with-nature project than, for example, in a dyke reinforcement. Adaptive management is part of the Marconi salt marshes: there is no predetermined final image and management and maintenance are intended to stimulate natural developments as much as possible. It is still unknown what the brushwood dams contribute in the long term if they are not maintained (and to what extent they are needed in the long term for the preservation of the salt marsh).