Structural works like dikes are mostly designed purely from a safety point of view. Optimization for enhancing ecological functions is not often considered in the design phase. In Natura 2000 area Oosterschelde, the dike upgrades should result in zero net change of nature value of each dike section compared to the original situation. The objective of ecobasins is to contribute to the ecological value of a dike and create a net positive contribution to natural value of the dike section. Construction of Ecobasins along dikes will improve biodiversity and bio-productivity . Other objectives are to create educational opportunities and to raise awareness of designers, constructors, students and visitors.
In 2008 the Project Agency Zeeweringen (established by Rijkswaterstaat Zeeland and Waterschap Scheldestromen) started a pilot project in cooperation with Rijke Dijken (Rich Revetments, in Dutch) on the Oosterschelde dike of the Koude- and Kaarspolder, northwest of Yerseke. In 2010 a second project was implemented in a dike section along the Oosterschelde between Wemeldinge and Kattendijke.
Within the pilot project near Yerseke, tidal pools (so-called ecobasins) are applied in the "toe" of the dike, i.e. the fortified link between the dike slope and the foreshore. The dike toe traditionally consists of loose stones, protecting the dike foreshore against erosion. In the case of the Koude- and Kaarspolder, the dike toe was partly renewed, strengthened and maintained. As a pilot experiment, ecobasins were designed and constructed as part of the renewal of the dike toe. Along the 1 km dike 10 short pools (ecobasin type 1, 2m length) and 2 long pools (ecobasin type 2, 20m length) were created. The basins were made waterproof with a sheet of asphalt underneath a layer of lava stone, so that they would not empty during low tide. Within the basins various stone types and sortings provide fixation for algae and shelter for juvenile fish and macrofauna.
The second project consists of much larger basins (5mx150m). These basins have been constructed with deep and shallow parts, were waterproofed with asphalt and the bed substrate was topped off with a layer of small lava stones (60-150mm). At some locations heaps of larger limestone (60-300kg) were constructed in the pools.
An analysis of the planned works and the ambient ecosystem is needed in the first step of the design process. On the basis of a set of conceptual designs, a feasible technical design has been realized and budgeted. A scan has been performed with the Project Agency to identify dike transects suitable for ecobasins.
In the pre-feasibility assessment, the following activities have been undertaken:
During the feasibility study, the Rich Revetment plan was further developed via:
In view of the influence of the envisaged ecobasins on the functioning of the flood defence system and the local environment, calling in physical and biological expertise on the local environment was important in order to have an effective design.
Important design parameters were the shape and the slope of the structure, the choice of materials, the size distribution and the porosity.
The technical design makes use of asphalt as a sealant to prevent leakage. Applying molten asphalt to create a pool with sloping edges proofed a complicated issue. Materials such as limestone and lava stone were selected to provide different substrates for species to attach to. Stones were loosely stacked to provide heaps to species to shelter from predators. These stones are of a size selection that is heavy enough to withstand the forces of wave impact.
The local climate and the local ambient ecosystem will provide the meteorological forcing (temperature, irradiation), hydrodynamic forcing (tidal range and current speeds), wave exposure and species diversity. The latter may be a limiting factor to the success of this type of project, because it needs a source from which the eco-basins can be colonised. Also note that the spatial scale of the structure and local ecosystem biodiversity will limit the choice of habitats that can be designed within a given project.
The design could be enhanced to provide more habitat for reef builders such as mussels and oysters (including associated species, such as crabs), or for macro-algae, which in their turn provide habitat to many fish and invertebrates. The foreshore allowing a design with dune or salt marsh vegetation could be achieved at the supratidal level. The design could be optimized to provide ecological functions for existing habitats in the ambient ecosystem. On the other hand, the same design principles could be utilized to reduce a priori the amount of habitat available for nuisance species, for instance in certain areas where they may reduce the quality of recreational activities. Filter-feeding animals such as mussels, for instance, are known to reduce the concentration of phytoplankton in the water column.
In 2008 the Project Agency 'Zeeweringen' started the design and construction of the Rich Revetments near Yerseke. In 2010 they constructed another set of ecobasins with adapted design and size on the dike of the Stormesandepolder. The applied research institute Deltares (through the Rijkswaterstaat WINN programme) was involved in the generation of concepts and pre-designs, the selection of suitable locations and the involvement of stakeholders.
The construction of the basins is an integral part of ongoing dike reconstruction works. Yet, a separate Natura 2000 license track had to be followed, because of possible negative effects on protected species. A project planning for the main dike reconstruction works was already underway, put the need for new licenses for the inclusion of the ecobasins on the critical path for the project as a whole.
Important questions in the design phase were:
The first pilot of the rich revetment ecobasin was delivered in 2008 on the dike section between Wemeldinge and Yerseke. The second set of ecobasins was delivered in 2010, on the dike section between Wemeldinge and Kattendijke.
The pilots have been monitored for a period of three year, 2008-2010. For detailed information on monitoring methods and results, see (in Dutch) Paalvast (2011).
In 2008 the basins were monitored in the period from July to October, this included a monthly visit to the site. During the visits the different species were counted and the basins were photographed. In October pictures were taken of algae and epifauna in the basins and in two reference situations (the traditional dike toe). The number of fishes and shrimps were estimated, but not determined by species. For the algea and animals that cover a large surface, the cover percentages on the stones and in the basins were estimated and translated to abundance. For the other animals an estimation of density was made and translated to abundance. In October 2008 the algae biomass was also determined.
In 2008 monitoring showed that the number of taxa in all short basins had increased. Organisms included ulvea, barnacle, tunicates and sponges. The number of taxa in the long basins was lower than in the short basin, which can be explained by the fact that the long basins are located higher in the tidal range and therefore were longer exposed to drying conditions. Results in 2008:
These results show that basins started to have a clear added value at this location.
In 2009 the basins were visited and photographed in May and October. The monitoring followed the same method as in 2008. In comparison to the monitoring results of 2008, the total number of species had increased:
In 2010 this result remained stable. Between the years shifts took place in abundance between various algal species. It was noted that some basins were silting up and others were leaking and dried up during low tide. As expected these leaking and drying basins contained less species diversity.
The following conclusions were drawn at the end of the monitoring period:
After construction the ecobasins have been monitored for a period of three years.
Reasons for monitoring are to analyse:
The monitoring covered a period of four years, from 2008 until 2012. Each year the results have been reported. No strategies exist for the specific maintenance of the basins as they form part of the dike. Dike maintenance is part of standard procedures of the Waterboard.
End of life
The ecobasins will have a long life-span, under the assumption that they are properly inspected and maintained. It is possible to re-use the stones and the asphalt for a new dike toe or other projects, if need be.
The basins may be prone to siltation, destruction by ice, leakage or vandalism. If a specific maintenance plan exists, the plan has address who is in charge of the basins functioning, who is legally responsible and who is bearing the costs of maintenance. Another concern may be that unattended visitors may get injured while visiting the basins, because this intertidal zone is mostly slippery because of algae growth. This may require a specific warning sign concerning risks of injury by the legal owner of the basins.
For an overview of the costs and benefits of ecobasins, the following indicators should be quantified:
Note that the Government makes no savings for rehabilitation or renewal at the end of the life cycle of a structure. In the Netherlands, for traditional projects in which the dike toe has to be replaced, costs of a standard dike toe of 5 m width are about € 100 the linear meter. The costs of constructing the first prototype ecobasins in the dike toe were about € 400 the linear meter. The total cost of the revetment works of which the toe is a part, are estimated at about €1000 per linear meter. The total length of ecobasins constructed per kilometer was about 60m. Less expensive ecobasins are possible, however, by optimizing planning and construction methods. Ecobasins sealed with foil instead of asphalt or prefab concrete, for instance, are expected to be cheaper. Furthermore, optimization between cost and basin-area is expected to be possible.
Other costs were not monitored in this case. The project has the character of an experiment, since the cost-benefit picture of mainstream application has not yet been completed. Therefore, it is difficult to give an estimate of the project management costs. Estimating these project management costs for a traditional dike toe project gives some insight into the magnitude. Based on pre-calculations by Rijkswaterstaat, the costs for project management are about 18% of total cost. Costs for constructing ecobasins were part of total cost of a complete revetment reconstruction scheme. On-site availability of equipment for stone placement and working with asphalt made construction of the ecobasins more cost-effective. The design requirement of the reconstruction works as a whole is to provide long-term (50 yr) protection against flooding. Maintenance costs are minimized for all aspects of the design, including the basins. It is concluded that additional construction cost of this type and this amount f small ecobasins are marginal compared to total costs of the revetment reconstruction. It is expected that larger-scale basins are cheaper per unit area, because a certain volume of stone is replaced by the basin.
In this case, quantifying the benefits is a lot more complicated than estimating the costs. Benefit quantification is the result of monitoring which was ongoing until the end of 2012. Results indicate increased biodiversity (factor 3-5) in the basins as compared with the surrounding intertidal toe area. Regular foraging of birds on organisms in the basins is observed. Therefore, effects on surrounding ecosystem can be expected. An inventory of species that utilize the basins could reveal that the basins provide benefits to ecosystem quality relevant for the Framework Directive or for species relevant for Natura 2000. This positive effect could be realistic for some red list bird species of the Netherlands (for instance Steenloper species, Turnstone).
The following lessons were learned from this project: