| Bfn Knowledge gaps mastic asphalt | |
|---|---|
| Context | Bfn Building for Nature project |
| Type | Outcome |
| Decompositie type | IOR |
Guidelines for design
Findings of inventories on revetments on lower parts of the dike have led to the improvement in understanding of the environmental conditions that are required for vegetation growth along the dikes. Based on these inventories, a list of lessons learned with respect to design has been constructed which can be used to optimize design of future dike revetments. Vergouwen (2015) provides an overview of lessons learned that is summarized in Table 1 and will be discussed underneath.
General conclusions for design guidelines:
- The type of substrate that is used in dike design is crucial for the ability of biota to attach. Large solid surfaces should be prevented and (micro)-structures should be created. This will offer the vegetation a suitable substrate for attachment and creates shelter. For example the placements of rubble stones at the toe of the dike, or the overtopping of mastic asphalt with lava stone creates such structures.
- The use of dark colours should be prevented. Dark colours can get warm in the sun which will significantly alter the environmental conditions. These conditions are usually not optimal for the present biota since these temperatures differ largely from the natural conditions.
- It is important to make a sustainable design that will not require many maintenance activities or includes material that will detach. The presence of small eroded material is often mentioned to be a limiting factor in the growth of vegetation on the substrate. Creating a sustainable design will prevent the communities to have to recover due to a new overlay. Furthermore, it will limit the disturbance to biota due to small material hitting their habitat.
- Steep slopes were mentioned as a reason for not achieving high community densities as a result of high wave action. These conditions could potentially be suboptimal to the biota of the artificial rocky shores in the estuaries of Zeeland, so they should be avoided in the design.
Furthermore, some general remarks with respect to design can be made:
- In order to scientifically determine which types of revetment are optimal for growth of vegetation, their environmental requirements need to be taken into account alongside these lessons learned. It is advised that when testing design, all parameters except for the substrate are kept equal in order to be able to make an honest comparison between design options.
- Shifts in species communities could provide insight in changes in driving environmental factors. However, as mentioned before changes in species cannot only be an indicator for changes in substrate since there are multiple parameters that can cause certain shifts. For example, the increase in Japanese oysters is likely due to an overall increase in the area and not necessarily due to a change in substrate. Since the cause effect relationship between the presence of species and the change in substrate in the field is hard to determine, it is wise to try to find general conclusions and observations on design and include these in experimental set-ups that can next be tested in a test location where all parameters except for substrate are equal.
| Lessons learned - guidelines for design: | Explanation: | Literature: | |
|---|---|---|---|
| 1 | Create holes and structure, prevent creation of a large solid surface and use porous and structured material | Shelter, suitable substrate for attachment by retaining water and microstructures, larger possibilities for ecological value | Meijer et al., 2011 Meijer en Didderen, 2014 Lock et al., 2009 Jentink, 2005 |
| 2 | Prevent the use of dark colours | Prevent high temperatures | Meijer et al., 2011 |
| 3 | Use a sustainable design solution and prevent using material that detaches, remove small loose material | No added disturbance through maintenance, provide recovery time, prevent loose material hitting biota with waves thus causing disturbance | Meijer et al., 2011 Meijer en Didderen, 2014 Jentink, 2005 |
| 4 | Prevent steep slopes | Higher wave action, different dynamics | Lock et al., 2009 |
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