It is concluded that a sandy foreshore can be a viable alternative to traditional means of dike strengthening in a lake environment, although some boundary conditions need to be met (Rijkswaterstaat & EcoShape, 2018c):
- Important variables in the viability of a sandy solution are the volume of sand required (i.e. how deep is the foreshore) and how far the sand needs to be dredged.
- The location and orientation of the waterline have a large impact on the amount of expected sand loss from the sandy solution and the resulting profile shape
- Another factor of importance is the effectivity as a defense in design conditions.
- The positive impact on recreation and nature habitat development can be significant with a sandy solution.
One of the things to take into account is the availability of sand for the construction as well as the maintenance of the sandy foreshore. The availability of suitable sand greatly influences the cost-effectiveness of the sandy foreshore. Sandy foreshores are a useful option only when there is a borrow location nearby (<5 km). In the case of smaller upgrade projects, it will be possible to use existing borrow locations. If a new borrow location is needed, the additional costs for research and permit procedures mean that the approach is useful only if a longer section is being strengthened with sand.
A sandy foreshore should preferably be perpendicular to the dominant wave direction. In that case wave reduction plays a major role but erosion as a result of net longitudinal transport remains limited. A sandy foreshore is also possible on more sheltered locations because longitudinal transport is limited here and the same therefore applies to maintenance. However, the potential wave reduction due to the sandy foreshore might then be less as well. Furthermore, in most cases the resulting profileshape will be different than the design profile due to natural processes. For the design it is recommended to use a profileshape that fits with the expected equilibrium profile according to the wave climate. With a shallow slope (less than 1:20) no large sand losses in cross-shore direction will appear.
Morphological lessons learned in the Houtrib Dike Pilot:
- The foreshore is more or less stable and there is almost no loss of sand
- A plateau has formed (unexpectedly), but does not enhance the stability of the foreshore
- The enclosure by the sheet pile wall and the existing Houtrib Dike itself have influenced the shape of the foreshore.
The manager of the flood defense needs to manage the uncertainties of the sandy foreshore since guarantees will be required to ensure that the foreshore stays in place and that it can continue to work effectively as a defense in design storm conditions. A sandy foreshore can be an alternative to dikes when the dike crest is not high enough or when a dike revetment is weak. However, a sandy foreshore is not a solution for piping. When macro-instability is an issue, a dune-for-dike solution (type a) may be an option.
A sandy foreshore in a lake environment requires an integrated design process that makes intelligent use of land and nature management objectives based on an approach designed to benefit nature. The creation of foreshores generates opportunities but also takes up space that is often already in use or for which there is a designated use. With regards to vegetation development, it can be pointed out that mixing Holocene material and grazing prevention measures have a positive effect on vegetation growth. However, research of the wave attenuation of reed plants around the water line is a challenge. Wave action quickly becomes too severe for the reed vegetation to thrive.
Vegetation lessons learned in the Houtrib Dike Pilot:
- Heavy wave attack hinders the growth of vegetation along the shore
- Mixing of clay (Holocene material) with sand at the start of the project has a positive effect on vegetation growth
- Grazing by birds has a negative effect on the vegetation