Some dikes - such as the dikes on our major rivers - are crucial in terms of flood protection for the hinterland. The assessment of dikes in the Rijswaard (Waardenburg, NL) showed that the crest of the dike was not high enough to eliminate the risk of flooding due to wave run-up and wave overtopping. However, raising the dike is far from straightforward and this section is particularly important due to its cultural and historical value. The Rivierenland Water Authority decided to call in Deltares to look at alternatives to the time-honoured method of raising the dike. The study took the existing vegetation in front of the dike into consideration and found that it has a potentially important role to play in terms of improving flood risk management. This is a fine example of Building with Nature and of using resources that are already in place.
Deltares looked at the existing woody vegetation in front of the dike - such as willows, mixed riparian woodland and hawthorn hedges in the flood plains - to see how much it can protect the dike from waves during high water. This information was used as a basis for calculations with the SWAN-VEG model to visualise the effect of vegetation on the wave patterns. It was found that the willows can substantially reduce waves and wave run-up on the dike.
The good news here is that the inclusion of existing vegetation in standard assessment methods has positive implications for dike management and maintenance in the future. The increasement of entire sections of dike may be less, or different approaches may be possible. The potential cost savings are significant.
An in-depth study is currently being considered to look at a larger section with the aim of seeing how vegetation in flood plains can result in wave reduction. Deltares is developing a validated method for both assessment and design that will allow for the proper calculation of the extent to which vegetation can reduce wave run-up and wave overtopping.
This successful study shows that the inclusion of existing flood-plain vegetation in the assessment of dikes affected by wave run-up and overtopping can reduce dike management and maintenance.
Here at the Rivierenland Water Authority, we think it is important to implement the principles of Building with Nature and to tackle complex dike upgrades in innovative ways. The Deltares study provides an interesting angle on alternatives to the traditional approach of raising and widening dikes.
In this project Deltares worked together with the following partner: