Operation and Maintenance
The Waterboard was involved in the project of Venema et al. (2013) in which the safety testing scheme was set up. This includes a monitoring program for the health of the willows and provides some guidelines for emergency situation, but not for the wave-reducing capacity in relation to the vegetation density, stem diameter and height.
Firstly, the new dike has a wider footprint than the ‘traditional’ dike. In the Noordwaard situation, a wider footprint has possible consequences for the maximum discharge through the area under design conditions. This is a critical aspect of the design, because of the overarching goal to reduce water levels upstream
The design includes the implementation of a willow tree plantation. Working with this living element is beyond the expertise of traditional dike engineers and necessitates expert input of biologists. Biologists could even be involved during the design and construction of the dike. Planning of dike construction is more complex compared to a traditional design as a sufficiently dense tree plantation takes time to grow (3-4 years).
A monitoring program needs to be set up to assess the evolution of the willow forest and its wave-reduction capacity. This can be supported by a full-scale experiment under design conditions in a big laboratory wave flume. In the current phase of the project the monitoring program in terms of wave reduction has not been made explicit (2014). Monitoring and maintenance of the health of the willow forest to ensure sufficient wave reduction is described in Venema et al. (2013).
In case the willow forest does not meet the requirements, there is a contingency plan: the new dike is designed to withstand significant wave overtopping, so with sufficient installed polder drainage pumping capacity the area remains adequately protected.
Management and maintenance
Maintenance is more complicated than in the traditional situation. The dike proper is designed to be “maintenance free”, except for regular mowing of the grass. However, the willow plantation necessitates yearly maintenance to guarantee sufficient density and tree health. Bi-yearly cutting of willow branches, replanting and the removal of the grass and branches will be managed by the Waterboard. Some uncertainty remains on the sensitivity of the tree plantation to pests, diseases, ice-winters, forest fires and instability under extreme wave forces. Therefore, contingencies against failure are implemented, mainly in the over dimensioning of the design. Furthermore, in the testing report of the dike (Venema et al., 2013), guidelines on how to deal with these phenomena and their associated risks are presented to the eventual managers of the dike.