Area pilot project Central Holland
The aim of this study is to contribute to answering the question of what can be done about the safety shortfall of dyke ring 14, Central Holland, now that in the third assessment round the flood defence systems along the Channelled Hollandsche IJssel and Amsterdam-Rijn Canal (so-called category C-dykes) have been declared substandard. A second aim is to provide the project group and the steering committee with input for the discussions on the new system of standards for flood risk management focused on the safety of Central Holland.
Results
The results of this study lead to the conclusion that raising the standard for a part of the flood defence system along the Nederrijn/Lek rivers is the most efficient solution to address the safety shortfall of dyke ring 14, now that the category C-dykes along the Channelled Hollandse IJssel and the Amsterdam-Rijn Canal have been declared substandard.
Raising the standard in this case would be necessary for the section running from Amerongen to roughly Bergambacht. The alternative – construction of an outlet structure in the western part of the Krimpenerwaard – would only slightly limit the length of this section. An outlet structure would help to limit the impact of any flooding.
The results of the sensitivity analysis show that the outcomes of individual calculations are highly dependent on the different assumptions made, particularly with respect to the breach depth, the breach growth, and the correction of the testing level. This pertains more to dyke ring 44 than it does to dyke ring 15. This is possibly due to the higher foreland lying outside the dyke in dyke ring 44 and the smaller size of dyke ring 15, in which the inflowing water volume is limited by the water level already achieved inside the dyke.
The conclusions of the study seem to be solid under the different assumptions. Of all the calculations made, only one combination of assumptions would lead to an adjustment of the conclusions for dyke ring 44. This concerns a fairly improbable combination of best-case assumptions. For dyke ring 15, the conclusions remain valid for all calculated variants.
The form of the discharge wave chosen is of essential importance for the final calculation of water levels in the flooded area and for the size of the flood. It is recommended that the same discharge wave be used as was used in determining the hydraulic preconditions, with a view to the intercomparability of the results. When interpreting the results, it should be borne in mind that this is only one of the many possible forms of the discharge wave. Caution would therefore be warranted when making statements about flood depths. When evaluating measures, it is recommended that both existing discharge waves be used to determine the sensitivity of the measures for the wave form.
When further studying and developing an elevation of the standard, it is important to focus on:
- Legal conditions for imposing a stricter standard to protect the dyke ring behind.
- New normative water levels and discharge waves for the river Lek for the current standard, such that recent adaptations in the riverbed are taken into consideration.
- Ascertaining whether raising the standard for a part of the northern Lek Dyke should also have consequences for the standard applied to the southern Lek Dyke.
- Drafting a preliminary design for dyke improvement based on available data on the height and construction of the dykes.
- Drafting a cost estimation based on the preliminary design.
- Giving attention to the waterfront of Schoonhoven, because of the artworks present, cultural-historical value and the difficulty of integrating a dyke improvement.
- Costs of maintaining the flood-defence function of category C-dykes.