Pilot Multi-Layer Safety Marken
Flood risk management on the island of Marken does not meet the current safety standards. Some of the primary flood defences have stability issues or are too low. In addition, at some places the stone cladding of the dyke has collapsed or come loose. In order to address these problems, a dyke improvement plan has been developed over recent years. This plan should result in a considerably larger dyke (in terms of both width and height). On account of the required space, of the impact on the landscape, and the cultural-historical values of Marken, the plan has garnered little support. Moreover, the costs involved have turned out to be quite high.
This has prompted Rijkswaterstaat, as the manager of the area, to commission a study into the potential application of the multi-layer flood risk management concept. In the first phase of the study into potential multi-layer flood risk management strategies, six possible strategies have been developed.
Results
The ambitions with respect to all the strategies are expressed in the following terms:
- Prominent: a tailor-made approach to Marken, befitting its specific (cultural) history, and its scenic and spatial qualities.
- Habitable: for an attractive living and working environment, with a clear perspective for action.
- Safe: flood risk management and assistance up to par.
The point of departure for all the strategies is “a minimum level of flood protection for everyone living behind dykes or dunes”. This minimum level can be attained by a smart combination of measures in the three layers of multi-layer flood risk management.
Some of the strategies developed achieve a minimum level of flood protection by layer 1 measures only (dyke improvement); other strategies involve a combination of measures from all three layers. To develop the strategies, an analysis has first been conducted of the current qualities, the taskings already in place, and the future opportunities. Subsequently, potential measures were devised in the form of “building blocks”. These individual building blocks were then combined, according to various lines of thought / visions for the island, into six distinctive strategies that explore all the options, including the less realistic strategies.
Strategy 1: Preservation of the current dyke profile, focus on layers 2 and 3
The main goal of this strategy is to preserve and improve the current qualities of the dyke surrounding the island. The dyke retains its highly characteristic profile, featuring a narrow and low crest. A major overhaul is followed by continuous maintenance and management. A minimum level of flood protection is achieved by a combination of layer 1, layer 2, and layer 3 measures.
Strategy 2: Moving along with settling
This strategy opts for short-cycle dyke improvement. The dyke will be improved every 12 years, following the review rounds. Each improvement involves a minimum of raising and strengthening. This eliminates the need for super-elevation, thus reducing settling.
Strategy 3: The new settling-free dyke
This strategy involves the construction of a new dyke on improved soil. The current peat layer will be replaced by sand in order to minimise settling. The new dyke will be located outside the current dyke. Its profile will feature a narrow crest and a steep inner slope. The new dyke will be approximately equal in height to the current dyke.
Strategy 4: Focus on the outside – foreshores
This strategy involves an improvement of the current dyke towards the outside. The inner slope will be mitigated in order to improve stability. As a result, the crest will shift to the outside. The dyke will be raised slightly. In addition, a raised maintenance path will be constructed on the outside of the dyke, covered with a layer of rubble. This serves as a stability bank and to achieve wave reduction. At a number of spots, this basic profile will be expanded with a wider foreshore in combination with Nature development.
Strategy 5: Focus on the inside – a dyke that can withstand overtopping
This strategy is based on a dyke that can withstand overtopping. This enables the crest to be kept low, thus maintaining a view of the water from the island. In order to make the dyke able to withstand overtopping, the inner slope will be mitigated, or the dyke will be covered with erosion-proof clay.
Strategy 6: Watery Marken – living high
Under this strategy, all the houses on the island will be made water-resilient or raised in the long run. This investment will be linked to the future restructuring of the housing stock. The focus will be on realising an extraordinary living environment. Living high, in a watery environment, on the only island in the Markermeer lake. The historic shipyards of Marken will serve as a basis for the designs.