Improving Flood Safety and Ferry Navigation in the Changing Environment of Holwerd

Abstract

Relative sea level rise and climate change represent long-term threats for the ecosystems and flood defence system in the Wadden Sea. Moreover, the flood-dominant tidal basins in the area lead to sedimentation in the navigational channels used by the ferry services that connect the mainland to the Dutch Wadden islands. This report provides flood safety and coastal zone management solutions for the village of Holwerd, in the province of Friesland, Netherlands. High morphological dynamics and biodiversity characterizes this environment. Within this context, the Holwerd municipality, Wetterskip and Rijkswaterstaat were identified as key players to this study from a stakeholder perspective.

Alternative preliminary designs were presented to increase the resilience of the local sea dike and to recommend solutions for improving the ferry connection between Holwerd and the island of Ameland due to current dredging operations considered as excessive and which caused ferry delays. Careful consideration was given to designs due to the Wadden Sea being a UNESCO World Heritage Site and therefore protected under environmental legislation. Therefore, minimal negative environmental and ecological impacts were strived for in design of both the alternative dike designs and ferry connection alternatives. The impact that these alternatives may have on the socio-economy of Holwerd was also assessed where applicable.

Some alternatives were initially eliminated due to fatal flaws and the remaining options were evaluated by means of a multi-criteria analysis. The main criteria considered were the environmental impact, practical implementation, socio-economic impact, safety risk and reliability. Costs were evaluated through a subsequent cost-value assessment. The Wide Green Dike concept was the selected preferred solution as a reinforcement design to withstand extreme wave heights and water levels with a 1000-year return period, including sea level rise projections for the next century. This was achieved by performing an extreme water level analysis and wave transformation using the software Delft3D Wave. The Wide Green Dike concept would therefore ensure that the Holwerd sea dike still sufficiently meets the Dutch flood safety standards in the future. In relation to the navigability of the ferry, channel bend cuts are considered the feasible short-term solution to reduce the sailing time by 10 minutes. The long-term solution, adjusting vessel fleet sizes, potentially decreases the current maintenance dredging volume by 20%. The beneficial use of the dredged material for dike reinforcement and salt marsh development are suggested as integrated Building with Nature solutions to provide additional ecosystem services. Environmental Impact Assessments were conducted to identify potential risks related to each of the preferred alternatives and provide mitigation measures to ensure appropriate project implementation.