Area report on the Island of Dordrecht
The Island of Dordrecht lies in the transitional area between sea and river. There is one dyke ring on the island, which is managed by the Hollandse Delta district water board. The dyke ring already has a dyke improvement ahead as a short-term challenge to take on. In WV21 (Flood Risk Management in the 21st Century) it is recommended that the protection level be raised to provide 2.5 times greater safety. For a number of areas on the island, the current LIR standard is also presenting a challenge. Dordrecht is a point of focus from the perspective of group risk as well. The municipality of Dordrecht’s ambition is to be able to cope with the consequences of extreme situations. By transitioning to the “Approach tailored to risk” strategy, the impact can be made manageable. The Multi-layer flood risk management strategy developed in this report is focused on impact limitation.
Results
In layer 2, the regional barriers can be used to create a safe harbour for partial evacuation within the dyke ring. In layer 3, the necessary preconditions for individual coping capacity can be created through smart linkage with public and private investments. This involves building smart shelters for people who need help, the local protection of vital infrastructure, and improved risk and crisis communication. In addition, the long-term tasking will be limited as much as possible (e.g. by risk zoning with building regulations). The “Approach tailored to risk” strategy also offers a relatively inexpensive alternative to improving or replacing the Voorstraat.
The historical harbour area requires extra attention in terms of flood risk management because of its cultural-historical value. This also makes the dynamics in the built-up area and open spaces extremely low, which means any future optimisation of individual, local measures (e.g. jacking up buildings) is possible only to a limited degree. An alternative to a new primary barrier is the creation of extra storage for river water in Lake Grevelingen. The effect of this solution in terms of a fall in the Mean High-water level is limited to 10 cm for Dordrecht.
The Staart, an area lying outside the dyke, has a relatively high urban character and has a relatively high elevation (a mean above 3.5 m+NAP [Amsterdam Ordnance Datum]). The Staart comprises residential areas and business parks. When the Stadswerven were redeveloped, considerable knowledge and experience was gained with respect to water-resistant construction, which can also be used to (re)develop other residential areas outside the dykes in an adaptive manner. For the business parks situated outside the dykes, further study will have to be done into the challenge presented by climate change and how this should be addressed.
With respect to the Island of Dordrecht dyke ring, a tailored approach on those dyke sections with the largest potential risk is more cost-efficient than improving the entire dyke ring. This solution requires a differentiation according to dyke section, making it possible to give certain dyke sections a much higher standard (up to a Delta Dyke level of say 1/100,000 per year).
Currently, there is no administrative framework for this area (so-called evaluation framework for delta dykes), which means a transformation of the Kop van ’t Land to a delta dyke cannot be included in the decision-making on the dyke improvement in Flood Protection Programme 2. This presents a significant bottleneck to tackling this dyke improvement properly all at one time (by linking) and making future improvements (elsewhere) unnecessary.
In addition to differentiation in the standard for the dyke, the formalisation of existing grey and green infrastructure, such as regional flood barriers, forelands and flood partitions, is necessary to provide made-to-measure work. Finally, through local measures in layers 2 and 3, a relatively inexpensive and manageable situation can be created for the Voorstraat should one or more of the partitions fail. To make this solution possible, a replacement decision is necessary. The Island of Dordrecht will function within the Delta Programme as a multi-layer flood risk management test case for the new flood risk management policy.