Coping capacity of the Island of Dordrecht - the effect of floods on vital infrastructure and vulnerable functions
The municipality of Dordrecht is exploring how the flood-resistance of the urban areas lying outside the protection of dykes can be maintained or increased. These areas host a large number of facilities that are crucially important for the functioning of the city of Dordrecht. Because these areas lie at a relatively high elevation (often at dyke level), the chance is small in most cases that they will be flooded by the river. But in view of climate change, the chance that such flooding might occur is increasing.
Results
A rise in the sea level and increasing volumes of river discharge will increase the risk of floods in those areas lying outside the protection of dykes. In areas inside the dykes, this is being anticipated by reinforcing the dyke ring. Currently, measures are being implemented (HWBP21) to ensure that dyke ring 22 meets the flood risk estimate of 1/2000. To achieve this, approximately 50% of dyke ring 22 is being reinforced. The flood risk estimate of 1/2000 for dyke ring 22 corresponds with a normative flood level of 3 m + Amsterdam Ordnance Datum. In the study, this water level was projected onto the areas outside the dykes as a normative flood.
Flooding in the areas outside the dykes is caused by a high level of seawater resulting from a storm blowing in from the north-west, in combination with high (but not extreme) river water discharges. The interaction with the level of seawater makes this area dependent on the tides, such that the peak of a flood coincides with high tide. This also determines the flow rates of a flood occurring outside the dyke: these will be relatively low due to the gradual rise and fall of the water level (approx. 0.8 m in 6 hours) in comparison with a dyke breach, which means the probability of casualties and extensive damage is relatively small (de Graaff & van de Veerdonk, 2012).
With respect to the area outside the dykes, the authorities are exploring the possibilities for inhabitants to remain in the area for a certain period during such a flood. The relatively high elevation of these areas greatly enhances their capacity to cope with a flood (Onderbouwing Gebiedsstrategie EVD, 2013).
An important factor that determines the coping capacity of an area is the functioning or failure of facilities that are necessary to ensure liveability in the area or on which many other functions depend, and whose restoration time, if they fail, is relatively long. Exposure to a certain flood depth can mean that important links in a network or functions are disrupted or fail altogether. This can have a possible knock-on effect on non-flooded areas and bring about a domino effect such that other (vital) networks are affected. So the question to ask is: where are these vulnerable links located and what are the consequences of exposure to a “normative flood”? Will social meltdown occur in the areas of Dordrecht lying outside the protection of dykes due to the failure of functions?
These questions lie at the basis of this study. To answer them, an investigation was launched with network managers and experts to ascertain the robustness of the network / facility and the possibility that the network / facility will continue working during a flood. This investigation showed that the links that are important at the regional level are extremely well protected against floods.
The urban areas of the Island of Dordrecht that lie outside the protection of dykes have a relatively high elevation in comparison with the areas located inside the dykes. The average ground level of the Zeehaven and the Staart is roughly at or above the level of the dyke (varying from approx. 3 to 4.5 m + Amsterdam Ordnance Datum). During a normative flood, these areas will not or will hardly be exposed to water from the river, which means the vital infrastructure and vulnerable functions can continue to function.
The Historical Harbour area has a relatively low elevation in comparison with the Zeehaven and the Staart (varying from approx. 1.8 to 3 m + Amsterdam Ordnance Datum). This means that the vital infrastructure and vulnerable facilities in this area are most vulnerable to flood water coming from the river. The maximum inundation occurring during a normative flood at the level of the built-up area amounts to approximately 1 m. To safeguard the area’s coping capacity, certain sewer districts must be shut off: the mixed system in such a situation does not have the capacity to process the water. As a result of shutting off the sewer, residents and users will not be able to discharge wastewater for a time. The 13kV power network can continue to function up to 3.2 m, but at the residential level there is a chance of a short circuit occurring at 3 m + Amsterdam Ordnance Datum due to water that flows into the homes.