Flood defences contribute to the ultimate aim of resilience by increasing the capacity to resist flooding, thus lowering the probability of floods. Most of the countries analysed in the STAR-FLOOD project have flood defences in place.
A challenges is to create a good knowledge base to design a ‘watertight’ system of defences. State of the art knowledge on hydraulic conditions in extreme events, geotechnical knowledge on embankments and the soil below and knowledge of constructions like dams and weirs is required. A complete overview of the system is needed, as the weakest link determines the strength of the defence system. Temporary flood defences or closable weirs or dams often form the weakest link in the system and thus require specific attention. Although they provide a lot of flexibility, the risk of human or technical errors during their installation and operation is significant.
Flood defences in general have a lifetime expectancy of 50 to 100 years. They should not only be designed to withstand current extreme conditions, but also possible future conditions. As required investments in new infrastructure are often significant, there should be agreement on their necessity and functions (now and in the future). The ideas of adaptive management as introduced in §4.3 can help to set up smart designs and investment plans.
A recurring theme that affects all STAR-FLOOD countries (with the exception of the Netherlands), is the lack of resources for financing flood defence infrastructure. The global financial recession in 2009 seems to have aggravated this issue. Constraints on financial resources seem to have the greatest impact on maintenance of defences. Shortfalls in funding are reported in Belgium, England, France and Poland. This may have serious implications for maintaining standards of protection.
Flood defence is a particularly dominant strategy in Poland and the Netherlands. In the Netherlands flood risks are very high as about 60% of the total area can be flooded from large rivers and the sea. Therefore, there is a system of permanent flood defence infrastructure. In Sweden and England temporary flood defence systems are also used and are only installed when a flood event is predicted. Below we describe two extreme practices: the Dutch elaborate system of flood defences versus the Swedish system using temporary defences which are linked to disaster management.
Being a low-lying delta country, the Netherlands has depended on a system of dikes for its protection against floods for over 1000 years. The management of the flood defence system and protection through dikes has been highly institutionalised. The flood defence approach is characterised by a clear responsibility division, explicit standards and regulations, and secure financing.
One of the integral purposes of the Dutch Water Act is to prevent, and where necessary limit, floods and water logging. A legal distinction is made between primary and non-primary (also referred to as ‘regional’ or ‘secondary’) flood defence structures. For the areas protected by primary flood defence structures, legal safety standards have been established. For (most) regional flood defence structures, safety standards have been established in provincial by-laws. The competent (mostly regional) water authorities must make an effort to achieve these standards and have specific instruments at their disposal. They can strengthen or relocate dikes and designate conservation zones.
Within the Water Safety Programme, part of the recently developed Delta Programme, new legal safety standards for the primary flood defences are being developed. These are likely to be based on the maximum risk of an individual dying in a flood in a certain location, group risks and potential economic damage.
Competent authorities have a wide margin of policy discretion to achieve the standards. Periodically they have to report the actual state of the defence system to the supervisory organs, namely the Minister of Infrastructure and the Environment and the Provincial Executives. Their reports must be based on pre-set hydraulic conditions and technical guidelines. Supervisory authorities can give legally binding instructions regarding the implementation of the duty of care for water safety, however, hierarchical steering generally takes place on a political level through strategic planning rather than on the implementation level.
Efficiency and knowledge base
The national and regional water authorities that are endowed with the implementation of the flood defence system are highly specialised organisations with a strong knowledge base. This ensures a sound implementation and maintenance of the structural measures, an ongoing adjustment and improvement of the flood defence approach. It also produces innovative technologies that can be exported. Cost-benefit analysis, cost-sharing arrangements and efficiency-based procedures are increasingly applied to achieve particular levels of protection in the most cost-efficient way.
Financing through taxes
Flood defences are primarily financed by national and regional taxes. Regional water authorities have their own system of taxation that entitles them to raise taxes to fulfil their obligations. They are a sectoral organisation with an elected board, but being a sectoral organisation they are relatively independent from general political whims (see also § 5.5.1).
In the Netherlands the system of a highly institutionalised flood defence strategy is undeniably important, as a large part of GDP is produced in areas susceptible to flooding. The democratically organised regional water authorities have grown organically over a timescale of centuries and the high dependence of the Netherlands on flood defence systems justifies their existence as a separate democratic entity with its own tax system. The system of standards, testing and strengthening of the defence system will be applicable in other countries where a defence strategy is adopted (STAR-FLOOD Deliverable 3.2, see §8.2.1).
Flooding in Sweden does not occur regularly and is relatively hard to predict. Events are often local and affect relatively few people each time. Therefore, in general, there is a lack of urgency among inhabitants, policymakers and politicians to deal with floods. The construction of permanent defence systems would in most places be too expensive to finance. Building permanent defences often also conflicts with aesthetic and economic values: people like to live close to water and see the water from their houses. Thus properties close to water have higher prices. Constructing flood defences may spoil views on the water and decrease house prices.
The temporary measures that can be taken include demountable defences and sand bags, as well as plugging storm-water pipes (to prevent the water flowing in the wrong direction) and pumping. The demountable flood defences are stored centrally at the Swedish Contingency Agency in Kristinehamn. These defences are an investment of the Swedish National Government which is more efficient as an investment has to be made only once for all 290 municipalities. The Agency has staff that are always ready to take action and work closely with the Swedish Meteorological and Hydrological Institute. At the moment of intense precipitation (80-100 mm in a few hours) there is barely time to prepare for a possible pluvial flood event, with spatial planning this risk is reduced as much as possible. Yet, in case of extreme river discharges, there is more time to take action and the demountable defences are used. Only three cities, which have a relatively high flood risk, have permanent defences in place (Kristianstad, Arvika and Göteborg in planning phase).
The approach with temporary measures can be used as a good example in sparsely populated regions in Europe in which the consequences of a flood event are low or regions in which the probability of flooding is low. A precondition for working with temporary defences is that there should be flood warning systems in place and sufficient lead time in the case of an approaching event to install the defences (STAR-FLOOD Deliverable 3.5, see §8.2.1)