Identifying and managing flood risks

The enormous damage caused by recurring floods, illustrates how important it is to deal with precautionary and long-term flood protection measures at an early stage. Since complete protection against floods is neither technically feasible nor economically viable (residual risk), dealing with floods and the risk posed by floods rather requires a comprehensive and river basin-wide management. This includes precautionary measures (e.g., land use planning, floodplain designation, flood-adapted construction, technical measures, behavioural precautions), flood preparation (flood forecasting, warning systems), event management (hazard prevention), and post-flood management, including reconstruction (Figure 1). For this comprehensive risk management to work, it is particularly important that water management, weather forecasting services, regional planning, nature conservation, agriculture, forestry, civil protection, and other stakeholders work closely together - throughout the river basin, regardless of administrative boundaries.

Since the Elbe flood in 2002, the term flood risk has become established in public discussion and in legal regulations. In this context, flood risk is a combination of the probability of flood occurrences (annuality) and the possible damage associated with this event (damage potential). Floods are divided into "frequently occurring events" (occurring every 10 to 25 years), "moderate" (occurring at least once in 100 years or less frequently), and "rare events/extreme events" (occurring once in 200 years or less frequently). The extremely rare flood events - so-called extreme events - are characterised by high flow rates and high water levels. Areas are flooded that are "normally" not affected by flooding. This differentiation of floods according to certain annualities is based on statistical evaluations of a river discharges observed over many decades (probability approach). The damage potential describes values (buildings together with interior furnishing, industrial plants, traffic infrastructure, movable objects such as cars) that can be damaged by a flood.

In the European Union, the uniform application of flood risk management has been ensured since 2007 by the Directive on the Assessment and Management of Flood Risks (2007/60/EC) - in short, the EU Floods Directive (FD). The requirements set out in the directive are transferred into national law through comprehensive regulations on flood protection and flood risk management in the Federal Water Act (Wasserhaushaltsgesetz - WHG). However, these federal regulations allow each federal state in Germany a latitude in interpretation. Thus, federal states can also standardise certain contents through laws and ordinances in order to enforce the law. In the field of flood protection in Germany, the 16 federal states are essentially responsible for designing detailed strategies and measures preventing flood damages.

In addition to inland surface water floods, flood risk management also covers coastal floods and floods from groundwater. It consists of the following three core elements, which are revised in a six-year cycle. This involves a review of how far the implementation of measures has progressed and whether new findings, for example adaptions to climate change, make it necessary to adjust flood risk management and planned measures:

(1) Flood risk assessment and designation of risk areas (§ 73 WHG).

Risk areas are defined as those areas in which adverse consequences of flood events for certain protected assets can occur. To assess flood risk areas, the damage potential of flood events on human health, the environment, cultural heritage, economic activities and material assets is considered in more detail (Schütte, 2021) and related to the statistical probability of occurrence (annuality).

- Floodplains (§ 76 WHG):

In addition to the designation of risk areas, the Federal Water Act regulates the designation of so-called floodplains, which has long been an instrument of flood management (Schütte, 2021). According to § 76 WHG, floodplains are areas between surface waters and dykes or high banks and other areas that are flooded or flowed through during floods of a surface water. At a minimum, floodplains are defined as areas where a flood event is statistically expected to occur once every 100 years. Flood damage can therefore also occur outside the defined floodplains, for example in the case of a flood event that statistically occurs only once in 200 years.

In floodplains, special flood protection regulations apply to buildings. For example, municipalities are no longer allowed to designate new construction areas in floodplains. This "ban on new construction" is a key element in dealing effectively with flood risks. This counteracts a further increase in damage potential. At the same time, less damage to flood-prone areas means a lower flood risk.

(2) Preparation of flood hazard and risk maps (§ 74 WHG)

Flood hazard and risk maps contain detailed information on the areal extent, water depth of flood events (visualization of the hazard posed by a flood) and their possible adverse consequences (visualization of the risk posed by a flood). Risk maps include, for example, information on the number of inhabitants potentially affected, the type of economic activity in the potentially affected area, and industrial facilities (Figure 2).

Here, the maps are prepared for flood events with three different probabilities of occurrence: (1) high probability - expected recurrence interval every 10 to 25 years, (2) medium probability - expected recurrence interval at least once in 100 years, (3) low probability - expected recurrence interval at least once in 200 years. By using these hazard and risk maps, citizens can identify existing hazards and risks from floods in their region.

(3) Preparation of flood risk management plans (§ 75 WHG)

Based on flood hazard and risk maps, risk management plans are prepared for each river basin district. They are intended to reduce adverse consequences of floods. For this purpose, the management plans contain all objectives in the risk areas and all measures that are necessary to achieve these objectives (prevention, protection, precaution). If measures are implemented along rivers, they are not allowed to increase the risk of flooding for other countries in the same catchment area.

Schütte (2021): Schütte, Dieter B. Hochwasserschutz und kommunale Daseinsvorsorge.  Wasser und Abfall 23, 13–18 (2021).