2.2 Environmental aspects of floods and floodplains

please provide general comments on section 2.2 here

Water quantity management, including extremes like floods or hydrological droughts, should always be considered together with its impacts on the environmental quality and with water quality management. As mentioned in the Blueprint to Safeguard Europe’s water resources (EC 2012c) water over-abstraction is the second most common reported pressure in the first generation of RBMPs. At the same time, the hydrological regime defines the physical habitat in and along water bodies. Flow requirements to reach a good ecological status thus go beyond minimum discharges during dry periods but have to take into account the full range of discharges: from base flows (including low flows) to flood regimes with different magnitudes, frequency, duration etc. This link between quantity and quality is clearly made in the guidance document on ecological flows in the implementation of the WFD (EC 2015c). The objectives on protection and conservation of freshwater-dependent ecosystems can only be reached when discharges and water levels can vary over weeks, months and years including flood events. Thereby it is possible that flow requirements for certain species or habitats following from the Birds and Habitats directives (BHDs) go beyond the ones to reach the good ecological status (GES) as defined by the WFD and should then be considered in the implementation of the WFD (EC 2015c).

Where a flood, according to the definition in the Floods Directive (EU 2007, Art. 2 (1)), is “the temporary covering by water of land not normally covered by water”, the main area of interest is the floodplain where this flood happens. The natural floodplain can be defined in different ways (see section 2.3) and four key questions define the role this area can play in mitigating or reducing flood risk:

• What is the use of the area?

• What is the hydrological regime?

• What is the connectivity of the water body (river) and the floodplain? and

• What is the water quality?

Many former natural floodplains are nowadays under pressure from urbanisation, infrastructural developments and agriculture. In order to reduce negative economic and human impacts along many rivers, protection or regulation measures have been implemented. These have the negative side effect that the amount of water that can be stored is limited. Due to soil sealing or soil compaction, the water retention capacity to reduce the amount of overland flow is reduced as well.

However, the impact of soil sealing is certainly not limited to the floodplain itself as it even becomes more relevant at the scale of the catchment where it affects the river hydrology. Both peak flows causing floods and the low flows in dry periods are becoming more extreme with a higher variation of water levels over time. Contrary to this process, we have to question to which degree the water level is managed within narrow boundaries to support navigation, hydropower or other economic activities. This has to be compared with the hydrological regime close to the environmental flow with variations over time and periods of low flows and floods.

Connectivity is often related to flood protection. Dikes, dams and other infrastructural measures prevent the water from entering the area, unless a major flood event happens and the infrastructure is overtopped or fails. As flood protection provides increased security, the areas behind flood protection infrastructure are often highly developed, which causes large economic and social consequences once the flood event is of a higher magnitude than the protection level. In addition, these protection measures have a strong negative impact on the environmental quality of the water body and the floodplain, limiting the ecosystem services the area can provide.

A last issue is water quality and pollution. This deals both with the quality of the water entering the floodplain and the pollution that happens when contaminated inert soils are brought in suspense or when polluting installations (industries, but also oil and sceptic tanks) are flooded. In chapter 3 we look in more detail to these different aspects and how natural floodplains can provide several ecosystem services.