Land use in Europe is more and more anthropogenic, which has an influence on flood risks. Deforestation and agricultural land use leads to faster drainage of water. Urbanisation and the related increase in paved area has the same effect. As a consequence extreme rainfall leads to higher peak discharges in the rivers, as well as more pluvial and flash floods. At the same time, many rivers have been trained: their flow profiles have been constrained by dikes and groyns and their original flood plains are used for urban and other developments. As a consequence, many rivers can accommodate less water than in natural circumstances.
The STAR-FLOOD results point out that all analysed countries to some extent face the challenge to reverse these processes; to create space for water, instead of taking it. This can be done in several ways. In §5.4 we describe examples that limit developments in flood plains and flood prone areas. Below we describe sustainable urban drainage systems, local measures that are implemented to store water in England and slow down runoff. Such measures are also implemented in Belgium, France and the Netherlands. Another good example of providing more space to the water is the Room for the River programme in the Netherlands.
In urban areas flash floods can occur due to excessive rainfall and runoff over the paved surface in the city. These types of floods are hard to predict, and it is hard to mitigate the effects of these floods. Sustainable Urban Drainage Systems (SuDS) are implemented in England, in order to store the water from these intense precipitation events temporary and give the water the possibility to infiltrate/percolate into the soil slowly. SuDS connect impermeable surfaces to the underground and in this way drain water from the paved surfaces in the city and prevent urban areas from flooding.
Since April 2015 SuDS are formally treated as an additional planning consideration which is required in new developments within the existing planning system in England. The developer is required to establish a maintenance regime that is best suited to the local flood risk, locality and type of development (Defra, 2014a). Local Planning Authorities must ensure that developments of 10 properties or more consider options for SuDS with a statutory duty to consult Lead Local Flood Authorities (according to the National Planning Policy Framework, as amended).
In some cities there are, so called, ‘local champions’, individuals who put efforts to encourage the uptake of SuDS measures within the city. However, there are a number of barriers to implementing SuDS, such as the perceived efficacy of certain options, conflicts in motivation between Risk Management Authorities and delays in national guidance. A useful addition to the current localized approach, would be a strategy for retro-fitting the SuDS for the wider urban areas (STAR-FLOOD Deliverable 3.3, see §8.2.1).
The traditional flood defence approach in the Netherlands uses dikes to protect the land behind it. When more safety was needed, dikes were raised. The introduction of integrated water resources management at the national level lead to new solutions: to (re)create room for the rivers to deal with higher peak discharges. Flood plains are enlarged by placing dikes further from the river, lowering flood plains and removing obstacles.
In the case of Lent, the city of Nijmegen took charge of the Room for the River project and created a project that integrates water safety and the construction of a new city district. The project entails placing the dike further from the river, excavating a new flood channel and creating an island in the river.
The Lent project does not show a change in the flood risk management strategy, as it is still meant to keep water away from people, but it does show a change to new integrated solutions involving water management and spatial planning. This change in measures correlates with a change in governance from sector-based to more integrated governance.
The decision for developing the Room for the River programme and about the water level reduction to be achieved on each location was made on the national level. This first collided with the plans of Nijmegen. Yet, the State stimulated regional and local authorities to come up with plans that would integrate local (spatial) development plans. After negotiations, Nijmegen embraced the new approach and developed it into a truly integrated plan.
Creating space for rivers can be done in many locations where this space is available and currently dikes separate the river from a part of its flood plains. Integrated projects, such as in the case study Lent, are especially useful when many different objectives are at stake: i.e. in urban areas. Municipalities are in general able to integrate various stakes and can therefore play an important role in such projects (STAR-FLOOD Deliverable 3.2, see §8.2.1).