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“Room for the river”, also known as “space for water” or “Ruimte voor de river” is a group of measures taken within the floodplain and involving natural and forced (polders) flooding areas.

For the Kamp catchment in Austria, the effectiveness and efficiency of “Room for the river” was compared with the one for retaining water in the landscape by micro ponds or afforestation (Francés et al. 2008). For afforestation, the reduction is higher in dry initial conditions than in wet conditions, because the initially available soil storage capacity and the available interception storage is higher. For retention elements on the slopes (micro-ponds) or in the channel network (dams) the wet conditions produce higher peak reductions.

In general, the potential additional storage capacity resulting from afforestation and micro-ponds will have a physical limit which can be exceeded only by intensive measures such as dams. If the potentially damaged values in the flooded area have a high risk exposure, the smaller and more frequent events can have a large contribution to the total risk compared to the exceptional events. The effect of retention measures in the landscape is much higher for small events than for large events. As a result the risk reduction for these types of measures can be higher than expected from the hazard reduction.

In the Kamp catchment, significant reductions in the flood peaks can be obtained when retention basins along the main stream are constructed and the flood plains are inundated. However, a lot of room is needed for these measures. The main advantage of the room for the river methodology is that the polders/retention basins can be designed in a way that there is no retention for small flood discharges which leaves the full storage capacity for larger floods at the time of peak.

The peak runoff reduction of “retaining water in the landscape” measures is a function of flood return period, reducing its effectiveness with the flood magnitude. “Room for the river” seems more effective for medium return periods (see figure 5.6). It may be useful to combine these measures with other positive effects such as soil conservation, sediment transport reduction and environment protection.

Figure 5.6 Estimated flood peak reductions for different measures in the Kamp catchement (Austria)

Source: Room for the river / Francés et al. (2008)
Note: "room for the river" method =retention basins and flood inundation along the river reaches, "retaining water in the landscape" methods = micro-ponds and afforestation

Source: Francés et al. 2008; CRUE et al. 2009