Action: Remove flood defence banks to allow inundation
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- One controlled before-and-after study from the UK found more bird territories and species on a stretch of river modified to allow inundation of river edges compared to a channelized section of river.
- One study from Belgium found that a combination of mowing and flooding resulted in increased plant species richness in meadow plots, but infrequently flooded, mown plots had more plant species than frequently flooded, non-mown plots.
Recent major flooding events have resulted in a change in European water management policies, from flood exclusion strategies to the use of former floodplains through reconnection with main rivers. This change will lead to an increase in flood frequency and may provide opportunities for the restoration of floodplain ecosystems.
Supporting evidence from individual studies
A controlled before-and-after study on the river Roding in Essex, England (Raven 1986) found that in 1982 there were more territories and more species of bird on a 3 km stretch of the river that was modified in 1979 to reduce flooding in the area compared to an adjacent 500 m stretch of river that was channelized in 1974 (52 territories of nine species on the modified stretch vs three territories of two species on the channelized stretch). The experimental stretch had one bank excavated to create a 0.3 m high shelf (a ‘flood beam’) just above the level of the main channel. This meant that the main channel continued to carry water during dry periods (at a rate of 2 m3/s) but during heavy rains, the beam would carry water as well (at up to 40 m3/s), increasing the width and the flow capacity of the river.
A study of two meadows in adjacent nature reserves on former natural floodplains of the River Demer, Belgium (Gerard et al. 2008) found that mowing and flooding meadows resulted in increased plant diversity. Mown, frequently flooded plots had higher plant species richness (average 16 species/plot) than non-mown, frequently flooded (10 species) or mown, infrequently flooded (12 species) plots. Overall, there was a significant negative correlation between species richness and standing crop. Data were obtained from two reserves: one frequently flooded (150 ha, flooded at least once a year for 5-50 days) and one (600 ha) in which part is infrequently inundated (about once every 5 years). In each reserve, 10 plots (2 x 2 m) were randomly selected in annually June-mown fields, and five in non-mown fields. Plant species composition was recorded in each in early July 2005 and standing biomass mid-end of July.
- Raven P. (1986) Changes in the breeding bird population of a small clay river following flood alleviation works. Bird Study, 33, 24-35
- Gerard M., El Kahloun M., Rymen J., Beauchard O. & Meire P. (2008) Importance of mowing and flood frequency in promoting species richness in restored floodplains. Journal of Applied Ecology, 45, 1780-1789