Sediment deposition and net phosphorus retention in a hydraulically restored lowland river floodplain in Denmark: combining field and laboratory experiments

Restoration of river systems allowing the transformation of former drained and dry riparian areas into riparian wetlands will increase the overbank storage of sediment and sediment-associated phosphorus ( P). Wetland restoration is therefore a cost-effective mitigation measure to reduce the sediment and nutrient transport to river systems. The studied floodplain of the River Odense was restored in 2003 by remeandering the river channel along a 6-km reach. The restoration project involved 78 ha of riparian areas that were transformed from mainly arable land to extensive grassland and wetlands. The aim of the study was to quantify and model sediment and particulate P deposition on restored river floodplains. The present study suggests that during a 47-day flooding period, the river floodplain is able to retain 9-14.8% of the sediment and 1.1-3.7% of the particulate P transported in the river. Incubation experiments further showed that a maximum of 11-25% of the deposited phosphorus can be released as dissolved inorganic phosphorus following deposition. The results from the best deposition model (R-2 = 0.42 for sediment and R-2 = 0.44 for particulate P) show that work should be done to further improve the performance of these models.