Linking floodplain hydraulics and sedimentation patterns along a restored river channel: River Odense, Denmark

Re-establishment of riparian freshwater wetlands often involves re-meandering of former channelized river sections by narrowing the channel width and depth, which creates frequently inundated floodplains. Inundations are important for floodplains in terms of erosion/accretion of sediment and for deposition of nutrients and organic matter. The links between floodplain hydraulics and deposition of sediment, organic matter and phosphorus were investigated along a 6 km re-meandered channel section of River Odense, Denmark. A 2D dynamic river and floodplain model was set up for the investigated floodplain area. The flow model was validated against in situ measurements of flow velocities and depths during a one-day field campaign in January 2012. Deposition of sediment was measured on 30 artificial grass mats deployed on the floodplain during winter 2011/2012. Total deposition of sediment, organic matter and total phosphorus was estimated by Kriging and amounted to 4.72 kg m−2, 0.65 kg m−2 and 11.4 g m−2, respectively. The predicted duration of inundations, distances to inflow, flow velocities, and information on microtopography were compared with sedimentation patterns on the floodplain. Three distinct flow regimes were predicted by the hydraulic model and were classified as small, medium and large inundation events according to spatial extent and inundation depth. The model predicts spatially and temporally changing zones of confluence on the floodplain due to variations in inundation depth. The position of these zones correlates with zones of high sediment deposition. Simple regression models described only up to 76% of the variation in sediment deposition. We conclude that use of dynamic spatial distributed 2D models is necessary if the links between floodplain hydraulics and deposition of sediment, organic matter and phosphorus are to be accurately described.