Ecohydraulics, Clermont-Ferrand, France
Within the Dutch research project ‘Valley wide meander restoration’ six streams will be monitored over a 2 year period. The monitoring program aims at understanding both the initial morphological and the ecological processes. The present study focuses on the morphodynamic developments that took place after the completion of two restoration projects. The morphology and hydraulics of the field sites are evaluated using sequential GPS-surveys, LiDAR based digital terrain models, discharge records, water height records and information from sediment samples. The Hagmolenbeek can be described as a conventional stream restoration project. The meandering planform was designed to coincide with the planform it had in history, before straightening of the stream. The cross-sectional channel dimensions of the stream were dramatically reduced, to guarantee a minimum flow velocity and water depth during summer discharges. The channel width reduced from 9 to 2 m, and the depth from 1.2 to 0.4 m. The constructed stream channel is surrounded by an inundation area. The initial morphological changes show to widen especially in the channel bends. The second project under study concerns the Hooge Raam, where a new channel is constructed parallel to the previous stream. The channel was designed to investigate the autogenous formation of a meandering channel from an initially straight channel. After construction of the stream, the bed morphology shows two separate responses. In the upper reach, both the banks and the stream bed become occupied by pioneer vegetation. Consequently, bars are formed from sediment retained by the vegetation. Along these bars, deep scour holes are formed. In the downstream reach, in-stream vegetation is barely present and the vegetation development is less pronounced. Upstream sediment input caused the formation of a sequence of alternating bars.