Influence of river channel deflector hydraulic structures on lowland river roughness coefficient values: the Flinta river, Wielkopolska Province, Poland

River channel deflectors are environmentally friendly low head hydraulic structures constructed from wood, stones and coarse gravel Their aim is to protect river banks against erosion, to change the direction of river flow making the river course naturally meandrous instead of artificially straight, to provide natural refugia for fish and invertebrates and finally to improve hydraulic discharge conditions. However, from the designing point of view they are difficult to represent in hydrodynamic models since they are constructed and fixed in the river bed differently. For this reason, in hydraulic models’ deflectors may be treated as modifications to the value of the watercourse bed roughness or reduction in the active part of the cross-section. Thus, to test the suitability of one-dimensional models for river restoration involving river deflectors, a 1-D numerical model of the lower section of the Flinta river (Wielkopolska Province, Poland) was prepared. To do that, firstly three channel flow deflectors were constructed and installed in the lower section of the Flinta river. Secondly, hydrometric measurements were continued systematically over three years. Next, based on field studies and field data, calculations were made using the HEC-RAS 1-D software. Several variants of the shape and the effect of deflectors were tested. On this basis, the variability of the roughness coefficient value for the Flinta riverbed after the implementation of deflectors was analyzed. The novelty of the research is that the obtained results show that river channel deflectors have a significant influence on the processes of morphological changes (such as: velocity distribution, grain size changes, tendency towards channel meandering, channel slope) of the riverbed and thus also on changes in hydromorphological parameters of river flow. The presented case study confirmed the potential for effective deflector operation for the velocity range of 0.10 – 0.30 m·s−1. The maximum changes of the river bed elevation after the deflectors had been introduced reached 0.10 m. It was also found that the effect of deflectors on roughness at high water levels was non-significant and did not increase the risk of flooding. For Q1% flow, the implementation of flow deflectors had no significant effect on the capacity of the studied river section. Only slight subduction of 0.02 – 0.03 m of the water surface in the vicinity of the deflectors could be observed compared to the model from before the restoration activities. The greatest impact of the channel deflectors is seen at low water levels and low flows that do not exceed average flow values. Finally, it was shown that the best solution to implement deflectors into a one-dimensional model is to consider their influence as a change in the value of Manning's coefficient. The obtained results can be considered universal in relation to typical transformed small lowland watercourses that have been straightened and regulated and are to be subjectedto the process of river restoration.