Model-based evaluation of restoration measures efficiency in the anastomosing section of the River Narew
Paweł Marcinkowski, Adam Kiczko, Tomasz Okruszko
Multi-channel rivers support diverse and productive ecological communities, and their rareness in the developed world places a great importance on their conservation. The design of side channel restoration projects remains poorly informed by theory or empirical observations. One of the last examples of anastomosing rivers in Europe is a stretch of River Narew in Poland protected as a national park. Park Authorities face a problem of side channels extinction in last few decades and therefore a dedicated plan of protection measures (submerged weir construction, dredging and mowing of side channels) was established aimed at anastomosing system conservation. The objective of this study was an ex-ante assessment of the impact of proposed protection measures on discharge distribution, flow velocity and sediment transport potential in the anastomosing section of the river, using a hydraulic, one-dimensional model. Among the tested measures, more invasive dredging and damming were significantly more efficient than less invasive vegetation removal. Compared to other restoration projects of multi-channel rivers (Narew and Rhône cases), supported by extensive post-restoration monitoring, efficiency level was comparable reaching on maximum tenfold increase in discharge and fivefold increase in reach-averaged flow velocity in restored side channels. Post-conservation alterations of hydraulic properties in the anabranches resulted in a significant increase in sediment transport potential for measures changing channel geometry. Such change could potentially ensure the stability of restoration eliminating the main cause of channels extinction i.e. excessive sediment deposition.
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