Driftwood retention with distributed obstacles

Generally, classical trashrack structures are applied for driftwood retention purposes. In widened river reaches wood can be retained naturally. As flow depth decreases with increasing channel width, the natural retention mechanism results from an earlier contact between the wood and the bed. Physical model tests were performed in order to investigate how the effect of natural driftwood retention in widened river sections can be enlarged with distributed obstacles.

In a reference test the natural retention potential was specified. In the beginning of the simulated hydrograph wood deposition could be observed, whereas with increasing flow depth, the wood is transported into the lower reach.

Model tests with obstacles were performed with small racks, which were distributed uniformly (Fig.1). For the tests with obstacles - compared to tests without obstacles – the retention rate increases significantly. Though, a complete retention is not possible. An improved retention was observed, where the obstacles are situated near the bank (Fig.2).