Large wood mobilization and morphological changes in an alpine river following the release of an ordinary experimental flood from an upstream dam

The work documents morphological changes and large wood (LW) mobilization in a river-reach located downstream of a dam along the Spöl River (Switzerland), following an 8 h experimental release of 1.2-year flood. The aim of the flood was to mimic a natural event able to wash out either or both the sediment and LW. Two field campaigns (before and after the event) were made to generate DEMs and to collect data concerning the river morphology and LW characteristics. The formers were used to produce a Dem Of Difference (DoD) while the latter to identify the LW load fluctuations. Statistical analyses were implemented taking advantage of a 2D flowing model software (HEC-RAS 5.0.7) in order to compute the dimensionless diameter (D* = Flow depth (m)/ LW diameter (m)) and to detect, along with analysis of resident and mobile groups of wood, a mobility threshold (D* = 2.77) by means of a binary regression model (60 % accuracy). The threshold was used to estimate different mobility rates according to different water discharge scenarios. Overall, the event produced slight widespread morphological changes all over the channel, with a modest sediment deposition (2341 m3). Although evidence of LW fluctuations was detected, the LW input and output were almost equal (3.6 m3 ha−1 and 3.9 m3 ha−1, respectively) generating a net difference of – 0.79 m3. The median of the mobile LW was considerably lower compared to that of the resident LW. The results obtained demonstrated that the original aim of the experimental flood, to transport out wood and sediment, was not achieved because the duration and magnitude of such type of flood was limited, inducing equilibrium in the LW load and generating only slight morphological changes.