Author(s): Virginia Ruiz-Villanueva; Javier Gibaja; Herve Piegay

Linked Author(s): Virginia Ruiz-Villanueva

Keywords: Driftwood; Woody debris; Raft; Spillway; Accumulation

Abstract: Large wood (LW) provides fundamental functions in rivers, by influencing hydraulics and morphology, LW produces diverse habitats and plays a relevant role in fluvial ecology. However, large quantities of LW transported during floods may accumulate at dams. Accumulated wood may create obstructions, reducing the discharge capacity of the spillway, and cause undesired higher water levels in the reservoir. Potential solutions to these problems are, to redesign or adapt the spillway to allow wood passage, or, to retain and extract the wood mechanically. Besides the LW-related management issues, dam reservoirs are also unique observation windows to analyse this wood. As there are not measuring stations to do so, wood accumulated in the dam reservoirs provides an opportunity to better understand LW dynamics in river basins, determining its origin and fluxes. The Génissiat dam located in the Rhone River in France, 50 km downstream from the Geneva Lake, is a unique example. The arriving LW at Génissiat comes mostly from two tributaries, the Arve and the Valserine, and it is fully trapped in the reservoir, because the dam has no gates on the surface, and the managing company (the National Company of the Rhone) must mechanically remove it. Since 1990s the dam managers recorded the amount of wood that was extracted from the reservoir and in 1999 two pioneer surveys at the wood pile were carried out to characterize the type of wood (i.e., wood size, wood status, tree species). Since 2013 wood surveys are done periodically and were enhanced to include measures of wood density and wood chemistry, and forest characteristics. The goal of these surveys is to infer wood origin (i.e., identifying where the wood comes from) and compute wood fluxes (i.e., wood volume and its relationship with water discharge), improving the understanding of wood dynamics at the river basin scale. In addition, we compared instream wood characteristics with those of green wood, to better understand the influence of aquatic environments in LW. As one may expect, we observed that in general larger floods contributed with larger wood volume, but we also observed that similar hydraulic conditions resulted in a wide range of LW transport rates. Peak discharge is not the best predictor and other variables such as the flood origin, duration, volume and frequency need to be considered as well as inter-flood period length or characters. Our results showed that LW quantity arriving at the reservoir depends on the contribution from each tributary, the wood availability related to antecedent floods and the forest stand status. Our findings do not only apply to the Rhone but can be easily extrapolated to other rivers and help designing management strategies. We thus call dam managers to collect, store and facilitate data.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221261

Year: 2022