Flow structure associated with wood in rivers

Abstract:

Wood is a relevant part of a river ecosystem, as it provides habitat for different species, and creates heterogeneous flow conditions and morphological structures. The use of wood placement for river restoration or natural flood management has increased in the last decades. To plan and evaluate river restoration projects including wood, it is imperative to understand how different wood placements affect the flow to create habitat.

Flume experiments were performed to quantify flow structure and wake dynamics associated with wood placements in rivers. The objectives were to quantify the effect of wood characteristics and different wood placements in the channel on flow structure and to derive design recommendations that provide desirable river conditions with respect to fish habitat preferences. The experiments were conducted with a single log and constant approach velocity. The experiments further considered the variation of wood diameter (submerged versus emergent), wood length, wood orientation to the flow, and wood placement (middle versus side of channel).

We observed that flow variability, defined as flow velocity and turbulent kinetic energy, increased with increasing wood length and wood diameter. Emergent logs placed in the centerline of the flume were characterized by the presence of a von Kármán vortex street, resulting in the largest turbulence variability. We further noted that the wake length, defined as the required downstream length for flow velocity variability to recover, depends on wood placement. Given the same wood dimensions, a log placed at the channel side results in a wake length of ~40 times its length compared to ~7 times its length for a log placed at the centerline. We aim to link the hydrodynamic results to fish habitat preferences and to derive design recommendations to inform restoration efforts.