Dynamics of Wood Transport in Streams: A Flume Experiment

Abstract

The influence of woody debris on channel morphology and aquatic habitat has been recognized for many years. Unlike sediment, however, little is known about how wood moves through river systems. We examined some dynamics of wood transport in streams through a series of flume experiments and observed three distinct wood transport regimes: uncongested, congested and semi-congested. During uncongested transport, logs move without piece-to-piece interactions and generally occupy less than 10 per cent of the channel area. In congested transport, the logs move together as a single mass and occupy more than 33 per cent of the channel area. Semi-congested transport is intermediate between these two transport regimes. The type of transport regime was most sensitive to changes in a dimensionless input rate, defined as the ratio of log volume delivered to the channel per second (Q log ) to discharge (Q w ); this ratio varied between 0·015 for uncongested transport and 0·20 for congested transport. Depositional fabrics within stable log jams varied by transport type, with deposits derived from uncongested and semi-congested transport regimes having a higher proportion of pieces oriented normal to flow than those derived from congested transport. Because wood input rates are higher and channel dimensions decrease relative to piece size in low-order channels, we expect congested transport will be more common in low-order streams while uncongested transport will dominate higher-order streams. Single flotation models can be used to model the stability of individual pieces, especially in higher-order channels, but are insufficient for modelling the more complex interactions that occur in lower-order streams.

Physical modelling of the combined effect of vegetation and wood on river morphology

“…Individual logs within the same cohort were released at approximately 3 second intervals to ensure uncongested transport conditions, as defined by Braudrick et al (1997). These inputs achieved a total input rate of 60, 120 and 180 logs per hour in the first 6 hours and 40, 80, 120 logs per hour in the remaining 12 hours of the experiment to flumes 1, 2 and 3, respectively (Table 1, for further information see Bertoldi et al, 2014).…”

“…Large (dead) wood has also been studied in the laboratory, mostly to investigate its effect on the flow field and to assess the conditions under which wood can be mobilized and transported (Braudrick et al, 1997;Braudrick and Grant, 2001;Bocchiola et al, 2006;Welber et al, 2013). The interaction between wood and bridges during floods has also been investigated (Schmocker and Weitbrecht, 2013).…”

Factors influencing wood mobilization in streams

“…The formula to determine the draft (D) of a cylindrical piece with radius (r) can be estimated from Braudrick et al [1997] as…”

Modelling woody material transport and deposition in alpine rivers

“…Theoretical models for woody material entrainment based on the balance of hydrodynamic (F) and resistance forces (R) on individual large woody material pieces have been developed by Braudrick et al (1997) and Braudrick and Grant (2000). Assuming the shape of the woody debris pieces being cylindrical, and neglecting the influence of buoyancy, the hydrodynamic force can be expressed as follows (Haga et al 2002):…”

“…The relation between the flow resistance due to the presence of large woody debris (LWD) and increased inundation frequency has been analysed in detail by Shields et al (2001). New insights into the dynamics of wood transport in streams have been achieved by flume experiments (Rickenmann 1997;Braudrick et al 1997;Grant 2000, 2001;Degetto 2000;Haga et al 2002;Curran and Wohl 2003;Bocchiola et al 2006). The interaction of woody material transport with protection measures has been investigated with a special focus on check dams (Bezzola et al 2004;Lange and Bezzola 2006) and on rope nets (Rimböck and Strobl 2002).…”