Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 159

Hassan and Church, 2000). Consequently, the overall transport distance of clustered sediments is less
than unconstrained particles (Laronne and Carson, 1976; Brayshaw, 1985; Reid., 1992). Since pebble
clusters may occupy 10 50 percent of the river bed (Hassan and Reid, 1990; Reid., 1992; Wittenberg,
2002b) their role in influencing equilibrium conditions on riverbeds, and, hence, channel sections, cannot
be ignored.
Wittenberg and Newson (2005) took a morphological approach to interpreting the initiation and
movement of gravel, using particle tracing to determine the movement of individual cluster particles over
a range of flood event magnitudes and durations. The experiment was carried out on the River South
Tyne, UK, in 1996. They used flow hydrographs measured nearby and also benefited from previous
studies of historical development, channel morphology, and sediment transport at the same site. More
than 30 clusters were monitored over a seven-month period during which a flood with a peak discharge
of 183 m 3 /s occurred. The clusters occupied 7 16 percent of the bed ˈ as shown in Fig. 3.42. Changes
occurred at each element of the cluster: stoss, obstacle, and wake. Wake particles are transported most
easily. Four processes were common: ķ scouring and removal of cluster particles; ĸ aggregation of
particles due to deposition processes, Ĺ in-situ modifications, namely dislodgement, without major
replacement or transportation; and ĺ formation of new bed forms.

Fig. 3.42 Spatial distribution of clusters (after Wittenberg and Newson, 2005)

At the cluster scale, stoss and wake particles moved for the relatively short distances for flows that
were insufficient to dislodge the obstacle clasts. Random movement of stoss particles occurred for
distances of less than 1 m and most of the wake particles were entrained. Nonetheless, similar-sized wake
particles and stoss particles exhibited different transport distances. Wake particles moved further
downstream compared to stoss particles of the same size. The movement length of a single clast is
limited and generally ceases when the clast is entrapped by an adjacent cluster. The bed area covered
with clusters does not exhibit profound changes; also, neither the surface grain size distribution, nor the
structure of the main bed forms altered.

3.4 Environmental Impacts and Control Strategies of Riverbed Incision

3.4.1 Environmental Impacts
The most disastrous consequence of riverbed incision is increased bank slopes and landslides. Channel
degradation may affect the stability of slopes. The case of the Fier River, a tributary of the upper Rhine