Geology Reference
In-Depth Information
10
3
Braided-Meandering
Transitions
braided
10
2
meandering
10
1
multi-thread channels (braided)
single-thread
channels
}
1.5 < P ≤ 1.8
P > 1.8
(P = sinuosity)
10
0
0.1 1 10 100
Median grain size of riverbed (mm)
Fig. 8.16
Channel pattern as a function of specific stream power during bankfull discharge versus mean grain
size of the river bed.
Specific stream power (W/m
2
), calculated from river slope and channel cross-section at bankfull stage. Sinuosity (
P
) is
the ratio of the length of the thalweg to the valley length. High sinuosity (
P
> 1.5), single-thread (meandering) rivers
define a separate field from that of braided rivers. Based on data from more than 200 rivers, this plot predicts that, for
a given grain size, the channel pattern is a function of specific stream power and that changes in water depth or slope
can cause the river to cross the threshold defining the different channel patterns. Modified after Van den Berg (1995).
localized uplift by folding the substrate beneath
a reach of the channel. Both approaches show
potentially diagnostic transitions in channel
forms. As slope increases while discharge
and sediment supply are held constant, initially
straight channels become more sinuous (Fig. 8.17).
Sinuosity continues to increase until a threshold
slope is attained, after which any additional
steepening of slopes leads to a braided river pat-
tern. Further experiments indicate that, for differ-
ent discharges, but with a constant grain size, the
transition to braided occurs at different stream
power, but at approximately the same slope
(~0.012-0.013) for each discharge (Edgar, 1973).
In these experimental results, increased sinuosity
should not be equated with meandering channel
forms, because truly meandering, single-thread
channels have only recently been created on
stream tables (Peakall
et al
., 2007), well after
these experiments were conducted. Nonetheless,
the tendency toward higher sinuousity as chan-
nel slopes steepen suggests that planform
changes represent one way in which rivers could
respond to tectonic deformation.
Although the reasons that rivers meander are
not completely understood, meandering rivers
tend to dissipate energy (through localized
bank erosion and sediment transport) in about
equal amounts along equal lengths of channel
and to minimize the total energy expended
along the river course. At the regional scale,
sinuosity has been shown to be a function of
both rock strength and the frequency of high-
intensity discharge (Stark
et al
., 2010). Weaker
rocks and more frequent storms cause higher
sinuosity. At the local scale, when a channel is
steepened, stream power initially increases,
such that a stream formerly in equilibrium
will be pushed toward a non-equilibrium con-
dition. By increasing sinuosity, the channel is
lengthened, and the slope along the thalweg
decreases, thereby counteracting the increase
in the valley gradient. As a consequence, the
rate of work done per unit length of channel is
