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continued to contain flow at each subsequent time step
was recorded (pixels that became dry, i.e., abandoned
or vegetated and then wet again were not counted). The
analysis demonstrated that vegetation slows the reworking
time of the bed compared to unvegetated braiding. This
reduction was attributed to a combination of slower
erosion rates due to increased bank strength and the
limited ability of opportunistic creation of new channels
in areas not occupied by the flow due to the deterrence
effect of plants (Tal and Paola, 2010).
Transition matrices were used to study feedbacks
between flow and vegetation establishment. To do this,
pixels representing dry riverbed in the binary flow maps
were further classified into three additional classes rep-
resenting different stages of vegetation cover (plant age
and density; Figure 13.12c). The three main stages of
vegetation cover were first visually identified in the
images, then their distinct colour signature was mea-
sured (Figure 13.13). The colour value of the plants at
different stages represented a combination of both plant
age and density: dense clumps of young vegetation could
have the same colour value as sparse patches of mature
plants. Using the classified images transition matrices
were computed for pairs of images at 5-minute intervals
during all of the high flows as well as between high and
low flows. The transition matrixes shed light on impor-
tant feedbacks between flow and vegetation and the role
of a fluctuating discharge. A key effect of the plants was
to colonise abandoned (dry) areas during low flow and to
deter the high flow from reoccupying these areas during
the next flood, leading to progressive reinforcement of
the low flow wetted width (Tal and Paola, 2010). The
analysis also demonstrated that vegetation colonisation
was a highly self-reinforcing process: areas where plants
survived early provided a more stable surface for new
plants to establish. As new vegetation established it fur-
ther stabilised these areas and increased their chances of
surviving subsequent floods (Figure 13.14). As a result,
vegetated islands expanded through time and merged
with other islands to form larger islands. This continuous
process of accretion and amalgamation of islands eventu-
ally resulted in the formation of a permanently vegetated
floodplain (Figure 13.10; Tal, 2008).
(a)
(b)
(c)
Figure 13.12
Example of part of a classified image (1 m
1m)
(a) original image, (b) binary image, wet is white, black is dry,
(c) area that was black in b classified into bare-sparse vegetation
(black), dense vegetation (dark gray), very dense vegetation
(light grey; Tal, 2008).
×
13.4 Bed topography and flow depth
Approaches to measuring bed topography range from
simple manual point-gauges to sophisticated laser and
ultrasonic scanners mounted on automated carriages
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