Geography Reference
In-Depth Information
is characterised by more than 43 km of eroded banks
over a total of 190 km of banks, the right bank being
significantly more eroded than the left, mainly between
km 30 and 47. This raises interesting questions about the
cause of bank erosion and a number of possible explana-
tions were investigated including the role of neotectonics,
engineering infrastructure and sediment delivery from
tributaries or coupled hillslopes. Some reaches are very
stable on both banks because of embankment and other
protection (e.g., km 50-60). This longitudinal trend can
be compared with active channel width (Figure 15.13d)
showing that unstable banks upstream of km 65 occurred
all along the channel course whatever the width which
demonstrates the strong disruption of the channel asso-
ciated with degradation following a period of intense
gravel-mining.
The segment procedure is useful for combining infor-
mation from different sources, each of them being coded
or measured for each channel segment. Information pro-
vided from the oblique air photos can then be linked
with other information provided from long profiles, field
measurements (e.g. grain size) but also aerial photos.
Lassettre et al. (2007) showed that the frequency of in-
channel wood pieces deposited in a river section increased
linearly with the local amount of wood introduced. The
information was calculated for 250 m long segments, the
wood pieces were identified from oblique helicopter pho-
tographs whereas the wood delivery was based on an
estimate of floodplain eroded surface area provided from
an overlay of two sets of aerial photos and an estimate
of wood volume from field work (Figure 15.14). This
approach suggested that the accumulation of wood is not
linked to upstream sources but to local ones, the sites
of in-channel wood sources and deposits being usually
the same.
From the examples presented herein, the benefits of
using close-range photographs are numerous. This is a
low-cost technology that:
•
can
provide
both
qualitative
and
quantitative
information;
•
generates original spatial information that may be
undetectable using standard, vertical aerial photography
because of the small, sub-centimetre scales of interest or
rapid changes in elevation;
•
can be used to obtain time lapse or triggered sequences
of photographs or videos over long time periods, thereby
increasing the temporal resolution of field observations
and avoiding the cost of lengthy fieldwork in remote or
dangerous areas;
•
can reveal the timing, location and amplitude of
significant events complementing other measurements
and
facilitating
expert
interpretation
of
processes
and/or forms;
•
allows reliable physical measurements when proper
ground control points are surveyed;
•
and provides valuable data for developing predictive
models when combined with other field measures.
Close-range image acquisition also has limitations.
Acquisition and analysis of oblique photographs requires
specific methodologies and a larger number of ground
control points than standard vertical aerial imagery.
Moreover, the spatial resolution and the angle of dec-
lination with respect to the surface are variable within the
image, depending on the distance between the camera and
the surveyed object. This implies that process detection
and measurements cannot be performed with the same
level of precision and resolution across the entire scene.
Measurements and observations are often made chal-
lenging by changes in image lighting and contrast (sunny,
cloudy, shadows) and this must be considered when
setting up the cameras, for example to avoid focusing
towards the sun or its reflection on snow or water.
When using time lapse pictures, continually changing
light conditions and the occurrence of sun reflections are
significant impediments to the possibility of automatically
extracting planimetric parameters. Furthermore, picture
acquisition is often limited to daylight, so that temporal
sequences do not include events that happen during the
hours of darkness. In this regard, there is a need to make
use of technologies for obtaining night-time images, for
example infrared sensors.
Most of the limitations listed above are not absolute and
new technological developments could help to improve
further the versatility of close-range camera systems.
15.5 Summary of benefits and
limitations
In this chapter we showed how close-range images
are complementary to other measurement techniques
for studying river corridor changes and processes. We
described and illustrated different techniques that use
close range photographs to maximise information at high
spatial or temporal resolutions and to monitor the occur-
rence of unpredictable events and to characterise their
biophysical effects.
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