Geography Reference
In-Depth Information
1.015
1.01
1.005
1
0.995
0.99
0.985
0.98
0.975
0.97
0.79
0.77
0.75
0.73
0.71
0.69
0.67
0.65
Distance from the source (km)
Figure 15.7
Pattern of the mean and confidence level (at
05) of two pebble roundness indices measured ''at-a-station'' on
samples of 100 particles along the main stem channel of the Progo from its source to the ocean : the Wadell index (rW) and the ratio
of perimeters (pebble/ellipse (rP)). Modified from Roussillon et al. 2009. In grey, significant break-points in the longitudinal trends,
identifying individual homogeneous river segments.
α
=
0
.
15.4.2 Monitoringfluvial processes
for recent developments) drastically limit the temporal
resolution with which data can be collected. In order
to maximise information in space and time, acquisition
of oblique photography can be a valuable solution for
monitoring temporal changes with an at-a-station setup.
Lane et al. (1994; 1996) and Chandler et al. (2002)
demonstrated that digital oblique terrestrial imagery is
an appropriate and affordable solution to the problem of
frequent morphological survey of river reaches. This is
particularly true when monitoring multi-channel rivers
that are characterised by complex inundation dynamics
(Lane et al., 1994; van der Nat et al., 2002; Ashmore and
Saucks, 2006; Luchi et al., 2007; Bertoldi et al., 2009)
and where morphological changes can happen on a
daily/weekly timescale (Milan et al., 2007; Ashmore et al.,
2011). A further valuable advantage of close range imagery
is the possibility to pinpoint the exact spatial and tempo-
ral occurrence of infrequent but rapid processes, or those
occurring in remote and dangerous environments.
Four examples are reported below that illustrate differ-
ent possible applications for oblique close-range imagery;
namely (15.4.3) subaerial bank processes, (15.4.4) braided
rivers inundation dynamics, (15.4.5) river ice formation
and dynamics, (15.4.6) characteristics of riparian struc-
tures and wood accumulation.
Historically, information about channel morphometry
has been obtained by planform mapping (using field sur-
veys, aerial photographs and published maps) combined
with surveys of channel cross sections. Such data are
highly valuable, but they capture only a snapshot, that is
strongly dependent on the water stage, and, in general,
a lot of information is not captured because of poor
temporal resolution. In addition, bed elevation surveys
are demanding of resources with the result that only a
small number of cross sections can be measured. This
type of data is therefore most appropriate for measur-
ing changes that occur over large areas or that are due
to a high magnitude process and will tend to overlook
details between cross sections or small changes due to
smaller magnitude processes. Moreover, 2D numerical
models require continuous and high resolution input
data and, similarly, produce continuous (in time and
space) information on the flow field, bed topography
and sediment transport, which needs suitable field data
for validation.
The recent adoption of airborne and terrestrial laser
scanning in river science may provide possible solutions
to some of these problems, but their costs, and the impos-
sibility to survey wetted areas (but see Smith et al., 2012
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