Geography Reference
In-Depth Information
series leads to discussion of the hierarchy of geomor-
phic features. Segmented patterns are common when
looking at channel planforms, which, for example, can
change abruptly from a large braided pattern to an
embanked channel. Similarly, in the case of a valley,
the cross section can change from a V-shaped valley
to an alluvial one. Such physical changes usually corre-
spond with changes in AGOs, making it possible to use
the aggregation/disaggregation process to detect forms
at the appropriate spatial scale for measuring meaning-
ful attributes by aggregating DGO-scale attributes (e.g.
descriptive statistics,) or by combining other raw data or
vectorial data.
The aggregation/disaggregation procedure is therefore
a practical way to develop systematic characterisations
of fluvial systems and, because other non-image based
data can readily be disaggregated, to combine imagery
with other sources of data. For example, data can be
extracted in a systematic manner from longitudinal pro-
files, LiDAR or DEMs to providing attributes such as
elevation, slope, drainage network and vertical channel
changes (Reinfelds et al., 2004; Jain et al., 2006). In addi-
tion, data from oblique aerial photos, field campaigns
(e.g. grain size distribution, hydraulic parameters) or
other GIS resources (floodplain or land-use polygons,
stream power/flow records using regionalized hydrolog-
ical models) (Williams et al., 2000; Dawson et al., 2002;
Nardi et al., 2004; Hall et al., 2007) can be incorporated
into the analysis.
Nevertheless, the series are not always characterised
by distinct segments, and other longitudinal structures
can also emerge. Rather than showing distinct homoge-
neous reaches of different length separated by clear or
transitional contacts, the longitudinal series can provide
a longitudinal trend (constant change in channel width
downstream) or a periodic organisation, well illustrated
by the sequence of particular meso-habitats, such as pools
and riffles. When looking at the longitudinal evolution of
the flow channel width or of the channel depth, a periodic
structure can be also observed as a proxy of the longi-
tudinal changes in the geometry of meso-habitats. Other
statistical procedures than the ones used to highlight
homogeneous reaches, such as the Fourier transform,
the spectral analysis, the spatial autocorrelation, or the
wavelet analysis, can therefore be used to highlight these
patterns. Moreover, the aggregation/disaggregation pro-
cedure applied at a network scale highlights a scaling
issue. A scale dependency is then evident when looking
at slope, discharge, active channel width and grain size
as all are interdependent, with catchment area, discharge,
width generally increasing when grain size and slope are
decreasing. However, there are usually exceptions to the
general scale dependencies, particularly for slope and
grain size. Such exceptions represent interesting points in
the network. Technical questions are then raised regarding
the removal the size effects and exploration of the prop-
erties of reaches. Finally, the aggregation/disaggregation
procedure opens new scientific questions on the nested
and organised nature of biophysical features along a flu-
vial corridor, allowing re-exploration of the theoretical
hierarchical framework of Frissell et al. (1986). We still
do not know if the entities observed at a given scale
level are nested in the ones observed at a higher level
or if the entities observed at different levels are in fact
independent.
11.4 Detailed examples to illustrate
management issues
In this section, we present three contrasting case-studies in
order to highlight how imagery, combined with additional
information, can be exploited for providing insights for
managers. First, we introduce a reach-scale retrospective
approach that combines multiple sources of information
on the Ain River collected in cooperation with local river
managers and funded by a European Life project. The next
two examples focus on network-scale approaches applied
to a single image set at time t, one based on continuous
data and the other on a discrete sample of reaches within a
larger network. Both studies were supported by the Water
Agency 'Rh one Mediterranee' responsible for the imple-
mentation of the Water Framework Directive in the
Rh one district that requires regional data and planning
tools for targeting conservation, restoration or mitigation
measures. All these works utilised
Institut Geographique
National (IGN)
aerial photograph resources. IGN covers
the national French territory, with pioneer photos in
the 1930s and more systematic surveys after 1945. Other
national sources of aerial photos, such as those of the
Inventaire Forestier National
(National Forestry Inven-
tory) are also available. All the territory is covered with
orthorectified digital photos available as Orthophotogra-
phy Database with dates starting in 1998. Older ones are
archived on paper and are not yet available in a digital form.
Scale is usually 1/20,000 with a spatial resolution of 50 cm.
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