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greatest potential for the approach is in the
upper right corner of the bivariate plot, where
stream power and sediment supply are high, and
where there is relatively little urban encroachment
up to the banks. Where stream power and
sediment load are low, whether the condition
be natural (reflecting catchment geomorphology)
or resulting from human-induced changes (such
as reservoir-reduced flow regime and sediment
load), the river does not have the capacity to
erode, deposit, and rebuild its complex forms,
at least not in time scales demanded typically
by the public for visible restoration projects.
Below dams, flow dynamics and sediment loads
can be partially restored through reservoir re-
operation and sediment augmentation (centre).
However, high stream power and sediment load
are not sufficient in themselves, because even
a potentially dynamic channel could not be
permitted to erode, deposit, and migrate if there
is a high degree of urban encroachment up to
the banks. In such cases, natural elements can
be incorporated into restoration schemes, but
the channel must typically be hardened to resist
erosion and undermining of infrastructure. In
highly urban settings, 'restoration' of straightened
or canalized channels can be likened to 'gardening',
in which desired elements can be incorporated into
the projects, but they are not sustained by present
fluvial process, and thus require maintenance.
In such highly urban settings, social benefits
may outweigh ecological (left side of diagram).
With high-value land uses (such as premium
vineyards) adjacent to channels that are not
highly dynamic, zones most vulnerable to bank
erosion can be identified and infrastructure set
back in advance of erosive high flows, an approach
termed anticipatory management , which plots in an
intermediate position, just below and to the left of
the espace de liberte approach (Figure 18.2).
Clearly, the espace de liberte is not a feasible
approach for river management everywhere.
However, it can be applied far more broadly
than it is at present, especially in North America,
where - even in rural settings where infrastructure
constraints are lacking and there is a high ecological
potential from a dynamic channel - meander bends
have commonly been hardened with boulders and
large wood (Kondolf, 2006).
The potential to give the river a corridor is
illustrated by two case studies in contrasting
settings, one in peri-urban Geneva, Switzerland,
the other in rural California, USA.
The Aire, Geneva: a
peri-urban river
The Aire drains a 100 km 2 catchment, flowing
northward from the Sal eve range and glacial
moraines in France onto the Plaine de L'Aire, a
broad basin through which the Aire is naturally
unconfined and alluvial (Figure 18.3). As it enters
the basin, the Aire flows northward over its
alluvial fan to the village of Lully, then flows
eastward at a lower gradient with a meandering
pattern, ultimately joining the Arve just above
its confluence with the Rh one, in Geneva. To
understand the constraints and opportunities facing
managers on the Aire, it is useful to summarize the
geomorphic context and history of interventions.
As the Aire flows onto the Plaine de L'Aire
(from the town of St Julien), it descends across
its alluvial fan with an active, braided channel.
This reach was shown on the 1810 cadastral
map as having a channel about 100 m wide,
within which a narrower lit mineur was shown
in the carte Dufour of 1871 (Figure 18.4).
Following a large flood in 1876, dykes were
built to create a sediment basin within the wide,
braided lit majeur ; the basin has been dredged
periodically to remove sediments accumulated in
wet years. Downstream of Lully, the first 3 km of
the single-thread, meandering channel (sinuosity
1.46), was straightened artificially and converted
to a trapezoid with concrete banks and bed in
the 1930s. This 'channelized reach' had a higher
gradient (increased from 0.005 to 0.007), draining
adjacent agricultural lands efficiently and routing
flood waters downstream more quickly, increasing
peak flood discharge. In the 1960s, the reach
of the Aire furthest downstream was put in
an underground culvert to permit building of
industries and a major highway over its former
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