Geoscience Reference
In-Depth Information
Table 18.1
Objectives of the revitalization of the Aire River, Geneva.
Reducing downstream flood risk
by using the floodplain to store flood waters through construction of two dykes
normal to the direction of flow, designed to retain flood waters, and in a future phase to construct an above-ground
stream channel downstream of the Pont Rouge, which will serve to convey flood waters in excess of the capacity of the
subterranean culvert.
Revitalization of dynamic channel processes
by removing concrete banks and armoured bed, and in places lowering
the adjacent floodplain to create a wider
lit majeur
.
Re-routing the river into a large northward meander bend
, which was cut off artificially from the river during the
canalization project of the 1930s.
Creating a meandering lit majeur
, with erodible banks, immediately south of the canal.
Establishing an 80 m wide wildlife corridor
along the channel by converting agricultural lands to riparian habitat.
Retaining the canal as a human artifact
, to serve as a trail and parkland, recreating the meandering channel to the
south of the canal. The canal will be partially filled in, its north bank developed as the continuous trail, with
recreational opportunities provided in the partially-filled canal. Human use will be encouraged in the canal and trail,
thereby taking pressure off the restored channel and riparian corridor.
Geneva landscape architect Georges Descombes,
to revitalize the river. The restoration approach
sought to allow the Aire to recreate its own
complex morphology, while providing access for
the large urban population nearby, and providing
flood retention (Table 18.1).
Revitalization of the Aire has been under way
in three phases since 2002, allowing the stream to
migrate as much as possible given infrastructure
constraints, restoring the channel to a meander
bend that had been cut off artificially, while
retaining portions of the artificial channel as a
human artifact, adapted to provide recreation for
users from the surrounding urban area. Artificial
elements such as poured concrete steps define the
banks at key points, stabilize the channel at critical
infrastructure points and provide human access
points,
develop and migrate. In response to concerns that
a featureless bed for the wide
lit majeur
would
lack habitat value and visual appeal, and calls for
cutting a pilot channel to provide some deeper
water for fish, the designers' proposal (Figure 18.6)
is an intriguing one: they propose to excavate
diamond-shaped depressions in the bed to create
an initial 'lozenge' pattern, which will provide
some initial bed complexity whilst presenting a
compelling landscape design. The expectation is
that this pattern will be a formalized beginning
to recovery of the braided pattern that existed
historically in this reach, while the traces of the
mechanical excavation will be erased over time,
perhaps gradually by a series of small floods, or
perhaps at once in a large flood.
Upstream of the flood bypass, the Aire
experiences a wide range of flows, including
frequent geomorphically competent flows. The
bypass system is designed to allow floods to
increase within the channel up to 45 m
3
s
−
1
,such
that the channel of the Aire still has energy to
erode and deposit. That the Aire has sufficient
sediment supply to build complex channel forms
is demonstrated by the rate of sediment deposition
in a deep 'pool' that was excavated to provide
a deep-water fish habitat (and which filled in
with gravel and sand within a year) and the
abundant and mobile gravel bars in the natural
reach downstream of the bypass, where slower
contrasting
with
the
dynamic
channel
nearby.
Dykes cutting transversely across the floodplain
along with channel control structures, will create
two
retention
basins
on
the
floodplain,
to
limit
flows
downstream
to
the
flood
bypass
m
3
s
−
1
,the
and
underground
culvert
to
70
combined
capacity
of
these
two
downstream
components.
In the final phase (affecting most of the
channelized reach) now under review, the
designers propose to create a wide
lit majeur
within which a narrow
lit mineur
is expected to
