Environmental Engineering Reference
In-Depth Information
Case example: River Caldew
It was concluded that the River Caldew is char-
acterized by a series of dynamic reaches featuring
bar growth (storing mainly coarse sediment) and
localized bank erosion (yielding mainly fine sed-
iment), interspersed with morphologically stable
reaches that transfer bothcoarse andfine sediment
downstream. This suggests that the dynamic
reaches could be allowed to evolve naturally so
that they storemore of the coarse sediment (which
is what causes problems in the urban reach) while
releasing fine sediment that is not involved in
sedimentation in the urban area. These findings
provided the basis for design of an iSIS Sediment
Model that established the potential for reducing
the frequency with which future maintenance
might be required in the urban reach by allowing
or enhancing morphological evolution of the
channel in the reaches naturally storing coarse
sediment upstream (Wishart et al. 2007).
The Caldew in Cumbria, UK, was the subject of
Environment Agency investigation following se-
vere flooding in Carlisle, in January 2005. Sedi-
ment issues arose because gravel accumulation
within the urban reach of the river was identified
as contributing significantly to flood risk. Oper-
ationally, deposition in a channel with a flood
defence function would usually be managed by
dredging, but as the river is designated as both a
Special Area of Conservation (SAC) and a Site of
Special Scientific Interest (SSSI) there is a pre-
sumption against sediment removal under the
Habitats Directive. Dredging may still be al-
lowed, due to the overriding public interest, but
it is now a requirement to demonstrate that any
work is performed in the most environmentally
friendly way possible and to show that a need for
repeated dredging continuing indefinitely into
the future is avoided. Hence, sediment dynamics
were investigated as part of a wider geomorpho-
logical assessment of 14 km of the River Caldew
(Fig. 5.5).
Limitations
Evenas a screening tool, streampower analysis has
marked limitations. Firstly, the tool should be
300
1
3
4
5
6
2
URBAN AREA
250
200
150
100
50
35 Wm
-2
0
Distance downstream (m)
Fig. 5.5
Longstreamvariation in specific streampower in theRiverCaldew.Modified fromBrookes andWishart (2006).
