Geoscience Reference
In-Depth Information
of extreme events; by 2100 there could be a
10-30% increase in the magnitude of UK rainfall
events, up to a 50-year return period (Fowler
et al.
,
2007). Equally pertinent to flood-risk assessment
is the conclusion that, for some time, Britain has
experienced a cyclic pattern of 'flood-rich' and
'flood-poor' periods (Werritty, 2002; Lane, 2008).
If a 'flood-rich' period coincides with extreme
events, the implications for affected catchments
could be severe, so the approach to flood risk
management will need to take account of this
apparent cyclic pattern.
It is not only water resources that are affected
by these extreme floods. Recent large floods in
north-east England affecting the River Wansbeck,
River Coquet, Glen/Bowmont system and River
Breamish in 2008, and the River Derwent and
River Cocker in Cumbria in 2009 have shown
similar patterns of sediment mobilization and,
with the exception of the Wansbeck, significant
alteration of channel course. In the majority
of cases this sediment re-mobilization involves
contemporary bedload of the rivers; however,
during the Derwent flood, several parts of the river
appear to have undergone a phase shift from a
sediment-limited situation to one where additional
sediment (in some cases re-mobilized from long-
forgotten industrial waste deposits) has re-entered
the system through bank erosion and channel
avulsion and temporarily shifted the channel out of
balance with its bedload (Plate 34). The 2009 River
Derwent flood emphasized the importance of local
sediment sources in affecting the morphological
response of the channel. The downstream effect
of this renewed geomorphological activity on
local communities was devastating - swathes of
agricultural land on the floodplain covered in
gravel and cobbles, undermined and collapsed
bridges, and road and rail infrastructure severely
disrupted.
Recent floods on the River Coquet and the
River Derwent in Cumbria have also confirmed
that morphological responses to high magnitude
events
synergistic and severe. Moreover, the implications
of these combined factors are commonly
overlooked in flood-risk mapping exercises
and are likely to be important in river systems
with high rates of sediment delivery and long-
term transfer of sediment to floodplain storage
(Lane
et al
., 2007). The non-linear response
patterns of the Coquet, Till and Bowmont/Glen
river systems (Northumberland) highlight that
attempts to reduce lateral channel migration
by bank stabilization measures are more likely
to exacerbate flooding. This is because channel
capacity cannot be maintained with the erosion
patterns observed during the last two large
floods, a conclusion supported by longer-term
monitoring of these rivers. Confounding this
further, climatic and geomorphic responses are
not closely coupled, so different parts of the
river systems will respond differently because of
significant time-lag effects (Schumm, 1977; Orr
et al
., 2008). Morphological response will also be
heavily influenced by local conditions such as
catchment gradient and lithology and by human
interference such as channel modifications and
floodplain development.
Generally speaking, a river channel adjusts
slowly or episodically in response to the major
drivers of river discharge (predominantly at
bankfull level) and fluvial sediment fluxes supplied
from the catchment or the channel upstream
(Thorne, 1997). In engineering terms, adjustment
is considered as 'instability', so its constituent
processes have been heavily controlled (Brookes,
1987; Sear
et al
., 1995). Many modifications
of river channel cross-sections carried out
for flood protection purposes also modify the
geomorphological behaviour and energy of
the flow, for example by increasing channel
depth and stream power (Brookes, 1987). Flow
regime, by contrast, is most heavily influenced
by impoundment, regulation and abstraction,
although catchment land-use, land-cover and
land-management are now also seen to have
a subtle but important influence, especially on
flow extremes and fine sediment concentrations
(Brooks and Brierley, 2004; Golosov
et al.
, 2004).
are
non-linear
(Phillips,
2003,
2006).
Consequently,
the
combined
direct
effects
of
sedimentation
and
climate
change
could
be
