Geoscience Reference
In-Depth Information
Over time, rivers in Britain have been heavily
managed through regulation, impoundment and
the drainage of floodplains. Post-war Britain
saw rapid expansion of urban development in
floodplain areas and the replacement of absorbent
floodplains with impervious surfaces. Given the
financial investment involved, simply retreating
from these developed floodplain areas is not
feasible, so ways need to be found of living with
the environmental and socio-economic risks. There
is a clear need for a more holistic approach to
'catchment connectivity' and to develop whole-
catchment assessment and modelling techniques
to assess flood risk so that flood-sensitive areas
can be targeted for management purposes (Brierley
et al. , 2006, 2010; Rice et al ., 2006; Brierley and
Fryirs, 2008). One element of a more integrated
approach is termed 'catchment consciousness'; this
reflects the view that land and water must be
considered together, and that the whole river basin
represents an ecosystem capable, when intact, of
supporting sustainable solutions to human needs
as well as reducing risk from extreme rainfall
(Newson, 2009).
The three dominant variables that determine
local channel morphology and behaviour
are channel gradient, the extent of channel
confinement and catchment hydrology
(Montgomery and Buffington, 1998; Church,
2002; Reinfelds et al ., 2003). As system change is
a function of the excess of force over resistance,
a truly process-based approach should be based
on the spatial distribution of driving and resisting
forces in the catchment. A critical issue is the size
of the disturbance and the river's ability to resist
or accommodate the impact of that disturbance
(Werritty and McEwan, 1997). Bank strength
is likely to be a significant factor (Eaton et al .,
2004), but characterizing bank resistance is highly
dependent on site-specific conditions and this
problem has not yet been resolved successfully.
A combination of: (i) system complexity (Philips,
2003); (ii) singularity or, put another way, the
paradox that it is easier to make predictions for
an assemblage of landforms than for a single one
(Schumm, 1991); and (iii) limited data (Downs,
1995) have so far prevented detailed analysis of
the sensitivity of many rivers to a range of different
driving variables.
A fluvial geomorphology
approach to assessing flood
risk
The recent floods in northern England have
also demonstrated a clear need for a fluvial
geomorphology-based approach to assessing flood
risk. The contribution of fluvial geomorphology
to solving complex inter-disciplinary problems
represents an opportunity as well as a challenge
because previously many policy decisions were
made without adequate consideration of the spatial
and temporal complexity of river systems (Dollar,
2004). The benefit that fluvial geomorphology can
bring is impartial insight and understanding of river
systems (Kneupfer and Petersen, 2002). This is
particularly important in considering the potential
impacts of climate change on flood frequency and
magnitude. It is also vital to understand and predict
consequent alterations to channel geometry,
especially for rivers undergoing long-term
aggradation or which are subject to in-channel
management such as dredging. In future it will
be important to undertake more catchment-wide
audits of factors influencing sediment erosion,
transport and deposition, and in particular to
examine the role of flooding as this is when the
majority of rapid change in river systems occurs.
In the UK, fluvial audit was developed to do this;
a combination of field survey, historical map and
documentary information, and scientific literature
are used to gain a comprehensive understanding of
the river system and its catchment (Sear et al ., 1995,
2003). Through this process, it is also important to
understand how river systems behave/respond in
the period between geomorphologically significant
events. This is because in some cases the responses
by a system that has not fully re-adjusted to a
previous event can be more severe than some of
the more immediate effects of the flood itself. So
far, there have been few studies that consider this
Search WWH ::




Custom Search