Environmental Engineering Reference
In-Depth Information
black bog ant (
Formica candida
), southern dam-
selfly (
Coenagrion mercuriale
), water vole (
Arvicola
terrestris
), marsh fritillary (
Eurodryas aurinia
) and
common frog (
Rana temporaria
), to name but a few,
whereas the otter (
L. lutra
), for one, travels long dis-
tances between rivers, lakes and wetlands to hunt and
rest. The diversity of habitats along the river corridor
has gradually declined in response to increased
urbanization and the decrease in forest floodplains.
EU Commission figures suggest that, among others,
45% of butterfly, 38% of bird, 24% of flora and 5%
of mollusc species are considered as threatened by
extinction (Halaham 2000). Many of Europe's rivers
now have little natural floodplain habitat with restricted
riverine corridors and a thin strand of open water is
often all that remains to connect this network. As these
habitats have reduced, the Biodiversity Action Plans
have tried to redress some of the issues of habitat
destruction and these plans have been introduced
across Europe with the aim of rectifying some of this
environmental damage. Within these plans specific
species associated with rivers and streams have been
identified.
suggested that the distribution of rainfall may change
and stormier conditions will prevail with associated
flash floods resulting in new flow regimes altering the
meaning of a one-in-50-year event. Since 1988, an
increase in the frequency and magnitude of flooding
in many parts of Scotland, for example, has led to
increases in the assessed risk of flooding. Marsh
et al.
(2000) showed that the risk estimate for a flood the
size of that which occurred in Perth, Scotland, in 1993
increased from one in 2000 years in 1988 to about
one in 100 years in 1994. Subsequent work examining
the effects of climate change in Scotland (Werritty
et al.
2002) indicated that, by the 2080s, floods
presently estimated to have return periods of 50
years may occur with return periods of as low as 17
years. Such research implies that previous embank-
ments and walls designed to a high level of protec-
tion are now woefully inadequate for major flooding
events.
11.4.2 Urbanization and development
In addition to these predictions of future climate
change, urban expansion continues and whereas his-
torically the floodplain was a wet area avoided by
industry and homeowners, with the development of
new, improved flood-protection techniques some of
this reticence has long been forgotten. Yet, a flood-
protection scheme is only as good as its design speci-
fication and many of those implemented to withstand
a one-in-50- or even a one-in-100-year scenario are
not a guarantee that these floods will be kept at bay
in the future. These schemes are therefore unlikely
to be sustainable economically in the longer term.
Furthermore it is unrealistic to expect to be able to
design flood defences to cope with an estimate of the
probable maximum flood either on the grounds of
finance or health and safety. There is now a great deal
of conflict between the pressure on local governments
to deliver increased housing and infrastructure, and
the acceptance that empty floodplains that are allowed
to flood can be an economic asset as well as being
of benefit to biodiversity. In the UK the government
document PPG25 (2001) states that planners should
avoid inappropriate development of the floodplains.
In many countries though, the scientific expertise
to prove accurately that new developments will not
11.4 Current disturbances and threats
Despite a growing awareness in the scientific and polit-
ical worlds that natural riverine environments are a
major asset to ecological diversity and that reconnection
of the rivers to their floodplains is one way to alle-
viate current uneconomically viable flooding problems,
there continue to be disturbances and threats to river
ecosystems that must be acknowledged.
11.4.1 Future climatic scenarios
Climate scenarios vary considerably and there is still
much discussion even within the Intergovernmental
Panel of Climate Change (IPCC) about both the pre-
cise rate and extent of future variation in temperat-
ure and precipitation. Nevertheless, there is general
consensus that there is likely to be an increase in global
warming. Some figures suggest that an average 4°C
increase in temperature throughout the world by
2080 could result in an estimated average increase in
river discharge of 20% worldwide. Furthermore, it is
