Civil Engineering Reference
In-Depth Information
O
Low fragility: resilient ('unbreakable'). Minimal or no adjustment
potential. Only minor changes occur, such as bedform alteration, and the
category or subcategory never changes to another one, regardless of the
level of damaging impact.
O
Medium fragility: local adjustment potential. It may adjust over short
sections within the vicinity of the threatening process. Major character
changes can occur or the category or subcategory can change to another
- but only when a high threshold of damaging impact is exceeded,
e.g. it may require a catastrophic flood, sediment slug or clearing of all
vegetation from bed, banks and floodplain.
O
High fragility: significant adjustment potential. Sensitive. It may alter/
degrade dramatically and over long reaches. Major character changes can
occur or the category or subcategory can change to another one when a low
threshold of damaging impact is exceeded.
Threshold points can also be important in assessing vulnerability. There is
increasing evidence that ecosystems seldom respond to gradual change in a
gradual way. Lakes often appear to be unaffected by increased nutrient concen-
trations until a critical threshold is passed and the water shifts abruptly from
clear to turbid. Submerged plants suddenly disappear and animal and plant
diversity is reduced - an undesired state from both a biological and economic
point of view. Substantially lower nutrient levels than those at which the
system collapsed are required to restore the system. The economic and
social intervention involved in a restoration is complex and expensive, and
sometimes irreversible. Studies of rangelands, forests and oceans also show
that human-induced loss of resilience can make an ecosystem more vulnerable
to random events like storms, droughts or fires with which the system could
previously cope. An ecosystem with low resilience can often seem to be
unaffected and continue to generate resources and ecosystem services until a
disturbance causes it to exceed a critical threshold. Even a minor disturbance
can cause a shift to a less desirable state that is difficult, expensive, or even
impossible to reverse (Swedish Environmental Advisory Council 2009).
There has been a considerable amount of work over recent years to identify
ecological thresholds, including such things as critical flooding frequencies
for wetland survival and for fish and water bird breeding. Saline intrusion or
structural compaction of aquifers are further examples.
Thresholds also apply to human activity. Urban water supply systems often
have critical dry sequences, which if exceeded, would result in system failure.
Walker
et al.
(2009) reported on a resilience assessment of the Goulburn-
Broken catchment in Victoria, which identified a range of known or likely
thresholds in the catchment relating to biodiversity, agricultural activity, the
local economy, and social value structures.
Trends in condition are important. If condition is declining, it is more likely
that an event will trigger a threshold change. On a river system, if connected
wetlands or instream drought refuges are reduced in number and area, then it
