Environmental Engineering Reference
In-Depth Information
100
80
60
40
More likely
Uncertainty (worst)
Un
c
ertainty
(
best)
20
0
0
10
20
30
40
50
60
70
80
90
100
Fig. 16.15
Probability of failure
Time (years)
chart.
allowances. Alongshore influences upon defence
failure, for example structures restricting the sup-
ply of sediment to a frontage, can almost entirely
be accommodated through the consideration of
foreshore levels. For example, if such restrictions
do apply then there is a need to account for that in
the foreshore levels or assessment of foreshore
volatility.
Figure 16.15 illustrates the required input, with
the solid black line indicating the best assessment,
and the degree of uncertainty shown by the dotted
and dashed lines, which are considered to approx-
imate to 5% and 95% confidence limits:
So, in this example, it is expected that the
structure is most likely to be effective for another
40 years, but it might collapse after 20 years, or
could last 60 years. During the period leading up to
that, the User has assessed that there is a 1%
chance of storm conditions exceeding design con-
ditions year-on-year and leading to its failure,
although this could be as much as 1.5% or as low
as 0.5%.
In other words, under the 'best assessment' the
graph is saying that the defencewill definitelyhave
failed by year 30, but recognizes that there is a
small chance that this failure could actually hap-
pen this year, next year, or at any point forward.
different levels of complexity. These components
can be determined by detailed modelling or em-
pirically from local knowledge and engineering
judgement.
Once both curves have been defined they may
be combined to obtain probabilities of erosion to
a fixed asset or spatial distribution of erosion at a
given time in the future. This is done under the
definition of three main scenarios as given in the
next section.
Definition of scenarios
There are three possible scenarios that canmodify
the erosion distance estimates. These are defined
by the different time estimates of when the failure
of the structure is expected, fromthe probability of
failure curve:
Scenario 1:
The first scenario is then constructed
with the erosion profile curves considering the
three time lags defined by the probability of
failure. The worst case is represented by using
the worst time of failure with the worst ero-
sion profile scenario. The intermediate erosion
profile is defined using the best estimate of the
time of failure against the best scenario for
erosion profile. Finally, the best scenario
for the erosion profile is combined with the
best estimate of the time for the structure
failure.
Scenario 2:
The second scenario is constructed
from the erosion profile assuming that the ero-
sion line stays in the same position but once
Range of techniques for determining recession
and probability of failure
The techniques employed to evaluate both the
erosion curves and the probability of failure have
