Is the consumer (population) ready or willing to accept an infrastructure service,
which now and then is out of use? Who would be willing to accept an underground
basement as a car garage which may get flooded in the middle of the night?
Sometimes, there might be ample warning, sometimes flooding is quite sudden.
After having considered the 100 year return period, even if a higher
T r ¼ 1000 years is taken, the equation for R and Table 10.3 show a 7.2% chance
(not to be neglected as being small) for a 1000 year flood to occur during a 75 year
span, i.e. within a man’s life span. Can 7.2% be considered negligible when
insurance policies are taken for life or car accidents, where even lower probabilities
are encountered?
It can be observed – by going down any column in Table 10.3 for the spans
N ¼ 5 years to N ¼ 500 years) – that the risk that an event is exceeded or reached
over a certain span of time, decreases with an increase in return period (first column
in Table 10.3). This result is often used in the design of huge structures. There is also
an increase in cost by considering the design of a structure for a long return period.
However, this should be done to be safe from calamities causing loss of life and
property. For example, when Malpasset dam in France and Vajont dam in Italy failed
in 1959 and 1963 respectively, there was a huge loss in human life and property.
Many Codes of Practice have chosen a return period of 50 years because the
average lifetime of most buildings and structures is around this value of 50 years.
This could well have been true at one time. However, there are other factors
which, in modern times, need to be considered:
(1) The use of better and stronger materials has increased the lifetime of the
buildings and structures, drains or bridge culverts. This implies a longer life
span, along the rows of Table 10.3.
(2) No owner is ready or willing to pull down his building after 50 years, if it is still
serviceable or not dangerous. Built in 1889 for the Universal Paris Exhibition,
the Eiffel Tower was to be dismantled just after. Today, after more than
200 years, with regular maintenance, it is still standing and visited by millions
of people annually.
(3) Who has seen or heard of a drain being demolished to be enlarged, except when
it has been proved to be really inadequate. Even if a local authority tried to do so,
it is very likely that adjoining structures would prevent this.
(4) Started in 1160, the construction of Notre Dame de Paris finished a century later.
There are many other such structures around the world, built not much later,
which are still standing today. Would any present day designer still consider a
50 years lifetime for such monuments?
(5) The cost of dismantling, pulling down, or demolition has become so high that the
owner (private or government) will be reluctant to really do so, unless it concerns
public safety.
The lesson to draw out from these examples is probably simple: structures have a
life span much longer than 50 years, and due consideration must be given to their
design and required resilience.
10.2 Climate Change Variables
289
which now and then is out of use? Who would be willing to accept an underground
basement as a car garage which may get flooded in the middle of the night?
Sometimes, there might be ample warning, sometimes flooding is quite sudden.
After having considered the 100 year return period, even if a higher
T r ¼ 1000 years is taken, the equation for R and Table 10.3 show a 7.2% chance
(not to be neglected as being small) for a 1000 year flood to occur during a 75 year
span, i.e. within a man’s life span. Can 7.2% be considered negligible when
insurance policies are taken for life or car accidents, where even lower probabilities
are encountered?
It can be observed – by going down any column in Table 10.3 for the spans
N ¼ 5 years to N ¼ 500 years) – that the risk that an event is exceeded or reached
over a certain span of time, decreases with an increase in return period (first column
in Table 10.3). This result is often used in the design of huge structures. There is also
an increase in cost by considering the design of a structure for a long return period.
However, this should be done to be safe from calamities causing loss of life and
property. For example, when Malpasset dam in France and Vajont dam in Italy failed
in 1959 and 1963 respectively, there was a huge loss in human life and property.
Many Codes of Practice have chosen a return period of 50 years because the
average lifetime of most buildings and structures is around this value of 50 years.
This could well have been true at one time. However, there are other factors
which, in modern times, need to be considered:
(1) The use of better and stronger materials has increased the lifetime of the
buildings and structures, drains or bridge culverts. This implies a longer life
span, along the rows of Table 10.3.
(2) No owner is ready or willing to pull down his building after 50 years, if it is still
serviceable or not dangerous. Built in 1889 for the Universal Paris Exhibition,
the Eiffel Tower was to be dismantled just after. Today, after more than
200 years, with regular maintenance, it is still standing and visited by millions
of people annually.
(3) Who has seen or heard of a drain being demolished to be enlarged, except when
it has been proved to be really inadequate. Even if a local authority tried to do so,
it is very likely that adjoining structures would prevent this.
(4) Started in 1160, the construction of Notre Dame de Paris finished a century later.
There are many other such structures around the world, built not much later,
which are still standing today. Would any present day designer still consider a
50 years lifetime for such monuments?
(5) The cost of dismantling, pulling down, or demolition has become so high that the
owner (private or government) will be reluctant to really do so, unless it concerns
public safety.
The lesson to draw out from these examples is probably simple: structures have a
life span much longer than 50 years, and due consideration must be given to their
design and required resilience.
10.2 Climate Change Variables
289