10.2.3 Frequency or Return Period to Be Considered
Can we rely on historical climatic data to carry out our design? Can we still use the
standards that are prescribed or advised by “outdated” codes or “state of the art”
practice?
A hazard (possible danger source) can be a human-induced process (building on
steep slopes) or naturally occurring event (floods, hurricanes) with the potential to
create loss. The exposure, to a hazard, of anything valuable to humans is considered
a risk – which is often taken as the combination of probability and loss. Thus, while a
hazard is well defined as a potential threat to humans and their welfare, the notion of
risk is probably debatable. On the one hand, a person takes a risk when he goes
fishing during rough weather (he may be drowned), while on the other hand, risk
(or consequence) is considered as the probability of occurrence of a specific hazard.
So, is risk the actual event of drowning, or is it the probability of drowning? In this
chapter, probabilities will be referred to as such.
In many sectors, the term return period T r (say, 50 year) for an event is often
considered. This does not mean that the given event (flood or other) will take place
every 50 years. Instead the return period means that over a long period (say
1000 years), this particular event will have occurred 20 times
(1000 years ¼ 20 Â 50 years). i.e. the average time between any two such events
is 50 years ¼ return period. The probability associated to this 50 year return period is
1/50 ¼ 0.02. (P ¼ 1/T r ). In fact, a probability of 0.02 for a hazard is a good rule of
thumb, but in some cases, the designer might decide that a 100 year return period
might be better. For example, a bridge (Bindra 1975) is a structure that is expected to
be in operation during a very long period. During the design of such structures, it is
natural to consider events (floods) with a return period of 50 years or even more.
There are so many world bridges that have been in existence for more than 100 years,
forget the 50 years considered during the design. Every structure is designed for a
certain design life (after which, it is assumed – usually wrongly – that it will be
replaced), during which, it is necessary to ensure that the structure is useable without
any danger to life and property.
10.2.4 Is a Return Period of 50 Years Acceptable?
The risk that such a structure fails during its design life has been explained by Mays
(2004) as follows:
Using historical data, it is possible to determine return periods (or probabilities) of
events (rainfall intensities, flood flows).
If an event occurs with a probability P, then
the probability that the event does not occur
= 1ÀP
and the probability (of no occurrence) in two successive years = (1ÀP)(1ÀP).
and the probability (of no occurrence) in three successive years = (1ÀP)(1ÀP)(1ÀP)
10.2 Climate Change Variables
287
Can we rely on historical climatic data to carry out our design? Can we still use the
standards that are prescribed or advised by “outdated” codes or “state of the art”
practice?
A hazard (possible danger source) can be a human-induced process (building on
steep slopes) or naturally occurring event (floods, hurricanes) with the potential to
create loss. The exposure, to a hazard, of anything valuable to humans is considered
a risk – which is often taken as the combination of probability and loss. Thus, while a
hazard is well defined as a potential threat to humans and their welfare, the notion of
risk is probably debatable. On the one hand, a person takes a risk when he goes
fishing during rough weather (he may be drowned), while on the other hand, risk
(or consequence) is considered as the probability of occurrence of a specific hazard.
So, is risk the actual event of drowning, or is it the probability of drowning? In this
chapter, probabilities will be referred to as such.
In many sectors, the term return period T r (say, 50 year) for an event is often
considered. This does not mean that the given event (flood or other) will take place
every 50 years. Instead the return period means that over a long period (say
1000 years), this particular event will have occurred 20 times
(1000 years ¼ 20 Â 50 years). i.e. the average time between any two such events
is 50 years ¼ return period. The probability associated to this 50 year return period is
1/50 ¼ 0.02. (P ¼ 1/T r ). In fact, a probability of 0.02 for a hazard is a good rule of
thumb, but in some cases, the designer might decide that a 100 year return period
might be better. For example, a bridge (Bindra 1975) is a structure that is expected to
be in operation during a very long period. During the design of such structures, it is
natural to consider events (floods) with a return period of 50 years or even more.
There are so many world bridges that have been in existence for more than 100 years,
forget the 50 years considered during the design. Every structure is designed for a
certain design life (after which, it is assumed – usually wrongly – that it will be
replaced), during which, it is necessary to ensure that the structure is useable without
any danger to life and property.
10.2.4 Is a Return Period of 50 Years Acceptable?
The risk that such a structure fails during its design life has been explained by Mays
(2004) as follows:
Using historical data, it is possible to determine return periods (or probabilities) of
events (rainfall intensities, flood flows).
If an event occurs with a probability P, then
the probability that the event does not occur
= 1ÀP
and the probability (of no occurrence) in two successive years = (1ÀP)(1ÀP).
and the probability (of no occurrence) in three successive years = (1ÀP)(1ÀP)(1ÀP)
10.2 Climate Change Variables
287