Flood frequency/ likelihood
Flood frequency or likelihood are often described in terms of average
recurrence interval (ARI) and annual exceedance probability
(AEP). These two terms
are defined below in two ways-I have given the strictly correct, engineering
definition (1), and a less formal definition (2).
| Definitions of Average Recurrence Interval (ARI) |
Definitions of Annual Exceedance Probability (AEP) |
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Source for technically correct definitions:
(1) Australian rainfall and runoff: A guide to flood estimation, Institution of Engineers, Australia , 1987, p. 7;
(2) the author.
Annual exceedance probability is usually expressed in terms of '1 in a 100', '1 in 50' etc. or 1%, 2% chance of occurrence in any one year, etc., and many individual rivers have been described using a flood frequency curve, an example of which is shown in Figure 5.2.
The bottom (horizontal) axis of the graph shows the annual exceedance probability as a percentage, and the left hand (vertical) axis shows the logarithm of the peak flood discharge (in cumecs). The diagonal line is the flood frequency curve, which indicates the estimated or expected probability of a given discharge for this particular river. Taking one point on the graph-the probability of a peak discharge of 10 cumecs (log 10 = 1.0) is 50% or 0.5. If the peak discharge was 40 cumecs (log 40 = 1.6 approx) the probability would be approximately 0.1% or 1 in 1000. The flood frequency curve is based on data from a number of floods over a period of years. It is usual, however, for actual recorded flows or discharges to cluster above and below this line.
So the means for describing flood frequency includes an element of the description of intensity (in this case, discharge in cumecs.)
An example of a flood frequency curve
Source: Australian rainfall and runoff - A guide to flood estimation, The Institution of Engineers, Australia, 1987. p. 212.
Flood extent
The extent of a flood is best described using a map. Pages 144-148 of Disaster mitigation in Asia and the Pacific show some of the types of flood maps that may be used to describe the flood hazard for an entire country.
You are also likely to encounter flood maps for particular sections of rivers. These should ideally show a number of pieces of information, including:
- one or more historical maximum flood levels;
- the flood levels for a variety of annual exceedance probabilities, typically 5% (1/20), 2% (1/50) and 1% (1/100) annual exceedance probabilities;
- some of the roads and structures in the area, or at least an indication of land use;
- a list and table of maximum historical flood events;
- an indication of the flood levels that are considered to be minor, moderate and major floods;
- an approximation of flow rates at particular points in the river for given flood levels.
So the description of the extent of floods includes parts of the description of intensity and frequency, and ideally should display this information graphically and in the form of a map.
Flood time-frame
The aspect of time-frame in relation to flood refers to a number of things including:
- how much warning time there is (i.e. the period between the time the flood is or can be first detected or predicted, and the time the information is disseminated);
- how much lead time there is (the period between the time the warning is received and resources can be deployed);
- the time of year that floods are more likely to occur;
- the length of time over which the flood will be causing damage and hampering response efforts etc.
Flood manageability
Manageability in relation to floods concerns the degree to which something can be done about the floods, from the point of view of prevention, preparedness, response and recovery. This will vary enormously from river to river, and from area to area.
This description of flood shows that you can describe some hazards reasonably thoroughly. But not all hazards can be described with such reasonable accuracy and certainty.
One of the things to be aware of when the characteristics of intensity, frequency, extent, time-frame and manageability do not seem to fit your chosen hazard/s, or if they appear incomplete in this-you should not try to force the subject of your analysis into a model that does not suit it. If these characteristics do not suit your hazard/s, add to them or remove some as you see fit. Use whatever descriptors are the most appropriate for your hazards.