Hong Kong Black Rain flooding; Hong Kong Black Rain flooding;

A fiscal measure for flooding

Flood mapping around the world uses the concept of return periods to decide how big a flood event is, but many people believe that this method is confusing and is actually outdated.

I think we should move to a fiscal measure for flooding, purely based on predicted flood damages since this will create a system that everyone can understand and significantly reduce the analysis needed for flood protection.

Return period is a statistical method used by hydrology and other sciences to establish how often an event occurs in a long dataset. So if an event of a certain size happened ten times in a 100 year dataset, it would have a return period of one in ten years. This does not mean that it will happen every ten years, just that a similar event ‘happened’ ten times in the last 100 year dataset we examined. Some of these events may have happened in the same year and then not again for 30 years.

So it is not really a generic measure of how often it happens, it is just a measure of how many times it happened in the dataset we are looking at. So whilst it is known as a measure of frequency, it does not tell us how frequently it will happen unless you refer to the original dataset.

Another confusing part of return period is that it is used to decide how big an event is, when in fact is it is a measure of how ‘frequent’ an event of this size or larger could be expected to happen (in a long dataset). I also used the term event, which itself is a subjective term as we can have ‘events’ within an event and if you change the start and finish time of an event, you change its return period.

It is difficult to find another instance of this type of measure being used, which is good news as it would appear to be similar to a doctor diagnosing a wound simply by the amount of pain the patient felt (ignoring the term fatal wound).

It is therefore not surprising that many people believe that the use of return periods is confusing. There have been other arbitrary measures around such as referring to previous actual events i.e. the 2007 flood in Hull, but these are only of use in areas that have previously flooded and only of use in that locality. 

I would suggest that we probably need to move to a fiscal measure for flooding that could be purely based on predicted flood damages. It is a lot easier to justify spending £1 million on a 1m high flood defence when the area behind the defence would be subject to £10 million of damages with 1m of flooding. I know we use cost benefit analysis on flood defences already, but this is based on providing protection to a certain ‘return period’. We should drop the return period and just make the financial assessments on a sliding scale of flood height costs against defence height costs. 

So if it costs less to protect an area than it does to not protect it, society should protect it. However, some may think this solution is too simplistic and prefer to stick with trying to justify using the frequently misunderstood concept of return periods.