Press, politicians and 100 year floods

This post covers basic probability theory.


The media and the politicians seem to be completely confused by the concept of flood frequency, particularly in the abuse of the concept of "a hundred year flood".

The use of that term creats the misguided idea that you get such a flood every hundred years, and that having had one a few years ago, you aren't going to see another one for nearly a century.

This is shows a complete misunderstanding of statistics and probability. Which for the people being evacuated from their houses you can partly understand —you can't expect them all to have studied maths to A-level or remembered the details. What is wrong is that the press keeps using the same term, along with "20 year flood", misleading the people. And the politicians, they are equally a bunch of Oxbridge PPE-graduates who don't have a single cartesian coordinate between them —but should at least have those science advisors to explain the basics. FFS, there is the whole "Royal Society" which is meant to explain science to royalty, and, given we still live in a feudal state, the crown's ministers, Cameron included.


A "hundred year flood" really means a "1% chance per year flood". Assuming that the effects of the previous year's weather has no bearing on its successors, the probability of having a 1% flood the year after a 1% flood is, wait for it: 1%. The probability of having one in the five years after is, wait for it: five percent. And in 15 years, it's 15%. So the fact that York is currently underwater for the first time since 2000, means that the the two-flood-in-15-year-event, which had ~15% probability, has occurred. Which is not impossible, even for a "hundred year event". In fact, when you start counting since, say, 1995, you are looking at the probability of two 1% floods happening in a 20 year period —which is actually 20%: 1 in 5.

For the curious, assuming that the flood events are entirely independent, it'd follow a Poisson Distribution

Except, certainly within a single winter, we know the events are not independent —if the ground is saturated from previous rainfall, the rivers bloated from previous storms, then the probability of another storm triggering a flood is higher. If the land is already full of water, then it only takes a little bit more to tip things over the edge.


That "hundred year flood" really means, then:

The meteorologists' model of rainfall over a single winter, of the volume and frequency of rainfall, predicts the probability of flood of a specific volume occurring at 1%.


The probability of  a 1% flood re-occuring may follow a poission distribution —and hence the likelihood of multiple floods happening within a few decades is actually quite high.

Floods do appear to be happening more often than even a Poission distribution would apply, so what does that mean?

Some hypotheses spring to mind

1. The rainfall model is correct and we've simply had the misfortune to have a rare-but-not-impossible series of storms.
2. The rainfall model is correct, but the estimates of probability of storms within a season are wrong —that is, bad historical data created optimistic estimates.
3. Year-on-year flood events are not independent.
4. Changes in the terrain: farming differences, the building of houses on flood plains, etc, changed the runoff of the system, so amplifying the effect of rain
5. The rainfall model is in fact wrong due to failures such as the failure to consider the impact of global warming on the evaporation of water, the actions and position of the gulf stream, and/or the fact that with warmer air, it falls more as a a liquid ("rain"), than in a crystalline form ("snow" and "hail").
6. There was a more pessimistic (i.e. accurate) estimate of rainfall, but managerial or political pressure discounted it in favour of one which played down the risks, reducing the requirements and cost of flood defences, and obviated the need to press for changes in the agriculture system within the catchment area

Note also that these hypotheses are not exclusionary. The model could have failed to consider global warming, been based on bad historical data, and not planned ahead for the conversion of flood plains into suburban housing estates —then been downplayed by politicians who disagreed with the answers.. Which, when you think about it, is entirely possible.

That's why the term "hundred year flood" is so bogus. More accurate is "a 1% flood based on a broken or pre-global warming model with incomplete data without considering urban sprawl, and probably downplayed for political reasons". Using the term "100 year flood" does nothing but create unrealistic expectations that the floods aren't going to re-occur, year-on-year.


Someone in the press could look at the model, the data, the politics and determine what's actually happened, then try and explain it in a way which doesn't use terms like "hundred year flood". Because the science is there, the maths is there —and someone needs to hold the politicians and the scientists to account.

[These photos are all from Jan 4, 2014, showing the Avon fairly close to breaking its banks. Avon Crescent was actually underwater in winter 1990

Bristol is not currently under water

As of 11:44, Bristol is not under water. It may well be at 20:45 tonight, or 09:20 Saturday morning

On Avon Crescent, the council are putting up a barrier with sandbags

 Somewhere we have some photos to scan of this road being under water in 1990. Hint, avoid living in places with names like "avon", "harbour", "river" or "mead" -as every so often the water comes back to remind you where the name came from.

The avon is full, though it is draining fast

 And it makes a good change from the normal mud-flat view of the area.

Presumably the A4 portway and Avon gorge cycle paths are places to avoid tonight.

If you do want a view of any floods tonight, although the Pump House is more photogenic, the Novia Scotia pub is a better place to drink beer. They often serve pints of Heavy on draught, to get the Auld Scotia feel.

A4 flood chaos

Here is a short video of what the portway looks like when it's underwater. As you can see, it merited live broadcast coverage from heart.fm, and blocking the bike path for such coverage. It is not often that outbound traffic is forced to swerve slightly to stay dry.


Further in to the city, signs of problems

This is one of those situations you don't want when you are doing work in your own bathroom: everything filling up with water. It's generally considered the kind of process failure where you start wondering whether or not the builders have liability insurance and who you know to call to fix things.

Give it some heart!

As we said yesterday, it was unfair that bicycles could carry along the flooded A4 Portway at full "race speed" while cars had to slow down from 40 mph -with even that speed limit a recent crime against commuters coming in from Wales, Avonmouth and Portishead.

Yet again, an anti-car council fails to recognise the A4 portway is a premier commute route into the city.

We do think that bicycles should slow down, not just because there may be risk to them, but to share the suffering, we, the motorist, must endure.

For this reason we are grateful for the heart.fm car, RY07DMV, cunningly placed in the middle of the bike path "reporting on a live news story". The car in front belongs to Bristol Council and they parked to one side of the path and behind some signs, showing unnecessary sympathy to people who do not pay their fair share of tax in the city.

By parking widely, RY07DMV manages to remove visibility for inbound and outbound cyclists, increasing the risk of collision, and hence the chance of a live news story being bigger than "puddle on A4"

We say: Give it some heart!

Breaking news: we can confirm that a representative of the Bristol Traffic Project will be at the Cornubia Pub tonight, starting at 7:30 to 8 pm, to tell off the cyclists of the Bristol Cycling Campaign and drink some of the beers that the pub has to offer. Normally we would demand, indeed expect unquestioningly that our representative would drive there before downing 18 pints of Hidden Quest Beer and heading home, but apparently the DVLA have just revoked our representative's license for reasons they refuse to discuss, and they will be forced to walk. We understand this is not through choice, and forgive them, as long as they do not get in the way of any cars trying to get over zebra crossings. We believe that this is more evidence that the DVLA is anti-motorist. In the US, even when you have a license revoked for Driving While Intoxicated you are still allowed to drive to and from school and work, yet in the UK once a license is revoked you are not even permitted to pop out for a few pints of beer.

A4 Portway Flood Chaos

Showing that we are the timeliest news outlet in the city, here is the breaking news warning everyone of a large puddle on the A4 portway, just where it goes under the suspension bridge.

Large vehicles are advised to go through at speed, as it is more fun, but to check their brakes afterwards.

Why the puddle? The River Avon is at full height, and the drains off the road are back filling. The bike path is clear but the road is suffering. Again, evidence of an anti-car council at work.

Only the swans and ducks are happy

Further in to the city, the harbour works are in a spot of bother.

There is a risk that North Bristol will become isolated from the Novia Scotia pub.

Wilderness near UWE

We've celebrated this before: a bit of the wilderness near UWE. A river crossing. Something that forces pedestrians to turn back, and ensures that the only people who cycle through are doing it on mountain bikes. When they get to their destination they can feel pleased with themselves, and justify the extra expenditure on suspension and disc brakes.

We car drivers like it because it makes commuting by bicycle more miserable.

Which is why we have some bad news. This feature is no more. This week the builders of the housing estate behind MOD Abbey Wood dug some ditches, and as a result, the water feature has drained away.

This is part of a local anti-car conspiracy. They should be turning this path into a road for more driving options between Lockleaze and UWE and its business park, just as they did for the Old Filton Road, but instead this: wasting money on tax-dodging cyclists. Worse than that, tax-dodging student cyclists!

The joy of autumn

Autumn is one of the South West's best times to be on a bicycle. The sun, the dappled leaves and the dry paths after the summer baking makes it very memorable.

Yes, some people may have memories of a nice September. But if you are a mountain biker, this month's rain lets you test out your skills every day, even on normally boring bike paths. Here is the Lockleaze to UWE path, which two days in a row have had a stream running from the new roads being built to the left of this picture, a stream going over the path at 4-5 inches depth, and then draining into the woods.

This makes it an excellent place to safely practise river crossing techniques