# Ever wondered what angle rain falls during a once in 100 year storm?

A 1 in 100 year storm could be a typhoon, a cyclone, a hurricane, or a tornado.

These storms have very strong winds, so what happens if the rain angle is actually horizontal?

If the rain was horizontal it wouldn’t hit the ground would it?

So does the rain just keep going until it eventually evaporates?

But that can’t be right because we have floods.

And if these events  are really once in 100 year storms, how come we can have one every year?

(refer to the comments below for the real answer). However continuing on for the rest of us….

The only logical answer is that the rain must come down somewhere, so once in every 100 years the rain comes down on your building.

At an angle of 63.4 degrees (2:1). Because that’s whats written in the Plumbing Code.

So it sort of takes it in turns, every 100 years your building is the lucky one, the structure at the end of the rainbow.

But not to worry, because if we use the plumbing code, we design all box gutters for this event.

We design for a 1 in 100 year storm with the  rain coming down at an angle of 63.4 degrees (2:1)

1. Jonathan says:

63.4° from horizontal, or 63.4° from vertical?

2. Christopher Starr says:

1 in a hundred years is actually misleading. In environmental sciences, it is actually a statical chance of 1 percent in any given year that can have a magnitude of a 100 year flood, which means that it can occur more that once every 100 years. It has been documented that it can occur back to back years.

• Hi Chris,
Of course you are absolutely right.
Engineers realized that this term was misleading to the general public (and the lawyers, and the media), so they changed it to Average recurrence interval (ARI); But this was just as misleading to the afore mentioned group, so we now talk in terms of AEP, Average exceedance probability. expressed as a percentage.

So an AEP of 1% represents 1 in 100, so there is a 1% chance of a 1 in 100 year storm occurring every year.
Similarly an AEP of 2% (2 in 100, or 1 in 50) means there is a 2% chance of a 1 in 50 year storm occurring every year.

So a 1 in “n” year storm is only a statistical average to give us a ‘design’ intensity.
The particular “n” chosen is based on risk analysis and likely damage.
eg an eaves gutter overflowing is likely to cause less damage than a box gutter overflowing, so a lesser design storm frequency is used.
An eaves is designed for a 1 in 20 year storm. (AEP 5%), and a box gutter for 1 in 100 year storm (AEP 1%).
If there is no likelyhood of any damage, as in some surface drainage systems where any surcharging will run off to the road or somewhere, the Code allows a design frequency of down to 1 in 2 years (AEP 50%).

The confusion is, a 1 in ‘n’ year storm is only a way of determining a “design storm size” and has theoretically nothing to do with the likely occurrence.

Hence the change to AEP, which is an attempt to combine the two terms.

However thanks for pointing that out, and encouraging me to clarify the point.
My blog is… well… tongue in cheek … you might say, for those in the know. But written in a form that attempts to point out the stupidity, and confusion, of the terms in a fun way.

3. James Dunstan says:

I am just wondering where abouts in the plumbing & drainage code it states the 2:1 rainfall?

• Hi James,
AS/NZS 3500.3-2015 Fig 3.4.2(B) diagram (a) and (b)
Angle is 2 vertical to 1 horizontal

4. Anonymous says:

2:1 is 26.6 degrees.

• 2:1 is 2 vertical to 1 horizontal. Like a pipe grade of 1 :200 is 1 vertical to 200 horizontal.
In hydraulics the first term is always the vertical component.
so 2:1 is just an easier way to say 1:0.5.
This gives an angle of 63.43 to the Horizontal.
In other words, 26.6 degrees is the angle to the horizontal if the grade was 1:2
and 63.4 is the angle to the horizontal if the grade was 2:1.