Help with rainfall term

[atrizzy]

Hi all,

I have a client that is specifying for a design that we use a value of '24 hr mean annual rainfall.'
This is a value being used in conjunction with a catastrophic event, and is intended to be less intensive than a typical 'extreme' type of rainfall.

However, any dataset (IDF curves, etc) I look at requires that we define a return period. The client's design criteria documentation does not expand on the meaning of '24 hr mean annual rainfall.'

Any idea what that could mean?

 

[TigerGuy]

It's the average 24-hour rainfall total for whatever period storm is required. It sound like they are looking at a 1-year storm. What do the design requirements from the local authorities say?

 

[atrizzy]

I found a govt guideline for stormwater design indicating that MAR (mean annual rainfall) should correspond roughly to a storm of 2.33 yr return period.
This seems reasonable enough, and I think is a good enough reference to approach my client with. Unfortunately, this client does not require adherence to local authorities... otherwise, the criteria might be simpler.

 

[1503-44]

2.33 yr return periods sounds about right for a 24 hrs storm, however that is a wholely inadequate rainfall to determine anything more than a rain ticket printing order.

It is completely insufficient to avoid drain backup and serious ponding problems on flat roofs, for example, if that's the issue here. A 1h rain can equal 60% of the 24h total.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[atrizzy]

Thanks 1503, understood. This rain is meant to coincide with, and mildly exacerbate, a very rare and catastrophic event.

 

[1503-44]

For that you need to look at the return frequencies. That's your indicator for the degree of severity of a catastrophic events.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

Houston example latest NOAA data

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

Chart is from

https://hdsc.nws.noaa.gov/pfds

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[atrizzy]

Thanks so much 1503, very helpful.

 

[zelgar]

Depending on what the project is, you may want to be certain on the frequency of the storm event. As indicated by 1503-44, there are a lot of different frequency storm event (well, not really, but that is how it's typically how it is stated) for storm event. A lot of regulations for a catastrophic rain event will typically be the 25-year, 24-hr storm event. Now, this doesn't mean that the storm will only occur once every 25 years, it means that the probability of the storm event happening every year is 4%= 4/100 = 1/25.

Additionally, since this is in a hurricane prone area, there may be requirements to use data that is more stringent than normally required (e.g., using the 50 or 100 year storm vs. the 25 year storm).

 

[TerryScan]

Have you asked the client for clarification since this is not a typical design criteria?