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Help with rainfall term

Help with rainfall term

Help with rainfall term

(OP)
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?

RE: Help with rainfall term

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?

RE: Help with rainfall term

(OP)
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.

RE: Help with rainfall term

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."

RE: Help with rainfall term

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

RE: Help with rainfall term

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."

RE: Help with rainfall term

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."

RE: Help with rainfall term

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."

RE: Help with rainfall term

(OP)
Thanks so much 1503, very helpful.

RE: Help with rainfall term

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).

RE: Help with rainfall term

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

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