NOAA Atlas vs Reality 1

[DPAJR]

Hi,

Has anyone out there had any luck convincing NOAA that in certain cases their rainfall Atlas might be a bit too aggressive. We have a project that requires a 100 year storm. The value from the NOAA Atlas for the area in question says the 100 year 24 hour storm value is 3.2 inches. The 10 year 24 hour storm is 2.2 inches. I used extreme care in transferring lat and long to the NOAA maps so misreading the map is not the problem

The area is desert. Rain gage data starting May 1, 1958 shows that the largest storm in 51-1/2 years was 1.88 inches on Jan 21, 1984. It seems unlikely that their isopluvials are accurate if there has not been even one 10 year storm in the last 50.5 years.

Alternatively has anyone ever convinced FEMA that the NOAA Atlas is not always correct?

I am not holding my breath but I was just curious.

Thanks

 

[SMIAH]

Was the largest storm of 1.88 inches in 24 hours?

Statistical analysis on more than 50 years of data should be adequate for 100 years.



Do you have access to the yearly peak 24 rainfall? If yes, you could perform such an analysis (e.g. with Log-Pearson Type III Distribution) and then contact FEMA with the results?

 

[DPAJR]

Hi SMIAH,

That is a great suggestion. As usual you are always ready to help. I have read a lot of your answers to other people's questions too.

Yes I have the daily 24 hour rainfall peaks for every day since May 1, 1958 except for a few rare days when the gage did not get read for some reason.

Can you recommend a reference for the Log-Pearson Type III Distr?

Thanks,
dpajr

 

[DPAJR]

Hi SMIAH,

I found several tutorials for log-Pearson.

Thanks,
dpajr

 

[cvg]

before you get too carried away, the values you quote are not un-reasonable for the US desert southwest (which is where I assume you are located). I did a little search and only found two gages at Death Valley and Barstow that were listed with less rainfall for the 100-year, 24-hour storm than what you list. For Barstow, the 100-year, 6-hour storm was 1.9 inches which is what you observed in 1984. Perhaps 1984 was approximately equivalent to a 100-year, 6-hour storm...

Quite a few areas which I would consider to be among the dryest in the country including Blythe, Brawley, Coyote Wells, Needles and Thermal all are listed with higher 100-year rainfall depths than what you have.


Bear in mind that you are looking at a single rain gage in the desert where rain gages are generally few and far between. Typical storms in the desert are small, intense thunderstorms which can very easily miss hitting your rain gage. NOAA uses all rain gages in the region, not just one.

Your 100-year rainfall could also occur in an hour, not 24-hours. You may want to use a 6-hour storm, not 24-hour.

You might try finding a similar gage nearby to see if you can validate those numbers. Or another option is to use the regional regression equations for the area and see if that corresponds to the runoff you are calculating.

 

[beej67]

I'm with cvg. Those isopluvials are based on all the rain gauges in a region, not on one gauge.

To get to the meat of your question, though, why do you need to convince NOAA at all? Are they your regulatory agency? This seems like an argument to bring to whoever's doing your engineering review, not to the "keepers of the maps." They're not likely to want to change their maps just for you, but you may be justified in adjusting your design storm based on historical data if you document your assumptions, do everything right, and get local government approval for your design procedure.

As always, your goals are to design things that you think are safe, that you can defend in court, and that other folks will buy into.

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