Rainfall Frequency Atlas - Update, general trends?

[Spartan5]

Hello all,
I am primarily involved in wastewater treatment and collection system design, so this is more of an idle curiosity question for you folks in the hydrological end of things.

My understanding of stormwater projects is that the designs are based on "design storms" such as a 10 year, 24 hour storm and that the this data is found in the rainfall frequency atlas published in the 60's. That analysis is based on data collected in the first half of the 20th century.

One aspect of climate change is the increase in frequency of "extreme" precipitation events. Say what you want about climate change, but it certainly appears (with the multiple '500 year' floods we've had and other trends I've noticed) that we are having more extreme precipitation events.

Here in central lower Michigan, I've had several 50 and hundred year storms in the past couple of years. The most recent storm was 6 inches in two days. This area was hit hard by the remnants of Lowell and Ike and many areas have already shattered records for September rainfall and single day records going back over 100 years. The single day record (for any day) near my hometown in SW MI was bested by almost 50%. All told they wound up with more than 11 inches of rain in 3 days.

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=iwx&storyid=18108&source=0

There was extreme flooding, and many detention/retention basin were overtopped resulting in property damage.

So... is there an updated atlas that is taking into account these trends? Are there design methods and requirements that are taking into account that a 10 year storm isn't a ten year storm anymore?

 

[MiketheEngineer]

Not sure - but you can add 6'' in 5 hours in St. Louis last weekend.

Things flooded that have NEVER flooded in known history

The charts definitely need updating

 

[RWF7437]

I believe you'll find the latest data here:

http://www.weather.gov/oh/hdsc/

You can also get on their mailing list to get updates.

good luck

 

[RWF7437]

Also:

"100-Year Storms

One of the more confusing phrases used in meteorology and hydrology is "100-year storm". The phrase implies that an intense rainstorm dubbed as an "100-year" event brings rainfall totals heretofore unseen for 100 years, and not to be experienced again for another century. This is a logical, but incorrect conclusion to draw from the phrase. A "100-year storm" drops rainfall totals that had a one percent probability of occurring at that location that year. Encountering a "100-year storm" on one day does nothing to change your chances of seeing the same amount of precipitation the very next day.

Intense rainfalls are typically geographically isolated. Therefore, increased population density and improved precipitation monitoring networks have increased the likelihood of capturing (measuring) heavy rain events. Also, improved communication has allowed faster and more complete transfer of weather information. When the neighboring county is walloped by a "100-year storm", we hear about it quickly. Invariably we will vicariously "experience" the event and wonder why "100-year storms" seem to be occurring every other week!

A 24-hour "100-year storm" for most Minnesota communities is roughly six inches. "

In other words, a 100 year storm has nothing to do with 100 years. It can occur 10 years in a row; Global warming or not. Or, it may not occur for 1000 years.

The probability that a "100 year storm" will occur during the life of a 30 year mortgage is about 24%. That is, there is about a 1 in 4 chance such a storm will be equaled or exceeded in that time.

good luck

 

[cvg]

yes, there is updated data available for many areas on the NOAA precip frequency server. Also, a basic misunderstanding is that a 100-year storm causes a 100-year flood. This is not always true, especially in urbanized areas. Where population growth has resulted in development with the resulting increase in hard surfaces, the amount of runoff from storms has increased. Channelization of rivers has also resulted in those waterways having less capacity then they originally did when they had large, undisturbed floodplains.

 

[LCruiser]

In the southwest, there is a "100 year storm" almost every year. The thing is, it's very small. You can think of a "100 year storm" happening over 1/100 of an area every year, on average.

As for the statement: "One aspect of climate change is the increase in frequency of "extreme" precipitation events", that is only conjecture at this point. The energy behind storms in the mid-latitudes is mostly the difference in density along the jet stream. With "global warming" the Arctic is supposed to heat up more than the equator, so that energy is decreased.

Mostly the problem is more development, and more of it in flood plains. Look at how stupid people were to rebuild in Galveston, and they will again (and again...).

Chalk your observation up to anecdotal information.

 

[ACtrafficengr]

LCruiser and RWF7437,

A lot of people don't have a good grasp of probability. Toss a coin and get heads 10 times in a row, and some will say that the chances of getting tails is higher than 50-50. They don't understand that in random events, previous history doesn't affect the next toss.

Perhaps if 100-year storms were called 1% storms, they might better understand the concept of a 1% chance in any given year, as opposed to once every 100 years. 5-year storms would be 20% storms, etc.

That being said, I've heard that in my area, what were once 5 year storm intensities can now be expected every 4 years.




"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[civilperson]

Think of the inverse of having a storm event. For instance a 1% chance of a storm occurring is a 99% chance of not occurring. Thus in sixty eight years there is a 50% chance of the 100 year storm occurring.