500 year flood data 2

[jwolfschlag]

I have a detention pond which I have sized for the 100 year flood + freeboard + water quality according to the city standards. The city has commented that they want to know how large the pond would have to be to hold the 500 year flood. I have no data on a 500 year flood, and haven't been able to find any. Does anyone know where I can find anything regarding a 500 year storm, or any storm greater than 100 year?

 

[rday]

Jen,

(I'm guessing it's you.)

I have never seen information published with that info. The few times I have seen it referenced in reports it was extrapolated.

I do have a couple of ideas though. You may try your state department of natural resources. Also, FEMA has a flood insurance program and produces maps that show 500-year floodplains. Here that info is based purely off stream flow, but your area might have rainfall data. There should be a local entity with that data, here it is the County Engineer's Office.

Rik D.

 

[jwolfschlag]

Rik! How are you? email me and let me know how you are doing (jwolfschlag@yahoo.com). You'll never believe that I'm married with a daughter now.
My project is in Denver, and there is no data from the City and County or Urban Drainage for anything greater than the 100 year storm (that I can find). And I've checked the firm maps and it's not in the 500 year floodplain. I don't know why the City is insisting on wanting 500 year flood conditions.

 

[Bayou]

The Western US Rainfall-Frequency Maps can be found at:

http://www.wrcc.sage.dri.edu/pcpnfreq.html

However there is not a listing for the 500-year.
How the 500-year is typically found is by plotting
the other frequency flows onto probability paper and
drawing a line to the 0.002 probability line.
Also if the stream is a studied stream, then the 500-year
flow should be listed in the city's FEMA FIS Volume 1.

 

[gibfrog]

I agree with Bayou. However, I would explain it a little differently, in a analytical rather than graphic mode. I would tabulate the rainfall versus Log(recurance interval), linearly regress the data to a straight line, check your correlation coefficient and extrapolate to the 500 year storm. It takes under 2 minutes with a TI-89, if you have your rainfall versus recurrance interval data.
I would report the pond size for the 500 year storm without any pond freeboard and with limited parking lot/local road flooding. Basically, I would toss all safety factors to the winds and concentrate on not flooding any habitable structures and not excessively flooding any arterial roadways.
Clifford H Laubstein
FL Certified PE #58662

 

[rday]

Jen,

I'm good, I'll e-mail you with more.

I would go with gibfrog's method. Excel will also do the math, in addition to producing a nice graph for the report.

Personally, I think designing for the 500-year event is overkill in most cases.

 

[dicksewerrat]

I think your City is just fishing for info. They probably have a project to mitigate some flooding and one of their elected officials has a hair where the sun don't shine. They will just tell the official the approx. factor of increase.

 

[rmw]

Wouldn't you have to go out in the mountains and read the indian petroglyphs (sp?) to get this data??

rmw

 

[RWF7437]

The National Flood Frequency program (available free from NRCS) will calculate the 500 year flood for both urban and rural watersheds.

Good luck

 

[cvg]

RMW - you're not that far off, geomorphologists do something very similar to determine historic flow patterns and flood limits for rivers and washes. Very handy when you have limited rainfall or gauge data

 

[treg]

You might try the state department of water resources, or the state enineer, or the division of dam safety. It might be stated as a fraction of the probable maximum percipitation (PMP).

 

[RWF7437]

My guess is that the County only wants to know the 500 year flood flow and water surface elevation so that they can make a reasonable guess as to what freeboard to require.

The simple extrapolation method suggested by others should be the easiest, and reasonably accurate, way to arrive at this.

Good luck

 

[enviroq]

Considering that the project is almost two years old now, I hope jwolfschlag has an answer by now!

 

[IRstuff]

Does it matter? That person hasn't logged on in over a year.

TTFN

 

[DwattedWabbit]

Of course, if it is this difficult to get non-existent information on 500-year flooding, whatever the engineer extrapolates maybe acceptable by the City, as long as he or she has something to show for the estimates. The City may only argue the estimates if the City engineer(s) has done some homework as well.