How do you determine flood velocity?
How do you determine flood velocity?
(OP)
I have a structure that is in a floodplain in CT. I am able to determine the 1% flood level, but I am at a loss as to how to obtain the water velocity for design. I would appreciate any advice.






RE: How do you determine flood velocity?
Never, but never question engineer's judgement
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
if you know Q and A, then you can calculate V
this is basic hydraulics, see chapter 3 in Brater and King - "Handbook of Hydraulics"
RE: How do you determine flood velocity?
Never, but never question engineer's judgement
RE: How do you determine flood velocity?
V=ds/(1 second) to
V=Square Root(g*ds)
Where ds is the stillwater depth and g is acceleration due to gravity.
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
If you know the structure is in a floodplain that is probably because it is shown on a Flood Insurance Rate Map ( FIRM ). That map is based on a Flood Study (FIS).
Get a copy of the FIS and the FIRM and plot the location of your structure on the map.
The FIRM, the FIS and all the data upon which they are based are public documents in the U.S. They should be available, at nominal cost, for viewing, copying or other purposes at your City Hall, County Offices, City Library and from the Federal Emergency Management Agency (FEMA).
You can review the FIS yourself and probably understand most of it, including the jargon contained in the remainder of this reply to your question. You don't need an expert to do this but you may want a colleague, or a government employee, familiar with Flood Studies to help you with it.
The FIS will tell you what flow was used to calculate the Base Flood Elevation (BFE). It may also tell you what the AVERAGE velocities are in your area of interest .
Treat these numbers as you would earthquake loads or wind loads. THEY ARE STATEMENTS OF PROBABILITY ONLY !! BFEs for example are only considered accurate to +/- 1 foot elevation.
Velocities are similarly approximate. They will be higher near the center of the stream and will be lower at the edges of the flooded area, in general.
If you, or your colleague, are familiar with the program HEC-RAS you can use it to calculate velocities anywhere in the stream and to calculate drag and other forces caused by the flowing water. Unless your structure is very large or expensive or otherwise "important" this detail will probably not be necessary to complete your structural design. If it is "important", like a Hospital or National Guard Armory we can only ask why anyone would put it in harm's way ?
If your structure is to be in a "floodway" you have many more problems to solve; including, explaining why it must be in such a place.
Use a healthy factor of safety (read, ignorance). Remember too that flooded streams often carry large, potentially destructive, debris with them. The so called 100 year flood is NOT the largest possible flood.
Good luck
RE: How do you determine flood velocity?
RE: How do you determine flood velocity?
There maybe requirements in the CT code on flood design. There is in the MA code for dwellings in flood areas.
RE: How do you determine flood velocity?
I have the FIRM, but I dont have the FIS so I will look for that.
The structure is not a building, so nothing in the building code helps.
Once I determine the velocity I can then determine the loads to ASCE7 and check that they are not more critical than the wind loads.
Many thanks
Craig Dolby
RE: How do you determine flood velocity?
Careful. It seems easily possible that high wind loads AND high water levels or velocities could occur simultaneously. So you want to consider the combination as a possible governing case.
good luck