How do you determine flood velocity? 4

[csd72]

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.

 

[COEngineeer]

flow / cross section area?

Never, but never question engineer's judgement

 

[csd72]

If only I knew the flow.

 

[JStephen]

Seems that whoever calculated the floodplane elevation would also have come up with that information at some point. Check with city, county, see if anyone's got any studies done in determining flood planes?

 

[cvg]

you need expert advice from a drainage engineer / hydrologist. From your post, it is not possible to come up with an answer. It is even possible that you have ponding with no velocity at all. What are you designing that requires a velocity? If a floodwall or something similar, you will again need a drainage engineer to assist with this as there will be issues with erosion, hydraulic performance, permits and submittals to FEMA etc. You might consider posting this with more information on the storm/flood engineering forum.

 

[civilperson]

Why not use Manning's equation for a estimation of velocity?

 

[cvg]

without knowing the flow rate, channel slope and roughness - Manning's equation is useless. Besides, if you know the flood elevation and the cross sectional area, you can calculate mean velocity just by using the continuity equation, Manning's is not needed.

 

[civilperson]

Channel slope can be an average between two locations, Chow shows various side slope values for differing vegetation condition giving Manning's N. "if you know the flood elevation and the cross sectional area, you can calculate mean velocity..." something lacking in that assumption.

 

[cvg]

civilperson - there is nothing lacking in the "Continuity" equation: Q=VA

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"

 

[COEngineeer]

Cant you just be very conservative and say Potential energy = Kinetic Energy, hence V = sqrt (2.g.h) ? I cant remember my fluid class

Never, but never question engineer's judgement

 

[JedClampett]

The commentary to ASCE 7-05 gives a range between two values:
V=ds/(1 second) to
V=Square Root(g*ds)
Where ds is the stillwater depth and g is acceleration due to gravity.

 

[cvg]

sorry, that just doesn't work...

 

[RWF7437]

"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."

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

 

[Robertmet]

Many of the floods we have seen recently are far greater than ever seen before or even imagined ! Some areas still permit building within the flood plain which is crazy..My first flood was in 1955 and still have vivid memories. One mile of a road washed away along with a bridge. A surviving bridge had a 5' diameter rock on the roadway !!

 

[BSE05]

FEMA 55 is a 3 volume set on flood design. It has alot of info.

There maybe requirements in the CT code on flood design. There is in the MA code for dwellings in flood areas.

 

[csd72]

Thank you all for your advice, particularly RWF7437,

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

 

[RWF7437]

csd72,

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