how do you determine flood velocity

huangyhg

how do you determine flood velocity?
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.
flow / cross section area?
never, but never question engineer's judgement
if only i knew the flow.
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?
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.
why not use manning's equation for a estimation of velocity?
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.
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.
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"
cant you just be very conservative and say potential energy = kinetic energy, hence v = sqrt (2.g.h) ?  i cant re  
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.