CMP has an average 'N' of .25 for the Manning's equation. Also the velocity at the pipe wall is zero and increases very nearly to the peak at the center by the square of the distance from the wall. Manning's will give you the average velocity. Draw a few diagrams and make an educated guess. her's an examplweof what i mean.
Manning's equation is 1.49/n times the hydraulic radiusto the 2/3 and the square root of the slope. If you have RCP with an 'n' of .13 and a measured or calc. value of 9 fps and put that water into a CMP with value'n'=.25 the water will slow down to ~4.5fps.  If your calculated velocity is near what the fish can swim through, then I would assume they would use the path closest to the pipe wall. If you are going to build a new 'fish ladder', I would make sure that the pipe is large enough to slow the water down to under 4 fps but over 2.5 fps.  

Thank you for the information. I will do as suggested and make a few diagrams.  I believe you meant a Manning's N of .025 for CMP and .013 for concrete pipe.  If you remember where you learned this I'd be glad to read more about it.

Russ

the assumption that velocity is zero at the boundary (pipe walls) is correct, however, the distribution is not linear.  The velocity increase very quickly as you move farther away from the boundary.  Note: the velocity normally calculated in mannings equation is the "average" velocity.  This takes the velocity distribution into account.  The actual velocity in the center of the pipe will be greater than this.  I would caution against assuming that there is a significant zone of slow moving water along the bottom of the pipe - sufficient for a fish to fit in.  Better to assume that the fish will have to negotiate through the pipe at the calculated average velocity.

Thank you,

But what IS that distribution ? Has it been measured or calculated by any one anywhere ? Still looking !

Yes, this has been done.  I have participated in this type of testing with high velocity flow through large pipes (72" steel waterline fittings). Unfortunately, I'm not sure if the work I performed was ever published.

Velocity distribution will vary with pipe size, type of pipe, flow rates, water temperature etc.  I'm not sure there is a single relationship that can be used.  The relationship will also change significantly if the pipe is not flowing full or is not circular.  However, as stated before, I believe that the velocity starts at zero at the boundary and increases very rapidly as you move away from the wall.  Unless you are looking at extremely small fish, I would assume that the zone of slow moving water is not large enough.  

If you wish to pursue, contact one of the research facilities around the country and maybe they can suggest a published paper which may give you an answer.  Try the following:
Colorado State University Engineering Research Center
University of Iowa Institute of Hydraulic Research
St. Anthony Falls Hydraulic Laboratory
Corps of Engineers Waterways Experiment Station

Or try the ASCE journal of hydraulics.
http://www.asce.org/
products and services ==> publications ==> journals

I found 3 papers published by ASCE which may be useful:

Flow Velocity Profiles in Gravel-Bed Rivers
by Vito Ferro and Giorgio Baiamonte
Journal of Hydraulic Engineering, Vol. 120, No. 1, January 1994, pp. 60-80

Momentum and Energy Coefficients Based on Power-Law Velocity Profile
by Cheng-lung Chen
Journal of Hydraulic Engineering, Vol. 118, No. 11, November 1992, pp. 1571-1584
Discussion: by B. A. Christensen, Member, ASCE, (Prof., Dept. of Civ. Engrg., Univ. of Florida, Gainesville, FL 32611) HY May 1994, pp. 666-669
Closure: HY May 1994, pg. 669  

Denil Fishways of Varying Geometry
by C. Katopodis, N. Rajaratnam, S. Wu, and D. Tovell  
Journal of Hydraulic Engineering, Vol. 123, No. 7, July 1997, pp. 624-631  

Thanks CVG,

You're leading me in the right direction.  I've also queried the Corrugated Steel Pipe Manufacturere's Association but so far, no reply.  Surely they know, if anyone does.

Russ Faust