Calculating wetted perimeter for pipes in Time of Concentration 5

[aerolz]

I need some help with calculating the wetted perimeter for pipes in my TC calc. I have pipes starting at 18" (upstream end) diameter, then 24" and 30" at the pond end.
I am somewhat confused by how to calculate for each pipe plus need to know if I average the combination of the three sizes for the TR55 worksheet. I would appreciate any help in a basic explanation or spreadsheet etc for a drainage dummy.
Thanks

 

[dicksewerrat]

Do you know the flows in each pipe? Because the flows give you the wetted perimeter.
1/2 full = 1/2 perimeter
you will need to look at the internal angle of the arc length of the dry perimeter. then convert to radians and there you go.

 

[TerryScan]

How did you size the pipes? Usually, one will have the velocity from the pipe sizing calculations. This is what we are really after here. Generally, the overall pipe slope does not vary greatly, and the velocity usually does not change much when the pipe size increases with increasing flow. I would normally take the average velocity in the pipes and multiply by the overall length to get the Tc. (significant slope changes etc. would have to be handled inidividually).

 

[bltseattle]

Hydrology is not a precise science; if the slopes are within a couple percent then I would use the median pipe diameter and average slope over the run. Assume something about the hydraulic radius- pipe full condition is the standard assumption made in all the drainage manuals that I use. Make sure you catch the major changes in pipe slopes. Usually small differences in diameter do not substantially affect the flow velocity. And ususally the shallow unconcentrated flow travel time component of the overall Tc greatly exceeds the pipe flow travel time, so focus on getting that part right more than the pipe flow.

 

[LHA]

Ditto, blt.

I have never had a reviewer reject an approximate pipe size and a typical slope for the channel flow calc, and just assume full flow, it won't change much. Unless the run is very long and flat, it will only contribute seconds anyway. Don't spend a lot of time on this, concentrate on calculating sheet flow accurately.

 

[aerolz]

Thanks to all that replied. I had made some assumptions related to using the median pipe diameter and average slope etc in doing my calculations, but I wanted some input from people who are more proficient in drainage.
I have someone checking my calcs now, to verify my design.
Again thanks.

 

[sarala]

I had some design calculation spred sheet, for the design of drainage network, with indian standards SP35, for calculating pipe sizes,at the same time checking the Max.possble dicharge, Minimum & Max. velocities aganist the slopes.


RAVI

 

[pmkPE]

The wetted perimeter is used to calculate the hydraulic radius of the pipe for various levels of fluid flow. R, the hydraulic radius = A of the fluid divided by the wetted perimeter, the contact "length" about the circumference.

The American Concrete Pipe Association published the Concrete Pipe Handbook which includes a chart of the Relative Velocity and Flow in Circular Pipe for any Depth of Flow. The chart shows the relationship between the flow, the area of flow, the hydraulic radius and the resultant velocity based on the depth of flow versus the proportion of value for full flow.