The use of Manning's Equation in gravity flow system
The use of Manning's Equation in gravity flow system
(OP)
Hello,
I am trying to determine if the Mannings's Equation in a gravity flow system takes into account various losses due to fittings (i.e., elbows, tees, diffusser header, etc.).
If not, would the applicable formula be the Hazen-Williams formula that takes into account these losses fittings? If so, how do you account for flow through the fittings when the pipe is not flowing full?
The system is gravity flow of water that will will be distributed via a header over the top of filter cells. I am concerned that the Manning's equation does not take into account the losses due to the fittings in the header and associated piping. Being gravity flow the pipes will not be at full flow. Any help with this would be appreciated.
Thanks in advance!
I am trying to determine if the Mannings's Equation in a gravity flow system takes into account various losses due to fittings (i.e., elbows, tees, diffusser header, etc.).
If not, would the applicable formula be the Hazen-Williams formula that takes into account these losses fittings? If so, how do you account for flow through the fittings when the pipe is not flowing full?
The system is gravity flow of water that will will be distributed via a header over the top of filter cells. I am concerned that the Manning's equation does not take into account the losses due to the fittings in the header and associated piping. Being gravity flow the pipes will not be at full flow. Any help with this would be appreciated.
Thanks in advance!
aumechengr





RE: The use of Manning's Equation in gravity flow system
Good luck
RE: The use of Manning's Equation in gravity flow system
Because the pipe (channel) is sloped, the additional head (hydraulic jump) created in bends, valves, etc. will not affect the hydraulic grade line for a long distance. You will have to calculate the hydraulic jump if you want the suface profile along the pipe.
Hazen Williams only works on full pipes and uses the pipe length (which is where the fittings come in), friction coefficient and the waterlevels on either side of the pipe. It does not care if the pipe is sloped or not. To include the fittings you must get a table to obtain an equivalent length of pipe for the fitting. Valves typically have a chart giving the headloss at various flows. You must combine this with the pipe headlosses for your calculation or backtrack into the equation for an equivalent length. Do not ignore these losses in your full pipe calculations. Not always will it; but, it can bite you big time if you do.
Now just because the water is flowing by gravity does not mean the pipes will not flow full. You will have to calculate their capacity and see what the actual flow depth is.
RE: The use of Manning's Equation in gravity flow system
Thanks for the information that you supplied. I did a little more research and came to the same conclusions that you both mentioned. Again, thanks for the help.
aumechengr
RE: The use of Manning's Equation in gravity flow system
Clifford H Laubstein
FL PE 58662