Could someone verify this cal regarding Manning's equation ?

[halo1]

There is a concrete-lined pipe that has a very small slope around 0.006 and at the end of the pipe there is a wall extremely close to the exit of the pipe. I want to design the wall so that it can withstand the force of water flowing out of a 42'' pipe. My method used to find the force is as follows:
1. Given: n=0.013, S=0.006, Velocity=24fps, diameter=42'' concrete-lined pipe.
2. Use Mannings equation(Q=Area*HydraulicHead^(2/3)*S^(1/2)) to find flow rate.
3. Then use the momentum equation F=mv where F=rho*Q*velocity

Could someone verify that this is the correct method to use when finding the force required by the wall to withstand the force caused by the flow of water ? I'm just a little stumped because im getting an extremely low pressure value of 2.6psi and it seems too small. Thanks.

 

[halo1]

also forgot to mention that for manning's equation there is a 1.486/n factor as well

 

[BRIS]

The method is partly corret. But I am not sure about your calculations.

For 42" (1.07m) dia and s= .006 with n=0.013 the velocity will be 8fps (2.5m/sec) not 24fps.


I assume that the the pipe is flowing full. In which case the slope used should be the hydraulic gradient not the pipe slope. That is the difference between the upstream and downstream water levels divided by the pipe length. you also need to deduct minor losses of about 1.5 v^2/2g.

The force is small.

Brian

 

[halo1]

thanks alot bri. The error was due to the wrong S. Should have been 0.06. Thanks for helping me see the error. =)