Normal depth vs Critical depth

[vicky2507]

Hi

I have the follwoing question

In which cases should I use normal depth as design parameter for gravity flow in pipes? I know that for culverts I have to make an analysis of the critical depth and determine the flow profile as critical or sub critical since the capacity of the pipe can not by calculated with manning equation.... When can I use the manning equation then?

 

[francesca]

You can use Manning's equation for open channel flow in culverts.

http://www.engineeringexcelspreadsheets.com/?p=535

 

[bimr]

The depth of water under conditions of steady, uniform flow is called the normal depth. From the energy equation, it can be shown that normal depth is achieved when S = friction slope. This relationship tells us that for uniform flow, the driving force due to gravity is balanced by the frictional resistance of the channel walls S. Mannings used this information to come up with the uniform flow formula that bears his name where n is the Manning roughness coefficient, frequently called Manning's n, V is in ft/sec, R is in feet, and S, which is technically the friction slope, is dimensionless.

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Defining_NormalDepth.htm

Critical flow is unstable because small changes in roughness or bed slope produce relatively big changes in depth. The water surface for flow at critical depth appears wavy due to these instabilities. It is poor engineering practice to design channels so that water flows at critical depth. The only time we want to intentionally force critical flow is either for flow measurement or flow control of some type.

In general, storm drainage systems shall be designed so as to maintain a minimum velocity of flow of 2.5 feet per second and a maximum velocity of 10 feet per second when flowing full. Check this requirement with your local agency having authority.

http://www.ctre.iastate.edu/pubs/stormwater/documents/2o-2openchannelflow.pdf