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I was trying to understand the difference between Drag coefficient Cd and Manning's N. There are many sources available online which provide values of Manning's N for different land covers such as Barren land, Developed Land open spaces, Developed Land medium intensity, Barren land, Deciduous Forest etc.

My question is:

  1. Are values for Drag Coefficient Cd published somewhere for different NLCD land covers? Or Manning's N can be interchangeably used as Cd for these land covers?
  2. If answer to 1 is No, then given the value of Manning's N how can one calculate the value of Drag coefficient Cd?

Thanks in advance for your time.

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I think you are confusing terminologies.

The Manning N values are representations of surface roughness that is used in the Manning equation, as it is applied to fluid flow in an a open or a closed channel. Basically it's a representation of how rough the walls are of fluid enclosing structures such as: aqueducts, rivers, flood plains, tubes, pipes, tunnels or corridors in buildings (for ventilation purposes, where the fluid is air). Other equations, such as the Atkinson equation use differing values of resistance.

Such resistances indicate how the roughness of a channel affects the flow of fluid through the channel.

The coefficient of drag applies to objects moving in a fluid, such as airplanes, rockets, stones thrown and cars moving in air and submarines moving in water, when totally submerged. Others might say that the coefficient of drag is more about how a fluid moves around an object that is totally immersed in the fluid. It's a matter of perspective. This is totally different to surface roughness of a conveying structure.

The skin roughness and the shape of an object and the way it is orientated within a fluid determine its coefficient of drag.

Considering only shape, imagine a cone moving through a fluid, such air or water. If the cone is moving so that the pointy end pierces through the fluid the coefficient of drag will be different to the situation where the blunt circular end of the cone pierces the fluid. Similarly, the long side of brick will have a different coefficient of drag to the short end of the brick.

If you are trying to look at flow of water over the surface of land and the land surface is covered with boulders and/or shrubs, it is easier consider such objects as elements of the surface roughness, and to use an average roughness value for that section of land, rather than as individual objects for which you need to calculate various coefficients of drag.

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  • $\begingroup$ Thanks Fred for your excellent explanation. Your statement about how to look at drag coefficient for fluids instead of solid bodies totally makes sense. I had seen in the literature the friction slope term of 1-D St Venant's momentum equation expressed as Cd* Vel^2/h. With some math, one can express Cd as g*n^2*h**(-1/3), where n is the Manning's coefficient, h is height of water and g is acceleration due to gravity. I agree that n needs to be a weighted average across the area under consideration. Here is the reference- (dumas.ccsd.cnrs.fr/dumas-00597434/document). $\endgroup$ May 2, 2023 at 15:53

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