Understanding of Hazel-Williams

[Iomcube]

Somebody please audit my understanding of Hazel-Williams eq. for gravity flow & pressure flow both have same form of eq. when it comes to finding velocity:
V = k C (D/4)^0.63 S^0.54

But for pressure S = hf / L

https://www.lmnoeng.com/hazenwilliams.php

For Gravity S is the slope of the pipe (unitless)

https://www.omnicalculator.com/physics/pipe-flow

 

[HTURKAK]

Essentially both sources use the same formula . D/4 is equal to hydraulic radius ( hydraulic radius R =The area /( wetted perimeter of the pipe) and Hydraulic radius R = D (diameter)/4.

Notice that , for gravity flow, Hazel-Williams eq. can be used for FULL PIPE FLOW . The item S ( head loss / L ) for pressure flow is equal to slope of the pipe for gravity flow..

 

[bimr]

If you are doing some basic calculations, the washington state calculators are probably more useful.

Link