How to calculate the Hydraulic Radius

[Van PD Tri]

Hi to all,

Can anybody let me know how HEC-RAS calculate the hydraulic radius?

It is clear that the calculated Hydraulic Radius (R) (from HEC-RAS) plus the lowest point (LP) of a cross-section does not equal to the water surface elevation. In addition, the difference between the the water surface elevation and the combination of R+LP change according to the hydraulic roughness applied at a cross-section.

Could any body explain more how HEC-RAS do the calculation?

Many thanks and happy new year.

Best regards,

Tri Van

[Van PD Tri]

Hi,

I have a check by myself and know that R = A/P. It is fine.

But the next question is how HEC-RAS calculate the water surface elevation.

Best regards,

Tri Van

[MattZ]

Please refer to Chapter 2 of the Hydraulic Reference Manual for the equations and calculation process.

A quick summary of the WSEL calculation for a steady flow simulation is this:

1) Start with a known water surface elevation at a cross section. This WSEL is from your boundary condition.

2) Assume a WSEL at the next upstream cross section (assuming a subcritical analysis).

3) Calculate conveyance and the velocity head. The conveyance calculation takes into account the wetted perimeter that you originally asked about.

4) With the conveyance calculation, determine the average friction slope.

5) With the results of the two previous steps, calculate the energy head loss between the two cross sections (equation 2-2 in the manual).

6) Then solve for the WSEL using the energy equation (equation 2-1 in the manual).

7) Compare the calculated WSEL with the assumed WSEL and if necessary repeat the calculation until the assumed and calculated WSEL match within your specified tolerance.