Normal depth option for HEC-RAS

Chris:

We submitted our floodplain study done with RMS on a 3-mile portion of the Rio Grande River to the International Boundary Water Commission for validation. They requested the HEC-RAS files, which I exported and sent. The reviewer noted that the downstream profile boundary condition (assumed subcritical) was "critical depth" and suggested that "normal depth" would be more accurate. However, RMS does not allow normal depth as a downstream profile condition.

How much difference do you expect between normal depth and critical depth boundary conditions? Why does RMS not allow the normal depth option, when HEC-RAS does so? I did not anticipate a significant impact on the calculated floodplain depth at our area of concern, which is the floodplain level at a constructed wetlands at Station 78 (7,800 feet upstream of the downstream beginning point). However, the reviewer ran the floodplain calculation in HEC-RAS, and calculated a 0.4 feet increase in floodplain depth when normal depth was used as the downstream boundary condition instead of critical depth.

Does this seem reasonable to you?

Thank you,

Merle Miller
Cape Environmental Management, Inc.
12037 Starcrest
San Antonio, TX 78247
(210) 377-2008

Dear Merle :

Note that RMS and RiverCAD do support the HEC-RAS Normal Depth Option. However, we call it the "Slope Area" Method. This is how it was called in HEC-2, and essentially is computed the same way. So, select "Slope Area" in the Profile Discharge dialog box, and define the energy gradeline slope. If you don't know what the energy gradeline slope is, you can click on the "Calculate" button and RMS will estimate the energy gradeline slope using the channel invert slope (a reasonable assumption).

A computed critical depth will be different than that of normal depth. A critical depth option should only be used where the downstream boundary condition actually passes through critical depth (such as spillway).

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com