I am looking at a Flood Risk Assessment that uses HEC-RAS to model the predicted levels of flooding on a piece of land. As shown in the attached image, the report lists a number of variables with abbreviated names and I would like to know briefly what each one means and which one of those values shows the predicted height of flood water.

My HEC-RAS data

I found this document: HEC-RAS Output Variables which gives the full names of the abbreviated variable names. But the actual significance of each variable still make little sense to me.

Thank you in advance for any help you can give me.

  • $\begingroup$ Does the program have a help menu / file - you should find info in there or an explanation of the algorithm / formulae used. $\endgroup$
    – Solar Mike
    Mar 18 '18 at 9:38
  • 1
    $\begingroup$ @SolarMike, thanks for your reply but I don't actually have the software myself, nor do I have any experience using it. I just have the printed report that it generated. $\endgroup$
    – Wes1324
    Mar 18 '18 at 9:54

HEC-RAS stands for Hydrologic Engineering Center - River Analysis System. It's an Army Corps of Engineers program for characterizing flow in river or other large open channel systems.

First of all, the output for your question is the channel data at one particular cross section in an open channel. In order from left to right:

Q = total channel flow in cubic meters per second

Min Ch El = elevation of the lowest point of the channel at that point (section)

W.S. Elev. = the elevation in feet of the water surface at that section

Crit W.S. = this is the elevation of the water surface at critical flow; higher than this and the flow is subcritical, lower than this and the flow is supercritical. Think of a slow meandering river (subcritical) and a rapid, shooting river (supercritical). Compare this elevation to the actual WS Elev to determine which type of flow it is.

E.G. Elev = The is the elevation of the Energy Grade Line, which is the sum of the actual water surface elevation and the additional head derived from the flow velocity. This number represents the elevation if the flow velocity were cleanly directed upwards at this point in the channel. It's important to determine factors of safety of nearby facilities if the flow should become obstructed at this point.

E.G. Slope = The is just the slope of the above Grade Line. It's related to the slope of the channel bottom and the velocity together.

Vel Chnl = The average flow velocity for the channel in meters per second. This is an average just to get a feel for the overall flow.

Flow Area = Looking at the cross section of the channel, the area of the flow.

Top Width = the width of the flow section measured at the top free surface.

Froude Number = This number characterizes the flow as subcritical or supercritical. See above for the meanings. If this number is greater than 1, the flow is supercritical, and less than 1 means subcritical. If it's very near or equal to one, the flow can be unstable, varying (surging) back and forth between sub and super - critical. In this case it's well below 1 so the flow will be well behaved, subcritical, meaning essentially gentle flowing.

For a given channel, there will be several of these cross sections with outputs for each section. Put together, they form a picture of the overall flow characteristics for the entire length of the channel being evaluated. Good results come from good inputs, and it takes some experience to know where to take your sections for these programs to yield good results.


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