Minimum Flow requirements

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
It's common knowlege that HEC-RAS cannot go "dry" at any time during a simulation. However, very low depths can be problematic as well. Very low depths can lead to compounding errors that can cause instabilities, and eventually your model to crash. There are a couple of tricks to get around this problem. Pilot channels and added base flow. None of you inflow hydrographs should start with 0 inflow, unless there is sufficient backwater in you system to prevent the upstream cross section from going dry. The trick with adding base flow is to keep it as small as possible, while providing the necessary stability. Try 1% of the peak of the hydrograph. You definately do not want to add a significant amount of volume to your model before the flood wave arrives-that will limit the extent of hydrograph attenuation you'll get.

To add base flow to an inflow hydrograph, simply add a minimum flow value in the box at the lower left corner of the flow hydrograph editor. Then, for that hydrograph, RAS will use the base flow amount any time the hydrograph flow is less than the minimum flow value.

HEC-RAS Short Course-Basic and Sediment Transport!

Next month I will be teaming with Gary Wolff to teach an HEC-RAS short course through River Restoration Northwest. The course will be in Tacoma, Washington, March 23-27. Gary will be teaching Basic RAS the first 3 days, and I'll be teching Sediment Transport with RAS that last 2 days. If you're interested, don't wait too long, last year we completely booked up the same course. You can get more information and register at http://www.rrnw.org/index.html.

Wide Flat Cross Section Bottoms

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
A common problem with the import of cross sections from GIS that were developed with a terrain model generated from LIDAR is the cross section with a flat bottom. This is a result of LIDAR's inability to penetrate the water surface. As a result, you get a flat bottom that represents the water surface elevation in the channel at the time the LIDAR was taken.

This will cause problems right from the beginning of your simulation, particularly if you are running low flows (low depths), and your model will most likely crash right away. You can see the result of the lack of bathymetric data in the profile plot.

To fix this, you'll need to get some definition to the bottom of your cross section(s), either by approximating the channel bottom, or by field surveys.

n Values in Graphical XS Editor

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
Until the new version 4.0 came out, the most you could do with n-values in the Graphical Cross Section Editor, was to move around their respective breakpoints. Now you are able to directly change the n value in the Graphical XS Editor. You do this by simply clicking on the n value on the plot. You'll see it change to an input box that allows you to edit it. When done, just click away from the box and your new n value is entered. Make sure you advance to the next cross section so that the change is saved to memory.

Severe Energy jump at junctions for unsteady flow

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
A limitation in HEC-RAS unsteady flow is that at junctions, RAS will project the downstream water surface elevation to the 2 upstream cross sections across the junction. Then the energy grade at each upstream cross section is back calculated. The result, especially if your cross sections are spaced far away from the junction is a depth that is too low at the upper cross sections, resulting in a large velocity head, and a huge jump in energy. To balance the energy from that cross section to the next one upstream results in a large increase in water surface elevation. This is a common experience at junctions in unsteady flow modeling and will look similar to this in a profile view. A tell tale sign of this problem is if you see the cross section on the upstream side of the junction go supercritical.

The way to fix this is to move your cross sections as close to the junction as possible (or add cross sections closer). This will minimize the negative affect of the junction limitation in RAS. Also, adding more cross sections between the cross section just upstream of the junction, and the next one upstream, will minimize any residual jump in water surface elevation while trying to balance the energy.

Breach Progression-Linear or Sine Wave?

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

I get this question a lot. Do I use a linear or sine wave dam breach progression? The concept of the sine wave breach progression is that the breach will initially grow at a slow rate (when the breach just begins to form, not a lot of head on the breach), then it will speed up as velocities and shear stress picks up through the breach. Then it will slow down again at the end of the breach as the water level in the reservoir reduces to a small amount over the breach invert.

There's no real research out there that I know of that addresses this. The way I look at it is if my reservoir draws down with my breach (slow breach/small reservoir case), then the sine wave progression makes more sense to me. If my reservoir remains high as the breach is forming (fast breach/large reservoir case), then the linear progression makes more sense to me (actually a combination of the two makes the most sense to me in this case-this can be done by manually chaning the values in the table on the side to start out as a sine wave and then transform into linear). In all honesty, I usually incorporate the breach progression into my sensitivity analysis, and eventually go with the more conservative of the two.

How to best model a junction.

Written by Chris Goodell, P.E., D. WRE
Copyright © RASModel.com. 2009. All rights reserved.
Here is an example of a junction.
With junctions, generally it is important to get cross sections as close to the junction as possible-even more so with unsteady flow modeling (http://hecrasmodel.blogspot.com/2008/12/unsteady-flow-and-junctions.html).

With this example, I would certainly add more cross sections upstream of the junction like so…


What I’ll usually do is draw an approximate “plane of confluence” (the yellow line), and then butt the new cross sections up to that line, without crossing over it. This plane of confluence will generally follow high ground between the two reaches, but technically should define the boundary between flow lines of one reach and flow lines of the other reach. The new cross sections will be missing the high ground on its “confluence” side, but that’s okay, because the other cross section (in the opposite reach) will capture that.

You certainly do not want cross sections overlapping or extending into another reach’s flow path. Sometimes you’ll find that interpolated cross sections might overlap at times. That’s okay as long as it is just a geometric schematic issue, and that cross sections were not really surveyed overlapped.

Crazy Energy Grade Line

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
Sometimes you may find in your unsteady flow simulation a nice smooth water surface profile, but then you turn on the energy grade line and it looks terrible. For example, in this steep section, the energy grade line is obviously incorrect. More times than not, this can be fixed very easily by adjusting the htab parameters. Sometimes in HEC-RAS, the HTAB parameter "starting elevation" is set too high by default. If you have a very low water surface elevation (i.e., well below the first HTAB computation point as shown in the figure), then HEC-RAS has a difficult time computing the energy grade line. This can create errors that may lead to instabilities. By changing the starting elevation for HTAB development to the invert elevation of the respective channel (and maybe providing more resolution by adjusting the increments and number of points), you can smooth out the energy grade line.