Written by Chris Goodell | Kleinschmidt Associates
Copyright © The RAS Solution. All rights reserved.
I’ve mentioned this before, but I am thoroughly impressed
with the robustness of the finite volume solution scheme used in 2D areas in
HEC-RAS. As long as your Courant Numbers
are in a good spot, you rarely get errors and instabilities in 2D areas. HOWEVER, the transition from 1D to 2D and
back is another story. In fact, the
majority of errors I get in 1D/2D models occur there (either at the cross
section to 2D area interface for inline connections, or in the cells adjacent
to the lateral structure for offline connections). In this post, I’m going to talk about the
errors next to lateral structures at 1D/2D boundaries.
Take this for example.
The model runs well with friendly blue bars everywhere. But I just can’t let go of the 5.154-ft
error. It only happens once and
obviously doesn’t cause the model to crash.
But it’s there, and a 5-ft error is a little more than I can stomach.
My usual go-to output for cross section errors, the water
surface profile plot, gives me no clues as to what is wrong. In RAS Mapper, I have what appears to be a
flooded situation around the time where the error occurs, but reasonable
looking results. I turned on the Update
per Screen option in the Velocity Layer Properties window, so that I could
optimize the velocity scale for this view.
Here I noticed that the maximum velocity is 11.5 feet per second (fps)
(which is about 3.5 meters per second).
This is way too high for this river, and in fact while hovering around
the centerline of the river, I get maximum velocities of around 2.8 to 2.9
fps. But since the Update per Screen
option is turned on for velocity here, that indicates that there is a hotspot
velocity of 11.5 fps somewhere in this view.
I just have to find it.
On closer inspection, I can see some lighter shades of blue
and green (i.e. higher velocities) in the floodplain adjacent to the lateral
structures (as highlighted in the white circles).
Knowing that cells adjacent to lateral
structures are typically going to be the culprits in 1D/2D models, I zoomed in to
the cells around the lateral structures to get a closer look.
I started with the circled area to the right,
since that was closest to the cross section that generated the numerical error
of 5.154 ft.
While zoomed in, it was hard to see at first, with the
terrain turned on, so I turned off the terrain and noticed this little sliver
of high velocity right at the boundary of the 1D river and the 2D area.
Turning the velocity off and the terrain back on, you can
see how the 2D area dips into the main channel just slightly, and in fact it
overlaps the lateral structure as well.
This reveals a very typical problem that you can run into with 1D/2D
offline connections with lateral structures.
For a given timestep, RAS will compute a volume of water going over (or
through in the case of gates and culverts) the lateral structure into a given
cell.
But if the receiving cell has a
small sliver of low lying area, like in this example, that relatively small amount
of volume could be enough to significantly raise the water surface in the
cell.
Perhaps enough so that it is even
higher than the water surface adjacent to it in the 1D reach.
This would then send water back the other
direction the next time step.
Besides the
numerical shock of a sudden and large rise in stage, the oscillating effect of
sending water back and forth can set up errors that persist, grow, and lead to
an instability.
Now if you’re using the weir equation on the lateral
structure, you could change your weir submergence decay exponent from the default
value of 1 to 3 (1 is the most accurate, 3 is the most stable).
This has a dampening effect on the oscillations
and errors you get from this situation.
Read more about the weir submergence decay exponent in the
HEC-RAS User's Manual on page 8-41.
You might also reduce the weir coefficient.
This will reduce the volume of water that
transfers from the river to the cell for a given timestep.
If there is no elevated feature represented
by the lateral structure (e.g. it is not a levee), you would want to use a very
low weir coefficient on the order of 0.1 to 0.5, as discussed in the
2D Modeling User's Manual on page 3-50.
However, in this case, it might be better to
just use the 2D equations over the lateral structure instead of the weir
equation.
Using the “Normal 2D Equation
Domain” (this is just a funny way of saying “Use 2D Equations”) is a relatively
new feature available in lateral structures.
But if your lateral structure does not represent an elevated terrain
feature (e.g. a weir or levee), then this might be the better option to
use.
However, in recognizing that the 2D area perimeter and the lateral
structure are poorly located, I will fix this first to see if that’s all that
is needed to solve the problem.
First, I’ll pull in the 2D area to just beyond the high
ground. This can easily be done in RAS
Mapper on the 2D Area Perimeter Layer while in edit mode. Next, I’ll pull over the lateral structure so
that it resides ON the high ground.
In the current version of HEC-RAS (as I type this post),
Version 5.0.7, you cannot edit lateral structures in RAS Mapper.
So you have to do it in the Geometry Editor
for now, using the Edit…Move Points/Objects command.
After moving the lateral structure, I
adjusted the location of the 2D perimeter again, so it is just inside the
lateral structure.
Here you can see a
much better placement of both the 2D perimeter and lateral structure.
And don’t forget, since I moved the lateral structure
placement, I have to re-extract the terrain onto it. Fortunately, HEC-RAS gives us a short cut to
do this with the Terrain Profile button.
After re-running, the error is gone and the results look
much better.
Notice the peak velocity in
the scale is back to a normal value of 2.6 fps. And the 5.154-ft error is gone!
