[Scott Miller]

Since you have ArcGIS available, it is good to know how to modify elevation grids. You can use the spatial analyst extension for this.

http://desktop.arcgis.com/en/arcmap/latest/extensions/spatial-analyst/enabling-the-spatial-analyst-extension.htm

In the spatial analyst toolbox is a tool called raster math. Without going much into how to use it, I’ll just point out a very useful formula.

[output] = con(isnull([input2]),[input1],[input2]

input1 would be the existing terrain
input2 is a modification you want to make to the terrain

The formula checks where there is no data in input2. It converts the null cells to input1 values, and puts input2 values where there is data in input2. You can create elevation values for input2 either as polygon or polyline vector data with an elevation value field. Convert the vector data to raster data, and just be sure to set the environment to have the same extent and cell size as the terrain raster.

I use this method to cut channels in elevation data to drain closed depressions. When I have recovered enough live storage this way, then I use the hydrology>fill tool in spatial analyst. Fill makes a prettier graphic for management.

This full technique is not needed for patching the terrain in HEC-RAS. You can create a polygon patch with an elevation field, convert that patch to a raster, and export it as a TIFF. RAS Mapper allows you to layer this patch on the top when you generate a new terrain. Use your original terrain source, and lay a small rectangle with lower elevation values across the embankment.

Sorry for the long post.

[Scott Miller]

It sounds like the connection that is being breached has an embankment profile that follows the terrain. When the breach forms, the breach goes lower than the terrain. Is this correct?

It may be that the developers did not write a method to handle the exception, so the model runs. The breach below the terrain may be draining the model. Try running the time of the breach at a short output step, and see whether the water disappears instantaneously, or drains out.

In order to fix the problem, I expect it will work to generate the terrain with lower elevations at the vicinity of the breach. Keep the embankment profile, but make sure the terrain at the breach is no higher than the bottom of breach.

[Scott Miller]

I think you would have to approximate it. Instead of creating a single internal boundary condition along the mesh cells on the bank of a reach, divide the source among cells along the bank. The inflow hydrograph can be scaled. Say, if you use the same hydrograph in 100 cells, scale the inflow by 0.01. There would be 100 boundary conditions, rather than one.

It gets further from what you are trying to do, though. I don’t expect you can have different internal boundary conditions in two adjacent cells, so the source cells would have to be separated by cells that do not have boundary conditions. Also, I expect that the internal boundary condition cells cannot share faces with a lateral structure.

Try it though, I haven’t yet. I am more familiar with external boundary conditions, where I assign a slope of the topography into the mesh, and flow enters the mesh at the lowest part of the section.

[Scott Miller]

Are you asking whether it is possible to control the flow across a lateral structure?

[Scott Miller]

1. Use Alt+PrtScn, if they’re on your keyboard, to capture screenshots of individual dialog boxes.
2. Make sure your computer is up to par. This may help: http://hecrasmodel.blogspot.com/2016/08/optimizing-your-computer-for-fast-hec.html
3. Take a look at the maximum velocities and the average cell size to calculate a Courant number. Is it reasonable for the equation set you’re using (dynamic wave vs. full momentum)?

Use the full momentum equation set for dam breaks.

Consider using refinement regions to tailor the mesh. It is probably reasonable to have larger cell sizes farther away from the break. It’s probably reasonable also to have longer time steps as the flood wave attenuates. Control the time step with the Courant condition (version 5.0.5).

Try writing output much less frequently and see if that speeds the model up. An I/O bottleneck will slow things down, especially with 2 million mesh cells.

[Scott Miller]

Ven Te Chow, 1959. Use it as a starting point. Match the friction values in Chow’s table to the land cover types and channel characteristics. Scale the friction factors across land covers and channel characteristics to calibrate your model.

[Scott Miller]

It would work to limit your lateral structure lengths to 500 m, but they can be longer since you’re not likely to have a lateral structure completely orthogonal to the elevation grid. I would expect the limit to be 2^0.5 * 500 m, or about 707 m, going entirely diagonal on the grid. But it seems I’ve made lateral structures longer than that limit. Make a center line and take a look at the station/elevation table. If it’s more than 500 rows, shorten it. Or you can edit out lines, but that tends to be time consuming.

As for adding a couple of millimeters… the elevation profile for an embankment is point to point, jagged. The tailwater 2D flow area elevations are discrete, or stepped, with a single elevation for each mesh cell. It is likely that the jagged line will cut the corners of several steps. The embankment must be at least has high as the tailwater terrain elevation. The geometry preprocessor will indicate where the problems are. Adding a little clearance reduces the number of corrections needed on the profile. There may still be a couple of station/elevation rows to raise.

Use your professional judgement regarding how to incorporate cross section survey. It’ll be up to you to defend.

[Scott Miller]

Look into the Version 5.0.4 Supplemental User’s Manual, section 2.3.

Or just draw boundary conditions inside the mesh. The RAS will distinguish between internal and external boundary conditions. The key thing now with 5.0.4 & .5 is that external boundary conditions must be drawn outside the mesh.

[Scott Miller]

If the lateral structures serve only to make the 1D/2D connection, then the “weir” profile will follow the terrain, right? Get this from the terrain profile button and transfer it to the embankment station/elevation table. Add a couple millimeters. The station/elevation table will take up to 500 station/elevation pairs. Consider how many elevation grid cells your lateral structures will cross over their lengths.

Put the ends of the lateral structures on high ground, or the 2D flow area may leak between the ends of lateral structures (someone verify this?). Any bumps on the 35 km profiles may do. If there are reaches of high ground that won’t get flow, skip em. No need for a lateral structure where the water wont cross.

It may help to keep the lateral structures about the same length. If you need to iterate 1D/2D flow (for stablity), the minimum flow tolerance ought to apply +/- equally to all lateral structures. Or figure which reaches will have the most 1D/2D flow, and make those shorter.

Take a look at the manual, or this Chris Goodell post and comments, regarding weir coefficients:
http://hecrasmodel.blogspot.com/2016/07/weir-equations-in-hec-ras.html

[Scott Miller]

I used Geometry Overrides to replace all Land Cover Manning values with n=0.08, except for on the 2D channel, n=0.028. The model did not appear to be any faster with less nominal heterogeneity.

I then reverted to the model without Land Cover, and increased the default 2D flow area Manning value to n=0.08. I defined a Manning override region on the 2D channel, n=0.028. The model is running fast again.

The values are the same in both approaches. What would it be about the Land Cover that is slow?

[Scott Miller]

I reduced the maximum cell size by half to 16 feet, and am using the Manning values without increasing them 25%. The uniform time step is back down to 2 seconds for developing a hot start file. The 2D flow area is iterating about 2 to 4 times instead of 14 or so times.

Slow 2D convergence is occurring during low flow conditions. The model is so slow, it will go slower than real time during a storm. Is it usually the case that applying Land Cover to specify Manning values makes 2D convergence more tenuous? Could it have something to do with the number of land cover types or granularity of the Land Cover map layer?

[Scott Miller]

This is actually a new topic.

Here’s the way I read the headwater position in the manual:

Let’s say you have already built a 1D RAS model, and it extends across the floodplain. You can draw 2D flow areas on top of the 1D floodplains, without having to rebuild your 1D model. Use the “Next to bank station” headwater position for this situation and draw the 2D area to match your bank lines or bank points (the edge of the main channel).

If you are building a model that you know is going to be a 1D/2D model, you do not need to include the floodplains when you draw the 1D cross sections. In this case draw the 2D flow areas to match the ends of the cross sections, and use the “ overbank” headwater position.

The 1D reach is always the headwater in RAS. The 2D flow area or storage area is the tailwater.

[Scott Miller]

You will need to leave enough of a gap between lateral structures so that no 2D cell has more than one lateral structure connected to it. You can add or move calculation points to accomplish this, or choose the ends of the lateral structure centerlines accordingly. I believe you can leave gaps as long as any high ground that does not become inundated along the center line.

I found that my model appeared to be losing flow between lateral structures. (About 2% error in the 1D reach. I could see particle traces in the 2D flow area accelerate to lateral structure gaps.) So I have reconstructed my lateral structures to end at the highest points along the flood plane. I believe this is like what Chris Goodell meant by “Do not have cells adjacent to the lateral structure that reside completely on the slope of the levee/berm/whatever you’re modeling with the lateral structure.” I would construct a short berm on the elevation model if there were otherwise no high ground.

For a point of reference, in my model the minimum flow tolerance has ranged from about 0.25 to 1.5 m^3/s for a reach about 1 km long on a stream that gets perhaps 12 m^3/s during an extreme event. I believe the minimum flow tolerance applies to each lateral structure.

[Scott Miller]

Thank you. In the figure the lateral structure is using normal 2D calculations.

[Scott Miller]

Hi Anthony.

I’ve dealt with some of the same things as you and think I can help a little. Take a look at this blog post regarding your first two questions: http://hecrasmodel.blogspot.com/2016/07/weir-equations-in-hec-ras.html

Regarding your third question take a look at this thread: http://hec-ras-help.1091112.n5.nabble.com/1D-2D-Flow-Exchange-amp-Minimum-Flow-Tolerance-td5485.html

Jarvus’ response to my question was helpful. Take a look at the mass-balance error in the computation log file under options. The 1D/2D flow errors you’re getting might not be so bad if your model is stable.

The lateral weir you show in your post is almost 1200 meters long. A lot of flow can pass across that boundary in a single time step. I think you could increase your minimum flow tolerance and 1D/2D iterations. (Your minimum flow tolerance seems incredibly low.) In the blog post linked above Chris Goodell wrote in reply to Toby:

As far as advice to reduce 1D/2D flow errors over lateral structures: Break up long lateral structures into multiple smaller ones. Be careful not to use too high of a weir coefficient. Do not have cells adjacent to the lateral structure that reside completely on the slope of the levee/berm/whatever you’re modeling with the lateral structure.

As far as I can tell from the 1D velocities in your post, the 10-second time step ought to satisfy a Courant condition for the size of your cells. I’m assuming the velocities in the channel are the highest.

Scott

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