Breakline and Cell Spacign Techniques at Roads/Linear Strutures in 2D

[InundationPete]

I am looking for input/suggestions regarding placement or appropriate techniques for using breaklines along roads (or levees, railroads, dikes) to obtain more accurate results throughout the top width of the structure in HEC-RAS 5.0.3. My normal daily application is modeling dam breaches for watershed-sized dams.

Initially, I had been roughly digitizing along roads in Geometry Editor using Google Hybrid imagery, then toggling that off to better align to centerline on the LiDAR-based terrain. Then I apply a breakline spacing to create “sub-mesh”. In my normal refined models the computation mesh is 50 feet with sub-mesh along structures of 25 feet. I use 100 ft to 200 ft mesh on preliminary model runs to determine approximate inundated zone, then tighten up the 2D flow area. On several models, the resultant depth output would abruptly stop at the interface of mesh cells on top of the road. And this is not at a structural blip, such as a median or curb. For the purposes of my analysis, I am interested in any overtopping of the travel lanes or centerline greater than 0.5 foot on the roads with traffic counts above certain thresholds. The results with breaklines along centerline gave irregular depth values for adjacent cells on the top width and the inundation areas did not look legitimate.

My go-to solution has been to digitize the breaklines on the downstream edge of the structure and still apply the sub-mesh spacing. For the county and lower-volume roads this technique was working to give satisfactory data and legitimate-looking inundation area completely across the top widths.

Yesterday’s challenge happened in my model at a US Highway. The 25 foot sub-mesh on for the breakline along the downstream edge resulted in cell faces approximately along the centerline. The elevated roadbed has an approximately 40 foot top width. See images. The depth values on the south lane is about 1.5 feet and then abruptly drops to 0 at the cell interface near centerline. In an attempt to help the model “see” this wider structure better, I digitized breaklines along the shoulders both upstream and downstream and applied the 25 foot sub-mesh. I am running the model now. Just thought I would request input while waiting.

[Lonnie A]

I think you are seeing the impact of the hybrid render mode. If you want to see the true model computed WSE you need to select the horizontal render mode (under tools in mapper). With the other ones you get mapping that interpolates from the computed WSE and can be misleading. Also note that there is a bug and when you create a static map it only outputs the hybrid render regardless of what was selected.

[cameron]

What does your terrain look like. where the breaklines are? Does the terrain pick up the road?

[InundationPete]

The terrain shows the road well since it is elevated.

[InundationPete]

I looked along the south lane of the road in the current results from using dual breaklines. Flipping between the horizontal and hybrid render modes does not show discernible differences. Inundation still is “stopped” exactly at the cell faces near the road centerline. New depths in that south lane are less than 1 foot.

[cameron]

what is your time step and are you using the full momentum equation? Do you have any water surface errors as the model is running?

[InundationPete]

With the 50 foot cells, I used 1 second computation interval. Maximum velocity result was under 28 ft/s. Using Full Momentum setting. I get WSEL errors on nearly every model.

[Lonnie A]

That is just what you’ll get with the mapping for the way RAS is handling 2D. The volume going over the crest isn’t enough to fill the cells on the downstream side to show flooding of the whole street. The flow going over the road flows out of the cells on the downstream side at a shallow depth. Remember RAS is basically doing interconnected storage cells. What you are seeing is something like this.

[cameron]

As I look more closely at the mapping, I agree with Lonnie on what is happening. So either you add more grid cells along the top of road or plot the Depth Max Extents which shows a cell fully wet if any portion of it gets wet.

[InundationPete]

Thank you for the sketch. That helped me better grasp the concept of what is happening with the rendering.

I ended up running the model with about six versions that had slight differences. This ended up being a basic sensitivity study to see what effects changes had on the results. The final version I am going to use for the official report used a computation mesh of 45 feet and 1 second interval. I set only one breakline for the highway and aligned it with the downstream top edge of the structure. This helped to get cells that spanned the top of the road with downstream faces parallel to the structure. See image below. This allowed the inundation/depth values to show up on the road and to not look as irregular at when the cell faces seemed to be stopping the flow in prior models. The various models generally converged with some inundation on that south lane, but did not consistently have more than the 0.5 foot threshold.

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