[cameron]

Just break it up into multiple 2D connections

[cameron]

Have you setup any Manning’s yet in Mapper?

[cameron]

Have you looked in the all of the guidance documents that come with HEC-RAS?

[cameron]

It appears your time step is too high. Are you using full momentum or diffusion wave for the solver?

[cameron]

That can be normal. There is a lot of attenuation in peaks as you go downstream.

If you took a hydrograph plot inside of the canyon before it spreads out, the peak would not have attenuated that much as it is confined. Once it gets to the open area it just spreads out and slows down.

[cameron]

You have to download some additional files at the location below for it to work.

http://www.hec.usace.army.mil/software/hec-ras/downloads.aspx

[cameron]

First off you probably don’t need that many interpolated cross-sections.

If you don’t care so much for mapping then it won’t matter what the interpolated cross-sections look like in plan view as the should be using the interpolated reach lengths.

There are two types of interpolation options, one that follows the channel centerline and one that doesn’t. It does not change the actual interpolation, it just changes the visual appearance. the one that follows the centerline will keep the banks visually looking correct.

[cameron]

What cell size are you using? It may be best to incorporate the logjams in the terrain and use breaklines and smaller grid cells near the logjams.

[cameron]

The solver you pick makes a big difference as well. If you use diffusion wave you may get more realistic results at the culvert than the full momentum solver. I have done several models that the only way to reach calibration is to use diffusion wave as full momentum would add additional losses.

As for instabilities, they usually occur for lower flows for culverts in 2D and once you have the bridge overtopped they go away.

The only way to fix these things are as others have suggested, bigger grid cells and smaller time steps.

[cameron]

SMS can also read HEC-RAS models now so your could follow the same procedure you used for SRH-2D

[cameron]

You can export out the mesh as polygons in Mapper.

Once this is done you could do a zonal statistic in ArcGIS to extract the wse for each polygon.

[cameron]

It would make sense to include them if they impact the wse. You would need to make sure to include ineffective flow areas in those sections following the expansion/contraction guidelines.

[cameron]

if now downstream boundary, water can not leave the system and it will just fill up like a bathtub.

[cameron]

Since you are failing at time zero, why not just set the time to fail? If you want to use wse, just pick something a little lower than the starting wse as it won’t matter.

[cameron]

depends on grid cell size and velocity. Follow the Courant Criteria.

[Author]

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