Model Efficiency – 2D Culvert at Issue

[Scott Miller]

Here is a comparison of hydrographs at this low gradient culvert – full momentum vs. dynamic wave. The dynamic wave simulation took much longer to finish a 3-day period than full momentum took to finish a 10-day period. There is more instability at the culvert for the DW simulation, but I have to assume it did not play too much of a role in the difference in model speed. The DW simulation listed thousands of 1D/2D flow errors, albeit moderate, downstream. The FM simulation listed none.

Dynamic Wave

Full Momentum

It’s a 24-inch concrete pipe. Five feet of freeboard above the crown.

[cameron]

What is the water surface profile comparison between the two solvers? I have found that for some models, the FM solver will add extra losses that don’t make sense. Just by switching between the two solvers, the wse changed over 12 ft at a USGS gage location for one study. The DW solver was within 1 ft of the gage and the FM was 12 ft too high.

My guess is that the FM profile is higher which makes the model more stable and gives less 1D/2D iterations so it runs faster. Is it correct is something you will have to check. I recommend doing side calculations to verify it is working correctly.

[Scott Miller]

It is worthwhile to compare the performance of equation sets. I had stayed away from full momentum, expecting that it would take longer than dynamic wave. As it turns out, FM is more stable and simulates far faster for the geometries and topography in this model.

The stages in the above hydrographs are nearly the same between DW and FM. The culvert flow is more stable with the FM equation set.

I did some spot checks of maximum water surface elevations along the valley (downstream from this culvert), and, sure enough, the FM equation set consistently calculates higher water surface elevations. The difference is not alarming, tapering from 0.2 foot, in a broad, flat headwater area, to below 0.02 foot below a 2D bottleneck reach. I have to resolve the difference with the expectation that FM is more accurate than DW. Gauges wouldn’t help with this one.

Thanks so much!

[hasansins]

Is there a way to plot culvert in 2d areas with water surface elevation in it over time?

[Scott Miller]

Yes. In the Geometry Data window, click on the 2D area connector that has the culvert in it. Choose ‘plot stage and flow hydrograph’. Headwater and tailwater time series are plotted, like in the graphic above.

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