Problems with interaction between 2D area and 1D reaches

[Scott La Vanne]

I’m using the October release of HEC-RAS 5.0 Beta. The geometry I created consists of a 2D mesh with 1D reaches upstream and downstream of the 2D area (see the picture below). The upstream end is the upper right and downstream is the lower left. The flow condition I’m running is a constant 20CFS inflow hydrograph at the upstream end for the duration of the simulation to obtain effective steady state conditions.

The simulation runs through without errors; however the results don’t make sense. The 2D area looks reasonable, but the 1D reaches are clearly off. The downstream reach water surface profile is far too high. The 20 CFS flow should be well within the banks and approximately match the inundation extent at the boundary with the 2D area. What actually results is a water surface profile far above expected levels and an inundation area all the way to the far extent of the cross section which should contain far greater events.

The upstream reach looks reasonable in the traditional method of viewing results (Profile plot, stage and flow hydrograph, etc.), but as shown in the picture below, doesn’t appear as an inundated area.

I’ve tried adjusting settings such as number of warm up time steps, the ramp up time, iterations between 1D and 2D, as well as varying initial conditions. I believe I created the connections between the 1D reaches and the 2D areas appropriately as well.
Has anyone seen this before, or does anyone have any thoughts on what might be causing this strange behavior?

Thanks.

[cameron]

Is this model something your willing to allow someone to look at? It can be difficult to diagnose just from a screenshot.

What is your time step and did you try using the full momentum solver instead of the diffusion wave? Did you turn on the 1D/2D iteration options?

[Scott La Vanne]

Cameron – I have tried using the 1d/2d iteration options. It did not occur to me to use the full momentum solver until you mentioned it. I’ve now ran the model with the full momentum equations and something still isn’t right with the downstream reach. I’d be happy to share the data though if you’re willing to take a look. I’ll send you an email shortly.

[Scott La Vanne]

For anyone else interested in this problem, below is the solution which Cameron was so gracious to help me discover.

The fix:

You have two cross-sections right next to each other (2042.102 and 2040.102) which are right next to each other, but the reach lengths for 2042.102 is over 2000 feet (should be 2 feet??). This reach length is the biggest culprit, others include the initial flow for the 1D areas and lowering the time step. I got it to run with 20 cfs and 100 cfs without a problem one I changed the reach length to 2 feet and adjusted the time step.

This reach length error creates a very long completely flat area within the reach, causing the high water surfaces. As I copied this from a purely 1D model before adding the 2D area, HEC-RAS must have recalculated reach lengths. The take home lesson from this is to verify your data hasn’t changed unexpectedly when copying forward data from another model, which I suppose is always good practice regardless of which program you’re using. I guess I was just too distracted with all this shiny new 2D stuff!

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