[Marulke]

Tony,
I am not sure exactly what your question is, but after looking at your screenshots I have some comments:

1) The bottom of the cross section at the upstream (US) end of the culvert is well above the culvert entrance. This is probably an error?

2) The high chord of the roadway at the US end is at approximately the same height as the US cross section. This is not impossible, but I had expected the roadway to be well above the ground level.

3) Does the distance from the US cross section and the length the culvert fit with the distance between the US and DS cross section? They dont have to be equal, but the culvert appears to be placed to far downstream.

Suggestion: Check your geometry again.

Hope this helps.
Marulke

[Marulke]

From the Profile Plot: Options-> Plans and select the plans you want to display

[Marulke]

I have noticed the same problem on several occasions. I have no solution, but I dont think the calculations are affected.

[Marulke]

1 Culvert
If the culvert has inlet control the capacity will only depend on the headwater. You can compute the capacity curve, Q = f(H), and enter it as a user defined gate curve in the Inline gate editor. (I haven’t tried it, just guessing.)

2 Stilling basins
Why do you want to model them? The design is already given in handbooks. You may need to model the approach flow to find the flow depth and velocity at the entrance to the stilling basin.

The tailwater level is also important, but is normally controlled from downstream (sub-critical DS reach), and can be modeled with RAS.

Flow inside the dissipators can not be properly modeled with HEC-RAS, but you don’t need it. However, it is convenient to include some simple representation of the basins so you get a continuous model.

3 Good tool?
RAS is easy to use and gives the main parameters you need for detailed design (velocity, depth). Using 3 D models is much more involved and may still not yield good results inside stilling basins, under gates etc.

[Marulke]

I don’t know how HEC-RAS handles the difference in Manning number between river bed and abutments for bridge energy loss compuations. I think, but I am not sure, that RAS ignores this.

Anyway, the flow distance under a bridge is normally quite short, and the friction loss small.

[Marulke]

How far downstream of your reach of interest does your model extend?

I would place the downstream boundary so far down that you capture any geometry that may control the water level at the bridge. I would also check that reasonable changes to the downstream boundary condition (slope, if you use normal flow) does not effect the slope.

You may also consider if inflow from tributaries downstream of the bridge may affect the tailwater level, for example other inflow to the swamp.

When running a flood hydrograph (un-steady) you should also consider the flood duration and total flood volume. If the swamp has a large storage capacity an will dampen the flood, a flood with longer duration (larger total volume) but lower peak may give the highest water levels. Try a steady run with the Qmax from your unsteady and compare levels.

[Marulke]

If you want to include the riprap in the geometry, I believe using a sloping abutment to represent the riprap is OK. RAS combines the US and DS cross section and the bridge geometry to make two CS inside the bridge. You may check them in the bridge editor to see if they represent the actual geometry.

However, I dont know how RAS handles the difference in friction between bridge (concrete) and river bed / riprap, when calculating the friction loss through the bridge.

[Marulke]

You can import several data formats, including CSV (but I have never tried to do it). Read chapter 6 in the users manual. You can also copy station – elevation data from Excel and paste it into the CS editor in RAS.

[Marulke]

Thanks Chris, I will try running the model with unsteady flow.

[Marulke]

HEC-RAS is a one dimensional model and generally does not account for forces due to change in direction. (The force balance method at a junction does.)
Would it not be safer, in terms of understanding the calculations, to do a manual calculation of the force balance equation?

[Marulke]

I don’t really understand the problem.

If the flow is subcritical, the water level is controlled from downstream. So, if you have the correct down stream water level (boundary condition) and the correct flow on the reach from the DS BC and up to where the tributary enters, you should get the correct water level at that CS.

Are you concerned about the detailed flow pattern and water levels just where the tributary enters?

Marulke

[Marulke]

Cant you add an extra cross section upstream the culvert, and set a flow change at that cross section? Or add 25 cfs as lateral inflow between two CS just before the culvert inlet.

Just an idea.

Marulke

[Marulke]

You can do do dam breach modeling in RAS.
However, RAS will not calculate the size of the breach opening or the breaching time. You will have to input the final breach dimensions (width, depth, etc.) and the breach formation time. It is entered from the Inline Structure menu (Geometric Data window).

[Marulke]

On the figure you show, is flow direction from right to left, or left to right.

Normally, on my HEC profile plots, the downstream boundary is to the left.

You could have mixed up cross section numbering so that the most downstream cross section was entered as the US section etc.

Or, I may be completely wrong.

Marulke

[Marulke]

Brian,
it is often feasible to compare energy levels, not only water levels. In sub-critical flow the water level drops as it passes through a constriction (for example caused by a part of the cross section being ineffective) because the flow velocity increases. (Jarvus writes about this.)

The energy level (energy grade) will (almost) always drop in the downstream direction. If you display the energy grade line on the profile plot, it may help you understand what is going on.

Regards,
Marulke

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