[Chris G.]

I think the +-0.1 meter is reasonable. While the water surface is lowering, RAS the river is continually adjusting to the change in water surface, and resulting change in storage. That shows up as a slight change in discharge. Once the water surface remains static for a while, the discharge at all cross sections should level out to an exact 100 cms.

If you are able to get to the end of your hotstart without fluctuations and instabilities, then copying the solution at the end of your hotstart run and using it as your initial conditions for your project run should also show no fluctuations and instabilities. Double check and make sure your hotstart is numerically stable-especially at the end of its simulation, and you are writing your hotstart at the END of the simulation. Also, make sure your boundary conditions (first inflow hydrograph value, initial flow, and downstream boundary) are all consistent with the last timestep of your hotstart file.

Hope this helps. Good luck.

[Chris G.]

That’s because hydraulic radius for the cross section as a whole is not equal to the sum of the hydraulic radii for the LOB, CH, and ROB. When RAS runs it’s computations, it sums the conveyances calculated for each individual sub section (LOB, CH, ROB) to arrive at the total conveyance. Then to get the weighted n value for the whole cross section, RAS takes the entire conveyance and backs out the n value with Manning’s equation. Because the total hydraulic radii is different in these two approaches, the weighted n value will be different.

[Chris G.]

Try using a series of overflow gates. In the flow editor, you can set them to any height individually to effectively simulate what you are talking about.

[Chris G.]

Hmmm. Try this. It may or may not be faster than reading manually from the stage/flow hydrographs.

1. Set your detailed output interval to the smallest setting, 1 minute.
2. Rerun your model. It may take a LONG time to post-process with a 1 minute detailed output interval.
3. Open up your summary output table and select All Profiles (except Max WS).
4. Copy an paste to a spreadsheet. Each cross section will have a number of rows, each representing a different profile.
5. Then add a column that takes the difference between the first WS El in the block of data and each successive profile.
5. Look in that row for when you get to 0.3 ft. That’s your arrival time.

Like I said, this may not be much faster than manually reading off the hydrograph plots.

Your other option is to write a program that automates this process, but that would be too difficult unless you have programming experience and you know how to control RAS externally.

Good luck.
Chris

[Chris G.]

I think you are on the right track with editting the geometry file directly. Definately save a copy first, then make your changes. It may take a little detective work to figure out what is what in the geo file.

Good luck.

Chris

[Chris G.]

That is correct.

[Chris G.]

If your model is georeferenced, RAS will maintain the cross section vertex coordinates for it’s position, regardless of the reach length. That could be why you are not seeing a change.

[Chris G.]

Both the momentum equation drag coefficient and the Yarnell Pier Shape Coefficient, K, are dimensionless. So whether you are working in US or SI units, they will be the same.

Chris

[Chris G.]

Yes. I would say in my experience enough persistence and creativity will allow you to model just about any kind of meander, if you have enough “doglegs” in your cross sections. However, even if you are able to draw cross sections through the heavy meander, it won’t necessarily accurately reflect flow lines at all flow rates. In that case, there’s just no getting around haveing two (or more) different geometries.

[Chris G.]

You could simply use a uniform lateral inflow (use negative flow values to simulate flow leaving the system), or even explore the use of the groundwater internal boundary option. In any case, RAS is not a ground water model and you really should have a good idea of what is leaving the system through the Karst, before you put too muchy stock into the RAS results.

[Chris G.]

If youre working in unsteady flow, the amount of volume that leaves a cross section can have a significant impact on the attenuation of your flood wave, and you should account for it. Letting RAS use the vertical wall of water does not properly account for volume going into storage. Whether you extend your cross sections to high ground to contain all of the flow (and storage volume), or you use a lateral structure to account for volume leaving depends on the behavior of your storage volume and how connected it is to the main channel flow. i.e. if the water surface elevations between cross section and storage are significantly different, use a lateral structure.

[Chris G.]

You could also use a lateral structure. The lateral structure has a flap gates option for culverts, which only allows one way flow. However, this is a simple binary operation in that it is open if the river is higher, closed if the sea is higher. There can be no other conditionals like “if the sea reaches 0.6 meters, close the gates”. In that case, I would certainly take Jarvis’ advice and use the gate rules scripting feature.

[Chris G.]

Make sure the hydrograph output interval on the unsteady flow analysis window matches the interval used for your input hydrograph(s).

[Chris G.]

I think this looks fine, as long as you skew the bounding cross sections as well. I don’t see where any cross section crosses the stream line twice. If you are modeling this for a high flow event, I would probably strighten out my stream centerline as it approaches the upstream bridge deck. It probably won’t hug the embankment then make a drastic left-hand turn like you have drawn. But the cross sections look good, in my opinion.

[Chris G.]

Great!

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