Compute Flow Based on Known WS

[WMiller]

I have a channel with a known cross section.

Is it possible to compute flow based on known upstream and downstream WS elevations?

Thanks in advance!

-Wesley

[Anonymous]

Yes.

You can make a model with a stage time series boundary at both the upstream and downstream end.

If this is a 1D model, you have to specify an initial flow just to get things up and running but after that RAS will compute the flow.

[Anonymous]

Thanks Jarvus, Having a little trouble figuring out how to set up such a model. Google isn’t being much help. Any advice would be greatly appreciated.

Thanks!

[ckatsoulas]

after you create a cross section in the geometry window you go into the “steady flow data” window to set your flow and then click the “Reach boundary conditions” box. The box that appears gives you an option to set and upstream and downstream boundary condition such as a known water surface (this is what you want), critical depth, normal depth or a rating curve.

[Scott Miller]

You can make a model with a stage time series boundary at both the upstream and downstream end.

If this is a 1D model, you have to specify an initial flow just to get things up and running but after that RAS will compute the flow.

Wesley,

This is fitting gradually varied flow to a prismatic channel, assuming uniform friction. Do I understand correctly? Or does the cross section vary?

[Anonymous]

Scott,

Exact scenario as follows:

Road follows a natural channel. Natural channel flowed out of it’s bank during a flood event and over topped the road (depth varied 0 – 2.5′, depending on road elevations).

There are USGS water level gauges on both sides of the road which recorded WS data at 15 min interval.

Following the flooding event, we surveyed and created a profile of the entire road edge (channel side), approximately 3 miles.

I imported the road profile into RAS as XS 2 (upstream) and copied that XS downstream 24′ (width of road) to XS 1. I then imported the time stamped WS information as the known boundary conditions for XS 2 and XS 1 as unsteady flow data. I was required by RAS to provide an initial flow to XS 2, to which I put an arbitrary value of 10cfs..

My hopes were to develop a stage discharge curve and also to estimate total volume which over-topped road.

However, the resulting stage discharge curve did not look correct (odd spikes which did not coincide with spikes in upstream WS, etc.). I tried changing the initial flow of XS 2 to 2,000cfs, which greatly affected the results (neither in a positive nor negative way, just different). Therefore, I do not trust my method or understanding of how to properly model the situation.

I’m still researching and trying, but any help would be appreciated!

[Scott Miller]

I’m reading that it is an unsteady 1D model with two cross sections 24 feet apart, the width of the road. The channel, crossing the road, is as much as three miles wide. Flow is lost from the main water course, without return flow along that length of the road, and that’s what the model is intended to get at. Is that correct?

[Anonymous]

Not exactly.

The road parallels the channel. The road is built on the right bank of the channel.

Water flowed over the road for approximately 3 miles of roadway.

Water flow was only in one direction (over the road), for purpose of this exercise.

[Scott Miller]

Got it. I was referring to the channel in the HEC RAS model. I don’t expect this approach will work well. There’s head to drive flow across the road, but there is also a downstream component to the flow, parallel to the road. 1D cross sections should always be cut perpendicular to the flow direction.

Here are two approaches to compare:

Broad crested weir. Solve analytically for flow across the road. Break the road into discreet weirs each with it’s own elevation. Adjust the road elevations or the gauge stage to account for the slope of the main water course. Figure cross flow is going to affect the weir coefficient. That may be enough to develop a stage-discharge curve and get to the answers.

Full 2D model. Include the main water course along the length of road. Use the stage-discharge for the water course side gauge to determine the flow in the main watercourse for the upstream boundary condition. Augment that, if needed, with the weir flow stage-discharge curve. Use the road profile for a 2D flow area break line. Elevation data as coarse as a USGS DEM ought to be usable at this scale.

I’m just not sure whether there’s sufficient drop on the land side of the road to model as a weir. I hope this helped.

[Anonymous]

Thanks Scott. Its a hard scenario to explain, but the flow is perpendicular to the XS’s.

Weirs were my initial thought, but I would have to break into a-lot of separate weirs as the overflow was intermittent across the 3 mile stretch or road and all “weirs” will have different elevation information and some become interconnected as the WS raises. Was hoping to use RAS to avoid the tedious task of modeling as weirs, but may have no other choice.

Thanks for your time on this.

[Scott Miller]

It would be good to know whether the two-cross section model would work. My initial thoughts, once I understood the model configuration, were about how few cross sections that is… and that there are issues with the river slope. Does a two cross section model with stage boundary conditions work?

An alternative to full 2D would be 1D main channel / 2D overbank. It’s probably the best way to use HEC RAS to track overbank flows. You have the road profile to use as lateral structure elevations. Calibrate up to the road becoming overtopped, and use your best flow or hydrologic data for higher flows. That lee side gauge ought to help.

[Anonymous]

The two cross section model with stage boundary “worked” in that RAS ran a solution. I just do not trust the solution. I’m moving onto a different plan for the time being, and may return to this some other day to try and model in RAS.

[Author]

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