[Chris G.]

Hard to answer without looking at the model. May I suggest you post the model to a dropbox or google drive site. That way, Forum members with some time can download and troubleshoot it for you.

C

[Chris G.]

By default, RAS will use the Manning’s n values from the bounding cross sections for the bridge internal cross sections. However, you have the ability to go into the bridge internal cross sections and change Manning’s n values, bank stations, and station-elevation points. Changing the n values for the internal cross sections will allow you to simulate concrete abutments. To access the bridge internal cross sections, go to the bridge/culvert editor, and under the Options menu item, select “Internal Bridge Cross Sections…”.

[Chris G.]

Usually this is just a schematic thing and doesn’t affect the computations. However, sometimes it suggests a problem with how you input your bridge geometry. Go though your bridge input data and make sure it is correct, consistent, and complete.

Good Luck-
Chris
@RASModel

[Chris G.]

Please try changing your Windows international settings to English (United States) when running RAS. That may solve this problem.

[Chris G.]

What I’ve done in the past is just downstream of the downstream storage area, I put a “dummy” reach with 2 cross sections. Put them very close together, but similar in size to the reservoir. Then add your downstream stage hydrograph to the downstream cross section of the dummy reach.

Chris G.

[Chris G.]

This may help.

http://hecrasmodel.blogspot.com/2013/05/skew-angle-for-bridges.html

[Chris G.]

Sounds like a bug to me. Maybe check to see if the moveable bed stations at the bounding cross sections are outside the bridge cross section limits.

[Chris G.]

The suggested “rule of thumb” by HEC and many others is to locate the bridge bounding cross sections as close to the bridge as possible without being on the bridge or approach embankments. In otherwords, put it as close to the toe of the embankment as possible.

[Chris G.]

I believe the particle size for the CSU pier scour equation is D95. Regardless of how it is labeled in the HEC-RAS GUI (D95), or the report (D90), the value you enter into that input box is what is used directly by RAS for the CSU pier scour equation.

[Chris G.]

I would check that cross section. Make sure the geometry and sediment data is all correct there. Can you see any output? The log output file may reveal what is going on. Perhaps you are getting an unrealistic large deposition of material at that cross section which causes the computations to halt. Hard to say. Run the data set in steady flow, without sediment, and see if it works okay. If nothing works, I suggest sending a bug report to HEC, because regardless of any input errors, the software shouldn’t abruptly halt without giving any error messages.

[Chris G.]

Unfortunately the Visual Fortran run-time errors really don’t tell us much. Any time you get a run time message, it is a bug and HEC will want to know about it. I suggest submitting a “bug report” to HEC through their website, http://www.hec.usace.army.mil.

Chris
@RASModel

[Chris G.]

Steve-
One of the most common reasons for a dam breach model blowing up right at the start of the breach is sudden increase in both the local and convective derivatives in the St. Venant Equation of conservation of momentum. In other words, stage and flow are changing quite suddenly over a short space and a short period of time and RAS doesn’t do well with this under default conditions. Addint more depth to the base conditions in front of the dam breach flood wave can help to stabilize this condition. You can add more depth by:
1. Providing more base flow.
2. Increase Manning’s n values.

You can also help to stabilize the model under these conditions by dampening out the local acceleration terms. You do this by checking the “Mixed Flow” box in the unsteady flow analysis window. Please read up on this before using it so you understand what’s happening. By the way, this sometimes makes things worse, but it’s easy to try, so see if it works.

Read these posts for more info…
http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html
http://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.html

My guess is the permanent ineffective flow areas, if set up properly, are not causing any trouble.

Good Luck
Chris
@RASModel

[Chris G.]

Doug-
Unless you manually enter a flow change below the culverts, in steady flow, RAS will use the same discharge for your entire reach. Since there were no changes downstream of the culvert, you will show no changes in results downstream of the culvert. Steady flow will only measure the increased/decreased stage upstream of the culvert. If you believe the existing or proposed structure creates enough of a backwater effect to significantly attenuate your discharge as it approaches and then passes through the culvert, then you should measure this effect by running HEC-RAS in unsteady flow.

Unsteady flow is always occuring in natural streams. The degree to which it affects your design/analysis efforts dictates which option you use-steady of unsteady. If you’re not sure, you should try both ways.

Good Luck-
Chris
@RASModel

[Chris G.]

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Subject: Re: Dam Break Model: Starting WSEL Issue
References: <[email protected]>
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It sounds like you have everything set up except discharge through the dam. Y=
ou can use either gates or pilot flow. If you set your pilot flow equal to y=
our initial flow, your reservoir will stay at whatever elevation you set it a=
t. You are correct to be worried about too low of a depth. 0′ definitely wil=
l crash your model. 0.5′ may be difficult too. Very low depths=3Dunstable mo=
dels.=20
Good luck.=20
Chris

Sent from my iPhone

> On Feb 11, 2014, at 10:32 AM, “Hawk [via HEC-RAS Bloggery Forum]” wrote:
>=20
> I have put together a dam break model that utilizes a storage area to repr=
esent the dam (essentially the model construction is the storage area, two c=
ross-sections ‘inside’ the dam and which the dam geometry is superimposed on=
, and 6 downstream cross-sections spaced at ~100′ intervals). I added 500 c=
fs of baseflow to the inflow hydrograph (represents less than 10% of the pea=
k inflow).=20
>=20
> The model is currently stable and running. However, I need to simulate an ‘=
empty’ starting condition within the reservoir. I have set the initial elev=
ation in the reservoir at an elevation which represents 0.5′ of depth (I fig=
ured starting with 0′ of depth would create a problem). I also set the two ‘=
internal’ dam cross-sections to start at this same WSEL (0.5′ of depth). =20=

>=20
> When I run the model the first time step essentially shows the WSEL within=
the dam as above the spillway elevation (this represents over 20′ of depth!=
). So immediately the inflow hydrographs starts spilling over and continuin=
g downstream. This obviously ignores any type of attenuation affect the dam=
would have. =20
>=20
> How can I simulate this ’empty’ starting condition in the reservoir? Is i=
t even possible given the that velocities are practically zero at the two ‘i=
nternal’ sections?=20
>=20
> Sorry for the lengthy post. Thanks for any help you can provide!=20
>=20
> If you reply to this email, your message will be added to the discussion b=
elow:
> http://hec-ras-bloggery-forum.1091112.n5.nabble.com/Dam-Break-Model-Starti=
ng-WSEL-Issue-tp571.html
> To unsubscribe from HEC-RAS Bloggery Forum, click here.
> NAML

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It sounds like you have everything set up except discharge through the dam. You can use either gates or pilot flow. If you set your pilot flow equal to your initial flow, your reservoir will stay at whatever elevation you set it at. You are correct to be worried about too low of a depth. 0′ definitely will crash your model. 0.5′ may be difficult too. Very low depths=unstable models. 

Good luck. 

Chris

Sent from my iPhone

On Feb 11, 2014, at 10:32 AM, “Hawk [via HEC-RAS Bloggery Forum]” <[email protected]> wrote:

I have put together a dam break model that utilizes a storage area to represent the dam (essentially the model construction is the storage area, two cross-sections ‘inside’ the dam and which the dam geometry is superimposed on, and 6 downstream cross-sections spaced at ~100′ intervals).  I added 500 cfs of baseflow to the inflow hydrograph (represents less than 10% of the peak inflow).

The model is currently stable and running. However, I need to simulate an ’empty’ starting condition within the reservoir.  I have set the initial elevation in the reservoir at an elevation which represents 0.5′ of depth (I figured starting with 0′ of depth would create a problem).  I also set the two ‘internal’ dam cross-sections to start at this same WSEL (0.5′ of depth).  

When I run the model the first time step essentially shows the WSEL within the dam as above the spillway elevation (this represents over 20′ of depth!).  So immediately the inflow hydrographs starts spilling over and continuing downstream.  This obviously ignores any type of attenuation affect the dam would have.  

How can I simulate this ’empty’ starting condition in the reservoir?  Is it even possible given the that velocities are practically zero at the two ‘internal’ sections?

Sorry for the lengthy post.  Thanks for any help you can provide!

---

If you reply to this email, your message will be added to the discussion below:

http://hec-ras-bloggery-forum.1091112.n5.nabble.com/Dam-Break-Model-Starting-WSEL-Issue-tp571.html

To unsubscribe from HEC-RAS Bloggery Forum, click here.
NAML

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[Chris G.]

Hi Orlando-

The junction lengths are measured from the cross section upstream of the junction, along the stream centerline, to the cross section below the junction. Read these posts and the RAS manuals for more information:
http://hecrasmodel.blogspot.com/2009/02/how-to-best-model-junction.html
http://hecrasmodel.blogspot.com/2008/12/unsteady-flow-and-junctions.html
http://hecrasmodel.blogspot.com/2011/05/modeling-junctions-for-unsteady-flow.html
http://hecrasmodel.blogspot.com/2009/02/severe-energy-jump-at-junctions-for.html
http://hecrasmodel.blogspot.com/2009/06/new-features-401-part-2junctions.html

[Author]

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