## Thursday, August 19, 2010

### How to use a Storage Area to define a Reservoir.

Written by Chris Goodell, P.E., D. WRE | WEST Consultants

It’s a little confusing, and not really directly covered in the manuals anywhere. But I get this question a lot. “How do use a Storage Area to define my reservoir in RAS?” First of all, make sure using a Storage Area and level pool routing is an appropriate way to model your reservoir. This is even more important if you are modeling a dam breach on this reservoir. Check this post first to make sure it’s okay.

Once you’re happy with the level pool assumption, draw in (or import)your downstream reach in the geometric editor. Then add in your cross sections. Next, place your inline structure (dam) at the upstream end of the reach. You’ll need to place two dummy cross sections upstream of your inline structure (and downstream of your storage area). These can be copies of the cross section downstream of the dam, but should be as close as possible to the upstream toe of the dam and close enough to each other to minimize the associated volume (relative to the reservoir’s total volume). Next draw (or import) your storage area upstream of the inline structure and it’s two dummy cross sections. It’s not uncommon to have the two cross sections reside within the storage area’s boundaries. That’s okay and it doesn’t affect the computations either way.

To make RAS recognize the connection between the upper dummy cross section and the storage area, you have to “move” the upper end point of the reach inside the storage area. To do this, go to Edit…Move Object, in the main geometry window.

Once in “move” mode, click and drag the upstream end point inside the storage area. RAS should then automatically recognize the connection. Make sure to uncheck “Move Object” in the menu once you’re finished. It should look something like this when done:

Make sure you provide some outlet flow, or your dam will overtop at the beginning of the simulation. This can be done by coding in gates, or by providing pilot flow. If you’re providing pilot flow, you must enter in an “Internal R.S. Initial Stage” for one of the dummy cross sections to set up your starting pool elevation. This is set in the Options menu item in the Unsteady Flow Editor.

1. Can a hydrograph be input into the upstream storage area?

2. Yes! It's entered in the unsteady flow data editor as a lateral inflow.

3. After computing the hydrograph looks plausible. The flow rises to the peak and drops down again. But shortly before the storage area is empty i get an negative flow and the water surface rises up a little again. And there is always a 'warning' after the computing because there is iteration between the dummy cross sections. can you help me?

4. Do you have a lateral inflow into the storage area? If not, adding that might help. If you do have one already, you might consider higher base flow. Also, the minimum elevation specified for your storage area in its elevation-volume curve should be about the same as the invert of the "dummy" cross section. If it is much lower than the cross section, you could cause the dummy cross section to go dry (and that might explain the reverse flow). Also, if it looks like you are getting very low water surface elevations in the dummy cross sections, you might want to try putting pilot channels in there. Good luck.

5. Chris, in this approach, can we model like this, suppose 4 canals come directly to a retention pond and one canal out from the pond.I think you can understand my point. Currently, I'm making a model like this, but model is unstable. I tried a lot to make stable the model.

6. Yes, you can do that. Check to make sure none of the canals are going "dry" during the simulation. Although a storage area can have 0 depth, cross sections cannot.

7. Hi there, I am new to HEC-RAS and having some problem with the connection between a reservoir and my stream. This post seems pretty close what I am doing, I think!

So, I have a small stream and I have modeled the stream with XS and culverts and its working fine. Now there is a small pond area at the side of the stream which I like to include in my model. The pond is not directly on the stream, its just beside the stream but there is a connection to it. So I added a storage area and tried to connect to the stream but seems I have to use a lateral structure. If I input a lateral structure at my stream (close to the stream station where my pond is situated), Now how the water flows from the stream to the pond area? It seems like its not connected. Do I need to make another reach from the storage pond to my main stream (as a second reach)? Then input the lateral structure on that reach? Then I think I need XS of that reach as well? Little confused, need help! Thanks in Advance --- Sharif

8. Hi Sharif-

Take a look at this post: http://hecrasmodel.blogspot.com/2012/11/connecting-river-to-off-channel-storage.html

Hope it helps.
Chris

9. I am also new to HEC-RAS, should the reservoir show up on the x-y-z plot? I have a storage area, and it appears to be connected to the river reach, but it never shows up in any of the cross sections

10. Storage Areas do not show up on the XYZ plot, unfortunately.

11. THANK YOU SO MUCH!! I HAVE BEEN TRYING TO FIGURE THIS OUT FOREVER!!!

12. I am trying to run an unsteady analysis witha storage area upstream of a dam but everytime i start the simulation i get the following message. i have a eleation-volume curve that has increasing valeus of elevation and of volume. my first volume is zero. what am i doing wrong?

HEC-RAS Error - Incomplete data, the following errors were found:
Storage Area: StorSverjesjoen
- Volume Elevation relationship does not have increasing volume.
- Volume Elevation relationship does not have increasing elevation.

13. I found an answer!! All the 100 rows must be filled in in the elevation-volume curve of the storage area!!

1. That's odd. You shouldn't have to fill in all 100 rows to define your elevation volume curve. I've done it many times with only a handful of rows. There needs to be the same number of elevation entries as there are volume entries. Perhaps that was the problem.

2. I got the volume error above and took a look at my stage-storage table. The first five rows had these values for volume: 0.0, 0.0, 0.0, 0.01, 0.01. Below that there were no repeated volumes. I tried just deleting the repeated values. This left empty rows, and I got both the volume and elevation errors above. Then I cut and pasted the table values upward, so there were no empty rows. I no longer got the elevation or volume errors.

3. Scott, I believe you and I experienced the same problem. I believe there was a rogue row of stage-storage data in your HEC-RAS table. If that is the case, filling all 100 rows incidentally solves the problem, but the underlying problem doesn't require you to fill the entire table to make it work.

If you were to select all cells in your problematic Elevation Volume Curve using the select all button (top left cell exterior to the value containing cells) and paste the table into a spreadsheet, I believe you would have found a row of data below the perceived end of your storage table.

When I did this for what I thought was a 3 row storage table, I found the culprit was an entry in row 23, well below what is immediately visible without scrolling in the HEC-RAS application. In general, if you receive 'Volume-Elevation relationship does not have increasing elevation' (or storage) for what seems to be a perfectly increasing table, this could be the problem.

14. Hi Chris,

I am having some problems modeling a dam break currently. The dam itself is located in the middle of a city. Just upstream of the dam is a lake that connects to the dam through a short river(~6000 feet). Currently I am trying to model the lake as a storage area using a storage vs stage curve. This storage area is then connected to my first upstream cross-section in my model(to simulate the lake outlet). In my unsteady flow data I have a stage hydrograph as my boundary condition at my 1st cross-section which uses stages that have been measured from a 1/2 PMP flood.

So couple questions from this:
1) Am I modeling this setup correctly? This is only the 2nd dam break I have ever done so I could be wrong.
2) Do I need an upstream boundary condition at my first cross-section, or does the stage vs storage curve from the reservoir take care of that?

My model is currently crashing instantly suggesting a problem with my initial conditions but I am not able to pinpoint where so I am assuming something is wrong with my storage area.

Any help would be greatly appreciated and on a side note this is an amazing help site that I have used constantly since I started my job, thank you so much!

1. It could be that the stage-storage curve and your stage hydrograph boundary condition are in conflict with each other. For your setup, I suggest having a boundary condition at the downstream-most cross section in your model (downstream of the dam-normal depth is a good choice) and a flow hydrograph set as a lateral inflow to your storage area reservoir. That's it. By trying to put a stage hydrograph upstream of the dam but below the storage area, you are forcing a water surface elevation that RAS perhaps doesn't like and then your model blows up. I think it's always better to leave those internal stage boundary conditions out of the model. Instead, put them in as observed stage hydrographs and calibrate your model to them.

If your model is still crashing, make sure:
1. First time step inflow, initial conditions flow, and flow through your dam are all consistent.
2. Model is not going "dry" anywhere.
3. Time step is sufficiently low (<1 minute to 30 seconds for a dam breach)
4. Run a steady flow plan and see if that reveals problem areas. Look for lots of red marks on your profile plot, which suggest critical, or close to critical depth. This could mean your n values are too low.
5. Start simple-just cross sections, then add in structures one at a time. Finally put in the breach. Run the model in between each addition.

If you're still having problems, send me the model and I'll see if I can diagnose the problem.

Good luck!
Chris

15. Am attempting to model through a small upper reservoir (storage area), down a reach through the lower on-stream reservoir to the roadway below.
Have both storage areas digitized and imported from HEC GEO-RAS. Get the following error upon computing unsteady flow:
"Lateral Inflow at -RS: was specified in the unsteady flow boundary data but was not used because this location does not exist in the current geometry file"
Know the model recognizes the upper reservoir, as it prompted to join the reach to the reservoir upon moving the last point into the bounds of the reservoir. How do I made sure the second reservoir is recognized by the model? Do not have a junction, only one stream.
Any help greatly appreciated.

16. A quick check of the initial conditions tab in the unsteady flow editor (assuming you are doing unsteady flow) will reveal if RAS recognizes your storage area or not. All storage areas require an initial water surface elevation and will be listed in a table at the bottom of the initial conditions tab.
You could also go to the geometry window and select Edit...Move. Then move the end point of the reach into the middle of the storage area. RAS will give you a message saying indicating it recognizes that you want to connect the storage area to the reach.

17. Could you provide assistance please?
I am doing a dam break. The reservoir is the most upstream point in the watershed, and there is a stream downstream of it. I'm debating whether this is a lateral or an inline structure. I'm leaning towards Inline because it is in line with the stream downstream of the dam, and it is definitely not lateral. I have 4 x-sections downstream of the dam, and 2 x-sections upstream of the dam (within reservoir). I drew in my storage area (reservoir) so that it overlaps the upstream-most x-section so that it recognizes the connection (as you described), and entered the stage-storage table. I made an Inline structure and entered in the breach data (have it set for overtopping). I entered my unsteady flow boundary conditions: at the upstream-most x-section, I used a Flow Hydrograph obtained from a HEC-HMS model (and set min flow to 100, because my range goes from 0 cfs to 40,000 cfs), and at the downstream-most x-section I used Normal Depth. Finally I set my unsteady flow initial conditions: Initial flow=100cfs @ upstream-most cross-section, and Initial Elevation is 1216 (normal pool of the reservoir). I go to run the unsteady flow and the computation bar is almost entirely red. I have no idea what to do.

1. 1. I think it would be better to use a lateral inflow to your reservoir, rather than an inflow to the cross section for your u/s boundary.
2. What is your computational time interval? should be seconds, not minutes or hours.
3. Are your cross sections spaced correctly? Use Samuels Equation or Fread's Equation.

There could be a number of things causing your model to crash right away. Take a look at your initial conditions profile (check box in the upper right hand corner of the WS profile plot) and see if that looks okay. If not, where are the problems? Go check those out. Look in your log output file and check the QDiff and ZDiff tables and see where the discrepancies are between your initial conditions profile and your first time step profile. Keep at it! You'll figure it out.

Good luck-

Chris
@RASModel

18. Does anyone know if the volume in-put for a storage area is cumulative or incremental? When inputting volume for a storage area for the elevation vs storage curve.

1. It is cumulative.

19. I am attempting to carry out a dynamic unsteady state simulation to model the dynamic response of a located mid-way pond in an ephemeral wadi stream that is subjected to a sporadic flooding hydrograph. The question is to identify the part of the flood surge that will succeed to pass by the pond and to enter the downstream wadi reach located d/s of the pond and to identify the part of the hydrograph that will be intercepted by the pond.
In order to do so, I used the cross section approach to deal with the pond (not the storage approach) because I do need more flow details to be calculated at the pond sections. I started with a dry pond initial condition however I figured out that Hec-ras automatically fill the pond initially and thus no part of the incomming flow is intercepted by the pond despite that the pond has a significant volume to intercept at least 50% of the comming water volume. Can some body guide how to deal with that issue.
If hec-ras can not model initially dry sections then how does it work with the inundation mapping which basically means wetting drying areas?

1. If there is no outlet flow at the inline structure, then RAS will automatically fill the pond for the initial conditions. To start with a low pond elevation, you must define some outlet flow (gates, pilot flow, etc) so that you have a balance between the inflow (upstream flow hydrograph) and what leaves the dam. Setting an initial conditions stage at the cross section just upstream of the inline structure will help. Read this post for more help: http://hecrasmodel.blogspot.com/2009/09/initial-reservoir-elevation-pilot-flow.html

2. To answer your second question, RAS cannot have a completely dry cross section. You need some water in the main channel at all times. However, the overbanks/floodplains can be dry or wet. Also, storage areas can be completely dry.

3. Greeting Chris - I am currently modeling a 6 sq. mi. watershed with urban/rural mixed characteristics. The routing is normally mostly dry until a storm event. I have noted your comment above where you state that RAS cannot have a completely dry cross section, water is required in the main channel. Is it acceptable to lower part of a cross section 0.25 ft creating something like a 1 ft. X 0.25 ft. triangular main channel with bank stations on either side, inputing a small <10 cfs initial flow condition, and ending the simulation before any reach runs dry?

4. Yes. That's one way to do it. An even easier way is to use pilot channels. Geometry window...Tools...Pilot Channels...
Initial flow (also called baseflow or minimum flow) is also a good idea too. Many times in situations like yours, I'll use a combination of pilot channel and baseflow to get the desired effect. Good luck!

20. I am modeling a small stream with a sedimentation basin at the mouth and a downstream weir using quasi-unsteady flow analysis. After inputting all the cross-sections, the inline structure and the storage area, I keep getting this error message: Storage area "sed basin" -does not have initial elevations specified in the steady flow data. But I am not using steady flow... Please help!

1. That's annoying. I believe you found a bug. I tried the same thing with the MBex data set and got the same error message. I could not find a workaround. Even tried adding an initial stage for the storage area in the steady flow editor, but it didn't work. The only thing I can suggest for now is to simulate the storage area as a separate reach with cross sections. I'll forward this on to HEC and hopefully they'll get it fixed for the full release of 5.0. Thanks-
Chris
@RASModel

2. Bug message sent on to HEC. I'll reply again once I hear back.

3. Turns out Sediment Transport in HEC-RAS does not handle storage areas. You can have lateral structures though, just not storage areas. HEC is working on a more useful error message. So, for now, you'll have to either omit the storage area all together, or model it as a separate reach with cross sections.

21. Hi,
I need advice on this problems :
1.Why in the cross section, there are no storage area cross section. I had joint the reach and the storage area using lateral structure and the XS profile only show the reach end with the lateral structure. In fact, in geometric, I draw the storage area.
2. How do we connect the reaches when there are storage area between them? I use reach 1 upstream the storage area and reach 2 downstream the storage. it seams like the simulation/profile end before the storage, as my problem in No.1.

1. Sorry, but I don't understand your questions. Can you please rephrase them? Thanks.

22. Hi Chris, I have a simple question. I appreciate that if you could respond to me.As you know in Hec ras there are two ways to define the storage 1-Area time depth method. 2-Elevation versus volume curve. If I don't have these kind of data, how can I define the reservoir? what is your approach in such situations?
Cheers,
Mike

1. You'll have to approximate it somehow. At the very least you should be able to measure the surface area of the reservoir (Google Earth) and then approximate the depth. That will give option 1. Still, that is a crude approximation and it is much better to try to obtain an elevation volume curve.

23. That information is required for storage areas. If you don't have it, measure the surface area of the reservoir (this can be easily done in any GIS software application and even in Google Earth), and approximate an average depth. This is not the best method but it will work. Would be better if you had bathymetric data so you could generate an elevation-volume curve. See if you can find a "pre-dam" topo map. That would work as long as there hasn't been a considerable amount of sedimentation in the reservoir since it was constructed.

24. Good afternoon Chris! I have been trying for quite some time to rid my breach model of errors. This is my first project modeling a breach and have used the USACE HECRAS Dam Break Study Manual, HECRAS User Manual and also Colorado DNR HECRAS Guidelines to help me model. I am in Ohio but I have not been able to find many resources on this matter over here, even after contacting ODNR.

I have an unsteady flow model with a simulation time window set for 24 hrs: 29JUL2015 12:00 to 30JUL2015 12:00, computational interval set to 30 sec.

The Lake is the storage area. I used the elevation vs. volume curve with my normal pool elevation set as the lowest elevation (this is also my initial elevation in the boundary conditions). The boundary condition is lateral inflow hydrograph. I used the inflow data from a HydroCAD analysis output for the 24 hr probable maximum flood event. The maximum flow is 3000cfs so I used a minimum flow of 160cfs to keep the cross sections from drying. I made sure this was the same value at initial time step. I have Internal RS initial Stages checked in the unsteady flow data.

My last cross section downstream is set to normal depth.

I have 2 dummy cross sections upstream of my dam (placed 15’ apart) with the stream connected to the storage area. The minimum elevation is my pool/initial elevation for these two sections. I have used cross section spacing according to Samuels equation for the remaining downstream sections. I’ve even tried interpolating to see if it makes a difference. Manning's value 0.05 at banks and 0.03 for channel.

For the dam I input an inline structure. The weir data includes an emergency spillway at the end of the dam which is 6ft lower than top of dam. Pilot flow set to 160 cfs. I put the lake bottom elevation as the final bottom elevation of breach (20 ft lower than pool elev). I set the Failure mode to overtopping, starting at 29JUL2015 13:00. Other breach data 3:1 side slopes, 100 ft wide bottom.

The computation blue bar completes the Geometry Processor section but never starts at the Unsteady flow simulation. My error is:
A change (or known value) in WSEL or EG at a node at river station 10 in reach 1 caused a WSEL below the bottom of the cross section. Check your data for errors.

RS 10 is my most upstream section connected to the storage area. The profile plot shows my first dummy cross section upstream of the dam having an astronomical spike in EG Max WS El. up in the billions, which makes all the other cross sections appear to be near 0. I tried raising my minimum flow elevation to 500 cfs with no luck. I even tried to space the dummy sections farther apart. I also tried your suggestion to run in steady flow with cross sections only. The upstream condition was set to storage and it wouldn’t let me run with that so I had to delete storage area and ran with a rating curve and it seemed to work fine, even with the inline structure added next.

I’ve tried moving sections around and changing start elevations and flows and have yet to get the computations to complete. Any advice would be greatly appreciated on what to try next.

Thanks!

1. Sorry not to respond directly to you in here. I trust your model is still humming along nicely with the work we did to fix it?

25. Hi Chris. I need to model the sedimentation process of a reservoir. Can I use HEC-RAS to model this or do I need another model? I've heard of HEC-6 but I haven't been able to download it.

Thanks.

1. You certainly can. RAS actually works very well simulating reservoir sedimentation.

26. Hi Chris,

I am new to HEC-RAS. I am trying to model a river between two very large lakes. However, there is no control structures at the upstream or downstream of the river reach.

1) Should I model the lakes as storage areas? If yes, then "placing the last HEC-RAS cross sections somewhere in the storage area and using a Known Water Surface Elevation as the downstream boundary condition to enter the storage area's water surface elevation" - is this a correct approach? Do it need to assume a control structure at the upstream or downstream for that purpose?

2) In the same model, there is a small ponding area in the middle of the river, more like a in-between storage (significantly small compared to the lakes). There is a partial sluice gate (does not extend across the whole X section) downstream of the pond area. Does modeling this small storage as ineffective area in the X section a correct approach? Or should I go for off-the channel storage and connect with a lateral structure (there is no such structure there)? Another option I can think of is discontinuing the river and placing a storage area - which I am less inclined to adopt since this is a ponding area within the river. Can you suggestion which one of these would be a better approach?

27. Hi Chris,

I am new to HEC-RAS. I am trying to model a river between two very large lakes. However, there is no control structures at the upstream or downstream of the river reach.

1) Should I model the lakes as storage areas? If yes, then "placing the last HEC-RAS cross sections somewhere in the storage area and using a Known Water Surface Elevation as the downstream boundary condition to enter the storage area's water surface elevation" - is this a correct approach? Do it need to assume a control structure at the upstream or downstream for that purpose?

2) In the same model, there is a small ponding area in the middle of the river, more like a in-between storage (significantly small compared to the lakes). There is a partial sluice gate (does not extend across the whole X section) downstream of the pond area. Does modeling this small storage as ineffective area in the X section a correct approach? Or should I go for off-the channel storage and connect with a lateral structure (there is no such structure there)? Another option I can think of is discontinuing the river and placing a storage area - which I am less inclined to adopt since this is a ponding area within the river. Can you suggest which one of these will be a better approach?

1. Personally, if I had the data, I would make the entire thing a single reach with cross sections. No strorage areas. If you do want to do storage areas, you should consider using 2D areas instead. They will better capture any hydraulic gradient and secondary flow patterns. You can connect storage areas or 2D areas directly inline with a 1D reach. No need to place controls at the connection. Just make sure that your storage areas/2D areas don't force the 1D reach to go dry at any time in your simulation. cross sections cannot go dry-your model will crash. With regard to the middle ponding area, I think either of your first two suggestions would work. It just depends on how much detail in your results do you want in that area.

2. Thanks Chris. Just let me reiterate, you mean treating these huge lakes (actually Great Lakes) as part of the reach, thereby taking several X sections of the lake and then putting known water elevation as a boundary condition. Am I right?

3. Yes. You could do that. But making them storage areas would probably work just fine since they are so big relative to the river reach. Usually the deciding factor is how fast the water level changes in the lakes, and how much detail you want in your results in the lakes. You should read this post: http://hecrasmodel.blogspot.com/2010/03/dynamic-versus-level-pool-reservoir.html
Although it compares level pool routing (storage areas) with dynamic routing (cross sections) for dam break simulations, the concepts discussed carry over to conventional modeling exercises as well. The bottom line is dynamic routing will always be more accurate. But under certain conditions, level pool routing will produce similar results to dynamic routing. My guess is the Great Lakes would fit into this category. If you want to send me your model once you have it constructed, I'd be happy to take a look and offer suggestions. Sounds like an interesting model.

28. Thanks a lot, Chris. I appreciate your insights and would like to have your opinion/suggestions once I get the model running.

29. hi, am modelling a large floodplain. how do i ensure parts of cross sections "depressions" don't fill up before the preceding depressions fill up.(

30. Hi Chris,
I have followed the procedure described here to connect the reach to an upstream storage area (Moving object and a dot appears at the connecting point).
1. Do I need to connect the reach and storage area again using storage area connection editor at the top of the geometric data editor (pg 173 User Manual) and then a choose a rating curve/weird?
2. If not, how can I ensure flow from the storage area to the reach? Is it enough to assign a hydrograph at the unsteady flow editor without following the procedure described in 1?
Thanks.

31. 1. No. Moving the stream centerline inside of the storage area is enough for an inline connection.
2. The stage in the storage area and the reach are intertwined when connected. Whatever flow is entering the storage area will be attenated and then passed to the first cross section in the connected reach. This is all done by HEC-RAS automatically once you make the connection.

32. Hi Chris,
I have constructed my model and run a steady analysis. The cross-section downstream of the reservoir is getting a very low flow (though I have assigned a specific flow to that reach in the steady flow editor). In reality, there is no inline structure between the reservoir and the river reach so I am not being able to assign pilot flow to the model. Would you be interested to have a look at it?

1. I'm a little busy at the moment to take a look at it. Double check your n values. Make sure they are high enough and you don't have any typos (i.e., too many zeros).

33. Checked already. The n values are obtained from a previous study and is 0.0327-.022. Is there anything else I can check? Can the upstream reservoir be a problem?

1. Those n values are very low for low flow conditions. It is likely the previous study used those n values for flood levels. At lower flows, you'll need larger n values.

34. hey!! chris, i wanna know how to take boundary conditions for dam break analysis

1. Typically people use a PMF or some other extremely rare design flood event along with a sunny day event (typical conditions at the reservoir).

35. What does mean the error "No records were found (the data returned all -902's"?. That's appears when running the post processor

1. Usually that happends when the model goes unstable and doesn't finalize an output file as a result. Then when RAS tries to find certain output, it can't and so it returns "-902" values, which are considered null values in RAS coding.

36. What does mean the error "No records were found (the data returned all -902's"). That´s appear when running the post processor. It´s relative to the storage area.

37. The error states Lateral structure has user defined HW connections.The user defined list of RS-weir Sta's has more XSs than are necessary to cover weir profile.When I look at the lateral structures editor there is only one XS that at the end of the weir

38. I'm running an unsteady flow file with a lateral structure adjacent to a levee. The error states as follows: Lateral structure has user defined HW connections. The user defined list of RS-weir Sta's has more XS's than are necessary to cover the weir profile. There must either be A XS that is exactly at the end of the weir profile. OR one and only one XS that is past the end of the weir profile. When I look at the lateral structures editor there is only one XS that is at the end of the weir profile. Not sure what to do here. Using Version 5.0.1.

1. You must have more than one XS that is beyond the end of the lateral structure's length defined in the user-defined HW connections. I suggest using the default HW connections if possible.

39. Hi Chris,I have a question.
How can I model a gate with constant water level in the upstream of the gate (as a reservoir) and the flow is free.

40. Hi Chris,how can I model a gate with constant water level in the upstream of the gate ( as reservoir ).

41. Try using the Navigation Dams boundary condition.

42. Try using the Navigation Dams boundary condition.

43. I'm trying to set up a floodplain storage area in the middle of a reach and can't seem to find that particular setup in HEC 5.0. Can you guide me to where I might find a solution for this?

1. It's very similar to this post on modeling a reservoir with a storage area, only you don't have an inline structure. Just move the end points of your reaches into the storage area to initiate the connection.

2. I am modeling a similar situation - a stream connected inline with a storage area. Will the flows from the stream automatically enter the inline storage area or do I need to insert a flow hydrograph to the basin with the flows from the stream? Also, is an inline weir necessary? Thank you

3. For inline connections, yes, flows will automatically enter the storage area, provided you have properly made the connection. No additional hydrograph is needed. And you don't need an inline structure.

4. For inline connections, do you have to define two separate river reaches to initiate a connection to the reservoir? Or can this be modeled with one river reach passing through the reservoir on each end?

5. They need to be separate. One river/reach on the upstream side, and a different one on the downstream side.

44. Hi
i was trying to analyse a steady flow and when i run analysis i faced some errors such as :
(A weir line has not been entered for this storage area connection. If there is to be no weir flow, enter a weir line with an
elevation above expected stages.
The "start" part of the connection must reference a storage area or cross section.
The "end" part of the connection must reference a storage area or cross section.)

1. It sounds like you have a storage area connection, but have not entered the required input data for it. Go to the SA/2D Area Connection editor and either delete the connection, or enter the required input data.

45. Hi chris,
Finally my unstudy model started running after many trails,i have observed that there is no breaching affect on downstream sections after Dam breach. These sections are running only with hydro-graph which was given in the storage area just upstream of inline structure. It is really tankful, if you suggest a solution to my problem.

1. Make sure everything is connected properly as discussed in this post and make sure that "Breach this structure" is checked in the breach editor.

46. Hi Chris,
Thanks for quick reply. Yes, I did it, both the Storage area and River reach at start is properly connected and also Breach this structure was be selected.

1. Hmmm. Not sure what's happening then. I'd have to see the model to know.

2. Hi Chris,
once again thanks, ok i will share my model with you. how can send it to you.

47. Hello

48. Hello, I used HEC-GeoRAS to import the geometry to HEC-RAS, and I draw a Storage area on hecgeoras, although I can't connect it to the reach... how can i do it? Thank you

1. Once you have the storage area and the reach in the geometry editor, select Edit...Move Objects. Then click and drag the end point of the reach inside of the storage area you want to connect to. That should do it.

2. Thank you Chris, I was able to do it, although now I'm having trouble with the DSS output from the unsteady flow simulation. When I get the peak flow time for each station, it doesn't seem right. If you have some time to look at it I'd appreciate it. Not sure if it's a computation time or geometry problem. Thank you, best regards.

3. You need to run a longer simulation. Also you need to have much tighter cross section spacing, especially where there are large gaps between cross sections.

49. Hi Chris -

I am modeling a reservoir in which its inflow is direct from a stream and its outflow is regulated by an 18" circular culvert and a earthen dam/spillway. At low flows, the culvert is plenty to route flow to the downstream reach. How can I best model this, given that inline structures do not contain circular culverts?

Thanks,
Steve

1. Steve- In RAS Version 5.0.3, you can now put in circular culverts. But for any outlet that is not included in HEC-RAS, you can always use a "user-defined" gate and just put in a rating curve. Hope this helps.
Chris

50. Hello Chris.

I would like to know if I can have cross sections beneath the storage area. I don't now much about hec ras and I've always seen that there are only 2 cross sections with the storage area, so can I have a storage area and the cross sections beneath it in its full length?

1. I suppose it could be done. Not sure why you would want to though. Make sure you aren't double-counting volume!

51. Hello Chris,
after i draw my reservoir area and connect it to my reach (upstream), the reservoir automatically connects with my last cross section downstream. Is this normal?

1. No. Not normal. Is your reach properly drawn in the correct direction (from upstream to downstream)?

52. Yes, it is. I have imported my reach and my cross sections from autocad civil 3D.

1. I'm at a loss then. Not sure what could make that happen. If you are able to solve the problem, please share your solution. Thx.

2. Aparently, i couldn't figure out how to import the cross sections through civil 3D so i created manually a reach and copy-pasted them.

3. I am trying to make a dam breach analysis, but i can't quite figure it out and my unsteady flow simulation goes unstable. can i please send you my project to check it out?

53. Hi, Chris

I've been trying to do sedimentation analysis of a dam reservoir.
(catchment=3600 km2, annual rainfall=600 to 700 mm)
I'm using a inline structure (weir) to set a dam with a open spillway.

It works but the reservoir water level is always more than or equals to full, because of the above setting.

However, the river is a seasonal river and the water level is not always full even in the rainy season.
It is plausible that water level is lower than that and the spillout from the weir equals to 0.

Do you have any ideas about how to simulate reservoir sedimentation in a seasonal river on HEC-RAS?

Is setting weir a correct approach?

54. Chris,
I've used a storage area with a lateral structure to model an off-channel raw water reservoir. The model seems to be working well but when I breach the lateral structure my flows in the downstream river are lower than I would expect. I am using a dam breach time of about 1.5hrs and the volume to be drained would end up being 2,290ac-ft (99,752,400cuft)and over the 1.5hr dam breach I would expect approximately 18,472cfs to be flowing at some point (just based on simple math, if I do a broad-crested weir calculations I come up with a similar number) however, the model doesn't show more than approximately 9,700cfs flowing in the river. How do I know that I'm seeing the correct flow or what do you think could be the reason for the low flow?

1. I are considering submergence effects? That would effectively reduce your weir coefficient and could account for less flow than you might think.

55. Hi chris, I have a question, I try to make a flow modelling system from reservoir to spillway using a lateral structure in hec ras, but i little bit confuse about how i can Connect the flow from reservoir to lateral stricture and flow from lateral structure to reach?
I'll appreciate if you can help,
Thanks

1. Lateral structures must be placed in the river and assigned a river stationing. They are set up to flow from the river to a storage area (or another river, or a 2D area). Flow can go the reverse direction over a lateral structure, if the reservoir is higher in head then the river that the lateral structure is in. It will just show up as "negative" flow.

56. Hi Chris,

I am new to hec-ras and have noticed something that I am not sure is a bug or the intended behavior in ras.

I have noticed that when I set a pilot flow through the inline structure (dam) as well as an initial flow in the inflow hydrograph to the reservoir that is equal to the pilot flow, that this causes the max water surface to be lower than it should be for the resevoir/dummy xsections/dam. If I increase the pilot/initial flow value then the max water surface profile gets even lower. For design purposes, I need to know whether a specific storm will overtop the dam, and this approach causes my water levels to be artificially low.

To fix this I thought I could use a pilot flow of 0 and an initial condition of 0, and then add in an initial flow value in the xsection just downstream of the dam. In this way the two dummy cross sections have water as they are connected to the water from the reservoir initial elevation and the downstream elevations have water in them from the initial condition I set for them. The model runs but and at first I thought I had solved the problem. However, when the pilot flow is 0 this causes no water to flow from the dam, even though it is clearly rising above the spillway and even overtopping the dam. The water just keeps on going up until the simulation ends.

Any insight? I can send a link to my model if it would help.

1. Make sure you set your Internal River Station Initial Stage. This is discussed in the last two sentences of the post.