Tuesday, July 31, 2012

How to draw cross sections.

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2012. All rights reserved.

Cross sections must be perpendicular to the flow lines at all locations.  And they cannot intersect with each other.  That is why it is common to see cross sections snap at different angles outside the main channel (we call this doglegging).  The trick is to keep them from intersecting, while also staying perpendicular to flow lines.  In the figure below, the dark blue line represents the main channel.  The brown lines represent the edge of the flood plain.  The light blue lines are my impression of the flow lines through this terrain, if water were flowing appreciably in the floodplain.  The green lines are cross sections.  Notice that the cross sections are drawn so that they are not only perpendicular to the main channel, but also to my perception of the flow lines in the floodplain.  It can be very helpful to draw these flow lines before cutting cross sections. 
clip_image002
It takes a little bit of practice to do this correctly, and most of the time some trial and error, but as long as you remain perpendicular to the flow lines and don’t intersect, you’ll have a good set of cross sections. 
Where it can get tricky is at a junction.  The following RAS Bloggery article will help with junctions.  http://hecrasmodel.blogspot.com/2009/02/how-to-best-model-junction.html

86 comments:

  1. Do the cross sections always have to begin at the same point on a line? I.e 0 is always the same distance from an imaginary centreline down the middle of the river.

    If this start point varies how does HEC-RAS know how the channel meanders?

    Also where are LOB and ROB downstream reach lengths measured from?

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  2. Hi Robert. Great questions. No...the cross sections do not have to always begin their stationing the same distance from the imaginary centerline. One cross section could start its stationing at 500, and the very next one can start its stationing at -200. Most people will start their stationing at 0, but you don't have to. Some people like to call the "0" point the channel centerline, and so all station points to the left the centerline will have negative stationing and all points the right will have positive stationing.
    Now...how does HEC-RAS know the meandering characteristics of the river? The ONLY effects of meandering that HEC-RAS accounts for in the computations is the varying reach lengths (LOB, Channel, and ROB) for a given cross section. For example, if you have a river bending to the left, the LOB reach length will be the shortest and the ROB reach length will be the longest. Sometimes you have a fairly straight floodplain, while the main channel meanders around within that floodplain. I that case the main channel will have longer reach lenghths than the overbanks. The instructions for measuring the LOB and ROB reach lengths is to follow the perceived "center of mass of flow" of the respective overbank. I'll use contour maps to help guide me, but in the absence of good terrain information, I'll usually measure about 1/3 the distance from the main channel to the waters edge.

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    Replies
    1. why am i getting the same flood extent for different discharge values in hec ras/ ras mapper

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    2. Thank you for all of the resources you provide, Chris!

      I was wondering, do you still apply this 1/3 rule if the water's edge is within the main channel portion and not over the overbank station?

      For example, on one side of the XS, the overbank station is well defined and there considerable flow within the overbank. On the other side, the main channel bank is steep and high with no grade break until the very top, say like adjacent to a roadway, and no flow is in the overbank portion?

      For the steep bank side, is the OB length irrelevant because it never reaches the overbank station? Or is there some kind of doglegging considered while the flow is within the main channel?

      I think it would be the latter but wanted someone knowledgeable to weigh in.

      Thank you in advance!

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    3. You're welcome! You are correct in your assumption, if the water never gets to the steep side bank, then what you do for the left overbank flow line is irrelevant. It will never compute flow in that overbank. You could even cut the cross section off at the top of that bank and not even have a left overbank. Just put the bank station on the end point. Good luck!
      -Chris

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  3. Thanks for your response Chris this is all very helpful info.

    Just double checking is it only the LOB ROB and CHANNEL that affect the calculations not the stationing (i.e if you had one channel centreline at station 100 and the next cross section centreline at 200 but the LOB and ROB are equal then the calculations would not be affected)

    Also I am currently modelling a long stretch of river which bends at almost a right angle. In order for cross sections to not overlap the cross sections must cut across the channel parallel and then change direction to go vertical and avoid overlapping? Will this give an accurate representation of the floodplain and flooding? Or can the river be split into different sections.

    Thanks again for your help

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  4. If I had a bridge and a road crossing the main channel at a skew, would I make my sections adjacent to the bridge, parallel to the bridge, or would I still have them be perpendicular to the flow at the channel?

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    Replies
    1. I have the same question, a worked example of a skewed bridge and defining the cross sections to the approach would be very helpful. The manual has a small amount of information on this and shows the basic plan but in RAS I cannot get the plan view to look anything like how it is in real life and cannot be sure of the results.

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    2. Good idea. I'll work on a post for skewed bridges. Thanks for the suggestion

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  5. Your bridge bounding cross sections (Sections 2 and 3 in RAS terminology) should be parallel to the bridge. AND you should skew these cross sections (as well as the bridge) by going to the Options menu item in the Cross section editor.

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  6. I'm having a hard time placing bridge bounding cross sections (Sections 2 and 3) parallel to my bridge; because they end up crossing the channel twice. Is there a way to post screen shots?

    I read through the skew bridge blog but I'm unclear on how to proceed.

    Please note that it is a railroad crossing where I am expecting that my high flow (500-year) will be at least 3 feet below the low chord. So I am not worried about weir flow conditions.

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    Replies
    1. If you post a question in the forum, you can insert an image.

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    2. In the forum, I'm not seeing the "insert image" option. Should I use a particular web-brower? Do I need to be Logged in?

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    3. When you select "New Message or reply to an existing message, you'll see a text box to write some text. Above that text box is a button that says "Insert Image". Maybe you have to be logged in. Not sure.

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  7. Hi all,

    I found it tricky to draw cross sections perpendicular to the flow line in some cases that I don't know the floodplain extent.

    Can we draw the cross section perpendicular to the contours on the left and right banks instead?

    K Nguyen

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    Replies
    1. That would be a good approximation if you are unable to draw flowlines, but theoretically, it should be perpendicular to the flowlines, not the contours. Flowlines are not always perpendicular to contour lines, particularly in areas contraction and expansion.

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  8. Hi,
    I did imagine that the LOB and ROB data scewed the next (downstream) cross section relatively to the modelled by decreasing and increasing the distance to the next LOB and ROB. So that is not the case? The cross section itself is doglegging?

    But how do you model a cross section like the bottommost in the example picture above? On the right-hand-side of the main river, the cross section snaps twice.

    Caroline

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    Replies
    1. LOB, Main Channel, and ROB reach lengths simply give RAS a length over which to apply the friction loss. Doglegging teh cross sections provide the most 1-D hydraulically appropriate set of station elevation points. Two different things.

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    2. So how do you get the cross section to dogleg? Is there a certain feature in HEC-RAS where you can do that, or does that have to be defined from Autocad before exporting them?

      If it's done through Autocad, how do you export those cross sections from CAD to HEC-RAS? What I've done before is expot an alignment line with cross sections 20' apart for example, and they come into HEC-RAS as straight lines across the stream and floodplain. Am I supposed to draw the cross sections (perpendicular and doglegged) then define them as their own profiles and export them?

      Thanks Chris G. for your response, I've read many of them and they're all helpful.

      -Emad

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    3. Doglegging takes place in your GIS application. We use ArcMap, but I believe AutoCAD can do the same. Getting the cross sections into HEC-RAS from AutoCAD requires some 3rd party application, which I'm not familiar with. In ArcMap, we use HEC-GeoRAS to do this.

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  9. When creating river banks and flow paths with GeoRas, how do you handle a bridge or abutments that come into the reach? Do you draw the flow paths over the bridge as if it was not there and same for the river banks? Or do you follow the contouring around until the opening?

    ReplyDelete
    Replies
    1. If you expect the bridge to be overtopped in your simulation, draw your flow paths and bank lines up and over the bridge deck, just as you would expect the water to flow. If everything is contained within the bridge opening, then draw your lines through the bridge opening.

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  10. I've been modeling (HEC-2 and HEC-RAS) too long....Need to retire. Recently, I was sitting in a live conference of HEC-RAS / HEC-GeoRAS was being presented. The presenter had a skewed bridge, so instead of utilizing the Skew Routine in HEC-RAS, the presenter, when cutting the X-Sect at the US and DS sections of the bridge, cut the X-Sections perpendicular to the flow In and Out of the bridge, and allowed his cut-line to cross the road. This was surprising to me. I've always paralled the road @ toe of slope US
    and DS. Would like some discussing about this method. "When cutting x-sections, should you cross the road that the bridge is located or keep the x-section parallel to the bridge?"

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    Replies
    1. That technique is not recommended. You are correct that keeping your bounding cross sections parallel to the bridge deck is a better way to represent the hydraulics. However, out in the overbank, I've crossed over the roadway approach to keep the cross section perpendicular to my perception of the flow direction (this is common when the roadway approach bends significantly relative to the alignment of the bridge deck. As I always say, when in doubt, keep your cross sections perpendicular to the flow lines.

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  11. Hi,
    Is there anyway to calcualte the area of each cross section in HEC-RAS?

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    Replies
    1. There are a few different ways. If you are interested in the flow area (i.e. the area of the cross section that is wet, you can get that from the profile output table. If you want the area of the entire cross section (end point to end point), you can get that from the hydraulic property plots/tables (View...Hydraulic Property Plots), and select Total Area as the variable. Also, and this is less precise, but you can measure the area with the measuring tool in the cross section plot. Just hold down the Ctrl key and click points around the perimeter of the cross section. Then when you've completed a polygon the shape of the cross section, release the Ctrl key and you'll see an Area value pop up.

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  12. Hi,

    I am trying to remeander a reach of a brook, and I am trying to get HEC RAS not to show me a straight line in the geometry data editor, something more similar to the image at the top of the page. I have been trying to import it from AutoCAD and ArcGIS, unsuccessfully though. I have also tried to import the GPS coordinates of a few keypoints along the reach (from a CSV) but it is not working either.

    Is there a way I could get the reach looking not like a straight line but how it is planned to look?

    Thank you!

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    Replies
    1. Yes, you have to set it up in GIS and then import. The GeoRAS manual steps you through the process very well and should help you do this.

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  13. Thanks for the post, Chris.
    I am undertaking a assessment for a creek for existing and proposed cases. In the post case, two sections of the creek will be re-aligned, due to encroachment of a new road. The re-alignment of the creek is pretty much shift the creek about 2 meters and straighten the creek. My question is, do we normally keep the same XS lines for both the existing and proposed cases? if we do, that means some XS lines won't be perpendicular to the flow direction in one of the case? but we don't keep the same XS lines, it will be hard to compare the water levels between the existing and proposed case? (which is the main purpose my modelling).

    ReplyDelete
    Replies
    1. In these situations, I realign the cross sections so that they are perpendicular, but I try to keep the intersection of the cross sections with the stream centerline at the same river stationing. That way, they will be consistent on the profile plot and you can do a direct comparison. But you are correct, it will not be entirely consistent, since the cross section orientations have shifted from existing to proposed. You just have to keep that in mind and communicate that clearly to your client and other stakeholders.

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  14. Hi Chris,
    We are developing the cross cut line for a large river basin in Hec-Geo RAS. Due to meandering nature of river line the automated process of XS line construction, generate irregular cross cuts, which need to be correct manually for Hec RAS run.
    Do we have any other automatic way to generate cross cut for such large river basins?

    Thanks,
    Prince

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    Replies
    1. I don't. I've always drawn in my cross section cutlines by hand.

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    2. Thanks for your prompt reply.
      I wonder, if we could find a approach for it. Otherwise it would be time consuming exercise for large basins.

      Many Thanks,
      Prince

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  15. Hi,
    I am using HEC_GeoRAS in ArcGIS for extracting the geometry data. Now, the river I am modeling does not have much of a floodplain so I have the dark blue as the river/flow path centerline and dark brown bank lines. But when I extract the data to Hec-RAS, LOB and ROB reach lengths are missing. I have extended my cross-sections beyond the bank lines. Can this be a reason for it? I have already went ahead with the modeling. Is there a way this can be corrected now?

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    Replies
    1. Make sure you have drawn flow lines for the overbanks for each river/reach and that you have assigned a subsection to them (LOB, ROB).

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  16. Hi,
    How do you change the view angle of the whole schematic plot ?..for presentation i need north of the plan to be towards top of my sceen

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    Replies
    1. You can't in HEC-RAS. But you can export an image and rotate it in your favorite image handling software.

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  17. whether negative sign can be used in HEC-RAS geometric data at meandering portion of river when cross section criss-cross each other.

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  18. how to enter centerline of river in HEC-RAS from AutoCADD file of GIS file.

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  19. negative numbers can be used in cross section stationing. However, surveyed cross sections, or cross sections cut in GIS should never criss-cross or overlap. Sometimes, just due to the schematic representation in HEC-RAS, your cross sections may criss-cross in the geometric data editor, but they shouldn't criss-cross in a georeferenced project.

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  20. Hi Chris,
    I have had a few problems with generating cross sectional data from Inroads into HEC-RAS v4.1.0. The first problem is that I am trying to use our survey alignment for stations within HEC-RAS but since we have station equation adjustments the cross section stationing within HEC-RAS does not match like it should. It appears that the .geo file that is generated takes cross sections at intervals based on the linear geometry rather than our station values (our survey data is adjusted about 90-95 feet depending on its location). The other issue I am having is that the export does not seem to recognize arc lengths for the stream alignment and makes a line connecting the 2 points that formed the arc. Do you happen to know of any fixes for these problems?

    Sincerely,
    Randy

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    Replies
    1. Randy, I don't have experience with Inroads, so I'm afraid I can't help with that. As far as arc lengths, best way around that problem is to use more vertex points to define your stream centerline.

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  21. Hi Chris,

    I don't know if my last post went through so sorry if this seems repetitive. I am currently having issues with generating geometry from inroads into hec-ras and am looking for some assistance. The first problem that I am trying to rectify is the stationing that appears within HEC-RAS. I am trying to have our survey stations be the same in HEC-RAS but our stationing has a few station equation adjustments along it (our stationing is adjusted 90-95 feet depending on its location). The generated .geo file appears to take cross sections at the given intervals based on the length of the alignment rather than at our station markers yet the "Generate Water Surface Data" recognizes our station naming when asking for information regarding the alignment. The other issue is that the generated geometry does not recognize arcs within the alignment and substitutes them with a line connecting the start and end of the arc. If you know of any fixes for these that would of great help.

    Sincerely,
    Randy

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  22. Hi Chris. Is is typical to have cross-sections on one reach intersect with cross-sections on another reach? My gut answer is "it depends on the flow regime and how close these reaches are to each other."

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    Replies
    1. No. You should avoid this. Overlapping of cross sections (unless purely schematic) causes overestimation of volume.

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  23. Hi Chris. Is it typical to have cross-sections on one reach intersect with cross-sections on another reach? My gut answer is that it depends on the flow regime and how close these reaches are to each other.

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  24. Hello Chris when i create the tributary in HEC-RAS it give message that these section are not geo referenced, so when export to arc-gis ,the downstream reaches of the junction do not export to arc???

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    Replies
    1. Anything not georeferenced will not properly export to ArcGIS. You must make sure everything is georeferenced to be able to map.

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  25. Hello Chris when i create the tributary in HEC-RAS it give message that these section are not geo referenced, so when export to arc-gis ,the downstream reaches of the junction do not export to arc???

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  26. How does one draw a long section in Hec Ras

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    Replies
    1. Same as a short one. Just longer.

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    2. What I mean is I have a pipeline about 40 km the pipe crosses about +- 20 dry streams, which I used Hec Ras to run my floodplains now I need to show the flood level for each river on my vertical alignment (civil 3D) where the pipe crosses the river.

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    3. Better to model each stream individually then, rather then with one long cross section.

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  27. Ok thanks Chris, how to do I find the flood level @ intersection where my pipe crosses the stream, because all cross sections have different flood levels.

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    1. If you have a cross section located where the pipe crosses the stream, you can get a flood level from that cross section. Maybe I don't understand what you're trying to do.

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  28. Hei Chris

    I am working with HECRAS 4.1. I want to ask you one question about calculating downstream reach length, LOB, Main Channel, ROB. The downstream main channel length is well understood. but what does downstream reach lengh for example LOB mean ? Is it distance measured between two consecutive left bank stations or between water edges ? And how are they measured in Autocad where c/s are produced ?

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    1. The overbank (LOB and ROB) reach lengths should be measured along a line in the respective overbank that follows your interpretation of the center of mass. So it should be somewhere between the bank station and the edge of water. It is a very subjective thing to do, but fortunately, it is not very sensitive to the results, normally.

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  29. Chris,

    I have a river that runs parallel to a road (pretty much), then crosses the road (almost perpendicular), and continues on the downstream side parallel to the road.
    I am struggling with how to draw my bounding cross sections. On the upstream side I have been drawing the x-sects perpendicular to flow which resulted in the x-sects crossing the road. If I draw my bounding x-sects (sects 2 and 3) parallel to the road I end up doglegging section 3 across the road such that I don't cross an upstream cross section, and also to contain the flow within the cross section (I suspect that the WL will exceed the top of road).
    Question 1. Is it acceptable for the x-section to cross the road?

    If I do this then I also end up doglegging and crossing roads on the downstream side 3 more times (where I have 3 crossings pretty close together).
    Question 2. Would it be better to draw the bounding x-sections perpendicular to flow (and hence crossing the road at both bounding sections 2 and 3) at the upstream culvert, thereby eliminating the need to cross the road at the next 3 downstream culverts?

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    1. Yeah, sometimes you have to cross over roads. That's okay as long as you are keeping the cross sections perpendicular to flow lines.

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    2. Thanks Chris.
      Do you have any suggestions on how to enter the road deck elevation if my bounding cross sections are not parallel to the road?

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    3. It's tricky. Try to keep the road deck on top of the bridge at least while in the main channel. In the overbanks, if the approach road turns significantly, you can cross over it. The bounding cross sections should likewise be parallel to the bridge. Make sure to skew the bounding cross sections. All other cross sections should be kept perpendicular to flow lines. Dog leg them as necessary.

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  30. Chris,

    I am assisting in a widening/restoration project that is looking at incorporating an existing water quality basin that runs parallel to the creek and I was curious as to the best method for modeling the additional area since there will be ineffective flows that should only fill up when the remaining separation is over topped and then doesn't recede back into the creek. It is almost like a storage area but there will be an area above the ineffective area that will flow at certain wsel. I have included a drop box link with the existing condition cross section and the proposed version. I also include some comments regarding my proposed methods. Any help would be much appreciated. https://www.dropbox.com/s/w3fdyvgtk5446xh/Cross%20Section%20Info.pdf?dl=0

    Sincerely,
    Randy

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    Replies
    1. You might want to consider breaking out the area to the side and model it with a storage area. You can connect the cross section(s) to the storage area with a lateral structure.

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  31. Good afternoon.
    I have a question about the Flow Path Centerlines. What is the most correct way to draw these lines?

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    1. The center flow line should generally follow the stream channel invert. The overbanks should follow your approximation of the center of mass of the respective overbank. A common method is to go 1/3 from the bank station to the water's edge.

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  32. Hello Chris,

    I have looked around, and can not find a reasonable explanation for this. Why can't two cross sections intersect?

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    Replies
    1. Quite simply, if you overlap or intersect cross sections, then you will be double counting volume and your results will be in error.

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  33. Dear sir,
    Firstly I would like to thank you for all your useful advices that I have already read.
    I am trying to make a project using Arc 9.3 Hec-GEOras 4.3.93 and Hec-Ras 5.0.1 and is about dam failure.
    I have a lot of questions and I would appreciate if you could help me. I will use photos to my questions so it will be easier for you to help me. How can i upload here one?

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    Replies
    1. The forum would be a much better place to submit photos and to solicit help for your model. https://hecrasmodel.blogspot.com/p/hec-ras-bloggery-forum.html

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  34. Hi Chris,
    Thanks for this amazing blog. I am new to this world and I have learned a lot while reading your comments/replies. But I cant find solution to my own problem.
    I have developed a hec ras model for the steady state but I cant get it to run. It stops computing when it is developing "cross section interpolation layer". The model has 520 cross sections along four tributaries of total 42 km in length. The model works fine for 2-3 km reach in each tributary but the model stops computation, if I try to run it for the entire tributary. I cant ungerstand why only a few km reaches are runing when I have used the same approach for obtaining cross sections throughout tributaries. Any idea what could be going on?
    Thanks for your help! Nits

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    Replies
    1. Not really sure what's happening there. You might try to use a slightly different max spacing and reinterpolate.

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  35. Hello Chris,

    Thanks a lot for all your pro-bono HEC-RAS work. I am fairly new to this software and have some fundamental questions.

    In response to a query above regarding overbank flowpaths, you responded stating that they should approximately follow the center of the mass of flow and that generally is 1/3rd the distance from the channel centerline to water's edge.

    1)If you are modeling a previously unmodeled stream, what's the best way to judge where the water's edge would be?
    2) I have read varying opinions on where the bank stations should be set but I haven't been able to come to a conclusion. I found two very short forums a) http://www.eng-tips.com/viewthread.cfm?qid=343019 and b)http://hec-ras-help.1091112.n5.nabble.com/bank-stations-td190.html that briefly discuss this topic but it would be very helpful if an expert like you can parse the information.

    Could you please browse through those two links and provide a short summary of how to locate bank stations?It would be great if you are able to write a separate detailed article on this topic. Thanks,

    VB

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    1. Hi VB-

      1. That's where the 1/3 comes from. It's actually 1/3 the distance from the bank station to the edge of water. You'll have to take your best guess as to where the edge of water will be. Don't worry though, the results are typically not overly senstive to the overbank reach lengths. All the action is happening in the main channel.
      2. The most accurate way to locate the bank stations if you want to stay true to the conveyance distribution method used in RAS is to locate them where you have significant change in conveyance between the main channel and the overbanks. No one knows where this is exactly without running the model first, so typically modelers will put them at the obvious change in grade (or top of bank). A change in roughness is also a good place to key in on. And typically, your main channel width shouldn't change drastically from one section to the next. Reviewing the comments on the threads you link to, they're going into a lot of detail and discussion, most of which seems logical and there's certainly a lot of "industry standard" practice being quoted, but if you want to know for sure you have it correct, put it at the change in conveyance. If you want to double check where this is, do a velocity distribution with a lot of small slices and run the model. For most cross sections of typical rivers/floodplains, you'll see clearly where this is. But like with most things in computer modeling, there are always exceptions to the rule, so don't take it as gospel. I like to tell people to understand how the computations are being made, know your modeling objectives (i.e. what questions are you trying to answer), and use common sense, and you'll get it right every time.

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    2. Chris- Thank you for your detailed responses. It helps to know these things so that I don't start doubting my model-setup if it doesn't conform to one school of though or the other.

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  36. Dear Chris,
    I am working on flood delineation using HECGeoRAS with ArcMap 10.2. while doing flood inundation in mapping unit I faced error stating "system.Runtime.IteropService.COMException(8x80041098):ERROR010316 Unable to open the input raster:....Error in Executing grid expression at ESRI.ArcGIS.SpatialAnalysi.RasterConditionalOpClass.SetNull(]GeoDataset ConditionalRaster.IGeoDataset False Raster) at HECGeoRAS.clsMFloodGridIntersection.CalcDiffGridMonolith(IGeoDatasetPWSDS,String sUniqueDepthGridName) how can I overcome this problem? Thanks

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  37. Hi Chris ,while studying backwarer study on Hec Ras we came across problem like length of few cross section is about 15km the reason for this is that dam located in such a area where one side(say left) is elevated while other side(say right) is amost flat,getting ground level above 3to 5 mtr.of dam/full reservoir level is quite difficult also lots of tributaries associated with main river. How perform backwater study in such a scenario by using Hec Ras.

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    Replies
    1. If you're running a steady flow model, and most of the 15km portion of the cross section is ineffective flow, you could ignore it (omit the ineffective portion from your cross section). Tributaries can be included as their own reaches, connected to the main stem with junctions, or as lateral inflows.

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  38. hi chris,

    i've been wandering around as i encounter a problem while trying to Export RAS Data in Hec-GeoRAS but not much help. the problem says c:\Program Files(x86)\HEC\HEC-GeoRAS\bin\NewRasControl.xml and in the dialog message box "target location not specified..."fyi i've re-drawn my xs for times and it fix other problems but not this one. could u plis advice where i probably went wrong? Also is there a specified distance between two xs and i extended my xs very far away from flowpath....is it okay? i really appreciate if u could help..fyi im using arcmap 10.4 and hec-georas 10.2..thank u so much

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  39. Dear Sir,We are Working With Project Where We Have to estmate backwater level by using HECRAS v4.1.0 but we are facing a problem as main rivers accompanied by nearly 17 tributaries (10 tributaris on right side,7 tributaries on left side of river respectively) we have to study upto 24 km.it seems that cross section of river goes into tributaries &tributaries cross section goes into main river in objective to get GL 3 to 5 mtr.above The FRL.Also is it possible that manning value 'n' is same upto 24km? Please help us in this regard -bvs

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  40. i have done finishing cross sections and exported to HEC-RAS. but the cross section profile in hec-ras i.e the left and right bank are not unique. How to edit that cross section geo,etry in HEC-RAS.

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  41. Hi, Chris! I am working on a dam break project and I've recently received the survey data. Problem is that, once I've georeferenced, the elevation points do not follow a straight line. Some points are displaced. I've uploaded a print screen in the imgurl to illustrate.

    http://imgur.com/JFBjRYc

    Can you tell me if there might be some kind of problem by inserting this kind of cross section? Because I've run the model and have checked the results and apparently there is nothing wrong, however, I'm not 100% sure that it's ok.

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    Replies
    1. Matheus- That is strange. I've never seen that before. Must just be a graphical thing since the results look okay. You might consider manually moving the points into position if you want it to look more true to reality.

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  42. Hi Chris,

    I just wanna ask. What if I have an existing project and I need to work on it again but with bigger extent of study (i.e. river basin considered increased/enlarged thus extent of hydraulic study for the main channel and tributaties will be farther upstream), can I still use the existing centerline, banklines, & flow paths shapefiles from the previous project and just extend it further upstream (up to the portion of channel included in the new&bigger basin) when I digitize the model with HEC-GeoRAS in ArcGIS?

    Do you (somehow) know if this process will cause any problem that I might encounter?

    Thanks for the insight! I appreciate your help.


    -Dylan

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    1. Dylan, yes you can do that. Just make sure that where you have increased your model extents, the river stationing, reach lengths, etc are all consistent with what you already have in your model. Should work fine though. Give it a try!

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    2. Thanks for the response. I'm not sure I quite understand what you mean. Because I thought those parameters will change since the area of study (channel, flow paths,bank lines) will extend farther upstream and I'll have to repopulate the attribute table for this parameters before exporting them to hecras. Am I right? Also will it work for xs lines as well?

      To be honest the reason I'm not entirely sure about this process is because I'm still waiting for a more refined DEM so I havent tried doing it yet. I'm mostly relying to instinct and answers from those who have encountered and done this before.

      Thanks

      - Dylan

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    3. Yes, you are correct. Once you've defined your new model extents, just go through the GeoRAS steps from start to finish. That will redo all of the attribute tables and prep your data for exporting to RAS.

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