tag:blogger.com,1999:blog-1675184707067447729.post2992690274805628760..comments2023-07-12T07:12:04.697-07:00Comments on The RAS Solution: How to draw cross sections.Chris G.http://www.blogger.com/profile/00354834185663924786noreply@blogger.comBlogger114125tag:blogger.com,1999:blog-1675184707067447729.post-51718719098961920712020-02-08T07:00:49.980-08:002020-02-08T07:00:49.980-08:00Hi John. In the river, definitely keep your bound...Hi John. In the river, definitely keep your bounding cross sections parallel to the bridge. Out in the floodplain, where you go from parallel to the roadway approach to perpendicular to flowlines will depend a lot on the terrain, the degree of skew of the flowlines to the roadway. Generally, if the skew is quite low (<20 degree approx), I'll keep the cross section parallel to the roadway approach. More than that, I'll generally bend the cross section back to being perpendicular to the downstream flow direction. This is definitely a very subjective exercise. There is no "right" answer. But just keep a couple rules in mind and try to stick to them as much as possible.<br />1. Draw cross sections perpendicular to flow lines. This may require you to dogleg your cross sections if the flow lines are not parallel with each other. <br />2. Keep your bridge bounding cross sections parallel to the bridge deck. Skew the bridge deck and bounding cross sections if they are not perpendicular to the flow lines.<br />Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-30579078894416142602020-02-07T20:52:25.343-08:002020-02-07T20:52:25.343-08:00Hello,
First of all I would like to thank you for...Hello,<br /><br />First of all I would like to thank you for this blog, it helped me demystify the meaning behind a lot of the HEC-RAS capabilities and guided me in correcting and fine-tuning my first simulations. But I am still new to HEC-RAS and I have a problem to solve, one that I do not know how to approach. I am trying to simulate a bridge to be built on a bend of a stream. The problem is that the bridge is also on a road bend, exactly at the intersection with the stream. So there are two intersecting arcs, I am puzzled how to draw my cross sections in order to define the bridge piers. Should my cross sections be arcs parallel to the road – bridge arc? Or is this not acceptable? If my cross sections are straight lines perpendicular to the stream centerline then I cannot define exactly the piers beginnings and ends. The bridge is to have 3 piers and the cross section is going to be a two stage river channel. I will try to provide pictures as an example.<br /><br />https://drive.google.com/file/d/1P-pfwI_d0kbPUqxiK-Ag6e0w96CCbqPc/view?usp=sharing<br /><br />https://drive.google.com/file/d/1WFsYLfWQrkgbMLqdvhBNPduhor3-K5he/view?usp=sharing<br /><br />Also, I have run the simulation in HEC-RAS with arced cross sections and I get the following messages and notes for various cross sections but specifically for the one at the bridge:<br /><br />“The velocity head has changed by more than 0.5 ft (0.15 m). This may indicate the need for additional cross sections.<br /><br />The conveyance ratio (upstream conveyance divided by downstream conveyance) is less than 0.7or greater than 1.4. This may indicate the need for additional cross sections.<br /><br />The energy loss was greater than 1.0 ft (0.3 m). between the current and previous cross section. This may indicate the need for additional cross sections.<br /><br />Multiple critical depths were found at this location. The critical depth with the lowest, valid, energy was used.”<br /><br />I can add extra cross sections at the other parts of the stream but not in the bridge area, is there any way to overcome that and have HEC-RAS solve it without warnings?<br /><br />Any help would be greatly appreciated.<br />JohnAnonymoushttps://www.blogger.com/profile/14255007796687282802noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-5062865862170325432020-01-28T13:10:46.135-08:002020-01-28T13:10:46.135-08:00When in doubt, always draw cross sections perpendi...When in doubt, always draw cross sections perpendicular to flow lines. The direction of flow lines are determined by you, so it takes close examination of the topography and things that "get in the way of the flow" like bridge roadway approaches. You've identified the tricky part of bridges-when to transition from the parallel bridge bounding sections to those same sections being perpendicular to the flow lines. typically I make this transition around the bank station, so the main channel is parallel to the bridge, the overbanks are perpendicular to the flow lines. Of course, every bridge is different so use your judgment. Understanding the basis (and limitations) of 1D modeling, and the computations behind it, will help guide you.Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-1017855244868967922020-01-28T09:44:38.134-08:002020-01-28T09:44:38.134-08:00Hi Chris,
I have a question about x-secs crossing...Hi Chris,<br /><br />I have a question about x-secs crossing the roadway that approaches the bridge. I’ve read earlier questions and comments, some of the replies could be more specific or clarified. Comments I’m looking at were posted 02/20/15 and 11/14/17.<br /><br />I have a stream in a valley with a bridge crossing perpendicular to flow. Before and after the bridge, the approaching road is parallel to the stream/flow. Think of a harsh “S” shape. Originally I drew x-secs with middle portions perpendicular to flow and parallel to bridge, and outside portions parallel to the road and sometime parallel to stream. At the middle part of x-secs flood events will overtop the road, I have appropriate ineffective flow areas defined here. On the outside parts of the x-secs, I believe flood events don’t cross the road. In this case what do you recommend the cross sections do? Cross the road? Keep original shape but add ineffectively flow areas (temporary or permanent)?<br /><br />Additionally, the DOT has suggested x-secs that only remain perpendicular to flow and crosses the roadway several times. Where the suggested x-secs intersect the road it’s overtopped during certain flood events.<br /><br /><br />Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-4334299753186141312019-04-12T06:00:51.207-07:002019-04-12T06:00:51.207-07:00It is Steady Flow model so it looks like the flood...It is Steady Flow model so it looks like the flood flow lengths are most appropriate. Thanks for appeasing my conscious with your advice!<br /><br />Regards, TylerAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-11603575339950244452019-04-10T15:52:32.432-07:002019-04-10T15:52:32.432-07:00typically what is done is if you are looking at an...typically what is done is if you are looking at an event simulation (low flow to high flow back to low, you would maintain the main channel reach length following the low channel and then set your overbank reach lengths to go with the high flow. but if you are strictly focusing on the 100 year flood, then I would set the main channel lengths equal to the flood flow lengths. I think this is the most appropriate way. Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-15435889190904523862019-04-10T14:25:44.139-07:002019-04-10T14:25:44.139-07:00Hi Chris,
I'm hoping you can clarify that pe...Hi Chris, <br /><br />I'm hoping you can clarify that pesky "generally" for me. I have a reach which is well-confined, containing a well-defined floodplain where the channel meanders considerably. As the focus of the project is the 100-yr event, a large proportion of the flow is expected to just shoot over the meanders. I have the overbank flowpaths drawn to reflect this. It has been suggested that I also use a "flood flow" main channel length which would approximate the downstream distances. This makes some sense, but I don't know if it is fully justified. My understanding of HEC-RAS's computation methods makes such an adjustment seemingly unnecessary, but I don't know if doing so is actually erroneous. Looking for literature guidance, I have not found anything to suggest deviation from the centerline - aside from pesky mentions of "generally".<br /><br />Regards, TylerAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-27980137066812857802018-08-16T12:11:26.390-07:002018-08-16T12:11:26.390-07:00Hi Chris,
I am not sure I understand your stateme...Hi Chris,<br /><br />I am not sure I understand your statement “I'll usually measure about 1/3 the distance from the main channel to the waters edge.” What do you mean by “the waters edge”? A side of main channel, or edge of the target flow in your HEC RAS model, such as 1:500 flood? <br /><br />Please clarify it.<br />Westhttps://www.blogger.com/profile/14027840397254644387noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-83662288830420595122018-07-04T10:46:28.895-07:002018-07-04T10:46:28.895-07:00Hi Chris,
is there a method (in RasMapper/Hec-RAS ...Hi Chris,<br />is there a method (in RasMapper/Hec-RAS 5.0.4) to draw XS perpendicular to river center lines?<br />Thanks in advance!<br /><br />Andrea Conca<br />Anonymoushttps://www.blogger.com/profile/00100206570718353548noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-42383012311996835322018-06-28T21:51:32.017-07:002018-06-28T21:51:32.017-07:00I entered the CS and bridges However when running ...I entered the CS and bridges However when running a steady flow. The water level is in no contact with the bridge and the water is above the bridge and out of alignment with the bridgeAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-44622484718949236462018-06-28T21:49:30.260-07:002018-06-28T21:49:30.260-07:00I entered the cross sections and bridges. However,...I entered the cross sections and bridges. However, the water is in no contact with the bridge, it is higher and out of alignment with the bridgesAnonymoushttps://www.blogger.com/profile/05923232972109992508noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-57066434784333816402018-04-20T07:34:13.972-07:002018-04-20T07:34:13.972-07:00dear sir,
we are trying to dam breach analysis. we...dear sir,<br />we are trying to dam breach analysis. we have face to problem that inline structure is always displaced from its original place and it is located near second or third cross section.vijay A.patilhttps://www.blogger.com/profile/18054352217429673535noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-65181291419885170132018-02-14T08:34:41.304-08:002018-02-14T08:34:41.304-08:00Prince-
The cut line itself has no bearing on the...Prince-<br /><br />The cut line itself has no bearing on the computations. However, the station-elevation data does. When you change your cross section cut line length in GIS and then re-extract it to RAS, the station-elevation data will change. If the flow area is impacted by this change, then the results will change. If you manually change the cut line coordinates inside the GIS menu in the RAS geometry window, nothing will change with the computations, since the station-elevation data won't change. However, there is an option to extend the cut lines and sta/el in the GIS Tools menu. That will change the sta/el data so the results may change. <br /><br />But perhaps your question is really "what will happen to the channel water level depth if we increase or decrease the channel width". Typically if you increase the width, the water level will go up locally, but will show a decrease in the backwater upstream of the change. If you decrease the width, the water level will go down locally, but will show a rise in the backwater upstream of the change. Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-64493588705305932002018-02-14T00:52:07.512-08:002018-02-14T00:52:07.512-08:00Hi Chris,
What will be effect on channel water lev...Hi Chris,<br />What will be effect on channel water level depth when we increase or shorter the length of cross cut line?<br />ThanksPrincehttps://www.blogger.com/profile/08735583929876992187noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-16456286058953050012018-01-16T15:18:41.596-08:002018-01-16T15:18:41.596-08:00Hi Tejas-
All I can say is double and triple chec...Hi Tejas-<br /><br />All I can say is double and triple check that you haven't missed a step in the process as outlined in the GeoRAS manual. You can always check the attributes tables and make sure all of the data is there. A common mistake people make that will prevent LOB and ROB reach lengths from coming over is forgetting to designate them LOB and ROB in GIS. Also, if they don't intersect cross sections, lengths cant be computed. Keep trying...you'll figure it out.Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-41808624291403092192018-01-16T11:20:09.910-08:002018-01-16T11:20:09.910-08:00Respected sir,
I have digitised the river reach in...Respected sir,<br />I have digitised the river reach in HECgeoRAS with stream centre line, bank lines, flow path centre lines and XS cut lines as the RAS layer. The geometry is exported to HEC RAS sucessfully but when i look into hec ras at each and every CX the value of ROB and LOB is missing!!! however the channel length is specified..<br />pls help me outTejas Jadhavnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-81658561737081208152017-12-19T08:01:02.927-08:002017-12-19T08:01:02.927-08:00Best not to put them on or over dams, but instead ...Best not to put them on or over dams, but instead lay them parallel to the dam crest at the upstream and downstream toes. Then insert an inline structure between those two cross sections to represent the dam.Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-78857973145930767822017-12-18T13:48:21.079-08:002017-12-18T13:48:21.079-08:00Hi Chris,
What is the proper way to lay cross se...Hi Chris, <br /><br />What is the proper way to lay cross sections over dams? Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-79418236060493564532017-12-02T09:55:52.417-08:002017-12-02T09:55:52.417-08:00Hi,
i am extracting manning n values for crosssec...Hi, <br />i am extracting manning n values for crosssections in arcmap and it gives the message that N values are extracted successfully but the manning table is empty. i can't figure out the problem.khansa Gulshadhttps://www.blogger.com/profile/11842793272376754933noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-18814913310791993082017-11-17T01:37:42.096-08:002017-11-17T01:37:42.096-08:00Oh I see! The main reason why I'm doing it is ...Oh I see! The main reason why I'm doing it is because the cross section data from bathymetric survey is quite a lot since the main channel is long and there quite a number of tributaries. Aside from that, the channels are not adequately represented in the DEM of the catchment because of their depth. I was kinda hoping that by directly importing the cross section from CSV files will somehow speed up the process. Nonetheless, thanks for the insights! I appreciate them a lot! I'll try doing it thru GIS if I still can't make it work.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-50006744851495491072017-11-16T15:13:56.160-08:002017-11-16T15:13:56.160-08:00To be honest, not too many people use the import b...To be honest, not too many people use the import by CSV file method. That would explain why it is so buggy. I haven't done it myself, I like to import using GIS format. If I were to try importing by CSV, I'd probably do one river/reach at a time.Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-79925140416983673812017-11-16T01:56:02.081-08:002017-11-16T01:56:02.081-08:00Chris,
Thanks for the response and tips! Follow u...Chris,<br /><br />Thanks for the response and tips! Follow up question, I just thought about after you mentioned referring to the manual because it wasn't see there. If I have different files of the surveyed data of the main channel and several tributaries, should I merged them into one CSV file before importing to hecras or should I make individual CSV file for the main channel and each tributary?<br /><br />Thanks a lot and more power to you and this BL!<br /><br />-AlexisAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-23139952563409439622017-11-15T14:50:02.712-08:002017-11-15T14:50:02.712-08:00Alexis-Not sure why that is happening. All I can ...Alexis-Not sure why that is happening. All I can suggest is that you follow the directions closely in the GeoRAS manual for creating your geometric elements in GIS and importing them to RAS.Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-77634800503224304232017-11-14T19:11:16.682-08:002017-11-14T19:11:16.682-08:00Hi Chris!
Thanks for the reply! I think you under...Hi Chris!<br /><br />Thanks for the reply! I think you understood my question differently seeing that I didn't mention that my surveyed data has complete details, meaning there is stationing as well as the northings, eastings, and elevation. Lucky me! :)<br /><br />What I'm actually having problems with is because when I tried importing the data points, all the cross sections are seem to follow the correct positioning/location of each cross section only the main channel and tributaries doesn't have nodes/junctions. Also, warning/error message says cross sections are not georeferenced.<br /><br />I totally understand that this is the case because I haven't established the centerlines yet. However, when I tried digitizing the centerlines in ArcGIS from a DEM and exporting it to HEC-RAS, it still doesn't seem to fix it. The warning/error message is still there and the stationing as well as the reach lengths don't adjust themselves. And I'm quite confused because this is the process that the reference manual states when creating the model geometry from csv file format.<br /><br />I hope I made my problem a bit clearer this time. Thanks for your help!<br /><br />- AlexisAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1675184707067447729.post-90228589132006168332017-11-14T08:23:52.312-08:002017-11-14T08:23:52.312-08:00You've highlighted one of the more difficult s...You've highlighted one of the more difficult scenarios for laying out cross sections. Yes, you can bend your cross sections so that they remain perpendicular. However, you want to be careful about crossing over the approach roadway. Make sure you have a good feel for whether or not water will overtop the roadway during the simulation. If so, you'll want to avoid having your cross sections intersect your road profile. Unless you think water will be moving away from the bridge, in the direction of the road, then you can do that. Chris G.https://www.blogger.com/profile/03272464763887890080noreply@blogger.com