Hec ras cross section spacing

Boundary geometry for the analysis of flow in natural streams is specified in terms of ground surface profiles cross sections and the measured distances between them reach thyrocare.

Written by Chris Goodell October 8, Either way, approaching an unsteady HEC-RAS model especially a dynamic one as a beginner with little experience and understanding of how to stabilize it can cause significant delays in your project and worse, completely blow up your budget. Although the model ran to completion without crashing, it had unacceptably high errors. The following lists out the courses of action taken to stabilize the model. The links following some of these items will take you to more information about that particular technique. Cross Section Spacing. The initial spacing was way too coarse.

Hec ras cross section spacing

Cross-sectional cut lines should be created to capture the entire extent of flooding anticipated by the dam break scenario. As in any hydraulic modeling study, cross sections must be laid out to accurately describe the channel and floodplain geometry. Cross sections are laid out perpendicular to the anticipated flow lines of both the channel and the floodplain, during high flow conditions. Cross sections also need to be added immediately upstream and downstream of: tributary inflow locations; dams and other inline structures weirs, drop structures, or natural drops in the bed profile ; bridge and culvert crossings; levees and other types of lateral hydraulic structures. An example of a cross section layout is shown in the figure below. In addition to describing the physical changes and hydraulic structures within the channel and floodplain, there are also numerical considerations for adding or removing cross sections. In general, cross sections spaced too far apart will cause additional numerical diffusion of the floodwave, due to the derivatives with respect to distance being averaged over too long of a distance. See an example of artificial numerical diffusion in the figure above. In this example, the channel is a rectangular channel on a constant slope, with a constant Manning's roughness. The only change in the example is the cross section spacing. Additionally, when cross sections are spaced far apart, and the changes in hydraulic properties are great, the solution can become unstable. Instability can occur when the distance between cross sections is so great that the Courant number becomes much greater than 1. Another way to say this is that the cross section spacing is not commensurate with the hydrograph being routed and the computational time step being used i. Maximum Cross Section Spacing. A good starting point for estimating maximum cross section spacing are two empirically derived equations by Dr.

Can you tell me what can be the reason?

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Written by Chris Goodell December 2, Written by Chris Goodell, P. All rights reserved. So the question was, which depth do you use? His switch to bankful depth for his final equation is two-fold. One for convenience a depth characteristic is easier to measure in the field than hydraulic radius , but also because bankful depth typically yields the maximum hydraulic radius for typical flood flows in a natural channel. That being said when running extreme events in HEC-RAS, such as a dam breach, more consideration for a larger depth should be given. Though conventional wisdom has been that closer cross sections for implicit solution schemes like RAS make for a more stable model, this is not necessarily true with HEC-RAS.

Hec ras cross section spacing

Written by Chris Goodell October 8, Either way, approaching an unsteady HEC-RAS model especially a dynamic one as a beginner with little experience and understanding of how to stabilize it can cause significant delays in your project and worse, completely blow up your budget. Although the model ran to completion without crashing, it had unacceptably high errors. The following lists out the courses of action taken to stabilize the model. The links following some of these items will take you to more information about that particular technique. Cross Section Spacing. The initial spacing was way too coarse. A visual check alone of the geometry schematic and profile plot should encourage you to investigate a finer cross section spacing.

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Kind Regards Sam Chris Goodell on January 7, Hey Sam- If our model is stable, go to the water surface profile plot and check the max ws profile and see what was actually computed for the headwater elevation of that bridge. Rather it uses hydraulic equations specifically defined for those structures. Current page. That is okay. They should extend across the entire floodplain and should be perpendicular to the anticipated flow lines. Solution solver went unstable, iteration 2 at 15JAN Dam 1 Danny Fread Fread, and P. I was attempting to create a junction to join a section of reach downstream of my main channel that was to be added below the most DS XS of the main channel an afterthought for a railway crossing that was not included in the model to begin with. Hi Chris, sorry for the inconvenience, I have an error too in my unsteady flow simulation. Can you please provide me with your opinion?

One-dimensional 1D hydraulic modeling is based on open channel flow principles and calculations. The physical properties of open channels can vary widely, especially in natural channels. For this reason, it is very important to ensure that the cross-sectional geometry in your HEC-RAS model represents reality as closely as possible.

The geometry process is OK, but in the unstable flow simulation I get the following message in red:. You mention that "cross section spacing too close can overestimate energy loss and precipitate significant error" Do you have a sense of when these errors start to occur, i. Thanks for sharing! If you ever do get a chance to test the threshold of application about the equation's results, I'd love to see the results and get that up on the blog. In general, cross sections spaced too far apart will cause additional numerical diffusion of the floodwave, due to the derivatives with respect to distance being averaged over too long of a distance. Upon running the model, during the performing of the unsteady simulation, i have encountered these error of "RasUnsteady Then apply the probably fairly severe meandering adjustment to the mannings coefficient for that stretch of the river, and see what it gives you at those two sections. But if you wish for it to go away, you could check your elevation-volume curve. Samuels Samuels, End points of a cross section that are too low below the computed water surface elevation will automatically be extended vertically and a note indicating that the cross section had to be extended will show up in the output for that section. Joe on June 29, Good evening Chris, I had a model routing a dam breach flood wave about 5 miles down a river. Also, if that is correct, how often should I do a new cross section?