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.

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.

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 lengths. Cross sections are located at intervals along a stream to characterize the flow carrying capability of the stream and its adjacent floodplain. They should extend across the entire floodplain and should be perpendicular to the anticipated flow lines. Occasionally it is necessary to layout cross-sections in a curved or dogleg alignment to meet this requirement. Every effort should be made to obtain cross sections that accurately represent the stream and floodplain geometry. An example of laying out cross sections is shown below in the figure below. The general approach to laying out cross sections is to ensure that the cross sections are perpendicular to the flow lines. This requires an estimation of what the flow lines will look like in the overbank areas away from the main channel. One option is to draw a stream center line down the main channel along what is perceived to be the center of mass of flow. The same thing should be done for the left overbank and the right overbank. The assumed flow paths for the channel and overbank areas are shown as dashed lines 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. All of my structures were built in except for one and the model was running smoothly.

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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. This blog post will list some guidelines for drawing cross sections. In the past, obtaining accurate geometric data required a significant amount of time and money. This is because somebody would typically need to go out and survey cross sections. While this may still be necessary in some cases, light detection and ranging LiDAR makes it a lot easier to obtain topographic data. It is important to examine the topographic data you are using for your model. You may need to manually manipulate some cross sections to depict certain features that were not picked up by the LiDAR. Large rivers with relatively flat slopes can be modeled with cross sections spaced as much as 2, feet apart.

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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Matias on February 4, Hi Chris, sorry for the inconvenience, I have an error too in my unsteady flow simulation. You could cut several sections around the semicircle, and have them all terminate at the center point of the circle. That version is referenced in my grad open channel flow text Sturm which I also highly recommend. The River Station identifier may correspond to stationing along the channel, mile points, or any fictitious numbering system. Interesting blog, I learn a lot of new things. Where abrupt changes occur, several cross sections should be used to describe the change regardless of the distance. I've always found it to be helpful to have a discussion with the reviewer before you get too deep in a project if possible to discuss general approach and expectations. They have photos of flood plains with n values assigned to them also, which I've found quite helpful. Danny Fread Fread, and P. Thanks for the timely response Chris. It is a theoretical derivation of spacing based on the inherent numerical errors involved with linearizing the St. 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.

Written by Chris Goodell May 19, Each post will discuss a feature in the menu items, starting with the Tools menu item in the Geometry Schematic. Cross Sections should be selected early on in your project to adequately define changes in geometry, roughness, bed slope, and discharge.

Particularly for a dam breach model, where the presumed errors of the input data probably far outweigh these small numerical errors. Taccioman on February 7, The maximum xsec wsel error was 0. See message below please…. Could you come up with the solution? It seems to be different for every model. Anyway, to answer your question, not really. After this, the red bar starts running so I am kinda blocked on how to proceed to correct these errors. Your cross-sections should bend to always be perpendicular to the flow. I recently had to completely do over a model that someone else had had three stabs at. There we have it. If you can think it up, it can be done. Log In Come Join Us!