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.

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.

Hec ras cross section spacing

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. And, in fact, cross section spacing too close can overestimate energy loss and precipitate significant error, and eventually numeric instability. The bottom line, and I mention this every time I go over cross section spacing with students, is use an equation that seems to work for the rivers you work on to get a good starting point for spacing. Then adjust from there as necessary. This is the length of channel affected by some disturbance downstream.

If that's not it, I'm afraid I won't be of much help. Is it possible that you are getting errors with your computed water surface that aren't showing up in the computation window?

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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.

Hec ras cross section spacing

Cross sections are developed based on the location layout of cross section lines and properties from other layers such as the River, Bank Lines, and Terrain layers. Cross sections should be laid out perpendicular to where water will flow in the channel and overbank areas. Therefore, most cross section lines should be created from a minimum of four points the end points and points at the edge of the main channel. Cross sections will also be visualized when looking in the downstream direction; therefore, they should be created from left to right when looking downstream RAS Mapper will automatically flip the line to have the correct orientation. There are many considerations when developing cross section data for orientation, locating, and spacing, but keeping in mind that the cross sections should represent a smooth transition in geometry elevation and area and properties conveyance, surface roughness, etc is paramount. Use the terrain, river centerline, bank lines, flow path lines, inundation mapping, and other data to properly place cross section lines. Elevation data will also be automatically extracted. Cross section properties will continue to be updated each time a cross section cut line is edited. Therefore, the river station value will be updated as you edit the cross section layout, each time you finish editing the feature close it.

Is everything ok pure taboo

All in all, an error of 0. It's an exercise of "guessing" a water surface elevation to solve the equations then comparing the resulting water surface elevation with the "guess". The program considers the left side of the stream to have the lowest station numbers and the right side to have the highest. These two equations represent very different methods for coming up with spacing. This one was easy to spot in the profile plot because of the very nice smooth profile plot followed by a sudden spike in the energy level green dashed line at the upstream end. Fread's equation implies smaller streams and steeper hydrographs will require tighter cross sections. Up to data points may be used to describe each cross section. The River Station identifier may correspond to stationing along the channel, mile points, or any fictitious numbering system. Particularly for a dam breach model, where the presumed errors of the input data probably far outweigh these small numerical errors. In general, large uniform rivers of flat slope normally require the fewest number of cross sections per mile. And, in fact, cross section spacing too close can overestimate energy loss and precipitate significant error, and eventually numeric instability. Your model is unstable for initial conditions.

Written by Chris Goodell May 19,

That is okay. Written by Chris Goodell, P. Ben Cary,. A good starting point for estimating maximum cross section spacing are two empirically derived equations by Dr. It's good to have an approach in mind, and it's even better to be able to defend it, but your reviewer might tell you to scrap it and do it a different way, and you're honestly basically stuck with doing whatever they tell you. January 18, This is the length of channel affected by some disturbance downstream. The purpose of the study also affects spacing of cross sections. The following error occurs.. See an example of artificial numerical diffusion in the figure above. You're sharp, and I value your opinions quite a bit. But essentially, you are looking for places and times where any hydraulic parameter changes abruptly.