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, hec ras cross section spacing. Cross sections are laid out perpendicular to the anticipated flow lines of both the channel and the floodplain, during high flow conditions.

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.

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. The idea being that in the planning stage of a project, you could decide how long to make your model. This length through a rather elegant numerical analysis of the St.

Notice the energy spike is not completely gone, but it is much better. Register now while it's still free! And, in fact, cross section spacing too close can overestimate energy loss and precipitate significant error, and eventually numeric instability.

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 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. This may break linkages with other RAS data that depends on the river station.

Hec ras cross section spacing

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. Also, cross sections are needed to properly define the expansion and contraction zones around bridges, culverts, and inline structures. Each of these warning messages suggest that more cross sections will provide a better solution. Interpolating cross sections is a quick and convenient way to satisfy this relationship.

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Is it possible that you are getting errors with your computed water surface that aren't showing up in the computation window? Check your input data, especially your initial conditions. It's easy to join and it's free. If that's not it, I'm afraid I won't be of much help. Tullow 1 from R. Your article was very helpful. A visual check alone of the geometry schematic and profile plot should encourage you to investigate a finer cross section spacing. The numbering system must be consistent, in that the program assumes that higher numbers are upstream and lower numbers are downstream. I have readjusted the cross sections also based on the warnings given in the outputs , but it doesnt really solve anything. In ras mapper I correctly exported depth layer. Username or Email Address.

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.

Thank you for your time. I was wondering if it is possible to actually model the waterfall in HEC-RAS or if it can only be used as a downstream critical depth boundary. An example of laying out cross sections is shown below in the figure below. However, have come accross some interesting scenarios. Typically the second peak is the peak of the inflow hydrograph as it passes your dam. Join Eng-Tips Forums! Cross section data are traditionally defined looking in the downstream direction. However, if there is chanses within the sections, as shown in your pdf, then the cross section spacing would decrease. 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 this example, the only change made to the model was that cross sections were interpolated at very short intervals 5 feet.