Hec ras cross section spacing

Written by Chris Goodell October 8, Either way, approaching an unsteady HEC-RAS model especially hec ras cross section spacing 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.

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. These lines will not only help in drawing the cross sections perpendicular to the flow lines, but they also represent the path for measuring the reach lengths between the cross sections. Cross sections are required at representative locations throughout a stream reach and at locations where changes occur in discharge, slope, shape, or roughness, at locations where levees begin or end and at bridges or control structures such as weirs.

I keep getting huge errors. Solution solver went unstable, iteration 2 at 15JAN Dam 1

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.

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.

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.

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Occasionally it is necessary to layout cross-sections in a curved or dogleg alignment to meet this requirement. Typically the second peak is the peak of the inflow hydrograph as it passes your dam. Upon running the model, during the performing of the unsteady simulation, i have encountered these error of "RasUnsteady That is generally how errors begin in RAS. If the cross sections are too close together, then the derivatives with respect to distance may be overestimated, especially on the rising side of the flood wave. Make sure your initial conditions are set up properly. It can definitely be done. No one is here to clarify my doubts. The thinking being that the meandering coefficients are in there for a reason, you might as well try to match their results. Tullow 1 from R. 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.

But essentially, you are looking for places and times where any hydraulic parameter changes abruptly. Thanks Anonymous on April 14, Is 'feet' used as the unit of Depth in this equation? I'd' like to export also velocitys layer, than i create the layer "Stored save to disk — Raster based on Terrain" Now: in Hec-Ras 5. The next thing to ask, is what regulatory hoops you're facing MNcivil1. Venant equations into a four-point implicit finite-difference scheme. I'v drawn the profile along the river and checked the plot for WSE, but i am not able to understand why the WSE drop is there. Antonio Francipane on November 18, Hi Chris, thanks a lot for your blog. HEC-Ras interpolates between sections. The water surface profile plot should reveal this. The waterfall does not have to be a downstream boundary.