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?

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

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. Geometry Schematic. Profile Plot Samuels equation suggests anywhere from 15 ft to 50 ft spacing depending on what bed slope you use. I interpolated to 50 ft for the entire reach. Notice in the following profile plot of the downstream end of the reach how the water surface at the boundary cross section is below critical depth the red dot. This creates an overestimation of the water depth at the next upstream cross section, which in turns creates some instability over the next several timesteps.

The only change in the example is the cross section spacing. Solution solver went unstable, iteration 2 at 15JAN Dam 1 It's good to have an approach in mind, and it's even better to be able to defend it, hec ras cross section spacing, 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.

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.

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.

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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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. I attached a guess markup. Justin Bousquin on June 12, 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. What are the numbers on those contours? Can anyone tell me what's the problem? In general, large uniform rivers of flat slope normally require the fewest number of cross sections per mile. The program adds additional wetted perimeter for any water that comes into contact with the extended walls. This method is indifferent by 5. And, in fact, cross section spacing too close can overestimate energy loss and precipitate significant error, and eventually numeric instability. Download page Cross Section Geometry. I added more cross sections by interpolating the steep slope at the upstream end of the reservoir 10 ft spacing. Two heads are better than one, most times. See message below please….

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. It is a theoretical derivation of spacing based on the inherent numerical errors involved with linearizing the St. Most data sets have a "window" of workable spacings. Stationing must be entered from left to right in increasing order. Can you please provide me with your opinion? A visual check alone of the geometry schematic and profile plot should encourage you to investigate a finer cross section spacing. In ras mapper I correctly exported depth layer. I tried redrawing a small sample of my river geometries in RAS mapper but I there are still large errors. My model craches from the initial iteration. The general approach to laying out cross sections is to ensure that the cross sections are perpendicular to the flow lines. Could also be the computation interval is too large follow the Courant Number guidance in the manual. RAS will use the empirical structure equations inline structure, bridge, culvert, etc to compute from downstream to upstream. Anonymous on January 7, I have been able to clear the error. Accordingly, the cross sections would have to be recut. That is generally how errors begin in RAS.