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malsmadi

cross section manipulation - what is acceptable and what is not

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This is a judgment question. When editing cross sections, I am faced with situations where the placement of the DEM-generated channel seems incorrect because there is a deeper dip in elevation on either side, anywhere from 50 to 300 meters. Should I shift the channel banks in order to bound the shallower channel or keep them where they are and ignore the other channel? The purpose of the analysis is unltimately delineating flood zone.

Also, should the editing of cross sections be done in WMS or HEC-RAS?

Thanks.

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Malsmadi,

I'm not sure what you're asking about the DEM-generated channel. Are you saying that you have a surveyed channel and the channel survey data is not extracted as part of the DEM? If this is the case, I recommend going through a tutorial that describes how to work with a combination of DEM-generated cross sections and cross section survey data. This tutorial can be found on the WMS learning center tutorial site here: http://www.aquaveo.com/software/wms-learning-tutorials

The tutorial I would recommend going through is called HEC-RAS - Managing Cross Sections.

When delineating flood plains, WMS creates a water surface elevation surface from the HEC-RAS results and subtracts the DEM elevations from this surface to get flood depths and flood elevations. So the depth of your cross section or the position of your thalweg and banks do not impact your floodplain as much as the geometry of your cross section and making sure your RAS model is setup right.

I always recommend editing cross sections in WMS rather than HEC-RAS. If you edit cross sections in RAS, the cross section changes are not merged back into your WMS model. However, if you edit the cross sections in WMS, the cross section changes are saved with your WMS project. Also, WMS will not recognize any new cross sections added to your HEC-RAS model when the HEC-RAS solution is read.

Chris

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Chris,

Thanks for your answer. What I meant by DEM-generated channel is the channel obtained through flow accumulation from the DEM. Some of the resulting cross sections have a dip which represents the channel where I expected to be but some distance on either side, within the floodplain, there is an even deeper dip in the cross section. Should I shift the left and right banks in order to bound the deeper channel or no? Thanks.

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Malsmadi,

It depends on lots of things such as how much discharge you're trying to model, how much resolution your DEM has, etc. If your DEM does not have enough resolution to include your main channel, you probably need to get higher resolution data by either surveying your channel or estimating your channel geometry from your knowledge of the area. Maybe you could find a higher resolution DEM also.

Let me know if I can help with anything else.

Chris

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Chris,

As always, thanks for your help.

What is considered a good practice when creating cross sections in WMS? In particular, how far out do you extend the cross sections on either side of the channel? The ultimate goal of this study is to creat flood maps.

Do we extend all the way out to include what looks like the floodplain even if it is a few kilometers wide? The downside I see with this approach is that you run the risk of grabing other local channels that may be deeper than the main channel which will divide the flow. However, if you do not extend it far enough, you run the risk of truncating the flood zone sides inappropriately where the actual flood may extend beyong what you considered as the limits of your flood zone through the cross sections ends.

Thanks.

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Malsmadi,

When building a RAS model, you need to extend your cross sections to the extent of your flooding, even if it is a long distance that's maybe several kilometers. If there are other lower elevations along the cross sections that are not a part of your main channel, you could assign these as ineffective flow zones in HEC-RAS and RAS will assume a circular flow pattern in these areas. If there are flows in these local areas that are deeper than the main channel, you might want to consider modeling these areas separately from your main channel or you could include these flows in your model. HEC-RAS has several tools that account for various flow situations. The HEC-RAS user's manual and applications guide, installed with RAS, are a good resource. There are also various online discussions about HEC-RAS.

If your modeling becomes very complicated, you could consider using a 2D hydraulic model such as TUFLOW. Aquaveo develops the SMS software that has a good interface to TUFLOW. Here is more information about this model: http://aquaveo.com/software/sms-tuflow

Let me know if I can help with anything else.

Thanks,

Chris

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