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W Beck

Threshold Q for overbank flow - advice on determination?

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Hello,

I am attempting to use WMS / HEC-RAS to determine the threshold discharge for overbank flow in a third-order watershed. This is part of a floodplain sediment storage study. I am running a series of flows through the model and trying to determine the minimum discharge required to initiate overbank flow. I am assessing the overbank threshold visually, using flood depth contours (generated within the _fd dataset). I assume the presence of any flood depth contour ≥0 represents overbank flow in my WMS model (see attached image 1). I also assume that stream reaches without contours present are not exhibiting overbank flow. I should note that I inserted field-measured cross sections into my model. I have three questions:

1.) Can you offer any advice regarding visual (or non-visual) methods of determining the incipient point of overbank flow? I assume if there was a way to indicate top of banks in relation to flow depth, this would be helpful (e.g., a non-planform view of the cross section and associated flood depths). For image 1, the contour range is 0 (min) to 1 (max), with 5 intervals.

2.) The flood depth contour polygons often exhibit a width that is nearly equal to the channel width in my watershed model (Image 1) - so, how can I be sure the flood depth is not being calculated within-channel? Again, I'd like to know if the flood depth is occurring in the floodplain and not in-channel.

3.) I have noticed that many "isolated" flood depth contours appear around cross sections, even at flows that I would not expect to be out of bank. Any thoughts on this? See image 2.

Again, I am interested in determining the minimum discharge needed for overbank flow. Any advice on this non-traditional use would be greatly appreciated!!! Thank you.

 

 

Image 1.PNG

Image 2.PNG

Edited by W Beck

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W Beck,

Regarding question 1, you might try using the HEC-RAS program itself to visualize the water surface elevations at each of the cross sections in a 3-D view.  There's an option in HEC-RAS to view the cross sections in an X-Y-Z perspective plot.  This shows all the cross sections in your model along with the RAS-computed water surface elevations at each cross section.  This plot should give you a good idea where your water surface elevations are in relation to the channel banks.  You can also look at each individual cross section in your model and RAS will show you the computed water surface elevation on the cross section plot.

Regarding questions 2 and 3, when you read a HEC-RAS solution into WMS, WMS just uses the water surface elevations computed by HEC-RAS and places a single water surface elevation (WSE) point at the intersection of each cross section with the stream centerline.  Normally, you would run a linear interpolation between water surface elevation points along the centerline to get additional WSE points between cross sections.  Then you would create additional WSE points along each cross section to increase the density of WSE points and improve your floodplain delineation results.  WMS basically just subtracts the ground elevations assigned to your TIN from the WSE point elevations assigned to your 2D scatter set to get your water depths in your floodplain.  In your case, you merged surveyed cross sections into your WMS-extracted cross sections, so your TIN elevations do not match your actual cross section elevations used in HEC-RAS.

The Flood Depth (_fd) contours displayed in WMS subtract the ground elevations on the TIN from the HEC-RAS computed results, so if the TIN elevations contain the channel, water depths will be displayed in the channel.  Further, if any RAS-computed water surface elevations are higher than your TIN ground elevations, there will be flood depths at these locations.

Hope this helps somehow,

Chris

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