Need a second pair of eyes on a couple of HEC-RAS files

[beej67]

I'm having a real dickens of a time getting a floodplain compensation plan to work. This is for a landfill, that occupies the west bank of the river shown in the attached files, from station 20870 ("section a") to station 29979 ("section u"). They have some fill they'd like to do in the floodplain, and have some compensatory cut they can offer. I've gone through five trials so far of floodplain compensation (the baseline and 'trial 4' are attached) and with each additional bit of compensation I provide, the flooding elevations go down in every section but one, and instead that section goes up. ("section d," station 22326) The more I cut upstream, the higher the flooding at section D. I'm not allowed to touch section D itself, nor any other downstream sections, due to wetland and buffer issues.

The only thing I can figure, is in the steady state model, D acts as a kind of choke point, and streamlining the upstream sections causes water to pile up at D. I'm somewhat unclear why this would happen at D, though, instead of further downstream at the bend. Any ideas?

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[Ryb01]

Hi beej67,

Agreed on showing the increase in water surface elevations. I'm also seeing an increase at X-Section 22845. The model is showing a decrease in the E.G slope (proposed vs existing) at these sections which would make sense for the water surface increase.

An item for consideration….

As the model is showing a decrease in E.G slope, thus resulting in decreased velocities and increased water surface elevations, the conveyance in the right overbank maybe something to look at further in terms of in-effective flow areas and/or levees depending on the flow scenario your interested in. Might be pulling straws here but the model looks solid otherwise.

 

[beej67]

Thanks so much for your time.

Yes, the next most upstream section also had an increase in my most recent modeling, which included bathymetrics. Prior to the most recent data being added, we were going off assumed river invert geometry, and only exceeded existing flooding on one section. Now it's two, with the tailwater at D effecting E.

So how do I go about lowering the WSEL at sta 22326 and 22845, without manipulating those cross sections? As I say, every time I increase the sections upstream, the problem gets worse instead of better. I haven't ever seen a case like this crop up in the dozen-ish times I've done this sort of work. It seems unintuitive that increasing the floodplain would make the flood elevation worse. Should I try to introduce an artificial choke point (fill more in the floodplain) upstream of D? Is that what you're referring to when you mention a levee?




Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ryb01]

Hi Beej67,


May have found the issue.....please see the attachment below. Looks like the cross section geometry has changed quite significantly from existing conditions. The reduction in conveyace area would certainly be attributed to the increase in water surface elevations. Not sure if the geometry is correct but this may be a good starting point. Hope this helps.

 

[beej67]

Certainly it has - that's the encroachment from the landfill cells, and the compensatory excavation. The thing that's driving me crazy is that even if you swap the existing sections from A-E (sta 20870-22845) in for the proposed ones in the proposed model, so the section geometry is identical, the "FU100" HGL at D (sta22326) is still four tenths higher. Same flows, same geometry at and below the section, four tenths higher flooding. I need to formulate a better understanding of how floodplain compensation upstream can drive an HGL higher downstream, and how to mitigate that with the design. There's not a hydraulic jump, Froude numbers are pretty steady through the whole stretch.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com