HEC-RAS Scour 2

[Hunt1999]

I am analyzing several bridge crossings for scour using HEC-RAS. I am getting contraction scour values of around 30 ft., but my water depth is only about 10 ft. during the 500 year event. I believe the 30 ft. of contraction scour is too high, but I can't figure out what is wrong. I have delineated the flood plain using ineffective flows for contraction and expansion. I have use the HEC-18 equations to determine the lengths of the expansion and contraction reaches.

One issue I noticed with HEC-RAS is that the Width Values used (W1 & W2) are top widths, which does not match the HEC-18 guidance which implies using the Bottom Width of the channel...

Does anyone have any other ideas of what might be going wrong for me??

 

[gbam]

First, Recalc by hand following HEC-18 and compare your results. Also double check your sediment size to make sure you are not using too small of a particle size. I have computes contraction scour in the 10-15 foot range and when added to pier scour get around the 30-40 foot range. Is the 30-foot result for total scour? Just may want to double check your input and results.

 

[Hunt1999]

Great, thanks for the info. I recalculated using the HEC-18 equations for Live-Bed Scour. One issue is that the state that I work in requires you to use Live-Bed Scour only, because they do not trust the Clear-Bed Scour results. The big issue that I was having with some of these bridges is that if you use the full upstream section prior to contraction, the flow rate is very low, because the velocity is extremely low (0.5 ft/s). For the Live-Bed Scour Equations to be appropriate in this area, the particle size would have to be >0.01 mm. Since I do not have any soils data for these streams, I can not be certain of my particle size. What I am tempted to do is use a particle size of 0.075mm (ASTM cutoff for Sand vs. silt/clay). THis gives me a critical velocity of about 1 ft/s for most of my streams. I can then choose an approach section with a velocity close to that... Do you have a particle size that you typically assume when you do not have soils data?

 

[gbam]

Your small particle size seems to be the culprit. Have you tried a soils report from NRCS. Check out this website

http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx.

You may get lucky and have at least some justification for the size. Typically here in Southern Arizona we use 0.4mm for a rule of thumb when no data is available. We have dry washes here and it is no trouble to get a sample sieve it. Maybe your flood control or state agency can help you with a standard particle size.

 

[cvg]

why aren't you obtaining the samples and checking the actual grain size? This is a critical piece of data necessary for scour analysis. At the very least, you should gather samples yourself and visually try to identify the size, but preferably this goes to the soils lab for proper grain size analysis.

 

[Hunt1999]

Thanks for the information. We will try to get the soil sample to determine the D50 values.

One other question, when determining your Expansion and Contraction Lengths, do you use the equations from HEC RD-42 (equations 17-20)? For wide basins, the contraction and expansion reaches become fairly long. In our state, DNR is the jurisdictional reviewer for submittals, and their guidance is to assume the effective flow contracts at a 1:1 making the Contraction Length = 3*(Effective Flow Width), and water expands at a 2:1 making the Expansion Length = 6*(Effective Flow Width). Is this similar to your methodology, or would you use the HEC RD-42 Regression Equations? Sometimes I do get strange results using the RD-42 Equations...

 

[gbam]

Contraction and expansion guidance is provided in the HECRAS documentation. Typically, I use 1:1 contraction on the upstream side. The contraction is set from the new watersurface elevation just inside of bridge/culvert and extend outward at a 1:1 upstream until you reach the existing water surface elevation or floodplain width. Similarly, I use the expansion downstream in the same mannor. From the contracted width to the expanded floodplain. I am a bit old school, as I was taught to use the 4:1 or acceptable 3:1 expansion rates. My understanding now is that a 1:1 is now suggested as the expansion rate. Follow your local guidance. Your lengths may be different for each side of your floodplain as there may be greater conveyance on one side.

I place the most upstream bridge xsec at the furthest point of contraction intection with floodplain. Hopefully that makes sense. I will then remodel the existing condition with that xsec in place and then compare results.

Have some fun with this; I really enjoy hydraulic modeling.