Dave F.

HY-12 Question

7 posts in this topic

I was using the HY-12 and the results at access holes show the hydraulic grade line and the energy grade lines the same. This does not match a hand calculation check using HEC-22 procedures.  Am I doing something wrong or is there and issue with the program?

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Thank you Dave for your question.  There is no issue with the program.  Nor would I characterize that you are doing anything wrong.  But perhaps there's been an update in HEC-22 that you've missed.  The latest version of HEC-22, is the 3rd edition, published September 2009.  One of the main sections updated was the access hole procedures.  On page 7-24 (page 234 of the pdf) there is this paragraph:

Quote

 

Access Hole Energy Gradeline

Knowing the access hole energy level (Ea) and assuming the access hole invert (za) is the same elevation as the outflow pipe invert (zi) allows determination of the access hole energy gradeline (EGLa): EGLa = Ea + Za (7-30) As described earlier, the potentially highly turbulent nature of flow within the access hole makes determination of water depth problematic. However, it is not an unreasonable assumption to use the EGLa as a comparison elevation to check for potential surcharging of the system. Research has shown that determining velocity head within the access hole is very difficult - even in controlled laboratory conditions.(96) 

 

In more simple words, inside an access hole, we can accurately determine what the HGL will be.  So in HY-12, we assume the HGL to be the same as the EGL (as determining the velocity head is very difficult, even in controlled laboratory conditions).  

Does that answer your question and concern?

Thanks,

--Eric

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Eric,

Thanks for your response.  That answers my question.  I just wanted to make sure that the model was working correctly,  That will allow us to explain the results in our presentation of the HY-12 model in the FHWA Urban Drainage course that we are teaching next week in San Antonio. 

Dave 

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Eric,

When looking at access hole losses, I can't match the angled inflow losses.  In my example, I have two pipes entering the access hole, one 1.5 ft. diameter pipe at 83 degrees from the outlet and one 2.5 ft pipe entering 150 degrees from the outlet.  The inflow discharges are 4.08 and 15.92 cfs respectfully.  The outlet pipe is 2.5 ft. and has a total discharge of 20 cfs.  Using the HEC-22 equations I get the following:

Thetaw = sum(QjThetaj)/sum(Qj)=(4.08*83°+15.92*150°)/20=136.3°

 

Cq=4.5(sumQj/Qo)cos(Thetaw/2)=4.5*20/20*cos(136.3°/2)=1.67

 

Hq=Cq(Eai-Ea)=1.67*(3.85-3.80)=0.084 ft.

Model gives Hq=.208 ft.  The benching flow losses appear to match my hand calculations, but the angled flow losses don't seem to match.  I have another access hole in my example, with a slight angle loss and it also is higher in the model than the hand calculation gives.

Is there and issue with my calculations that is different than the model?

Thanks,
Dave

 

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This question is a bit more involved and I'd like to be able to see the model that you are using and step through the calculations and compare them to the HEC-22 manual and your computations. 

I recommend that you contact technical support and they'll work with you to get the model transferred over and we can follow up from there.  http://www.aquaveo.com/technical-support

Thanks,

--Eric

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Eric,

I think I found the problems.  I had a bearing reversed in my data. I was also checking using the option for having the model compute the bearings and lengths for the links and it was giving some incorrect lengths and bearings for the links.  But once I exited the WMS, and restarted, and changed my incorrect bearing the results in my model matched my hand calculation.  The version where I had the model calculate the bearings recalculated the correct bearings and lengths and then the both models matched.   So I appreciate your help on the matter, and I think everything appears to be working correctly now.  It appears that in making data changes, the HY12 model created some internal issues, we've had similar problems with HY8 and found that exiting the program will reset things and then the model works correctly.  So I won't need to send you my data files. 

Thanks,
Dave

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I'm glad you were able to get it worked out!!   If you find the steps that make those problems that exiting the program will fix, send those into FHWA and we'll get them fixed.

 

Thanks,

--Eric

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