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Hydraflow vs SSA results

19 REPLIES 19
Reply
Message 1 of 20
bf
Advocate
4359 Views, 19 Replies

Hydraflow vs SSA results

Ran a drainage design through SSA, then exported to Hydraflow and ran.  The results were markedly different.  So I exported the Hydraflow to XML, imported to SSA, configured necessary parameters.  Reran.  Same problem.  Anyone have this happen?

 

19 REPLIES 19
Message 2 of 20
Matt.Anderson
in reply to: bf

That is to be expected.

 

Hydraflow performs a standard step method using the Peak Q. 

 

SSA performs a hydrograph routing conserving mass and momentum. 

Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)
Message 3 of 20
bf
Advocate
in reply to: Matt.Anderson

Thank you Matt.  I may have other replies following.

Message 4 of 20
Paulns2
in reply to: Matt.Anderson

Please explain in more detail. run ana analysis in hydraflow and ssa using the same idf curves and i find that my system floods in hydraflow and i end up having to use larger pipe sizes. However the same system analysed in ssa indicates that the now larger pipe sizes are too big since there is extra capacity. I am trying to optimise my network and the difference between the two software is befuddling.

 

Regards,

 

Paul

Message 5 of 20
Matt.Anderson
in reply to: Paulns2

Paul -

 

Hydraflow Storm Sewers follows the procedures outlines in HEC-22 for storm sewer systems.  The process starts at the downstream outlet to a system, stepping iteratively up a network in search of a balance of the HGL and time.  The rainfall intensity for the each system is overall system intensity for the upstream area.  This results in a peak Q for a pipe.  It is a very conservative design method.

 

SSA uses the EPA SWMM hydraulic calculation engine which uses the Saint Venant equations to balance continuity and momentum in the system.   This is a time-dependant system and is typically startes from the upstream moving downstream, however, the solutions do allow reverse flow, attenuated system storage, and pumps - items that Hydraflow and the Standard Step method does not support.

 

 

Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)
Message 6 of 20
Paulns2
in reply to: bf

Then what's the use of hydra flow if the results cannot be trusted. In that case it's better to start and finish the design in SSA even though SSA does not have the automatic design capability. Is the auto design capability in the pipeline for future SSA iterations?

Regards

Paul
Message 7 of 20
Matt.Anderson
in reply to: Paulns2

Paul -

 

The methods using by Hydraflow are still valid, and are simple to the point that many still utilize Excel spreadsheets to accomplish these calculations.  In many jurisdictions, this is all that is required.

 

 

Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)
Message 8 of 20

Matt,

you may have already answered my question below before, but would like to ask again to get your thoughts:

"if SSA is set to a link routing method of Steady Flow, is that not the same as the standard step method?"

Message 9 of 20
Matt.Anderson
in reply to: bf

SSA's Steady Flow Routing is Q in = Q out hydrograph routing.

 

It is NOT Standard Step Backwater calculations.

Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)
Message 10 of 20
fcernst
in reply to: bf

It sounds like you are trying to size pipes using Rational Method hydrographs in a dynamic routing simulation model like SSA. This violates the assumption the Rational Method is based on (rainfall intensity constant over the storm duration, rainfall uniformly distributed over the watershed, etc)..

 

To size pipes from multiple upstream catchments using the Rational Method, you need to implement a system tc and weighted CA methodology  (McCuen) . This is what a storm sewer analysis program like Hydraflow does. 



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 11 of 20

Hi Matt,

For both hydraflow and ssa, TOC for each subcatchment is just for subcatchment itself, not cumulative TOC. Is that correct?
Because in the output, hydraflow provides cumulative TOC data but SSA doesn't. I doubt if we need to calculate the TOC ourselves by adding pipe traveling time and surface traveling time in SSA.

Message 12 of 20

Derrick -

 

The ToC either manually added or calculated from the various parameters in SSA or Storm Sewers is the subcatchment Time of Concentration.

 

Hydraflow Storm Sewer will add pipe travel time and adjust the overall system ToC at the next structure.  This only produces a peak flow.

 

SSA routes the triangular hydrograph down the pipe, but will not adjust the rational intensity.  This produces a hydrograph.

 

 



Matt Anderson
Product Manager
Message 13 of 20

It seems Hydraflow is more conservative but SSA is more realistic. So we can say SSA is safe?
Message 14 of 20

Is SSA safe?   Sure.

 

Reminds me of the 'all models are wrong, but some are useful' saying attributed to the statistician George E.P. Box.  

 

 

 

 

 

 

 

 

 



Matt Anderson
Product Manager
Message 15 of 20
fcernst
in reply to: Derrick1234567

Neither one of those software offerings model structure losses per industry standard HEC-22 3rd Edition final design analysis mentodology.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 16 of 20
jpettijohnBV4DB
in reply to: fcernst

Does Autodesk have a reply to this comment?

Message 17 of 20
ChrisRS
in reply to: jpettijohnBV4DB

Sadly, the questions really are:

  • Does Autodesk still have staff knowledgeable enough to provide a meaningful rely to this?
  • Does Autodesk still have the staff capable of modifying the programs to address any issues?
  • Should we wait for this to be resolved in Infraworks, then ask for a port back to Civil 3D?

(post-purge, I fear that the answers are No, No, No comment.)

Christopher Stevens
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Message 18 of 20
wfberry
in reply to: ChrisRS

Chris:

I concur!

It is interesting that you bring up Infraworks, last winter I spent some time "playing" with it.  In Civil 3D I call myself a "Frequent Saver". If I make a mistake and get things all "whacked" I just close without saving and jump right back in.  With Infraworks it saves "on the fly" so when I look at my road and I have the left lane missing after I deleted something . . .. 

Well anyway that's my story.

 

Bil

 

Message 19 of 20
fcernst
in reply to: jpettijohnBV4DB

They should be able to update Storm Sewers to HEC 22 3rd Edition because they have done it in Analyze Gravity.

 

I don't care for the interface in Analyze Gravity. 

 

I was hoping they would keep the Storm Sewers GUI, update it to HEC 22 3rd Edition, and bring it into the Model Space design environment, having the model space Pipe Networks update dynamically to Storm Sewers results.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 20 of 20
moham137
in reply to: ChrisRS

i have posted a question about month ago still didn't receive replay !!

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