estimated storage hydrograph comparison

[bforr]

C3D 2019

In Storm and Sanitary Analysis - SCS method - the estimated storage hydrograph is a straight line from the beginning of the hydrograph straight to the cfs/time point and the estimated storage is all above this line.

In Hydraflow the estimated storage is all of the area between the pre and post hydrograph line.

Which is truly most accurate?

[bforr]

and in hydraflow express, the diff between rational and scs is the same; i.e. straight line vs area between two bell curves.

[nate.philbrick]

Hi @bforr, 

Great question! I see you are using Storm and Sanitary Analysis (SSA) and Hydraflow Hydrographs extension. Could you share the files you are working with? This will allow me to better address your question. If you want to attach pictures of each of these analyses as well that would be helpful for me and others viewing this post.

[bforr]

hey Nate.  Thanks for replying.  I know this is a long discussed topic.  Attached are the SSA and Hydraflow files also a couple of pdfs showing the hatched areas for calculated detention.  

Can only attach 3 items.  Will reply with Hydraflow file.

[bforr]

Here are the files.

Hydraflow Espress and SSA.

well it removed the SSA I had added .txt to.

[bforr]

Morning Nate.  Attached are the two files I am using.  Used SSA and Hydraflow Express only.  Have fun.

[nate.philbrick · Accepted solution]

Hi @bforr, 

Thank you for the inquiry. This is my knowledge of Storm and Sanitary Analysis (SSA), Hydraflow Express Extension and Hydraflow Hydrographs Extension:  

SSA: This is meant to be a quick design to get an estimate on the storage required. As you said, the area above the linear line represents the storage required. This linear line begins when the total inflow becomes positive (>0) and ends at the Max flow. The max flow is intended to be from the existing conditions of the site. The detention storage calculation is only based on the proposed conditions (or existing conditions). The software does not calculate the difference between the existing and the proposed conditions. SSA allows the user to get a ballpark estimate without adding weirs, orifices etc. to the design.

Hydraflow Express Extension:   This is meant to get a more detailed idea of the storage required. It still is an estimate. As you said, the area between the two hydrographs represents the necessary storage required. The outflow hydrology is an estimate generated by the program.

Hydraflow Hydrographs Extension: This gives the most detailed estimate for storage detention. Like Hydraflow Express Extension, the required storage area is between the two hydrographs. The user has the most control over the existing and proposed conditions and therefore the detention storage.

Hope this helps with your question!  

Please hit the Accept as Solution button if a post fully solves the issue or answers the question.​ Thank you for your participation, your accepting a solution adds value to the post for the community as a whole, and helps other members find answers to their issues.

[bforr]

Thank you Nate.  So the final word is don't ever use SSA for detention storage estimates as they are always well below what will be needed.  It would be a huge pain to tell my boss this size pond for conceptual layout, then have to double it when  you really get to designing.  Ha you know how developers love to hear they have to lose a lot or 5.  Probably not a bad idea to remove that functionality from SSA.  

[bforr]

orrr add the full comparison to SSA.  hmmm sounds like an idea post.