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"Hi &#64;bforr, \n \nThank you for the inquiry. This is my knowledge of Storm and Sanitary Analysis (SSA), Hydraflow Express Extension and Hydraflow Hydrographs Extension:  \n \nSSA: 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 (&gt;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.\n \nHydraflow 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.\n \nHydraflow 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.\n \nHope this helps with your question!  \n \nPlease 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."
<https://forums.autodesk.com/t5/civil-3d-forum/estimated-storage-hydrograph-comparison/m-p/9065698>