Why do the sum of my peak flows below a diversion structure add up to more
than the flows above the diversion structure?

Around a diversion structure it can sometimes seem as though Hydra® is creating flows out of nowhere.  If you are modeling a sanitary or combined system, including stormwater and/or inflow and interflow (RDII, or rainfall-derived infiltration and inflow), and doing a worst-case scenario analysis on a design storm, this is an expected result of the way Hydra’s storm shuffle feature works.

The overall purpose of the storm shuffle feature is to find the worst-case scenario by shifting the storm hydrograph and miscellaneous hydrograph (representing rainfall-dependent infiltration) to a time that would cause the “worst case” (highest possible flows) in relation to the sanitary and infiltration hydrographs together.  This overall worst-case combined flow is represented by the black “Design Q” hydrograph in the Hydrograph Details window, though the individual hydrographs are still in their original time spots.  So you have to study the Hydrograph Detail to really understand what’s going on.

Hydra does this shuffling across time at EVERY entity to find the worst case — including immediately downstream from a diversion structure.  The storm and misc hydrographs may hit at different times in relation to the sanitary and infiltration hydrograph at every entity.  If you spend some time studying the Hydrograph Details you’ll see that the storm shuffle is the cause of the difference in total flows around the diversion structure than what you might expect.  The correct amount of flow is being diverted, but the timing of each component of the flow shifts to find the worst-case scenario.  This is consistent with Hydra’s method, but does not make intuitive sense immediately below a diversion structure.

Alternative “Worst-Case Scenario” Analysis Method

If the difference in flow below the diversion structure is unacceptable to you because of design considerations, there is another alternative.

Use Hydra’s “Worst-case scenario” feature to determine the time of day for the storm to hit which would generally cause the highest flows throughout the system, or in the sewer lines that are critical for your project.  There may be several times of day resulting in highest flows throughout the analysis period, because of attenuation, variation in types of flows, diurnal curves, and other factors.  Take note of when the storm hydrograph begins for each.

Then, run one or more analysis — without the “worst-case scenario” feature, but instead using a storm delay  which you determined above.   Now the flows around the diversion structure will act as you would expect, and you will have accomplished a worst-case analysis for your purposes.