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Difference in output with Hydrocad vs ICPR, methodology or other?

Difference in output with Hydrocad vs ICPR, methodology or other?

Difference in output with Hydrocad vs ICPR, methodology or other?

My company typically models with ICPRv3 however I am trying to make a case for Hydrocad. I discovered it from this forum and to be honest it fits with how I learned water resources better than ICPR despite having used ICPR in school. Anyway, there is a project that we have been struggling to fix the previous design for and I took the chance to model with Hydrocad. The difference in results is quite large despite having essentially the same inputs so I am trying to determine a reason.

Project info: 1.95 ac with average CN of 86 and exfiltration trench under pavement with effective storage of 0.63 ac-ft. Control = 2.5', top of exfiltration = 5.75', pavement min = 7.35', pavement max = 9.3'.

We developed a stage storage table using a calculated effective exfiltration volume so we have a single table of cumulative volume vs stage in both models and the parking lot will essentially serve as additional storage. I did originally model Hydrocad with 6 different stage storage areas which I felt was more accurate but I had to provide the same inputs to try and eliminate that as a reason for the difference in the models.

Output is a v-notch orifice 6"(w) x 6"(h), no weir so we can determine the 25yr-72hr stage. I compared the stage-discharge tables in both models and they were essentially identical. Both are using the same rainfall depth and SFWMD72 storm, same UH484 hydrograph, same 10m ToC, same 0.5hr time increment.

The ICPR model just shows a higher cfs into storage at the peak rainfall time and a higher peak stage (8.38') vs Hydrocad (8.06') and I cannot figure out why that would be. The real curve ball is that I modified the time increment to 5 min on both models and ICPR was reduced down to about 7.40' vs Hydrocad staying at 8.06'. I know I cannot expect troubleshooting on the ICPR model but I do want to understand if there is another factor in the software I am missing. Perhaps ICPR is not including initial abstration like Hydrocad does? The 100yr-72hr storm with no orifice discharge was also different, with Hydrocad coming in with a lower stage and about 0.5 ac-ft less stored. I can't imagine how this is the case with the same stage storage inputs unless there is a factor such as initial abstraction missing.

I'm sure I could write many more paragraphs but I will end with a question on how Hydrocad considers the stage storage model with a subcatchment linked to a pond as my model is set up. The subcatchment area is 1.95 ac, the entire site area, and then the stage storage table in the pond reflects the entire site area entered as a stage and cumulative storage. From the appearance of other tutorial models, these areas seem to be considered separately and are frequently separated by a reach. Is this the most accurate way to model a scenario where there is a pond but also storage over the entire site must also be considered? I assume I can consider the pond to be the entire site and add additional stage storage areas but perhaps it should be done with separate nodes? Typically flow would be to the pond by direct rainfall and also by pipe systems but for the 25yr and 100yr storms we would expect the lake to be overtopped at some points. I just want to make sure that I can still consider the rest of the site for storage as well as the pond.

Thank you and looking forward to many more exciting modeling scenarios! My Hydrocad file is attached.

RE: Difference in output with Hydrocad vs ICPR, methodology or other?

Start by comparing the runoff hydrographs. Make sure you compare the total volume as well as the peak flow. This will verify that all the runoff parameters are the same.

Your current pond "routing" is a very simple situation. Since you're disabled the outlet (Multiplier=0) there is no outflow, so the entire 2.3 AF is retained, and the peak storage is reported as 2.3 AF. That's exactly what you expect with no outlet.

Regarding the subcatchment area, this should equal the entire area that intercepts rainfall, including the surface of the pond. For the pond, just provide sufficient stage-storage data to contain the entire routing.

Peter Smart
HydroCAD Software

RE: Difference in output with Hydrocad vs ICPR, methodology or other?

Thank you psmart! I did identify an issue by checking that total volume. Apparently ICPR is not very accurate with time increments at 30m, Hydrocad does not change nearly as much with those higher increments. That improved the model on ICPR greatly, but still there is some difference in the peak flow, Hydrocad shows about 15 cfs and ICPR shows about 12 cfs. I still need to compare both the rainfall and runoff hydrographs to each other but have been trouble getting access to the machine that runs ICPR. They use the same SFWMD72 for runoff so getting a look at the rainfall will hopefully tell the story.

RE: Difference in output with Hydrocad vs ICPR, methodology or other?

Remember that you can cross-check the runoff volume by evaluating the SCS runoff equation. The volume is strictly a function of the rainfall depth and CN value. The rainfall distribution has no effect on the total volume. Here's an online calculator: http://zonums.com/online/hydrology/curve_number.ph...

Peter Smart
HydroCAD Software

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