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Importing Hydrocad v. 3.01 - Peak flows inconsistent

Importing Hydrocad v. 3.01 - Peak flows inconsistent

Importing Hydrocad v. 3.01 - Peak flows inconsistent

I imported a Hydrocad file from version 3.01 (1994) into my current Hydrocad version 8.5, and the peak inflows (Q) for a Proposed Pond, went from 1.6 cfs (original report) to 0.64 cfs.
I have checked for any possible errors in the Subcatchment areas, CN's, Tc's, rainfall depth for the 10-yr and 100-yr events, etc. and all the parameters seem to be similar to what they used back then, and still the Q's (in and out) are different.
I haven't changed anything, just imported the file (project) and run it for the 10 and 100-year events.
Any ideas why these could be happening? Thank for any help.

RE: Importing Hydrocad v. 3.01 - Peak flows inconsistent

Where does the difference occur?  In the runoff or routing calculations?

Runoff and routing results are generally the same from version to version, and the current HydroCAD release can import data from any previous version back to 1986.

The only significant change occurred with the handling of overfilled pond storage in HydroCAD-7.  With earlier versions, HydroCAD would automatically extrapolate from the defined storage whenever the storage was overfilled.  A warning message was issued to call attention to this approximation. The message also recommended that additional storage be defined in order to avoid the extrapolation and provide accurate results.

Starting with HydroCAD-7, overfilled storage is no longer extrapolated.  A warning still occurs, but the storage is not extended beyond the defined amount.  This change was made in order to handle underground (closed) storage volumes, where extrapolation is not appropriate.

This change will cause a difference in pond routing ONLY if you exceed the defined storage and ignore the warning message.  If you define enough storage to avoid the warning, the results will be the same.

For additional details please see the status bulletin at http://www.hydrocad.net/versions/ssb/ssb0104.htm   To see all the bulletins that pertain to your specific release please select "Check for update" in the HydroCAD Help menu.

If this is not the issue you've encountered, please post further details on your situation.

Peter Smart
HydroCAD Software

RE: Importing Hydrocad v. 3.01 - Peak flows inconsistent

Thanks for your help.
In the original printed report (MS DOS based), they accounted for Exfiltration = 1 CFS. When I imported the old file into v8.5, the exfiltration was included as Base Flow: -1.00 in the Advanced Tab of the Edit Pond dialog box.
I changed that to zero and Qin=1.6 for the 10-yr event, which is the same as the one reported in 1994, so I think that was the "issue", however I would like to know how to include the "old" exfiltration rate (1cfs) so I get the same Qout they got back then.

RE: Importing Hydrocad v. 3.01 - Peak flows inconsistent

Early versions accounted for exfiltration as in "inflow loss" by subtracting it from the inflow hydrograph.  However, this had the desired effect only when the inflow hydrograph exceeded the exfiltration rate.

When you imported the project to HydroCAD 8.5, the old inflow loss value was converted to the new exfiltration mechanism, which acts as an outlet device on the pond volume itself.

If you want to replicate the earlier results, you can pass the pond inflow through a link, setting the flow threshold to the desired value.  I believe this will give identical results to the previous mechanism.

Peter Smart
HydroCAD Software

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