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HydroCAD Hydrograph Peak Issues in Link Nodes

HydroCAD Hydrograph Peak Issues in Link Nodes

HydroCAD Hydrograph Peak Issues in Link Nodes

In HydroCAD, I am using a Link to manually enter an exactly triangular shaped hydrograph. My problem is that HydroCAD uses a best fit line to "match" my hydrograph data and calculates a "rounded" hydrograph peak that is inevitably higher than my actual peak. Is there a way to stop HydroCAD from using a best fit, rounded type of a line instead of using a linear rising and falling leg?

In the attached example file, the peak should be 94.30 cfs at 0.38 hours, but HydroCAD says it is 94.33 cfs at 0.38 hours.


RE: HydroCAD Hydrograph Peak Issues in Link Nodes

HydroCAD fits a parabola to the three highest points in order to determine an interpolated peak flow and time-of-peak. Because your hydrograph is slightly asymmetrical, this produces an interpolated peak flow that is slightly higher than the largest single point.

There's no provision for disabling the peak flow calculation. It's always calculated and reported. But keep in mind this has no effect on the routing calculations, which are always preformed using the exact values you have entered.

However, you could reduce the number of decimal places in the flow values, so it only reports to 1/10 CFS.

As a side note, I'm curious as to why you're entering a triangular hydrograph, rather than generating the hydrograph within the software. This is a rather unusual application of the software, so perhaps there's a better way to address your underlying goal.

Peter Smart
HydroCAD Software

RE: HydroCAD Hydrograph Peak Issues in Link Nodes

Another option would be to adjust the value immediately before the peak so it is exactly the same as the value after the peak. The interpolated peak will then be the same as the highest value.

For example, if the three highest values are: 10.0 11.0 10.1

Change them to: 10.1 11.0 10.1

Peter Smart
HydroCAD Software

RE: HydroCAD Hydrograph Peak Issues in Link Nodes

Yes, this method is a little odd, but we are trying to follow the requirements of the jurisdiction we are working in. All hydrographs are supposed to be triangular, starting at 0 at time 0, peaking at the Tc Time, and then ending at 0 at time 1-hour. When I have multiple subcatchment areas with different Tc times, I have trouble getting HydroCAD to create the triangular hydrograph that I want for each subcatchment because the intensity values are different for each Tc. I've found that by using the Link node, I can force it to use the exact hydrograph that I want. Good to know that this is just a reporting issue and doesn't affect the calculations. The reviewers sometimes question the little details like the hydrograph peaks not matching other reports. Thanks for your suggestion of limiting the decimal places. Also the idea of matching the point values on either side of the peak is a good one.

RE: HydroCAD Hydrograph Peak Issues in Link Nodes

on another point, 94.33 cfs is only 0.03% different than 94.30 cfs, and the peak still occurs at the same time.

I would expect the end results would be darn nearly the same whether the triangular or interpolated hydrograph were used...

RE: HydroCAD Hydrograph Peak Issues in Link Nodes

peter246 - The routing always uses the exact hydrograph values that are entered. The only thing that is interpolated is the reported peak flow, and this has no bearing on any other calculations.

Peter Smart
HydroCAD Software

RE: HydroCAD Hydrograph Peak Issues in Link Nodes

Sounds like they're making you emulate the old, completely improper and scientifically nonsensical Hydraflow rational method...


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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