What is the Peak Flow?

Each HydroCAD summary and hydrograph plot lists the peak flow(s) attained during the routing. This is an interpolated value, and is generally somewhat higher than the maximum flow shown in the tabular hydrograph.  The peak value is obtained by taking the peak of a parabola fitted to the three highest points.  This is the same technique used by TR-20 for reporting peak flows.

Although interpolated peaks are reported for most parameters, some reports (such as the device flow breakdown) use non-interpolated maximum values, which may be slightly lower that the corresponding peak value. For details see "Peak Flow" in HydroCAD help.

Using the tabular Hydrograph

All calculations are performed at the time step (dt) specified on the Calculations Settings screen.  Since a small time step can produce a very large number of calculation intervals, HydroCAD will automatically "shrink" the tabular report to fit in a limited space, such as one printed page.  This process automatically selects a larger time step for the tabular report, omitting the intermediate values from the table.  To see more of the values, deselect the "shrink" option on the report screen.  (You can also select a custom table step on the Reports tab of the Settings|Calculation screen, without changing the calculation step.)

The tabular hydrograph will show the highest single value in bold text.  If two adjacent values appear in bold, it indicates that the highest point falls in between, and has been omitted by the shrink process.  Deselecting the Shrink button will show the intermediate values, allowing the single highest value to be shown in bold.

Peaks don't add up?

When adding multiple inflows, the hydrograph volumes will add directly, but the peak flows will add up only if they occur at the exact same time.  When the peaks occur at different times, the resulting peak may be much less than the sum of the individual peaks.  In some designs, peaks are deliberately sequenced in order to minimize the final peak outflow.  Of course, this does nothing to reduce the discharge volume, and downstream flooding may still be induced by the broader peak of a typical post-development hydrograph.

If you want to verify the hydrograph addition process, right-click the tabular hydrograph report and select "Inflows".  This will display extra columns for the individual inflows.  Deselect the Shrink option to see more intermediate values.  Now you can scroll down to the peak values (in bold) to verify the hydrograph addition.

Why does the device analysis report indicate a different flow?

The top of each summary report shows the actual runoff or routing results, including the interpolated peak flow.

At the bottom of each pond summary is a separate analysis of the outlet devices, which shows how the flow is apportioned between multiple devices.  This analysis is performed at the time step at which maximum flow occurs.  Since this is a non-interpolated maximum value, it may be somewhat less than the peak flow reported above.  The difference will be larger for narrow peaks and/or large time steps.

You can reduce the difference by specifying a smaller time step (dt).  This will generally cause the maximum value to increase, and more closely match the interpolated peak.

You can verify the device analysis results by looking up the maximum elevation on the stage-discharge report.

For further information, open the pond summary and click on the device analysis portion of the report, or read about the device analysis report.

Why is my peak runoff different than other programs?

There are several reasons why the peak HydroCAD runoff can be different than another program:

1) Make sure you're using the same calculation time step in both programs.  For accurate results the time step should generally be less than one-half the smallest Tc value in the project, otherwise the peak runoff will be underestimated.

2) For the Type I, IA, II, and III distributions HydroCAD uses a polynomial-based rainfall distribution, which has greater resolution than the tabular rainfalls used by most programs, and will therefore give a higher peak intensity. You can examine this behavior by downloading a tabular Type x curve from the rainfall library.

3) Finally, HydroCAD reports an interpolated peak flow, while other programs may just report the highest discrete point.

If you still have questions please see our complete document on comparing results.