HydroCAD® Stormwater Modeling - Since 1986
If HydroCAD isn't producing the results you expect, there are several items you should check:
Finally, remember that some variation is to be expected from one modeling system to another, particularly when different methodologies are employed. When comparing two scenarios (such as pre and post development) be sure to use a consistent modeling approach. Keep in mind that any technique is inherently more accurate in predicting changes in runoff than in predicting the absolute runoff levels.
Strange but true
The peak inflow isn't always the same as the sum of the individual peaks! The peaks will sum directly only if they occur at the same time. When two hydrographs are summed, the flow is added at each point in time. If the peaks doesn't coincide, they won't be added directly. This effect is most pronounced with "narrow peaks" (such as runoff hydrographs with a short Tc) and illustrates the value of "sequencing" the flows from a site. See illustration here.
Using a smaller pond outlet can increase the peak discharge! This is common with a low-flow outlet on a pond, which allows the early runoff to drain quickly from the pond, preserving the maximum amount of storage for detention of larger events. If the low-flow device is reduced, more water is detained early in the storm, making the pond less effective for later inflows. For all pond discharge issues, check the stage-discharge curve, and not just the hydrograph!
A small increase in rainfall can cause a very large percentage increase in runoff! The SCS runoff equation predicts that no runoff will occur below a certain rainfall threshold. For rainfalls slightly above this threshold, the fractional increase in runoff can be very large. For example, with a CN of 70, the runoff will increase from .08" to .24" as the rainfall increases from 1.5" to 2". Only with a CN of 100 is the runoff directly proportional to the rainfall.
A large increase in rainfall may cause only a small increase in the overall discharge! If the site has sufficient storage to detain the the additional rainfall, it may cause only a slight increase in the final discharge. This is usually the result of a well designed detention pond, or other natural storage within your model. (As always, be sure to observe all warning messages, which are designed to identify deficiencies in your model.)
My peak site discharge increased after I added a detention pond! It's all a matter of timing. Although a pond will generally attenuate the flow going through it, it will also delay the peak. This will change the timing in relation to any downstream inflows, which can actually cause an increase in the combined peak. In some cases, it's better to release some of the discharge immediately, so that the peaks from different sub-areas don't occur at the same time. For more information read about peak flow.
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