HydroCAD Stormwater Modeling - Since 1986

Pervious/Impervious Runoff Calculations


With HydroCAD 7.1 and earlier, a single composite Curve Number is calculated for each subcatchment.  This value is a weighted average of all the CN values in the subcatchment, including all pervious and impervious surfaces.

Starting with HydroCAD-8.0, the pervious and impervious area is reported separately for each subcatchment, and in the project-wide node listing.  This reporting does not change the runoff calculations in any way.  It simply reports the amounts of pervious and impervious area.  HydroCAD-8.5 adds the option disable the impervious reporting, if desired.

HydroCAD-8.0 also adds the option to calculate the runoff separately for the pervious and impervious portion of each subcatchment.  Since this procedure is widely used in conjunction with the SBUH runoff method, it is automatically enabled whenever SBUH is selected.  You can also change the setting manually on the Advanced tab of the Settings|Calculation screen.

HydroCAD-10.0 build 8 also adds the ability to perform a "weighted-Q" runoff calculation.  This procedure calculates the runoff separately for each CN and avoids any CN averaging.  Details here.

Pervious/Impervious runoff calculations

When using the option for separate pervious/impervious runoff, HydroCAD applies the SCS runoff equation separately to the pervious and impervious portions of each subcatchment. The combined precipitation excess is then used to generate the runoff hydrograph. The result is identical to using two separate subcatchments for the pervious and impervious portions (both using the same Tc).

You can verify the runoff depth using the SCS runoff equation. The key difference in the results is that the initial abstraction will be much less for the impervious area, so it will contribute more runoff. This will be offset by the lower composite CN value for the pervious portion. The combined results can be higher or lower than using the using a single composite CN, depending on the details. However, the hydrograph shape will always be somewhat different, since the pervious area generates runoff earlier in the storm than the composite CN.

Note: This option is disabled for the Rational Method, since the runoff is directly proportional to the C-value.

What is an impervious surface?

Pavement and rooftops are the most common impervious surfaces, and are commonly assigned a CN value of 98.  This value allows for some evaporative losses and slight retention caused by the roughness of the surface.  Higher values (99 or 100) can be entered manually if required.

Water surfaces (such as a pond or reservoir) are commonly modeled with a CN value of 98, causing them to be reported as "impervious" surfaces.  Starting with HydroCAD 8.5, the CN lookup table includes new entries for water surfaces that allow them to be classified as pervious or impervious, depending on project requirements.

Determining the impervious area

CN values of 98 or greater are normally reported as impervious. (For the Rational Method, C values of .95 or greater are reported as impervious.)  Everything else is pervious, except for surface descriptions that include the "% imp" text.  These are pre-weighted CN values, and the percentage value allows HydroCAD to apportion the total area between pervious and impervious surfaces, as explained below.

Understanding the CN lookup table

The CN lookup table uses the same values developed by the SCS and published in TR-55 Table 2.2.  (Also see Appendix A2 of the HydroCAD Reference Manual.)  Although most of the values are for homogeneous ground covers, the table also contains a number of pre-weighted CN values, such as "1/4 Acre lots, 30% impervious".  These CN values are a weighted average between CN=98 (for the impervious portion) and the standard CN value for "Open space in good condition".  (See the footnotes on Table 2.2.)  These are the CN values that will be reported in the pervious/impervious analysis.  If these assumptions don't apply to a specific situation, you should list the pervious and impervious CN values separately rather than using a pre-weighted value.

Manual entry of pre-weighted CN values

If you chose to enter a a composite CN value of less then 98 directly into HydroCAD (rather than having the program calculate the weighting), the entire area will be reported as pervious, even though it may contain some impervious area.  For correct reporting of the pervious/impervious area, you can use a custom surface description that includes the impervious percentage, such as "45% imp".  You could do this by editing the description directly on the area entry screen, or you could add custom entries to the CN.TXT file if you use the same values repeatedly.  However, the recommended approach is to enter each CN value on a separate line, and avoid the complications of pre-weighted CN values.

What about the Time-of-Concentration?

Separate pervious/impervious runoff is intended to be used in situations where the pervious and impervious sub-areas are interspersed along the flow path, such that they contribute to a common Tc value, which is used for the entire subcatchment.  In cases where separate flow paths and Tc values can be determined, more accurate results can be obtained by using separate subcatchments for the pervious and impervious areas.  This is consistent with the principle that composite CN values should be used only for interspersed runoff areas that cannot accurately be divided into independent sub-areas.

What about unconnected impervious areas?

For subcatchments with less then 30% impervious area, the standard CN weighting is automatically adjusted (reduced) based on the amount of unconnected impervious area. This adjustment occurs whenever an impervious curve number is used which specifies "unconnected" in the description. A special item is available in the CN lookup table for this purpose. Otherwise, all impervious areas are assumed to be connected, and subject to the standard CN weighting procedure.

When unconnected impervious areas are present, the composite curve number is calculated according to modified procedure used in TR-55.  This causes the weighting for the pervious CN value(s) to be increased by half the fraction of the unconnected impervious area, while the weighting for the impervious CN value(s) is decreased by the same amount.  Due to the higher value of the impervious CN, this causes a reduction in the final composite CN value.

Note: This is a new capability added in HydroCAD-9.  For full details see the HydroCAD Reference Manual or NEH-4 Chapter 9.

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