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Each subcatchment uses a Curve Number (CN) to characterize the runoff properties for a particular soil and ground cover. The CN value is a primary input parameter for the SCS runoff equation, as used by HydroCAD.
High CN values (such as 98 for pavement) cause most of the rainfall to appear as runoff, with minimal losses. Lower values (such as 58 for certain wooded areas), correspond to an increased ability of the soil to retain rainfall, and will produce much less runoff.
The runoff volume is calculated with the SCS runoff equation, as show at the right. The equation is simple enough to evaluate by hand, or you can use an on-line runoff calculator. Disclaimer: This link is an independent 3rd party web site. Although we're not aware of any issues with this site, you assume all risks associated with the use of the site.
Note that the combination of low CN and precipitation values will produce zero runoff. This is an expected result of the SCS runoff equation. If the term (P-0.2S) is negative, the rainfall is less than the initial abstraction, and there will be no runoff.
How do I determine the Curve Number?
The curve number is based on the hydrologic soil group and ground cover. The subcatchment entry screen in HydroCAD includes a "Lookup" button that lets you browse a table of CN values that is based on the TR-55 reference table.
How do I determine the Hydrologic Soil Group?
The HSG depends on the soil type. For a list of soil types and the associated HSG, click the "Help" button on the CN lookup table. Also see the soil information page.
For a breakdown of the soil groups within a given project, see the new Soil Listing report which was added in HydroCAD-8.5
In general, the most accurate results are obtained when each subcat is homogeneous, that is, contains as few CN's as possible. When you average several CN's into one subcat, the results are not necessarily the same as creating several subcats and adding them together. For example, a single subcat will have only one peak, while adding multiple subcats can even produce a multi-peak hydrograph!
For handling multiple curve numbers, the following options will be found on the Advanced tab of the Settings|Calculation screen:
1) By default, HydroCAD uses a single "weighted-CN" for each subcatchment. The weighted CN is calculated by taking the sum of each CN value multiplied by its fraction of the total subcatchment area. The sum is (usually) rounded to the nearest whole number, as described below. The rounded result is used for all calculations and reports.
2) Starting with HydroCAD-8 an "SBUH weighting" option is available to calculate separate runoff from the impervious and pervious portion of each subcatchment. This allows the SCS runoff equation to be applied independently to the pervious and impervious areas, and may produce a significantly different result than using a single weighted CN. Details here. This weighting method is commonly used with the SBUH runoff procedure.
3) Starting with HydroCAD-10.0 build 8, a "weighted-Q" option is also available. This procedure performs a separate runoff calculation for each CN value, and then combines the flows (Q) to produce the total runoff for the entire subcatchment. The end result is the same as using a separate subcatchment for each CN value. Although this option tends to improve runoff accuracy, it is not supported by most H&H programs and therefore is not commonly used.
When using a composite CN value, it is common practice to round the result to the nearest whole number. Although it is possible to adjust the rounding, using a fractional CN value may yield different results than other programs and make verification more difficult. It also complicates AMC adjustments, which are defined only for integer CN values.
Starting with HyroCAD-8, you may elect to use a full-precision CN value, without any rounding. This option is available on the Advanced tab of the Settings|Calculation screen.
Can I change the number of digits used for CN rounding?
Starting with HydroCAD-8, you can change the number of significant digits by customizing the "CN" parameter directly on the Settings|Units screen.
With earlier versions of HydroCAD you will need to create a custom units definition file. Find the line that begins with "CN=" and change the maximum value of "100" to "100.0". This will add an extra decimal place to the rounded CN value.
What about unconnected impervious areas?
Starting with HydroCAD-9 you can use a special procedure for adjusting the CN value for unconnected impervious areas. Details here.
The CN lookup table includes several pre-weighted CN values, such as "1/4 Acre lots, 30% impervious", which were developed by the SCS and published in TR-55 Table 2.2. 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". (Also see Appendix A2 of the HydroCAD Reference Manual.)
When using these pre-averaged CN values, make sure the impervious fraction is comparable to the actual lots being modeled. In some instances this may include driveways and adjacent roads, while other designs may exclude these area. Since the impervious portion is subject to considerable variation, the pre-averaged values should be used only when appropriate to a specific application. If the impervious portion is significantly different, you should enter the pervious and impervious CN values separately rather than using a pre-weighted value.
Water surfaces are typically modeled with a CN value of 98 or higher. HydroCAD-8.5 includes new entries in the CN lookup table that allow water surfaces to be classified as pervious or impervious, depending on your reporting requirements. For details, read about impervious surfaces.
TR-55 provides CN values for "gravel streets and roads" including the right-of-way, but it doesn't provide a CN for the gravel surface alone. However, the TR-55 values appear to be based on 30% gravel with CN=96 and 70% "open space" in poor condition. So 96 would be a reasonable value to use for the roadway surface alone, since it is highly compacted and has minimal absorption capability.
However, if the gravel surface is not fully compacted or contains significant voids, a lower CN value may be appropriate. Although there are no standard CN values for these conditions, a CN value can be estimated based on the following procedure.
For some conditions, such as a layer of sand over an impervious surface, you may be able to estimate the CN value by using the SCS equation for the potential maximum retention, as listed above:
If we calculate S as the available voids in the sand, we can estimate the CN value by rearranging the equation as:
For example, 10 inches of sand with 30% voids would have a maximum retention of 3 inches, corresponding to a CN value of 77. This approach may also be useful for roof gardens or other artificial soil profiles in which the total voids are known.
Modeling runoff from artificial turf requires the determination of an effective CN value, which is sometimes available from the turf supplier. However, the CN value you achieve will depend largely on the base material, so it is important to follow manufacturers recommendations carefully. In some cases, you may be able to estimate a CN value based on the potential maximum retention, as described above.
Modeling infiltration through artificial turf is a different calculation that requires careful consideration. If your goal is to model sub-turf storage, and the turf is expected to capture (infiltrate) all the rainfall, the "runoff" (infiltrating through the turf) could be modeled as a subcatchment with a CN value of 98. The sub-turf storage could then be modeled as a pond, using the appropriate voids for the sand or stone fill. (Storage could be maximized by using embedded chambers, allowing the "pond" to handle additional inflows). The final discharge could be through exfiltration into the surrounding soil, and/or specific outlet devices, such as perforated pipe (modeled as an orifice array.)
What about porous pavement?
Modeling runoff from porous pavement requires the determination of an effective CN value, which is sometimes available from the pavement supplier. However, the CN value you achieve will depend largely on the base material, so it is important to follow manufacturers recommendations carefully. In some cases, you may be able to estimate a CN value based on the potential maximum retention, as described above.
Modeling infiltration through porous pavement is a different calculation that requires careful consideration. If your goal is to model sub-pavement storage, and the pavement is expected to capture (infiltrate) all the rainfall, the "runoff" (infiltrating through the pavement) could be modeled as a subcatchment with a CN value of 98. For details read about porous pavement.
How can I tell what curve numbers are used over the entire project?
Starting with HydroCAD-8, you can select the new "Area Listing" report, which shows a break-down of all curve number usage throughout the current project. The report is automatically sorted by CN value, with a separate subtotal for each surface description used in the project. It also indicates which subcatchments contribute to each entry. See the sample PDF report here.
The report is especially useful for verifying that all ground areas has been accounted for, but not double-counted. It also lets you verify consistent soil classification between the the pre-existing and proposed analysis.
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