Composite Curve Number (Impervious Area)

Directly connected impervious area (roofs and pavement piped straight to the storm system) uses figure 2-3: CNc = CNp + (Pimp/100)(98 - CNp). UNconnected impervious area (that drains across pervious ground first) uses figure 2-4: CNc = CNp + (Pimp/100)(98 - CNp)(1 - 0.5 R), R the unconnected fraction -- which is why disconnecting downspouts onto a lawn lowers the effective CN. Figure 2-4 applies only under 30 percent impervious. TR-55 example 2-4 -- a pervious (lawn, good) CN of 74 with 25 percent impervious, half unconnected -- gives a composite CN of 78.5 versus 80.0 if all connected. The pervious CN (from land cover and hydrologic soil group) is the user's input. A design aid; the local drainage manual and the engineer of record govern.

Connected (figure 2-3): CNc = CNp + (Pimp/100)(98 - CNp). Unconnected, total impervious under 30% (figure 2-4): CNc = CNp + (Pimp/100)(98 - CNp)(1 - 0.5 R), R = unconnected fraction of the impervious area.

The NRCS TR-55 (Urban Hydrology for Small Watersheds, 1986) Chapter 2 composite curve number for impervious area, figures 2-3 and 2-4, by name; verified against TR-55 example 2-4 (CNp 74, 25% impervious, R 0.5 -> CNc 78.5).

TR-55 is a free USDA NRCS publication; the connected and unconnected composite-CN equations are published in it.

Estimate. AHJ and licensed professional govern.

Field names used by the API: pervious_cn, impervious_pct, connection, unconnected_ratio, composite_cn, impervious_add

• Connected impervious CNc = CNp + (Pimp/100)(98 - CNp)TR-55 figure 2-3
• Unconnected impervious CNc = CNp + (Pimp/100)(98 - CNp)(1 - 0.5 R), total impervious under 30%TR-55 figure 2-4
• Pervious CN CNp from land cover and hydrologic soil group (user-supplied)TR-55 Table 2-2