Stormwater Calculations - Mathematics and Statistics for the Environment - page 781

TABLE 25.5B
Runoff Curve Numbers for Other Agricultural Areas a

Curve Numbers for
Hydrologic Soil Group Cover Description

Cover Type Hydrologic Condition b A B C D

Pasture, grassland, or range—continuous forage for grazing Good 39 61 74 80

Meadow—continuous grass, protected from grazing and generally
mowed for hay

— 30 58 71 78

Brush—brush-weed-grass mixture with brush the major element Good 230 48 65 73

Woods—grass combination (orchard or tree farm) Good 32 58 72 79

Woods Good 230 55 70 77

Farmsteads—buildings, lanes, driveways, and surrounding lots — 59 74 82 86

a Refer to TR-55 for additional cover types and general assumptions and limitations.
b Average runoff conditions and I a = 0.2 S .
Source: Adapted from TR-55 Table 2-2c—Runoff Curve Numbers for Other Agricultural Lands.

The RCN determination assumptions include the following:

• The urban curve numbers, for such land uses as residential, commercial, and industrial, are
computed with the percentage of imperviousness are as shown. A composite curve number
should be re-computed using the actual percentage of imperviousness if it differs from the
value shown.
• The impervious areas are directly connected to the drainage system.
• Impervious areas have a runoff curve number of 98.
• Pervious areas are considered equivalent to open space in good hydraulic condition.

Note: These assumptions, as well as others, are footnoted in TR-55 , Table 2-2. TR-55 provides a
graphical solution for modification of the given RCNs if any of these assumptions do not hold
true.

The environmental engineer should become familiar with the definition of connected vs. uncon-
nected impervious areas along with the graphical solutions and the impact that their use can have on
the resulting RCN . After some experience in using this section of TR-55 , the designer will be able
to make field evaluations of the various criteria used in the determination of the RCN for a given
site. In addition, the designer will need to determine if the watershed contains sufficient diversity
in land use to justify dividing the watershed into several sub-watersheds. If a watershed or drainage
area cannot be adequately described by one weighted curve number, then the designer must divide
the watershed into subarea and analyze each one individually, generate individual hydrographs, and
add those hydrographs together to determine the composite peak discharge for the entire watershed.
Figure 25.8 shows the decision making process for analyzing a drainage area. The flow chart can
be used to select the appropriate tables or figures in TR-55 from which to then choose the runoff
curve numbers. Worksheet 2 in TR-55 is then used to compute the weighted curve number for the
area or subarea.

25.6.3.3.4 Runoff Equation
The SCS runoff equation is used to solve for runoff as a function of the initial abstraction, I a , and
the potential maximum retention, S , of a watershed, both of which are functions of the RCN . This
equation attempts to quantify all the losses before runoff begins, including infiltration, evaporation,