Stormwater Calculations - Mathematics and Statistics for the Environment - page 782

START

No Unconnected

impervious
area?

Ye s

Impervious
area
<30%?

No Ye s

Determine
previous
CN
(Table 2-2)

Determine
previous
CN
(Table 2-2)

Ta ble 2-2
assumptions
apply?

No

Ye s

Determine
composite
CN
(Table 2-3)

Determine
composite
CN
(Table 2-2)

Determine
composite
CN
(Table 2-3)

END

FIGURE 25.8 Runoff curve number selection flowchart.

depression storage, and water intercepted by vegetation. TR-55 provides a graphical solution for the
runoff equation. The graphical solution is found in Chapter 2 of TR-55: Estimating Runoff . Both
the equation and graphical solution solve for depth of runoff that can be expected from a watershed
or sub-watershed, of a specified RCN , for any given frequency storm. Additional information can
be found in Section 4 of the National Engineering Handbook . These procedures, by providing the
basic relationship between rainfall and runoff, are the basis for any hydrological study based on SCS
methodology. Therefore, it is essential that the designer conduct a thorough site visit and consider
the entire site features and characteristics, such as soil types and hydrologic condition, when analyz-
ing a watershed or drainage area.

25.6.3.3.5 Time of Concentration and Travel Time
The time of concentration ( t c ) is the length of time required for a drop of water to travel from the
most hydraulically distant point in the watershed or sub-watershed to the point of analysis. The
travel time ( T t ) is the time it takes that same drop of water to travel from the study point at the bot-
tom of the sub-watershed to the study point at the bottom of the whole watershed. The travel time
( T t ) is descriptive of the sub-watershed by providing its location relative to the study point of the
entire watershed. Similar to the rational method, the time of concentration ( t c ) plays an important
role in developing the peak discharge for a watershed. Urbanization usually decreases the t c , which
results in an increase in peak discharge. For this reason, to accurately model the watershed, the