§ 2.4.4. Drainage Area (A)

The size (acres) of the watershed needs to be determined for application of the Rational Method. The area may be determined through the use of topographic maps, supplemented by field surveys where topographic data has changed or where the contour interval is too great to distinguish the direction of flow. The drainage divide lines are determined based on topography, street layout, lot grading, building structure configuration and orientation, drainage system layout and other features that are created by the urbanization process.

Example 2-1

An urbanized watershed is shown on the following figure. Three types of flow conditions exist between the most distant point in the watershed and the outlet. The calculation of time of concentration and travel time in each reach is as follows:

Reach	Description of Flow	Slope (%)	Length (Ft.)	"n" value/Surface Type
A to B	Sheet flow (grass lawn)	1.8	50	0.3
B to C	Shallow concentrated flow (gutter)	2.0	840	Paved
C to D	Storm drain with inlets (D=3 feet)	1.5	1,200	0.015

 

For reaches A-B and B-C, the travel time can be calculated from Equations 2-3 and 2-5.

T _t(A-B) = 0.0070.24 × 50) ^0.8 /(3.44) ^0.5 s ^0.4
= 0.028/(0.018) ^0.5
= 0.21 min.

T ^t(B-C) = 840/(60(20.3282)(s) ^0.5 )
= 0.69/(0.02) ^0.5
= 4.87 min.

The flow velocity in reach C-D needs to be calculated from Manning's Equation, using the assumption of full pipe flow, as follows:

V ^(C-D) = (1.49/n) R ^0.67 s ^0.5
= (1.49/n)(D/4) ^{0.67 s 0.5}
= (1.49/0.015) (3/4) ^0.67 (0.015) ^0.5
= 10.0 ft/s

The channel flow travel time is calculated by dividing the length by the velocity.

T ^t (C-D) = 1200/(60×10.0) = 2.0 min

The total time of concentration is calculated by adding the component sheet, shallow concentrated and channel flow segments.

T ^c = 0.21 + 4.87 + 2.0 = 7.08 min

The runoff coefficients (C) for the three (3) areas are given as follows for the 100 year storm (1% annual chance event).

Area	Land Use	C	Area (acre)
LU1	Grass Area (fair condition, flat)	0.41	3
LU2	Commercial (composite of paved and grassed areas)	0.85	20
LU3	Industrial (composite of paved and grassed areas)	0.81	30
Total			53

 

The rainfall intensity (i) of the 100 year storm can be calculated using equation (2-8) together with the coefficients in Table 2-5 for a time of concentration of 7.08 minutes as 11.54 inches per hour.

The composite runoff coefficient (C) = (0.41 × 3 + 0.85 × 20 + 0.81 × 30)/53= 0.80

Thus the peak flow Q _p = CiA = 0.80 × 11.54 in/hr × 53 acre = 489 cfs