Stormwater Calculations - Mathematics and Statistics for the Environment - page 783

TABLE 25.6A
Roughness Coefficient ( n ) for the Manning Equation—Sheet Flow

Surface Description n Value

Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011

Fallow (no residue) 0.05

Cultivated soils

Residue cover <20% 0.06

Residue cover >20% 0.17

Grass

Short grass prairie 0.15

Dense grasses a 0.24

Bermuda grass 0.41

Range (natural) 0.13

Woods b

Light underbrush 0.40

Dense underbrush 0.80

a Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass,
and native grass mixtures.
b When selecting it, consider cover to a height of about 0.1 ft. This is the only part of the
plant cover that will obstruct sheet flow.
Source: Adapted from TR-55 Table 2-C-1—Roughness Coefficients (Manning's n ) for
Sheet Flow.

engineer must be aware of any conditions that may act to decrease the flow time, such as chan-
nelization and channel improvements. On the other hand, the engineer must also be aware of the
conditions within the watershed that may actually lengthen the flow time, such as surface ponding
above undersized conveyance systems and culverts.

1. Heterogeneous watersheds —A heterogeneous watershed is one that has two or more
hydrologically defined drainage areas of differing land uses, hydrologic conditions, times
of concentration, or other runoff characteristics, contributing to the study point.

2. Flow segments —The time of concentration is the sum of the time increments for each flow
segment present in the t c flow path, such as overland or sheet low , shallow concentrated
low , and channel flow . These flow types are influenced by surface roughness, channel
slope, flow patterns, and slope.
• Overland (sheet) low is shallow low over plane surfaces. For the purposes of deter-
mining time of concentration, overland flow usually exists in the upper reaches of the
hydraulic flow path. TR-55 utilizes Manning's kinematic solution to compute t c for
overland sheet flow. The roughness coefficient is the primary culprit in the misap-
plication of the kinematic t c equation. Care should be taken to accurately identify the
surface conditions for overland flow. Table 25.6A in this text and Table 3-1 in TR-55
provide selected coefficients for various surface conditions. Refer to TR-55 for the use
of Manning's kinematic equation.
• Shallow concentrated low usually begins where overland low converges to form
small rills or gullies. Shallow concentrated flow can exist in small manmade drainage
ditches (paved and unpaved) and in curb and gutters. TR-55 provides a graphical solu-
tion for shallow concentrated flow. The input information needed to solve for this flow
segment is the land slope and the surface condition (paved or unpaved).