Open Channel Hydraulics

General Tools

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Material	n
poured concrete	0.014
shortcrete	0.016
asphalt	0.014
rip rap	0.035
crushed rock	0.025
grass	0.03
fine sand,silt	0.02
sand,gravel	0.025
coarse gravel	0.03

Maximum Design Velocity of Ditches

Material	Velocity (m/sec)
Fine Sand	0.45
Silt loam or silt	0.45
Fine gravel	0.76
Sandy clay or clay	0.9
Sod	0.9
Stiff clay or ordinary gravel soil	1.25
Coase gravel,hardpan,shale,soft rock	1.8
Hard rock	3.0
Lined: Asphalt, concrete or gunite	3.0

Information % h₁ = m n = s = w₁ = h₂ = w₂ = m m m m w₀ = h_f = Q = m³/hr m

The capacity of ditches and channels is calculated using Manning's equation.

The minimum velocity of flow should be 0.6 m/s in order to prevent the settling of solids.

for concrete sewers, the maximum velocity should be 3 m/s in order to prevent continuous and grit erosion, for other materal, see following table.

Closed storm sewers should be designed to flow full for the design storm.

For circular shapes, wetted perimeter WP = 3.14 D, wetted cross sectional area = 3.14 D²/4

Reference:

Chow, Ven Te. Open-Channel Hydraulics McGraw-Hill Book Company. New York

Review ReportRoughness Coefficient for Ditches and Channels