General Functions
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Open Channel Hydraulics
Open Channel Hydraulics
Material n
poured concrete0.014
shortcrete0.016
asphalt0.014
rip rap0.035
crushed rock0.025
grass0.03
fine sand,silt0.02
sand,gravel0.025
coarse gravel0.03
Maximum Design Velocity of Ditches
Material Velocity (m/sec)
Fine Sand0.45
Silt loam or silt0.45
Fine gravel0.76
Sandy clay or clay0.9
Sod0.9
Stiff clay or ordinary gravel soil1.25
Coase gravel,hardpan,shale,soft rock1.8
Hard rock3.0
Lined: Asphalt, concrete or gunite3.0
Information % h1 = m n = s = w1 = h2 = w2 = m m m m w0 = hf = 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