Composite roughness

1. Irrigation and Drainage Engineering /Civil Engineering 2019-2020
Weighted Manning’s Coefficient (Composite
Roughness) in Compound channels
In the previous discussion, we assumed that the flow surface at a channel
cross section has the same roughness (n) along the entire wetted
perimeter. However, this is not always true. For example, if the channel
bottom and sides are made from different materials, then the Manning’s
n for the bottom and sides may have different values. To simplify the
computations, it becomes necessary to determine a value of n, designated
by (ne) that may be used for the entire section. This value of ne is
referred to as (the equivalent n) for the entire cross section.
Let us consider a channel section that may be subdivided into N subareas
having wetted perimeter Pi and Manning’s constant, ni , (i = 1, 2, · · · ,N).
By assuming that the mean flow velocity in each of the subareas is equal
to the mean flow velocity in the entire section, the following equation
may be derived:
The Horton composite roughness equation shown below is applied
for open, regular, and irregular channels such as natural floodplains to
compute the weighted Manning’s coefficient (Equivalent Manning ne):
Where:
n: Roughness Coefficient
P: Wetted Perimeter
R: Hydraulic Radius

2. Irrigation and Drainage Engineering /Civil Engineering 2019-2020
Ex 1: For irregular cross section with data shown below, compute the
composite roughness using Horton composite roughness equation. Is the
flow subcritical or supercritical? If knowing S=0.00012, Water level=29
m, Q=120 m3
/s
Start station End Station n
0 10 0.035
10 30 0.027
30 40 0.025
40 50 0.028
50 70 0.03

3. Irrigation and Drainage Engineering /Civil Engineering 2019-2020
H.W
1) For regular cross section with data shown below, compute the
composite roughness using Horton composite roughness equation. If
knowing S=0.00015, Water level=27 ft. What would be the discharge?
2) The Fig. below shows a cross section of a canal designed to carry
1590cfs. The canal is lined with concrete (n=0.014). Find the grade of the
canal in ft/mile Z=1.5 , Ans: s=0.000147 or 0.776 ft./mile
Start
station
End
Station
n
0 20 0.013
20 30 0.027
30 50 0.013

4. Irrigation and Drainage Engineering /Civil Engineering 2019-2020
3) For the composite channel shown in Fig. (). The bed slope is (0.0003).
Compute:
1- The discharge of the channel
2- The required freeboard