what is the limitation of manning equation, everything is described

1. By : Ramyar ibrahim salih
University of Slemani
Civil Engineering Department
2020-
2021
Manning equation
limitation

2. Outline:
 Introduction
 Background and history
 Manning equation
 limitation
 Refrences

3. Introduction:
 One the most commonly used equations relating Open Channel
Flow is known as the Mannings’s Equation. It was introduced by
the Irish Engineer Robert Manning in 1889 as an alternative to the
Chezy Equation., compute the friction losses in a channel .

4. Background and History
• The Manning equation was developed by Robert Manning in the late 19th
century. It is interesting to note that Manning had no formal training in fluid
mechanics or engineering. His background was in accounting, which caused
him to search for the simplest solution that best fit the data.
• The Manning equation is an empirical equation that describes the relationship
between the velocity in a conduit and the channel geometry, slope, and a
friction coefficient expressed as a Manning n. In its essence, the Manning
equation describes the energy balance between gravity and friction in a
conduit.
• It is an empirical equation because it is not based on first principles formed
from theories of science and physics, but rather it is derived from curve fitting
to observed data

5. Bighorn River in Montana – a Natural Open
Channel
Irrigation Canal Branch in Sinai – A man-made
open channel

7. Limitation
1. Because the Manning equation is an empirical equation, the values for Manning n are
derived from experiment and observation. There is no hard, fast rule or scientific method
to determine what Manning n to use for a particular conduition the equation is not
perfect, the N value is estimated
Concrete pipe has a Manning n of 0.013

8. The channel of a mountain stream like this with cobbles and
large boulders has a Manning n of 0.050

9. • A floodplain like the banks in this picture with medium to
dense brush in summer has a Manning n of 0.100

10. 2. Manning doesn’t work with pressure flow very consistently ,
The flow must be by gravity not force ,
the pipe is pressurized, then Manning’s equation should not be used,
3. Manning proofed with time to be valid, beware that the coefficient can
vary with time A detailed table providing Manning n values ranged from 0.03
to 0.045 for natural, 0.022 to 0.035 for excavated earth, 0.01 to 0.025 for
artificially lined channels and from 0.035 to 0.15 for floodplains is available in
books of channel Hydraulics

11. 4. For mudflow selection of n values for them and subsequent conventional
hydraulic analyses probably are not applicable because of the large sediment
load channel scour and deposition and Debris-flow that has a lot of waste and
organic materials , and vegetation.

12. 5. Debris- flow are composed of clay, sand, gravel, cobble boulders, and other debris
deposited at the foot of a steep channel where the channel gradient lessens, flow
spreads on the fan, and flow velocity is decreased sufficiently to cause such deposits.
6. The passage of a flood can cause dramatic changes in channel geometry and
roughness in a very short time
7. Although scour is a function of a number of flow, channel, and soil properties, the
most dominant factor seems to be stream slope. `

13. Debris-flow

14. Reference:
 https://www.h2ometrics.com/manning-equation/
 https://www.jstor.org/stable/30066402?seq=3#metadata_inf
o_tab_contents
 https://www.slideserve.com/howell/manning-s-equation
 https://personal.utdallas.edu/~brikowi/Teaching/Applied_M
odeling/SurfaceWater/LectureNotes/Manning_Roughness/Li
mitations_Extensions_Mann.html
 https://pubs.usgs.gov/wri/1985/4004/report.pdf

15. Thank You