Question

Discuss why the Manning’s coefficient of roughness may be different for the same bedding material at low flows

Discuss a practical application where an engineer may design an open channel system that utilizes artificially roughened channel sections

Explain what channel conveyance is and the effects culverts and roughened channel sections have on it

In your own words describe what the Manning’s n-value is and why it is so important to identify the correct value when calculating velocity using the Manning’s equation

Answer 1

Reason for different manning's coefficient of roughness for same bedding material:

Apart from bedding material mannings coefficient varies considerably in low flows, the governing factors are as below:

1.Roughness of banks

2.Cross section of channel

3.Sharp bends

4.constrictions

5. side to side shifting of low water channel.

apart from above, folowing factors also affects

1.obstructions

2.irregularity

3.vegeterian

4.Meandering

Practical applications where artificially roughned channels are designed:

The turbulance promoters are used to enhance the performance of rectangular channel which used for turbine blade passage cooling.

Artificially roughnening is done to avoid scouring, trap of vegeterian and energy dissipation purpose.

also practical purposes served to make artificially roughnening to channels are as below:

By roghnening flow velocity is restricted to

1.protect the banks,

2.navigation,

3.water intake and

4.sediment transport deposition

5.Fish passage

channels conveyance and effects on culverts and roughned channels.

Channel conveyance s nothing but a quantitative measure of the discharge capacity of a water course. It is related to the total discharge to a measure of the gradient slope of the channel.

Headwater depth will be more if conveyance is supposed to be kept constant.

Conveyance will be reduced in roughned channels.

Mannings n value and importance of correct n value while calculating velocity:

V=(1.486*R2/3 *S1/2)/n

Where,

V- velocity of flow

R-hydraulic radius

S-slope of energy grade line

n is mannings coefficient of roughness

Its important to calculate mannings roughness of cofficient's correct value because coveyance is totally depends on mannings constant as follows

K=( 1.486*A*R2/3)/n

Where K is conveyance

if we don't have the correct value of mannings coefficient of roughness, we can not predict the conveyance and velocity which have many practical applcations

Hence , its important to calculate correct value of manning's cofficient of roughness.