Kennedy’s Silt Theory

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Kennedy’s Silt Theory

According to what I know, Kennedy’s Silt Theory states that the vertical eddies rising from the bed of an alluvial channel are responsible for keeping silt particles in suspension. The theory defines a critical velocity that will prevent silting or scouring. Kennedy’s equation relates the critical velocity to the depth of flow. Later, Kennedy modified the equation to account for different silt grades by including a critical velocity ratio parameter. The theory provides a design procedure that involves iteratively selecting a trial depth and checking if the calculated mean velocity matches the critical velocity. If not, a new trial depth is selected until the velocities match.

Assumptions Regarding Kennedy’s Silt Theory

The information provided states the following assumptions regarding Kennedy’s Silt Theory:

  • The eddy current is generated because of friction between flowing water and the roughness of the canal bed.
  • The quality of the suspended silt is proportional to bed width.
  • The theory is applicable to those channels which are flowing through the bed consisting of sandy silt or same grade of silt.

Critical Velocity Based on Kennedy’s Silt Theory

The information provided states that the critical velocity is the mean velocity which will just make the channel free from silting and scouring. The velocity is based on the depth of the water in the channel. The general form of critical velocity is as follows:

  • Vo = Critical velocity
  • D = full supply depth
  • C & n: Constants which found to be 0.546 and 0.64, respectively.

The equation is rewritten as follows:

  • Vo = C * D ^ n

The information provided states that the equation was further improved upon realization that silt grade influences critical velocity. So, a factor termed as critical velocity ratio was introduced and the equation became as follows:

  • Vo = C D ^ n m

Where:

  • m: critical velocity ratio which equal to actual velocity (V) divided by critical velocity (Vo)

Numerical Problem:

Limitations of Kennedy’s Silt Theory

The information provided states the following limitations of Kennedy’s Silt Theory:

  • Trial and error method used for the canal design using Kennedy’s Silt Theory.
  • There is no equation for bed slope assessment, so the equation developed by Kutter used to compute bed slope.
  • The ratio of channel width (B) to its depth (D) has no significance in Kennedy’s Silt Theory.
  • There is not perfect definition for salt grade and salt charge.
  • Complex phenomenon of silt transportation is not fully accounted and only critical velocity ratio (m) concept is considered sufficient.

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A Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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