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Erodible Channels

Erodible Channels. Tractive Force Method. The Tractive Force. When water flows in a channel, a force is developed that acts in the direction of flow on the channel bed. This force, which is simply the pull of water on the wetted area is known as the tractive force.

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Erodible Channels

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  1. Erodible Channels Tractive Force Method

  2. The Tractive Force • When water flows in a channel, a force is developed that acts in the direction of flow on the channel bed. This force, which is simply the pull of water on the wetted area is known as the tractive force. • The channel is eroded if the resultant of forces tending to move the particle is greater than the resultant of forces resisting the motion; otherwise it is stable.

  3. Unit Tractive force • Distribution of unit tractive force over the channel perimeter is not uniform (maximum value is on bottom and on sides) (Chow, 1959)

  4. Critical Stress • The shear stress at which the channel material just moves from a stationary condition is called critical stress, • Critical stress is a function of material size and the sediment concentration.

  5. Tractive Force Ratio (Choudhry, 2008)

  6. Angle of repose • The effect of the angle of repose should be considered only for coarse non-cohesive materials. • For cohesive and fine non-cohesive materials, the gravity component causing the particle to roll down the side slope is much smaller than the cohesive forces and may be neglected. (Chow, 1959)

  7. Permissible shear stress for non-cohesive material • Permissible critical shear stress shown in figure for straight channels. • Recommendation is made to reduce permissible critical shear stress by 10% for slightly sinuous channels, 25% for moderately sinuous channels and 40% for very sinuous channels. (Chow, 1959)

  8. Design ProcedureTractive Force • Selection of a cross section such that the unit tractive force acting on the channel sides is equal to the permissible shear stress for the channel material. Then we check that the unit tractive force on the channel bottom is less than the permissible stress.

  9. Example • Design a straight trapezoidal channel for a design discharge of . The bottom slope is and the channel is excavated through fine gravel having particle size of 8 mm. Assume the particle are moderately rounded and the water carries fine sediment at low concentrations.

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