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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Lecture 8: Design of Erodible Channels CEM001 Hydraulic Structures, Coastal and River Engineering River Engineering Section Dr Md Rowshon Kamal H/P:

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering 1. Channel must carry design flow/discharge (Q d ). 2. Velocity in the channel must not be high to cause scour. 3. Velocity in the channel must not be low to cause deposition. Hydraulic Parameters used in Design of Unlined/Lined Channels 2

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Minimum Permissible Velocity This is the lowest velocity which prevents both sedimentation (deposition) and vegetation growth. Recommendations by French (1985) Prevent from sedimentation – 0.61~0.91m/s Prevent from growth of vegetation – 0.76m/s 3

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering 1. Maximum Permissible Velocity Method This is one of the methods we use to design a channel. Special committee on Irrigation Hydraulics (ASCE) formed this method. Design criteria: Mean Flow Velocity < Max. Permissible Velocity 4

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Maximum Permissible Velocities Material Maximum Permissible Velocity (m/s) Clear Water Water carrying colloidal silts Fine sand, non colloidal Alluvial silt, non colloidal Stiff clay, very colloidal Fine gravel Coarse gravel

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Maximum Permissible Velocities (cont) Above values will be changed if; 1. Reduce values by 25% for sinuous (meandering) channels. 2. Increase by 0.15 m/s for depths greater than 0.91m. 3. Reduce by 0.15 m/s if channel carries abrasive material. 4. Increase by 0.3 – 0.6 m/s for channels with high silt load. 6

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Example 02 Design a trapezoidal channel (side slope 1:2) to carry m 3 /s on a bed slope of Use Maximum Permissible Velocity Method. Assume the following:- Coarse gravel in water carrying colloidal silt Depth to be greater than 1.0 m Mannings coefficient n =

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Answer Ex-02 From Table in slide no 5: Corresponding allowable velocity = 1.83m/s This may be increased by 0.15m/s because channel depth assumed to be greater than 1.0m, Modified allowable velocity = 1.98m/s 8

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Answer Ex-02 From Mannings formula Cross sectional area Wetted perimeter Cross sectional area Wetted perimeter y b 1 z 9

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Answer Ex-02 Substituting for the side slope, Negative value is not possible Design depth and width Solve for y and b 10

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering 2. Permissible Tractive Force Method (Shear Stress Method) Most rational and widely used method. Based on the consideration of equilibrium of particle resting on the bed with drag and lifting forces balanced by the submerged weight of particle. 11

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Permissible Tractive Forces by USBR USBR recommends the following values for boundary shear stresses: Course non-cohesive material (D > 5.0mm) Fine non-cohesive materials (Refer Example 2.4 – Next page please!) Cohesive sediments – Not covered by USBR D 75 in mm 12

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering For Fine Cohesive Materials 13

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Adjustment for Sinuosity Degree of SinuosityCSCS Straight channels1.00 Slightly sinuous0.90 Moderately sinuous0.75 Very sinuous

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Allowable Shear Stresses for a Trapezoidal Channel b = k b gyS 0 s = k s gyS 0 b 1 z y τ s and τ b - Max Bottom & Side Shear Stresses k s and k b - Depend on y, b, z 15

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Allowable Shear Stresses for a Trapezoidal Channel (cont) k s and k b Factors If Tables need to be used Design Conditions For bottom For sides or 16

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Bank Stability in a Trapezoidal Channel W Wsin FDFD Q Wcos Force normal to the side Forces acting on the particle: 1.Drag force ( F D ) 2.Component from weight ( Wsinθ ) 3.Friction force opposing R (Wcos θtan ϕ ) Sand particle 17

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Bank Stability in a Trapezoidal Channel (cont) At incipient motion, resultant force, R will be equal to friction force. (1) For side 18

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Bank Stability in a Trapezoidal Channel (cont) For the bottom θ=0; Combining (1) and (2) gives; (2) 19

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Bank Stability in a Trapezoidal Channel (cont) For finer materials, θ=0 ; i.e. Cohesive forces are much greater than the gravity force. 20

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Example 03 Design a trapezoidal channel to carry 125.0m 3 /s on a bed slope of The channel is to be excavated in coarse alluvium, containing moderately angular stones with d 75 of 50.0mm. The angle of friction for this material is 40º, which is also its angle of repose. 21

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering (i) Define the terms maximum and minimum permissible velocities. (ii) A river 30.0 m wide and 4.0 m deep and of a regular rectangular cross-section carries a discharge of m 3 /s through country with a bed slope of If the bed material is coarse alluvium having a D 50 size of 10.0 mm and specific gravity s = 2.65, estimate the total transport load using the Ackers and White formula. Question 04 22

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School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Thank You

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