School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Lecture 10: Threshold Motion of Sediments CEM001 Hydraulic Structures, Coastal and River Engineering River Engineering Section Dr Md Rowshon Kamal H/P:

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering 2

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering The topics we cover under River Sediment Transport are: 1. Threshold motion of sediment 2. Design of channels in erodible material 3. Modes of sediment transport Content 3

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Deposition and filling of reservoir in hydropower plants. Blockage of water inlets (river navigation) in irrigation schemes. Wear and tear of water turbines. Attach organic and toxic to sediment particles hence effect water quality. Problems from old polluted sediments on ecosystems of the water course. Why Do We Study River Sediment Transport? 4

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Bed load - Particles roll and slide along bed with occasional jumps into the main stream. Sediment transport mechanisms Depend on U, s and d 50 5

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Saltation load - Particles bounce or hog along the bed due to the impact of bouncing particles. Sediment Transport Mechanisms (cont)

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Suspended load - Particles suspend in water due to the turbulent velocity fluctuations. - Wash load can be considered as a part of suspended load. Sediment Transport Mechanisms (cont) 7

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Initiation of Particle Motion & Erosion Low discharge (or velocity) No particle movement i.e. flow condition is similar to a fixed bed Discharge Certain value Random motion of individual particles i.e. initiation of sediment transport. Condition is known as incipient motion/ threshold of motion or critical motion Discharge > Certain value Appreciable sediment transport 8

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Initiation of Particle Motion & Erosion (cont) Threshold of Motion - Movement of single particle, - Movement of few particles, - Movement of bed, - Sediment transport rate tends to be zero. 9

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Average Boundary Shear Stress (τ o ) W x FLOW W sin y1y1 y1y1 y2y2 A τ 10

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Average Boundary Shear Stress (τ o ) Force acting in the direction of flow Hydraulic radius, R = A/P Sometimes S is denoted by S o P=2y 1 +y 2 11

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shear Velocity (u * ) It has the units of velocity (m/s), however it is not a physical velocity 12

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Analysis (1936) Particle will begin to move when the combined drag (F D ) and lift (F L ) moment equals the weight (G) moment. 13

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Analysis (1936) cont Taking moment about Point of Contact, From geometry Combining and multiplying by 14

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Analysis (1936) cont For simplicity, we can assume Where At high Reynolds numbers pressure forces >> viscous/skin friction force. F D will act through the centre of the particle (b 1 =0, α 1 ) 15

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Analysis (1936) cont Let G = Submerged weight ( N.B. G / in lecture notes), Let F D = Drag force, 16

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Analysis (1936) cont Critical value =

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Diagram

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Shields Parameter for Larger Particles Governing conditions 19

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Mannings and Stricklers Formulae Mannings formula Stricklers formula d is in metres 20

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Example 01 An irrigation canal is to be constructed to pass 3.0m 3 /s along a line having a slope of The sides are to be banked and protected with grass (this tells us that we dont need to worry about the stability of the banks). Calculate the minimum width of canal if the bed material consists predominantly of: i) Gravel d 75 = 50.0mm ii) Gravel d 75 = 4.0mm (N.B. This is the d 50 size i.e. 75% of the particles by weight are smaller. The bed tends to become ARMOURED with this size of particle) Smaller particles at top of bed are transported away – leaving the larger particles which then protect the smaller particles beneath d 75 is used because it takes into account armouring) 21

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Thank You 22