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Similar to Chapter 9:Uniform flow in mobile boundary channel
Chapter 9:Uniform flow in mobile boundary channel
- 1. Chapter 9: Uniform flowin mobile boundary channel 1 Hydraulics Er. Binu Karki Lecturer
- 2. Rigid and mobileboundary channel
- 3. Rigid Boundary channel •Channel whose bed and banks are made up of non-erodible material. • Can resist erosion satisfactorily • Resistance to water depends only on nature of boundary surface and it can be determined. • Dimensions can be calculated by using uniform flow equations i.e Manning and Chezy’s equation can be used
- 4. Mobile Boundary Channel •Boundary is made up of loose soil which can be easily eroded and transported by flowing water. • E.g. river, unlined alluvial channel • Resistance depends not only on boundary surface but also on condition of bed and banks • Uniform flow formula is not sufficient for design. Stability depends on properties of channel material.
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- 6. Alluvial Channel • Ittransports water as well as sediment • Sediment and channel boundary material has same properties • It is stable if sediment inflow=sediment outflow, channel bed remains constant • Shape, longitudinal slope and cross sectional dimensions depend on discharge, size of sediment, and sediment load
- 7. Shear stress distributionon channel boundary
- 8. Shear stress distributionon channel boundary
- 9. Shear stress distributionon channel boundary
- 11. Incipient motion condition •At low discharge, with low depth of flow and small shear stress, sediment will be at rest • With increase in discharge, bed particles just start to move, this condition is critical condition/Incipient motion condition • In this condition 1. Shear stress=critical shear stress 2. Bed particle diameter=critical particle diameter
- 12. Incipient motion conditionvideo!!
- 13. Introduction to tractiveforce approach • Average shear stress on channel bed at which sediment particle just begin to move is critical tractive stress/critical shear stress • Approach assumes “Sediment particle resting on bed is in equilibrium due to balance in between 1. Drag and lift forces exerted by flowing fluid 2. Submerged weight of particle” • More rational and widely accepted
- 14. Introduction to Shieldsdiagram • According to Shield, the tractive force exerted by the flowing water on the sediment to cause motion is equal to boundary shear, τ0 • Tractive force/resistance to motion of single sediment grain= Dimensionless ratio=entrainment function or non dimensional shear stress Significant parameter in bed load movement d- grain diameter Ƴs and Ƴw-specific wt.of sediment and water
- 15. Introduction to Shieldsdiagram • Shear Reynold’s number( Rn),
- 16. Formation of riverbeds based on shear stress • τo > τc ,bed particles start to move in direction of flow • τo= avg. shear stress on bed of alluvial channel • τc = critical value • Different features form on bed under different conditions of sediment size, fluid and flow condition. • Characteristics of different bed forms =regime of flow • When τo< τc ,no sediment motion, bed remains plane, rigid boundary channel
- 17. Types of bedforms Ripples: • τo > τc , sediment particles start to move and small unsymmetrical triangular undulations appear on bed known as ripples. • Length of undulation<0.4m • Height of undulation<40mm • Sediment size<0.6mm • Sediment move by sliding and rolling • Smooth water surface • Formed when Fr<1(subcritical condition)
- 18. Types of bedforms Dunes: • With increase in discharge, ripples grow in size with flat upstream and steep downstream • Water surface not smooth • Some particle may remain in suspension • Length of undulation>0.3m to …m • Height of undulation>30mm to … cm • Sediment erosion in u/s • d/s movement speed <<mean stream velocity • Formed when Fr<1(subcritical condition)
- 19. Types of bedforms Transition: • With further increase in discharge, dunes get washed leaving small undulation • Small discharge increment may cause formation of sinusoidal waves (standing waves)on bed and water surface These two type of bed form are designated as transition • Formed when Fr is relatively high • Unstable bed form • Offers low resistance to flow
- 20. Types of bedforms Antidunes: • With further increase in discharge, sediment transport intensity increases. • Symmetrical bed and water surface waves appear • Sediment move d/s, crest of bed wave move upstream These undulations are Antidunes • Waves grow steeper and break • Occurs in supercritical flow • High sediment transport rate • low resistance to flow