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- 1. HIP Sharjah Water and Power Business Group Flexible Lining for Flood Channels 14 May 2009 Waqar Ahmed
- 2. FLEXIBLE LINING FOR FLOOD CHANNELS •Types of channel lining •Advantages of channel lining •Design considerations • Manufacturer’s Guidelines •Introduction to ChannelPro •Construction of Gabions •Placing of Gabions
- 3. TYPES OF CHANNEL LINING CHANNEL LINING RIGID LINING FLEXIBLE LINING GABIONS LOOSE RIP RAPCONCRETE GROUTED STONE PITCHING MATTRESS BASKETS
- 4. BASKETS MATTRESS STONE PITCHING TYPES OF CHANNEL LINING
- 5. ADVANTAGES OF LINING The main advantages of providing the lining to any flood channel are: • Stabilization of channel bed and bank slopes • Prevention of soil erosion • Pre determination of roughness coefficient value • Increase in capacity of channel • Promote vegetation / Improve aesthetic (Flexible lining) • Safety of valuables
- 6. DESIGN CONSIDERATIONS i Hydrology Flood frequency analysis ii Hydraulics Slope of channel Shape of channel Velocity of flow function of discharge, size & slope Choice of lining Velocity Economics iii Components of Design Average stone size, D50 Thickness of lining, T Filter Design
- 7. DESIGN CONSIDERATIONS Wadi Hailu Flood Channel Design •Road Starting from mountain toe •Expected flood of the order of 122 m3 /s (Flood Frequency Analysis) •Channel Width = 30m (Hydraulics) •Depth of flow = 1.5m •Velocity of flow = 3.5m/s •Stone Pitching Design USBR recommends the following formula for determining the size of stone that will not be dislodged under turbulent flow conditions D50 = (V / 4.915)2 Using V=3.5m/s, D50 will be 0.5m. The specific gravity of the stones was assumed to be 2.65 (i.e. density of 2,650kg/m3). If less dense stone is used, then the stone size should be increased correspondingly.
- 8. DESIGN CONSIDERATIONS The grading of the stone pitching should be as follows: •Maximum stone size = 1.5D50 (0.75m) •Minimum stone size = 0.5D50 (0.25m) •Not more than 40% of the stone should be smaller in size than D50 •The thickness of the pitching should be 1.5 times the stone D50 size. It is usual to place the pitching on a filter layer, to prevent fines being washed out from the embankment during the flood recession.
- 9. DESIGN CONSIDERATIONS Scour DepthScour Depth The Lacey empirical equation may be used to compute the depth of scour. The design scour depth below bed level (D) is given by: Design scour depth (D) = XR – Y [metric units] Where: X = scour factor dependent on type of reach (1.25m) Y = design depth of flow [m] (1.5m) R = 1.35 (q2 /f)1/3 (1.5m) q = the maximum discharge per unit width [m2 /s] f = Lacey’s silt factor
- 10. DESIGN CONSIDERATIONS Scour Factors Type of Reach Mean Value of Scour Factor "X" Straight 1.25 Moderate bend (most transitions) 1.50 Severe bend (also Shank protection at spurs) 1.75 Right angled bend (and pier noses and spur heads) 2.00 Nose of Guide Banks 2.25
- 11. DESIGN CONSIDERATIONS Lacey’s Silt Factor Soil Type Lacey's Silt Factor "f" Large boulders and shingle 20.0 Boulders and shingle 15.0 Boulders and gravel 12.5 Medium boulders, shingle and sand 10.0 Gravel and bajri 9.0 Gravel 4.75 Coarse bajri and sand 2.75 Heavy sand 2.0 Fine bajri and sand 1.75 Coarse sand 1.5 Medium sand 1.25 Standard silt 1.0 Medium silt 0.85 Fine silt 0.6 Very fine silt 0.4 Clay 5.0
- 12. Design scour depth (D) = 2m DESIGN CONSIDERATIONS
- 13. DESIGN CONSIDERATIONS USE OF GABIONS AND STONE PITCHING
- 14. DESIGN CONSIDERATIONS FILTER LAYERFILTER LAYER •Stone protection placed on embankments should be laid on a filter layer to prevent piping. When one filter layer is sufficient it is called a “graded filter”. When more than one filter layer is used, the coarser filter is placed on top of a finer filter (i.e. the permeability increases outwards), and the filter is called an "inverted filter". •The gradation of a graded filter should conform to the following guidelines established originally by Terzarghi: d15 filter / d85 soil < 5; d15 filter / d15 soil > 5; and d50 filter / d50 soil < 25 Where d85 is the sieve size which will pass 85% of the material, and similar for other percentages (d15 and d50)..
- 15. DESIGN CONSIDERATIONS
- 16. DESIGN CONSIDERATIONS Wadi Hailu Flood Channel Design (Scnario-2) Flood Discharge = 122 m3 /s Channel Width = 7m Flow depth = 2m Velocity = 5.7m/s According to USBR formula , D50 = 1.32m Corresponding Max stone size is 2m. A stone size of 2m for a channel bed width of 7m is not suitable. For such conditions we refer to Gabion Mattress.
- 17. DESIGN CONSIDERATIONS For design of Gabions we can refer to different Manufacturer’s Guidelines available for the selection of suitable mattress. Terra Aqua is one of them. Which gives a range of mattresses to be selected depending upon the flow velocity. From the table below it is evident that Gabions of thickness 450mm is suitable for taking up velocity of 5.6m/s Product Thickness (mm) Stone Size (mm) D50 (mm) Critical Velocity (m/s) Limit Velocity (m/s) Reno 150 75-150 85-108 3.5-4.21 4.21-4.51 225 75-150 85-118 3.5-4.51 5.4-6.09 300 75-150 98-123 4.2-5 5.4-6.4 Gabions 450 100-200 148-188 5.8-6.4 7.6-8
- 18. DESIGN CONSIDERATIONS
- 19. DESIGN CONSIDERATIONS Critical Velocity The velocity at which Gabions will remain stable without movement of stone fill. Limit Velocity The velocity which is still acceptable although there is some deformation of Gabions due to the movement of stones within the internal components or cells.
- 20. GABIONS-DESIGN CONSIDERATIONS SHEAR STRESS ANALYSIS Sywb ..γτ = The revetment is considered stable when there is no movement of individual stones. The shear stress acting on revetment is calculated as below: Where bτ wγ = Shear stress on inverts = Unit weight of water y = Depth of water S = Slope Also, shear stress on banks or side slope is given as follows: Sywm ...75.0 γτ = mτ = Shear stress on banks or side slopes
- 21. GABIONS-DESIGN CONSIDERATIONS CRITICAL SHEAR STRESS The Critical shear stress acting on invert is calculated as below: And Critical shear stress on banks or is computed as: cτ sγ = Critical shear stress on inverts C mwsc dC )..( γγτ = ϕ θ ττ 2 2 sin sin 1. −= cs sτ = Critical shear stress on banks = Shield's parameter (0.1 for Gabion) = Unit weight of stone md = Mean size of stone rocks θ = Bank side slope angle ϕ = Internal friction angle of stone fill Where:
- 22. GABIONS-DESIGN CONSIDERATIONS STABLE LINING: 1. The revetment is stable when: cb ττ 2.1≤ 2. The lining on the banks or side slopes of channel is stable if: sm ττ ≤ 3. With limited deformation it may be acceptable if: sm 1.2ττ ≤
- 23. GABIONS-DESIGN CONSIDERATIONS ESTIMATION OF POTENTIAL DEFORMATION When the shear stress reaches the critical value of the condition of “initial movement”, the stone fill in the Gabion moves downstream inside each compartment. With the shear stress increasing and more stone movement development, the deformation of Gabion mattress occurs. To evaluate degree of deformation, a parameter Dz/dm is used. Calculated using the relation below.
- 24. GABIONS-DESIGN CONSIDERATIONS LINING DESIGN WITH WAVE ACTION Banks of channels, canals, irrigation basins etc. are subject to wave action generated by wind; and the banks of navigation canals and rivers are subject to wave action caused by vessels. The major parameter in designing the revetment to resist the wave action are the wave height and bank slope angle. 3cot ≤θfor θ τ cot.2 S m H = 2cot ≥θfor 3/1 ).(cot4 θ τ S m H = The equations are only valid for wind induced waves less than 0.9m and frequent waves 1.4m high.
- 25. GABIONS-DESIGN CONSIDERATIONS GABIONS Vs RIP RAP In accordance with the shear stress analysis, we conclude that “For any given hydraulic condition the average size of stones needed in Gabions is half that required for a dumped loose rip rap revetment”
- 26. GABIONS-DESIGN CONSIDERATIONS i Overall stability of Gabion Mattress lining depends on: Strength of double twisted mesh Thickness of lining Grading of the stone fill material used ii Thickness of Gabion Mattress depends on: Flow Velocity Sediments and Bed loads Gradient of channel Curvature of channel trace
- 27. INTRODUCTION TO CHANNELPRO FLOW VELOCITY V = R2/3 . S1/2 (Manning Formula) n Where V = Velocity (m/s) n = Manning Roughness coefficient R = Hydraulic Radius (m) S = Channel Slope AVERAGE STONE SIZE, D50
- 28. INTRODUCTION TO CHANNELPRO
- 29. CONSTRUCTION OF GABIONS
- 30. CONSTRUCTION OF GABIONS
- 31. CONSTRUCTION OF GABIONS
- 32. CONSTRUCTION OF GABIONS
- 33. CONSTRUCTION OF GABIONS
- 34. PLACEMENT OF GABIONS Laying at Channel Bed •Length dimension of unit is laid parallel to the water flow Laying at Channel Side Slopes •Length dimension of unit is laid perpendicular to the water flow
- 35. PLACEMENT OF GABIONS
- 36. PLACEMENT OF GABIONS
- 37. FLEXIBLE LINING FOR FLOOD CHANNELS •QUESTIONS
- 38. FLEXIBLE LINING FOR FLOOD CHANNELS THANK YOU

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