guide banks.pptx

GB

1. Guide Banks (or Guide Bunds) Rivers in flood plains submerge very large area during flood periods. When some structure is to be constructed across such a river (e.g. bridge, weir, etc) it is very expensive to construct the work spanning the whole width of the river.

2. Therefore, guide banks are constructed to imprison(send down) the flow of the water within a reasonable waterway. They extend both upstream and downstream of the abutments of the structure. They are generally provided in pairs (couple) symmetrical in plan. The main parts of a guide bank are: Upstream curved head Downstream head Shank or straight portion which joins the two curved heads Slope protection and launching apron Con…

3. w h ere P = Lacey’s Regime perimeter (m), Q = maximum discharge (m³/s) Generally, L≈ 1.1 to 1.25 P . River Training Structures Design Criteria for Guide Banks a . Length of Clear Waterway: to be provided between the guide banks or the abutments of the work P  4 . 7 5 Q 1 2

4. Con…

5. b. Length of Guide Bank: According to Spring , length of guide banks on the upstream side from the axis of the work should be equal to 1.1L and downstream side from the axis of the work 0.1L to 0.2L. Other formulas (e.g. Gales) depending on the discharge are also recommended: Upstream of axis of work:- 1.25 L ------------------for Q max up to 20,000 m³/s 1.25L – 1.5L ---------for 21,000<Q max <42,000 m³/s 1.5L--------------------- for Q max >42,000 m³/s Downstream of axis of work: 0.25L ------------------- for all sizes of rivers. Con…

6. c. Radius of Curved Heads: Upstream curved heads – sweep angle of 120° to 145°; R = 0.45L Downstream curved heads – sweep angle of 45° to 60° and half radius of upstream curved head; R 1 = ½ R d. Cross section of Guide Banks: Top width not less than 3 m Constructed of locally available material, usually sand (earthen, soil) Side slopes not steeper than 2:1 (H:V) Free board of 1.25 to 1.5 m above anticipated flood level Con…

7. e . Slope Protection for Guide Banks: Water face protected by stone pitching (each stone weighing T = thickness of stone pitching (m) Q = maximum discharge (m³/s) Con… 3 40 to 50 kg Rear face slope provided with vegetal cover to protect it against wind & rain erosion At both curved ends the pitching is done on both front and rear faces The pitching as recommended by Inglis is given by 1 T  . 06  Q  where

8. f. Launching apron: The slope of the guide bank may be damaged due to scour, which may occur at the toe of the bank with consequent undermining and collapse of the stone pitching. In order to protect the slope against such damage a stone cover known as launching apron is laid from the toe of the bank on the horizontal river bed Slope of scoured face is assumed to be 2:1 (H:V). Thus if D is the depth of scour below the bed of the river then the length of the scour face will be = (√5) D. Depth of scour below high flood level (H.F.L.) = K R s Con….

9. f = silt factor d = mean size of bed material (mm) The thickness of the apron in launched position is assumed to be 1.25T. Thus, the quantity of stone required per meter length of the launching apron will be (√5) D x 1.25T = 2.80 TD Width of launching apron = 1.5D (usually). River Training Structures  Lacey ´ s regime scour depth ( m )   Q = maximum discharge (m³/s) 3 1  Q  R s  0.47  f  f  1 . 7 5 d

10. Inglis and Joglekar gave the following values for maximum scour depth :- 1    f  Q  3 D s  1.9  1    f  Q  3 D s  0.95  1    Q  3 D s  1.3  f  Maximum scour depth downstream of bridge: Maximum scour depth around bridge piers: Maximum scour depth at nose of guide banks of a large radius:

11. River Training Structures