River Training & Protection works

1. River Training & Protection works Ramesh Pinjani Sr Prof/Bridges/IRICEN Pune

2. Lecture Plan • Various River reaches to decide nature of protection works • Suggested protective measures for different reaches • River Training & Protection works details • Flooring, curtain & drop wall • Guide bunds & apron • Marginal bunds

3. Lecture Plan (Contd) d) Closure bunds • Spurs • Assisted cut offs 4. Design of river training works through model studies Ref; 1. Indian Railway bridge manual 2. IRICEN publication

4. RIVER TRAINING & PROTECTION WORKS • Various Types of River/River Phases (Para 801) • Upper Reaches (Mountainous) • Submontane Reaches (Foot Hills) • Quasi-Alluvial Reaches (Trough) • Alluvial Reaches • Tidal Reaches

5. Types of Rivers • Hilly, sub-mountain, alluvial, coastal • Meandering, straight, braided • Aggrading, degrading, stable • Flashy, virgin

6. River channel in hills

7. UPPER REACHES (Para 802)(Mountainous Rivers) • Narrow, Deep Cross Section, Steep Slope • Bed Material – Rock, Boulders, Gravel • Rise – Sudden and Flashy • Water with high concentration of sediment load • Suggested Protective Measures • Protection to Piers by RSJ, fenders or Rails • Soil Erosion Control, arresting bed load • Chutes with paved apron at the entrance

8. Sub-mountain river channel

9. Submontane Reaches (Para 803) (Foot Hills) • Bed slopes 1 in 50 to 1 in 500 • Bed Material –Boulders, Gravel and sand • Floods – Sudden and Flashy : All these channels normally overflow during high floods and the river acquires very wide and shallow cross section. The rivers in this reach are prone to progressively raise their beds by sediment deposition. Such rivers are known as "Aggrading" type. • Suggested Protective Measures • It is not desirable to locate bridge in such reaches. However, if a bridge is to be provided, training measures in the form of marginal bunds, extending right up to the high ground in the hills are required to shift the point of aggradation downstream

10. Submontane Reaches (Para 803) (Foot Hills) • To reduce the erosive action on the marginal bund a) Suitable slope protection with boulders or concrete slabs, b) adequate toe protection in the form of two rows of in-situ concrete blocks or boulders in wire crates and c) boulders in wire crates forming flexible type apron may be provided.

11. Bed slopes 1 in 500 to 1 in 2500 Bed Material –Small size gravel and Medium sand. Channel–Generally well defined course Suggested Protective Measures Bridging such rivers normally involves constriction of River – Guide Bunds Hydraulic Model studies desirable Quasi Alluvial reaches

12. Alluvial Reaches (Para 805) Bed slopes 1 in 2500 to 1 in 25000 River flows on flat Bed of Material Alluvium (sediment deposited by flowing water) River Meanders in its Khadir ( a strip of low land with in which river meanders) River Bed is normally stable Suggested Protective Measures Guide Bunds – main objective being to guide the river near the bridge to its course Tidal Reaches At the confluence of river with sea, the tidal effects predominate, Constriction of waterway to be avoided

13. Types of rivers • Hilly, sub-mountain, alluvial, coastal • Meandering, straight, braided • Aggrading, degrading, stable • Flashy, virgin

14. Meandering River Channel

15. Meandering river

16. Island type braided channel When flow in river channel is insufficient to transport the eroded material,& gets deposited , thereby blocking the channel. Another channel then may be formed and in course of time river bed become a network of such channels with island in between. Such streams are called braided stream

17. Type of Rivers Aggrading River Aggrading: Rivers in this reach are prone to raise their beds by sediment deposition, due to reduction in velocity.

18. Degrading River Sinking of pier Degrading: lowering of bed by erosion due to higher velocity

19. Stable river Stable: No perceptible rise of lowering of river bed occurring over long periods

20. Flashy Rivers in the hills

21. VIRGIN RIVER Virgin: They have no outfall in the sea nor do they join any other stream. Such rivers after traversing some distance loose all their water by percolation & evaporation

22. River Training Works The necessity and suitability of River Training Works should be carefully assessed. For effective design of river training works, following useful details are to be collected: i) History of flow/channel Change of river course happens mainly due to meandering effect of the river. In case of meander, it is possible to know the maximum radius of curvature from which it should be possible to estimate the likely area of effect. Adequate studies and data keeping results in easier, cost effective and simpler designs of protection measures.

23. ii) Addition of any hydraulic structure on the regime of the flow should be taken note of, both on upstream as well as on downstream. On downstream side, effects are rarely beyond 5 kms. Such changes may be in the form of additional spans/works provided by highways on upstream side. This may also be due to construction of barrage/dam/water storage structures. These are considered as Railway Affecting Works and may have significant impact on Railway bridge. iii) Shift in flow channel because of meander: This should be studied particularly with reference to acuteness of the meander given by tortuosity ratio. iv) Any large scale deforestation on upstream will cause heavy sediment load leading to change in flow pattern.

25. PROTECTION AND RIVER TRAINING WORKS Following are type of protection and river training works 1) Flooring 2) Flooring with curtain wall (U/S side) and drop wall (D/S side) 3) Pitching and toe wall 4) Guide bunds 5) Apron 6) Spurs/grayons 7) marginal bunds

26. Flooring • Provided in bridge with small opening and shallow foundation • Provided to prevent scour around the substructure to avoid undermining of foundations • Stone or cement concrete flooring used for minor bridges ( generally 300 mm thickness) • Some times apron around the pier with filter bed is provided in place of complete flooring • Flooring should be inspected and maintained properly

28. Flooring with curtain wall & drop wall • Curtain wall and drop walls provided to protect flooring from high velocity and scour • If necessary a sloping apron be provided beyond drop wall to lead water way • Inspected and maintained properly particularly D/S of drop wall for any excessive scour to avoid wash away of entire protection work

29. In the case of natural channels flowing in alluvial beds where the width of waterway provided is not less than Lacey’s regime width,(SSC Para 4.6.3) the normal depth of Scour (D) below the foundation design discharge (Qf) level may be estimated from Lacey’s formulas as indicated below D = 0.473 (Qf/f)1/3 Where due to constriction of waterway, the width is less than Lacey’s regime width for Q or where it is narrow and deep as in the case of incised rivers and has sandy bed, the normal depth of scour may be estimated by the following formula : (SSC Para 4.6.4) D = 1.338 (qf2/f)1/3 Where qf is the discharge intensity in cubic metre per second per metre width and ‘f’ is silt factor as defined in clause 4.6.3. Depth of drop wall will be 1.25 times the normal scour depth. Floor should cover the entire width and length of abutment including wing wall

30. DROP WALL

31. Sloping apron beyond drop wall

33. SCOURING NEAR ABUTMENT

34. Scouring near Abutment

35. Need for stone Pitching: Approach bank of bridges are subjected to serve attack under following conditions • when flood level at Bridge is very high and there is spill beyond the normal flow channel • When stream meets major river D/S of bridge. In such cases flood in major river may raise, the normal water level of smaller stream • If deep borrow pits are dug at toe of approach bank, rain water flows through these pits causing parallel flow and erosion of bank

36. Pitching with toe wall • Some times pitching is provided on approach banks, guide bunds and spur to prevent erosion during flood • Toe wall is an important component of pitching provided at toe of bank • If toe wall gets damaged, pitching is likely to slip down • Periodical inspections and proper maintenance is essential to safeguard banks and guide bunds

37. PITCHING OF BANK TOE WALL AND PITCHING

38. Slope pitching & aprons ( with loose stones)

39. Slope pitching & aprons ( with stones in crates)

40. Geo-fabric laid below stone pitching

41. Guide Bunds These are training works, required to be provided to guide flow smoothly through the bridge opening when constriction of waterway at bridge site is adopted. For large alluvial rivers, guide bunds are provided when shifting of river course are noticed in vicinity of bridge (U/S) These are provided with suitable shapes, adequate length and added with curved heads to give protection to exposed faces against river attack

42. Type of guide bunds in PlanConvergent, Parallel, Divergent , Elliptical

45. Guide bund at a bridge

46. GUIDE BUND – shape & design features • Shape : The Shape may be divergent, parallel (straight) or elliptical depending upon site condition. • The upstream shank of guide bund is 1.0 to 1.5 times the length of bridge , while D/S shank is 0.25 to 0.4 times the length of bridge. • The tail bund on downstream side is provided to afford an easy exit to the water & to prevent formation of vertical whirl pools which gives rise to scour. The tail bunds are also curved at their ends and should be properly armoured. • The material used is river sand, clay is not suitable. • River face of bund needs to be pitched with stone laid on filter bed, while rear face of bund can be provided with clay blanket and turf over it. • Radius of curved upstream mole head = 0.45L (L is waterway width determined from Lacey formula subject to minimum = 150mtr and maximum = 600 mtr.)

47. GUIDE BUND – shape & design features (Contd) • Radius of downstream curved tail kept as 0.3 to 0.5 times radius of upstream curve head. • Angle of sweep of mole curve head = 120 to 145o and tail head 45 to 60o • IS 10751-1994 and IRC 89-1985 for design & construction of guide bund & launching aprons. • Minimum top width is kept as 6 m (adequate for machinery) • The side slope of bank is kept as 2:1 (minimum) • Top level of guide bund is fixed as = water level corresponding to design discharge (Q50) +Afflux + Free board ( 1 meter ). • Pitching be laid in wire crates to avoid slippage of individual stone. Individual stone should to 35 to 50Kg. • It Should be inspected properly for any damage of pitching and cross section be taken and compared for any settlement. Any damage or slips be attended immediately to avoid further damage during next flood.

48. GUIDE BUND

50. Guide Bund Apron The apron is provided beyond the toe of slope of the guide bund, so that when bed is scoured, the scoured face will be protected by launching of the apron stone or wire crate containing stone Apron pitching with successive floods, goes on sinking (launching) starting from the front. At final position it is expected to lie in same slope of guide bunds (above the low water level) up to the maximum depth of scour. This action is known as launching apron It is important to inspect the guide bund pitching thoroughly and any weak spot be attainted immediately to avoid further damage during subsequent floods.