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River bank retreat and failure
- 1. RIVER BANK FAULURE AND IT’S STABLIZATION BY – AVINASH SAHU
- 2. CONTENT INRODUCTION RIVER BANK FORCES CAUSING FAILURE CAUSES OF FAILURE BANK STABILIZATION CONCLUSION REFERENCE
- 3. INTRODUCTION Bank failure may be defined as the rapid collapse or retreat of banks Generally caused when forces holding sediment is not sufficient It depends on sediment type,layering,moisture content etc. It also depends on location and rate of erosion It is because of both natural and man made causes
- 4. RIVER BANK FAILURE Source- wikipedia.org/wiki/River_bank_failure
- 5. River bank In geography, the word bank generally refers to the land alongside a body of water. Can be divided into 3 zones- Toe zone Toe zone most susceptible to erosion It is located between ordinary WL and low WL Bank zone Bank zone above high water level but affected periodically by currents Bank zone gets the most human and animal traffic Overbank area
- 7. FORCES CAUSING FAILURE Parallel flow erosion Detachment and removal of grains/aggregates from bank face by flow along river direction Due to absence of surficial bank vegetation Impinging flow Erosion due to flow at steep angle to long stream direction Bank appearance similar to parallel flow but more scarred than parallel flow erosion In meandered bends
- 8. Freeze-thaw Ice wedging cleaves blocks of soil mass Thawing loosens blocks and weakens mass Wind waves Cause rapid level fluctuations Significant only for large rivers with long fetch
- 9. MODES OF FAILURE Hydraulically induced failure Geotechnical failure Tectonic failure Gravitational failure Seepage failure
- 10. HYDRAULIC FAILURE Non cohesive soils are most vulnerable to this Hydraulic toe erosion occurs when flow is in direction of bend and highest velocity is at outer edge Centrifugal forces raise the water elevation so that it is highest on the outer bends and as water comes down, there is a spiral effect on the toe which is erosive Worst effect on the point immediately downstream of point of maximum curvature For non cohesive sands , the currents remove the non cohesive soil and creates a cantilever overhang of cohesive material
- 11. GEOTECHNICAL FAILURE Due to stresses greater than the forces it can accommodate May be due to seepage or pore water pressure parameters Seepage in an upward direction reduces the effective stress When effective stress becomes zero , soil has no frictional resistance to deformation The condition of upward seepage along with zero effective stress is also called liquefaction Pore water pressure increase in a saturated bank reduces frictional shear strength and increases sliding forces
- 12. TECTONIC FAILURE Due to tectonic movement , there may changes in the valley floor slope which can cause drastic changes in the river bed Tectonic movement can also cause avulsion in which a river changes its old channel and forms a new channel
- 13. GRAVITATIONAL FAILURE Occurs due to weight component causing detachment They may occur by different mechanisms Shallow failure- when a layer of material moves along planes parallel to bank surface Usually in soils with low cohesion and when angle of bank exceeds angle of internal friction Sliding failure sliding and forward toppling of deep seated mass to the river channel Associated with low height , fine grained cohesive banks and occur during low flow conditions Combination of scour at bank toe , high pore water pressure in bank material and tension crack at the top
- 14. BANK STABILIZATION There are both direct and indirect methods Direct methods Riprap method Rigid armour Flexible mattress Gravity wall Retaining wall Sheet pile wall etc.
- 15. Indirect methods Dikes-permeable/impermeable Retards-permeable/impermeable 1. No bank preparation is required 2. Cost is low due to no need of land acquisition 3. Little environmental impact 4. Not very effective when geotechnical bank instability erosion from overbank drainage are the causes 5. May sometimes cause safety hazard if stream is used for recreation or navigation
- 16. DIRECT METHODS 1.RIPRAP BLANKET Most commonly used method in which riprap or large stone are used which are not easily removed from the bank Stones should be cubical in shape rather than elongated because cubical stones more resistant to movement Stones should have sharp , clean edges
- 17. RIPRAP BLANKET
- 18. RIPRAP BLANKET
- 19. CONCRETE BLOCKS Loose concrete blocks fastened together to form a mattress Different block shapes and placement techniques can be used Filter or granular underlayment often used Sacks or soil cement bags can also be used
- 20. CONCRETE BLOCKS
- 21. RIGID ARMOUR Has little or no flexibility to conform to bank deformation Placed directly on bank slope in fluid or chemically reactive state Asphalt,riprap,concrete,soil-cement Withstands high velocities Has hydraulic roughness and prevents infiltration into river bank Difficulty in placing underwater
- 22. FLEXIBLE MATTRESS Here individual materials which cant resist erosive force on their own are put in a flexible container to provide adequate resistance These are flexible and remains in contact with bed even after scour and can be easily placed underwater If properly anchored , it can also be put on steep slopes Concrete block,fabric,gabion mattress etc.
- 23. GABION MATTRESS
- 24. VEGETATION Very labour intensive but inexpensive Trees provide deep and dense root system and will increase the strength of soil mass REF: Stream Corridor Restoration: Principles, Processes, and Practices 10/98 by FISRWG
- 25. CONCLUSION Different forces responsible for instability of banks Modes of failure include hydraulic , geotechnic, tectonic, gravitational etc. Techniques for stabilization include rigid armours, flexible mattress, vegetation etc.
- 26. REFERENCES S.K. Garg. “Hydrology and Water Resource Engineering” National programme on Technology Advanced Learning-”River Training” Wikipedia Amiri-Tokaldany, E., Darby, S.E., and Tosswell, P. (2003). “Bank stability analysis for predicting reach scale and land loss and sediment yield.” Journal of the American Water Resources Association Cancienne, R. M., Fox, G. A., and Simon, A. (2008). “Influence of seepage undercutting on the stability of root-reinforced streambanks.” Earth Surf. Processes Landforms , 33(11), 1769-1786.
- 27. THANK YOU