3.Chapter Two.pdf
- 1. CHAPTER TWO DESIGN OF RIVER BANKS AND BED EROSION PROTECTION WORKS BY Amanuel B. Msc in Hydraulic Engineering
- 2. 2.1. Riverbank protection works ❑ Introduction ➢ Bank erosion is the most common problem faced in river engineering practices in many countries. ➢ This has been recognized as an awful threat to the society. ➢ So, control of erosion is very much important to save agricultural land, property and infrastructures like bridges, culverts, buildings etc. ➢ Such eroded banks are located alongside the rivers. ➢ Bank erosion is a natural process in stable rivers; however, it can become accelerated and exacerbated by direct and indirect human impacts.
- 3. Cont’d ➢ River bank destabilization causes: ✓ Direct: Livestock trampling, removal of riparian vegetation. ✓ Indirect: Channel incision, then widening from hydrologic alteration in watershed. ➢ Bank destabilization processes: ✓ Hydraulic: Toe scour -> cantilever failure or rotational slide. ✓ Geotechnical: Channel incision -> supercritical bank height and bank failure from mass wasting, or due to positive pore pressure in stream bank (Fischenich, 1989).
- 4. Cont’d Impacts of Bank Degradation 1. Societal Impacts ➢ Property loss from undermining structure ➢ Sedimentation of in-stream structures 2. Environmental Impacts ➢ Fine Sediment Loading ✓ Water quality impacts from fine sediment and attached nutrients (e.g., Sekely et al. 2002) ✓ Aquatic habitat fouling and eutrophication ➢ Channel Widening: ✓ As banks widen, sediment transport capacity decreases and aggradation may occur potentially smothering aquatic habitat ✓ Riparian habitat can also be damaged.
- 5. Cont’d Bank Stability Governing Equations ➢ Stability of non-cohesive sediment on an angled bank (Julien 2010). ➢ Planar (Wedge) Failure Analysis for Steep Banks (Terzaghi 1943) ➢ Effective bank cohesion and material friction angle can be measured in situ or estimated based on bank material properties.
- 6. Cont’d Bank Stabilization Objectives and Approaches ✓ Protect river banks from degradation ✓ Prevent lateral migration of alluvial channel when property at risk Two main approaches for river bank stabilization 1. Strengthening the bank ✓ Hard Approaches: Riverbank riprap & retaining walls ✓ Softer Approaches: Bioengineering and vegetation 2. Reducing Hydrodynamic force ✓ Flow control structures 1. Riprap
- 7. Cont’d ➢ Constructed against a bank/escarpment to protect it from erosion while absorbing wave and flow energy. ➢ Permanent ground cover structure made up of large loose angular stones. Cross section of Riprap stream bank (http://www.nfl.dfo-mpo.gc.ca/e0005488)
- 8. Cont’d Design methods for Riprap 1. Shear-stress method ✓ Effective rock size required for riverbank stabilization under applied shear stress is estimated from Lanes relationship. 2. Velocity method ✓ Effective rock size required for riverbank stabilization under applied critical shear velocity.
- 9. Cont’d Riprap gradation ✓ Size of representative of stability of riprap is determined by the larger size of rock as these are not transported under given flow condition. ✓ Riprap with angular stone is more stable. ✓ For poor gradation of riprap a filter is placed between riprap and bank material.
- 10. Cont’d Riprap Filter ✓ Used under riprap revetment to allow water to drain easily from bank without carrying out soil particles. ✓ Filter thickness should not be less than 6-9 inch. ✓ Opening of 25% to 30% is desirable to minimize clogging and reduce head loss. ✓ Two types: Gravel filter and synthetic fabric filters. ➢ Suggested specification for gradation of filter material size Julien, 2002.
- 11. Cont’d Riprap Failure and its Prevention 1. Failure ➢ Riprap can fail due to particle erosion, translational slides, slumps and side slope failure. ➢ Riprap should not be used on slopes steeper than 1V:1.5H. 2. prevention ➢ Upstream and downstream ends of structure should be tied into stream banks ➢ A launching apron is an effective revetment for riprap protection
- 12. Cont’d Types of Riparian failure Types of Riparian protection Diagrams From: Julien, 2002
- 13. Cont’d ➢ Flow control structures ✓ Flow control or hydrodynamic structures have the following attributes: ❖ Reduce hydrodynamic forces against stream banks ❖ Control the direction, velocity, or depth of flowing water ❖ Reduce the possibility of bank degradation by diverting the flow ❖ These structures generally have certain degree of permeability
- 14. Cont’d Soft(er) Approaches: Bioengineering ❑ Use of live vegetation and woody material for bank stabilization. ✓ Often less costly in terms of: ▪ materials (locally sourced), ▪ labor (often hand labor), and, ▪ once established, maintenance. ✓ Requires time (several seasons) to establish, but self maintaining and re- generating once establish.
- 15. Cont’d ❑ Pole Plantings and live stakes ➢ Pole plantings, or, provide an inexpensive approach to bank stabilization. ➢ Stakes can often be cut from on-site or nearby vegetation and are installed by hand. ➢ Live stakes (e.g. willow) generally require a shallow water table, often a feature of riparian areas. ➢ They require 1-2 years to establish roots and resist erosion.
- 16. Cont’d Comparing Hard and Soft Approaches ❑ Hard, Engineering Approaches ➢ Advantages: ✓ Durable, highly stable, can give rise to vegetation ✓ Local damages can be repaired easily ➢ Disadvantages: ✓ Need construction practice and restricted to some design parameter ✓ Need manpower, materials, equipment ✓ Comparatively costly ❑ Soft, Bioengineering Approaches ➢ Advantages: ✓ Long-term, re-generating protection ✓ Often less costly ✓ Potential for better environmental outcomes ➢ Disadvantages: ✓ May require time to establish ✓ Not always practical (requires, soil, water and mild slopes) ✓ Can cause damage later on via wind-throw of mature vegetation
- 17. Cont’d
- 18. Cont’d Figure; Countermeasures against River Erosion
- 19. Cont’d ❑ Bank erosion prevention works ➢ Revetments represent a typical example of this type of structure, it includes: ✓ concrete revetments ✓ Dry masonry revetments. ➢ These are applied to protect the related riverbank from erosion and infiltration by river current, preventing the collapse of the riverbank.
- 20. Cont’d ❑ Engineered Revetments ✓ Gabions and mattresses are rectangular wire box filled with small stones, stacked on steep slopes and provide higher resistance for the velocity range of 2-5 m/s in which small riparian stone are unstable. ✓ Sacks and blocks are filled with soil or sand-cement and are used for emergency stream bank protections ✓ Concrete mattresses are precast concrete blocks held together by steel rods or cables and used in large river for complete coverage of river bank with facilities for fines to pass through. ✓ Soil cement are concrete used in place of riparian stone in the bank to stabilize the embankment. ✓ These are economical but have lower strength, are impermeable and sensitive to temperature freeze thaw cycles.
- 21. Cont’d ❑ Retaining Walls Retaining walls are vertical structures use to prevent streambank erosion or failure. Typical example of retaining walls: ➢ Gravity walls are massive and failure of wall is resisted by weight of wall. ➢ Cantilever walls are with reinforced concrete base and designed to resist lateral and hydrostatic pressure.
- 22. Cont’d ➢ Sheet pilling walls are flexible bulkheads and used in soft soil and tight spaces
- 23. Cont’d Riverbed erosion prevention works Typical examples of riverbed erosion prevention works: ➢ Check dams (stepped dams and consolidation dams). ❖ Check dam is a small, sometimes temporary, dam constructed across a swale, drainage ditch, or waterway to counteract erosion by reducing water flow velocity.
- 24. Cont’d ➢ Groundsills are installed to protect the riverbed against erosion due to water flow and preventing the collapse of the riverbank. ❖ There are two types of groundsills; 1. Head type; stabilizes the riverbed by preventing: ✓ The riverbed erosion ✓ Movement of riverbed sediments ✓ Destruction or collapse of the riverbank. 2. Non-head type is a supplementary structure to protect the existing head type groundsill if ; ✓ Scouring and erosion still progress ✓ Riverbed degradation occurs at the transition point of the slopes.
- 25. Worked out example on bank stability