1. Bedload transport Jens Turowski, WSL Bedload transport Lecture for Hydrological Processes and Modelling 24th June 2011 Jens M. Turowski, Swiss Federal Research Institute WSL

2. Bedload transport Jens Turowski, WSL What is bedload transport? Two modes of sediment transport in rivers –Suspended load Particle weight supported by turbulent forces No or infrequent contact with the channel bed –Bedload Particle weight supported by the bed Constant or frequent contact with the channel bed Whether a given particle is transported as bedload or suspended load is dependent on hydraulics and particle size

3. Bedload transport Jens Turowski, WSL Importance of bedload transport

4. Bedload transport Jens Turowski, WSL Importance of bedload transport Engineering factors –Locks, dams, water intakes, bridges, etc Natural hazard –Bank erosion –Sedimentation Economic factors –Damage in extreme events –Loss of storage volume behind dams Scientific interest –Sediment transfer (landscape evolution) –Channel dynamics –Drives bedrock erosion

5. Bedload transport Jens Turowski, WSL Importance of bedload transport Natural hazard –During the flood event in Baltschieder 2000, more than 100‘000 m 3 of sediment were deposited in the village –During the flood events 2005 1/3 to 1/2 of the total damage of 3 billion Francs was caused by bedload Baltschieder, Vallais, 2000

6. Bedload transport Jens Turowski, WSL

7. Bedload transport Jens Turowski, WSL Importance of bedload transport Schweibbach, Eisten, Vallais: Damaged water intake due to extreme bedload transport

8. Bedload transport Jens Turowski, WSL Importance of bedload transport Scientific interest –Sediment transfer –Channel dynamics –Bedrock erosion

9. Bedload transport Jens Turowski, WSL Bedload transport as a process Interaction of several complicated processes –Turbulent water flow –Large number of particles (granular processes) –Interaction of both Difficult to measure and observe in the field –Few data available Difficult to simulate –Scaling issues

10. Bedload transport Jens Turowski, WSL Bedload transport as a process Three stages in the movement of a pebble –Entrainment / initiation of motion Threshold discharge –Translation Transport distance –Deposition Favourable environment?

11. Bedload transport Jens Turowski, WSL Bedload transport as a process Interactions between channel morphology, sediment transport and flood hydrology –Channel morphology and hydrology determine hydraulics –Hydraulics and channel morphology determine transport –Transport shapes the channel

12. Bedload transport Jens Turowski, WSL Predicting bedload transport Bedload transport predictions for the Erlenbach with three commonly used bedload equations. Measured transport rates Meyer-Peter and Müller (1948) Rickenmann (1991) Wilcock and Crowe (2003) Discharge Bedload transport rate per unit width / m 2 /s Time / min Discharge / m 3 /s Semi-empirical or laboratory-derived equations –Dependent on shear stress or discharge and slope There are dozens of equations available and it takes expert knowledge to decide which one is appropriate in which circumstances

13. Bedload transport Jens Turowski, WSL Predicting bedload transport Minimal knowledge about: –Channel bed slope –Channel width / cross section –Grain size distribution Many simple transport equations have the form: Shields number Einstein number Shear stress Examples: –Meyer-Peter & Müller equation (VAW, 1948) –Fernandez Luque & van Beek, 1976

14. Bedload transport Jens Turowski, WSL Bedload prediction in mountain rivers Unfortunately, we are not very good in doing bedload predictions for steep mountain rivers… Bedload transport predictions for the Erlenbach with three commonly used bedload equations. Measured transport rates Meyer-Peter and Müller (1948) Rickenmann (1991) Wilcock and Crowe (2003) Discharge Bedload transport rate per unit width / m 2 /s Time / min Discharge / m 3 /s

15. Bedload transport Jens Turowski, WSL Bedload prediction in mountain rivers Bedload transport predictions for the Erlenbach with three commonly used bedload equations. Measured transport rates Meyer-Peter and Müller (1948) Rickenmann (1991) Wilcock and Crowe (2003) Discharge Bedload transport rate per unit width / m 2 /s Time / min Discharge / m 3 /s Typical features: –Predictions are better at higher discharges –Observations show larger fluctuations at low discharges

16. Bedload transport Jens Turowski, WSL Bedload prediction in mountain rivers Why do these discrepancies occur? –Sediment availability –Topographic differences Macro-roughness Large-scale bedforms –Step-pool sequences Wide grain size distribution Bedrock control

17. Bedload transport Jens Turowski, WSL Large boulders Step-pool sequences Bedrock control Active hillslopes

18. Bedload transport Jens Turowski, WSL Alexander Beer, WSL Sediment supply

19. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Tagged pebbles Retention basins Indirect measurements Sediment budgets

20. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Tagged pebbles Indirect measurements Sediment budgets Retention basins

21. Bedload transport Jens Turowski, WSL Retention basins Measurements by volume differences Robust and easy Coarse resolution –Depending on size and method, minimum volume changes of ~10-100m 3 can be detected –Data points every few months to years Installation and maintenance expensive Rothenbach SteinibachBaltschiederbach

22. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Tagged pebbles Retention basins Indirect measurements Sediment budgets

23. Bedload transport Jens Turowski, WSL Sediment budgets Sediment transport from topographical differences –Surveying –Laser scanning –Lidar Can cover large areas (whole catchment) Time-consuming and expensive Not very accurate / indirect observation

24. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Tagged pebbles Retention basins Indirect measurements Sediment budgets

25. Bedload transport Jens Turowski, WSL Basket samplers Direct measurements –Obtain grain size distributions as well as transport rate High temporal resolution (10 min) possible Dangerous / impossible at high flows Time-consuming

26. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Retention basins Indirect measurements Sediment budgets Tagged pebbles

27. Bedload transport Jens Turowski, WSL Tagged pebbles Tag individual pebbles and follow their path through the stream –Passive tracers (paint, magnets, radio-active, RFID) –Active tracers (radio, RFID) Large numbers needed Dependent on recovery Time-consuming

28. Bedload transport Jens Turowski, WSL Measuring bedload transport Basket samplers Tagged pebbles Retention basins Indirect measurements Sediment budgets

29. Bedload transport Jens Turowski, WSL Indirect measurements Measure the effects of transport, rather than transport itself –Noise generated (acoustic measurements) –Impacts –Disturbance of a field (magnetic) –Sonar back-scatter High resolution measurements possible Installation expensive, but little maintenance Data directly available Calibration with direct measurements needed

30. Bedload transport Jens Turowski, WSL Erlenbach, Alptal (Kt. Schwyz): WSL Observatory Small catchment (0.7 km 2 ) with long-term observations (>25 years) Bedload transport is measured with all five methods

31. Bedload transport Jens Turowski, WSL Erlenbach, Alptal (Kt. Schwyz): WSL Observatory Indirect sensors: geophone system –Measures „impulses“ due to passing bedload –Calibrated with baskets measurements Geophone sensors

32. Bedload transport Jens Turowski, WSL Geophone calibration Larger scatter for smaller timescales Linear calibration works very well!

33. Bedload transport Jens Turowski, WSL Two floods in the Erlenbach 20th June 2007 9th September 2007 Time / min Discharge / m 3 /s Impulses Discharge Impulses

34. Bedload transport Jens Turowski, WSL Geophone measurements Erlenbach: Bedload transport measurement 2002-2010 1-Minute data 18990 data points 316.5 hours 128 individual events

35. Bedload transport Jens Turowski, WSL Geophone measurements Erlenbach: Bedload transport measurement 2002-2010 1-Minute data 18990 data points 316.5 hours 128 individual events Large scatter

36. Bedload transport Jens Turowski, WSL Importance of channel morphology Heterogeneous bed Sediment trapped behind boulders, logs and steps

37. Bedload transport Jens Turowski, WSL Importance of extreme events Large events influence bedload transport for several years Shift both in –Transport efficiency –Threshold of motion Large-scale changes in channel morphology From: Turowski et al., ESPL 2009

38. Bedload transport Jens Turowski, WSL Geophone measurements Erlenbach: Bedload transport measurement 2002-2010 1-Minute data 18990 data points 316.5 hours 128 individual events Threshold discharge?

39. Bedload transport Jens Turowski, WSL Threshold discharge Threshold dependent on local grain environment –Friction angle –Protrusion –Local hydraulics Flow Direction of motion? Upstream protrusion

40. Bedload transport Jens Turowski, WSL Threshold discharge Start and end of bedload movement From: Turowski et al., GRL 2011

41. Bedload transport Jens Turowski, WSL Conclusions I Bedload transport is an important process in –Natural hazards –Scientific investigation –Hydraulic engineering –Society / economy in general Despite more than 100 years of research, there are many open questions –Initiation of motion / thresholds –Translation –Deposition

42. Bedload transport Jens Turowski, WSL Conclusions II Bedload predictions for mountain streams are difficult –Empirical methods are available –Few data for validation / calibration Detailed field measurements can help to answer many questions –Well-instrumented observatories (e.g. Erlenbach) Interaction between channel morphology, sediment transport, flood hydrology

43. Bedload transport Jens Turowski, WSL Thank you for listening! Questions?