Bedload transport Jens Turowski WSL Bedload transport Lecture

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

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

Bedload transport Jens Turowski, WSL Importance of bedload transport

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

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

Bedload transport Jens Turowski, WSL

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

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

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

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?

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

Bedload transport Jens Turowski, WSL Predicting bedload transport • 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 Bedload transport rate per unit width / m 2/s Discharge / m 3/s Time / min 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 Jens Turowski, WSL Predicting bedload transport • Many simple transport equations have the form: Einstein number Shear stress Shields number Examples: – Meyer-Peter & Müller equation (VAW, 1948) – Fernandez Luque & van Beek, 1976 Minimal knowledge about: – Channel bed slope – Channel width / cross section – Grain size distribution

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 rate per unit width / m 2/s Discharge / m 3/s Time / min 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 Jens Turowski, WSL Bedload prediction in mountain rivers • Typical features: – Predictions are better at higher discharges – Observations show larger fluctuations at low discharges Bedload transport rate per unit width / m 2/s Discharge / m 3/s Time / min 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 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

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

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

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

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

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 -100 m 3 can be detected – Data points every few months to years • Installation and maintenance expensive Rothenbach Steinibach Baltschiederbach

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

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

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

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

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

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

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

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

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

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

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

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

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

Bedload transport Jens Turowski, WSL Geophone measurements Large scatter Erlenbach: Bedload transport measurement 2002 -2010 1 -Minute data • 18990 data points • 316. 5 hours • 128 individual events

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

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

Bedload transport Jens Turowski, WSL Geophone measurements Erlenbach: Bedload transport measurement 2002 -2010 Threshold discharge? 1 -Minute data • 18990 data points • 316. 5 hours • 128 individual events

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

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

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

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

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