(3) Sediment Movement Classes of sediment transported

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Kristopher Eaton
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9/17/15 (3) Sediment Movement Classes of sediment transported Dissolved load Suspended load Important for scouring algae Bedload

Big Thompson Flood (1978) Graded stream -- current sediment supply and transport) regimes are in balance Aggradation Sediment

the particles on the bed of a specified size) - How does shear stress change with depth? with slope?

1 9/17/15 (3) Sediment Movement Classes of sediment transported Dissolved load Suspended load Important for scouring algae Bedload (5-10% total load) Moves along bed during floods Source of crushing for benthic organisms, fish eggs, etc. Big Thompson Flood (1978) Graded stream -- current sediment supply and transport) regimes are in balance Aggradation Sediment supply > transport Channel aggrades Degradation Sediment transport > supply Channel degrades shear stress that is necessary to move the particles on the bed of a specified size) - How does shear stress change with depth? with slope? The ability of a stream to move particle of given size (for a given Q) (Example: The stream at 1000 cfs is competent to move cobbles ) - How does competence change with discharge, with slope? 1

2 Hjulstrom curve (Fig.3.8, Allan text) Erosion, Transport and Sedimentation for individual particles Erosion à When critical erosion velocity exceeded (compare sand to gravel) Why is sand the most erodible? Small size, low cohesion Why is clay least erodible? Very high cohesion Transport What is this? à Once eroded, continued movement if stream velocity > fall velocity for particle Why is clay most transportable? Small size once suspended suspended Bed load (larger particles) Hjulstrom curve (Fig.3.8, Allan text) Erosion, Transport and Sedimentation for individual particles Erosion à When critical erosion velocity exceeded (compare sand to gravel) Why is sand the most erodible? Small size, low cohesion Why is clay least erodible? Very high cohesion Transport à Once eroded, continued movement if stream velocity > fall velocity for particle Why is clay most transportable? Sedimentation Small size once suspended suspended à Once eroded, if stream velocity drops below fall velocity for particle Thought question: How can some particles continue to be transported when average velocity < fall velocity? Bed load (larger particles) 2

Streambeds are well sorted Distribution of particle sizes reflects shear stress

areas (erosional) finer grains in slow-flowing Which look like natural (sorted)

Movement of streambed during high flows maintains channel form What determines

Sediment size (Hjulstrom curve) Shear stress acting on streambed particles How do

3 Streambeds are well sorted Distribution of particle sizes reflects shear stress conditions during and after bed-moving flows coarse grains in faster flowing areas (erosional) finer grains in slow-flowing Which look like natural (sorted) streambeds? Movement of streambed during high flows maintains channel form What determines bed mobility? Sediment size (Hjulstrom curve) Shear stress acting on streambed particles How do we calculate when the stream bed will move? Recall for a particle on the streambed: -- Shear stress, τ ~ ΔU/Δy The critical shear stress (τ cr ) is that force that initiates incipient motion of a particle(s) on the streambed 3

4 But it is virtually impossible to calculate movement of a single particle in a mixed particle streambed!!! τ cr = critical shear stress Function of: - grain packing - gravity constant - particle density - fluid density - particle diameter - drag and lift forces What s alternative? Use average mobility of a larger area of the streambed. - often calculated for the REACH scale. Knighton 1998 Usually we re interested in whole streambed mobility Focus on the population of particles in a reach, and estimate the average shear stress acting on the bed and see whether it is sufficient to move the average particle size (median or D 50 ): τ = ρgrs Above equation simplifies to τ DS, so, shear stress acting on streambed is directly proportional to Mean Depth Channel Slope (or gradient) As shear stress increases, what happens to stream competence? What determines how depth and slope change with increasing Q? ΔDepth: depends on channel shape ΔSlope: depends on channel bedform 4

https://www.youtube.com/watc feature=player_detailpage&v= S4-9IF0 https://www.youtube.com/watch?

15 Knighton) Constrained channel Depth increases rapidly with Q energy doesn t dissipate laterally Unconstrained

5 feature=player_detailpage&v= S4-9IF0 v=axjgfowj3ka ΔDepth: depends on channel shape (Fig Knighton) Constrained channel Depth increases rapidly with Q energy doesn t dissipate laterally Unconstrained channel (e.g., with floodplain) Depth increases to fill channel Higher flows spill onto floodplain (energy dissipated laterally) For same Q and slope, which channel type has greatest competence? 5

6 Why do riffles have coarse sediment and pools fine sediment? Which has greater shear stress at LOW FLOW? (Fig. 1.12) Hint: Think about depth and slope and velocity profile! Riffle at low flow has shallow depth but high velocity and steep slope. Pool has greater depth, but low slope. Which has greater shear stress during HIGH FLOW? Depth? Velocity? Slope? Shear Stress? 6

7 Sediment transport in riffles vs. pools (deposition vs erosion!) At low flow, pools are depositional At flood flow, pools are erosional coarse grains move through pools, deposit at head of riffles as flood recedes Next question: When does sediment move? What flow levels are responsible for transporting sediment and maintaining the channel form that we observe? The "Dominant discharge (Q d ) is the flow level that moves the greatest total volume of sediment. There is a Q d for both suspended load and bedload. * The Q d is determined by how large the magnitude of the flow AND the frequency with which it occurs. Question: Is the maximum discharge in a river responsible for moving the most sediment on an annual basis? 7

Frequency-Magnitude Concept Q d is dominant flow a channel forming flow The

channel occurs on average every 1-3 years.

5-10 years, depending on channel type Applies best to gravel-cobble rivers with

8 Frequency-Magnitude Concept Q d is dominant flow a channel forming flow The discharge that moves the most suspended sediment, (and thus maintains the ) channel occurs on average every 1-3 years. The discharge level that moves most bedload occurs every years, depending on channel type Applies best to gravel-cobble rivers with floodplains Suspended sediment load Freq. of Q Approx. Bankfull Q Example hydrograph: Bedload Occurs every yr Bankfull Discharge is an estimate of Q d 8

9 How do the 5 components of the flow regime vary among these hydrographs? Think about high flows that act as a disturbance to the bed. Frequency? Duration? Timing? Rate of change? 9

(3) Sediment Movement Classes of sediment transported

(3) Sediment Movement Classes of sediment transported Dissolved load Suspended (and wash load ) Important for scouring algae Bedload (5-10% total load Moves along bed during floods Source of crushing for