Sedimentology Lecture #6

Sedimentology Lecture #6

Exercise 1: Fenton River Floodplain Exercise Discussion: What information can these floodplain samples give us about past floods on the Fenton River? Floodplain of the Fenton River 100 90 80 Cumulative Frequency 70 Floodplain 1 Floodplain 2 Floodplain 3 Floodplain 5 Floodplain 6 Floodplain 8

60 50 40 30 20 10 0 10 100 1000 Grain Size (Microns) Look through these sediment transport slides to: create a flow chart using different equations. Complete the calculation using your spreadsheet. 10000 Methods:

1. This is floodplain sediment. Therefore, it represents fully suspended sediment deposited during many different floods. 2. The coarsest fraction (d95) in my grain-size distribution represents the coarsest sediment the flows were able to suspend. 3. Now calculate the settling velocity of the coarsest fraction. 4. Use u* versus ws of suspended load to estimate the shear velocity of the transporting fluid 5. Assume a drag coefficient Cd of 0.002. Use relationship between shear velocity and depth averaged velocity to estimate the depth-averaged velocity of the transporting flow in the river? 6. Estimate the boundary shear stress. Make some predictions: 1. What was the finest particle size that was transported as pure bedload associated with a flood of characteristic magnitude? 2. What range of particle sizes were transported as incipiently suspended load? Estimating settling velocity from empirical relationships Depends on: 1. particle size 2. density of sediment 3. density of flow 4. viscosity

5. gravitational acceleration 0.020 0.018 0.016 Settling Velocity (m/s) 0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000 0.00000 1000.00000

2000.00000 3000.00000 Grain size (micron) Why is it not linear?? 4000.00000 5000.00000 6000.00000 grain Settling diameter Velocity (m)m) (m/s) 32 0.001

63 0.004 88 0.007 125 0.012 177 0.019 250 0.031 350 0.048 500 0.073 710 0.103 1000 0.135 1410 0.169 2000

0.207 Exercise 2: Fenton River Gravel Bars Note: Using the Fenton river pure bedload from the Gravel Bar and the fully suspended load from the floodplain deposit allows you to constrain representative values of basal shear stress, shear velocity and depth averaged velocity for the river based only on its sedimentary deposits (potentially a longer record of floods than that provided by river gauges). Method: 1. Create a google sheet to build a grain size distribution of the clasts at the surface of a gravel bar in the Fenton River. Measure the smallest visible axis to do this. This represents the intermediate axis of the particle. 2. Assumption: This surface armour was left behind by the most recent depleting flood. Beneath this layer, the deposit consists of gravel and sand mixed together. Sand was winnowed away at the surface but gravel stayed behind, suggesting that the gravel at the surface was therefore right at the threshold of motion. The critical shear stress required to move these clasts was equal to or slightly greater than the shear stress exerted by the fluid. 3. What is the d50 of the grain size distribution. 4. Use the modified Shields diagram to estimate boundary shear stress. Estimating critical shear stress of a sediment particle from empirical relationships Depends on: 1. particle size 2. density of sediment

3. density of flow 4. viscosity 5. gravitational acceleration 6. Bed roughness (Use the modified Shields Diagram on next page. Relating tb to u Force applied by moving fluid to bed tb Areabed t b = Boundary shear stress u* = Shear velocity Boundary shear stress can be related to the mean flow velocity, by Cd = hydraulic drag def

def def coefficient 2 2 t b u* Cd u gH sin H = Flow Depth Summary of Relationships between fluid and sediment Key connections between solid and fluid phase t cr t b ws u* Experimental Results: Pure Bedload: t b

> tcr & ws/u* > 3 Incipient Suspension: 3 > ws/u* > 0.33 Full suspension: ws/u* 0.33 Reconstructing hydraulics from strata Empirical relationships: For pure bedload tb tcr ws 3 u* (Bagnold, 1941) (Nino et al., 2003) For fully suspended load ws

0.33 u* (Smith, 1977) grain Settling diameter Velocity (m)m) (m/s) 32 0.001 63 0.004 88 0.007 125 0.012 177 0.019 250

0.031 350 0.048 500 0.073 710 0.103 1000 0.135 1410 0.169 2000 0.207

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