Sediment transport. Sediment transport. Boundary layer stress. τ : This lecture

Transcription

1 Sediment transport GEO3-4306: oastal Morphodynamics Sediment transport This lecture background modes of sediment transport cross-shore transport longshore transport sediment balance oundary layer stress u τ : z 1

2 Shield s parameter τb θ = ( ρs ρ ) gd50 1/ 3 ( s 1) g * = d 2 50 ν s = 2.65, g = 9.81 m/s 2, ν = 1 x 10-6 m 0.5 /s, d 50 = 250 x 10-6 m 1 * 6 1 θ = 0.05 Sediment transport s soon as waves feel the sea bed, sediment will be in motion Waves stir the sediment Transport modes ed load (grain-to-grain interactions) Suspended load ( in the fluid turbulence versus gravity) Sediment transport Moving sediment can be organized in small bedforms (e.g., ripples, mega-ripples) 2

3 Example of wave ripples in the shoaling zone Sediment transport q = u * c sediment flux = velocity * concentration Processes relevant to cross-shore sediment transport E 3

4 Location (deep water) symmetric waves inactive bed transport is zero Processes relevant to cross-shore sediment transport E Location skewed waves ripples on sea bed 4

5 Processes relevant to cross-shore sediment transport E Location skewed waves bound infragravity waves sheet flow (flat bed) Location skewed waves bound infragravity waves sheet flow (flat bed) 5

6 Shoaling zone (1) Skewed waves stir N transport sediment Near the bed, c in phase with u 1 onshore transport Higher up in the vertical, c much lower and phase shift between u and c 1 no or small offshore transport Overall effect: onshore transport Net effect of bound infragravity waves? 6

7 Shoaling zone (2) ound infragravity waves transport sand stirred by gravity waves Large concentrations under high waves in the group coincide with bound infragravity trough (offshore infragravity orbital motion) Overall effect: offshore transport Shoaling zone (3) Skewed waves: onshore transport ound infragravity waves: offshore transport onshore >>> offshore Processes relevant to cross-shore sediment transport E 7

8 Location asymmetric waves undertow 8.7 m 3.6 m 2.9 m 2.4 m 1.7 m 1.0 m very large sediment concentrations under plunging breakers 8

9 symmetric waves Onshore transport Same mechanism as for skewed waves? Why / why not? 9

10 Large sediment concentrations and undertow 1 transport direction? reaking wave zone reaking, asymmetric gravity waves stir sediment Large concentrations (breaking-induced turbulence) Sediment transport: onshore by asymmetric waves offshore by undertow In general: few breaking waves 1 onshore many breaking waves 1 offshore Processes relevant to cross-shore sediment transport E 10

11 Location E infragravity waves (undertow) u c u*c Swash zone (during storms) Water motion dominated by infragravity waves Infragravity waves stir N transport sediment Large concentrations (breaking-induced turbulence) Sediment transport: unclear field experiments: onshore and offshore Potential offshore contribution by undertow In summary E : no transport : little transport (skewed waves and ripples) : onshore transport in shoaling zone (skewed waves) : on/offshore transport in breaking zone (asym. waves/undertow) E: on/offshore transport in swash zone (infragravity waves) transport rates increase 11

12 In case of rip currents Sediment is stirred by gravity waves, transported by currents Skewed waves only play minor (onshore) role in between the rip currents Other mechanisms not too important (undertow does not exist!) longshore sediment transport Gravity waves stir sediment reaking-induced alongshore currents transport the sediment littoral drift ross-shore integrated formula q : ρg H br sin α b cos α b Shore-normally incident (α = 0), transport is 0! Transport increases when wave height increases! Transport is maximum when α = 45 12

13 Sediment balance ed level changes are a result of gradients in the sediment transport rates q q x y z + = x x t mass balance equation (Exner s equation) Sediment balance if you want to know how an area will change, determine input and output difference between input and output is change in transport 13