Concepts & Phenomena in coastal and port sedimentation R. Kamalian Ports and Maritime Organization of Iran and PIANC-Iran 7 9 December 2015
What is the problem? Many harbours have sedimentation problems (see GE and statistics) Several nearby-shorelines have erosion problems (see GE) Difficulties in finance for maintenance dredging Lots of re-evaluations and re-designs Have a look at Iranian harbours (G.E.)
What to do? Try to minimize harbour siltation / sedimentation at design time when re-studying existing harbours
How? Need to Understand the related phenomena in depth Learn engineering solutions around the world Trace related research works (and involve) Learn existing and new computation/simulation techniques
Some definitions
Effective Phenomena Sediment types? Sediment source? Sediment transformation mechanism?
Sediment Type Sediment may be Sand Silt Clay Mixture of sand and silt Mixture of silt and clay (mud) See sand closely Clay vs. sand See grain size distribution See fall velocity diagram Different sediment types are transported by different mechanism What about our shorelines? (G.E.)
Sediment Sources Mostly from inland by rivers Somewhere cliff erosion From inland Other sources Sometimes dead corals The upstream beach (works for some decades) The nearshore sea bed Dredging, construction
Sand transport mechanism Pure current cannot transport sand considerably Two transport mechanisms are important: 1. Transport by breaking wave 2. Transport by non-breaking wave Moves mostly as suspended load, in the surf zone the most important one. acts rapidly Stokes drift, very slow, much smaller by order of magnitude Considered as long-shore and cross-shore transport
Sand suspension mechanism Two suspension mechanisms 1. Suspension by breaking wave 2. Suspension by bed ripples the most important one. acts rapidly very slow, smaller by order of magnitude, still important out of breaking area Add ripple picture
Sand transport by wave breaking wave breaks at nearshore (see) Longshore sed. transport wave breaking causes sand suspension (see) wave breaking oblique angle also causes longshore current (see) longshore current moves the suspended sediment (see) Sediments settles rapidly when it comes out of breaking area (see) Out of breaking zone, sand moves slowly along wave direction (Stokes-drift)
Sand transport by non-breaking wave Lagrangian particle movement is not symmetric under wave vel. field (see) Bed-ripples help near-bed suspension (see) Also non-linearity of wave, phase lag in velocity and suspension cycles, (see) Sand moves slowly forward in wave direction Enough bed slope can stop this mechanism Out of breaking zone, sand moves slowly along wave direction (Stokes-drift)
Cross-shore sand transport Cross-shore sed. transport is as important as longshore transport In breaking zone, sand moves seaward (undertow flow) see Before breaking, sand moves slowly shoreward (Stokes drift) see One or more sand bars grow up within some hours see A new stable beach profile develops after some hours see Long-shore transport is concentrated on the sand-bar see
How littoral drift acts? sand transport occurs during storm/wave events, not at calm conditions The process is as following: during the storm: 1. Nearshore sand moves seaward to generate sandbar (cross-shore sed. transport) 2. Sand moves parallel to the shoreline by a distance (above sand-bar, breaker line) after the storm: 3. Swell waves bring the sediment back to the shoreline during several days to weeks Littoral drift
Silt transport mechanism More or less similar to fine sand Comes in suspension easier than sand Less fall velocity than sand (for example 2 mm/s instead of 2cm/s) Does not settle rapidly when comes out of the wave breaking zone Pure strong current can make it suspended Strong current can transport silt without wave breaking Fine sand behaves like silt (more or less)
Sedimentation vs. Siltation Different type of deposition in access channel see Different type of deposition at harbour entrance see Different bed slopes (much more gentle for silt)
Mud transport mechanism Cohesiveness plays an important role in erosion stage No need to wave breaking Current is sufficient for suspension Very low fall velocity Suspended when current speed becomes larger than a critical value Deposits when current speed becomes less than another critical value
review an old movie in this regard
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PERCENT FINER SAMPLE NO.3606 NO. 200 100 CLAY SILT SAND GRAVEL 40 U.S. STANDARD SIEVE SIZES 20 10 4 IN. 3/8 3/41 2 3 6 10 15 20 95 85 75 65 55 45 35 25 15 5 100 90 80 70 60 50 40 30 20 10 0 0.004 0.01 0.062 0.1 2 4.75 10 19 100 508 mm
PERCENT FINER SAMPLE NO.3599 NO. 200 100 CLAY SILT SAND GRAVEL 40 U.S. STANDARD SIEVE SIZES 20 10 4 IN. 3/8 3/41 2 3 6 10 15 20 95 85 75 65 55 45 35 25 15 5 100 90 80 70 60 50 40 30 20 10 0 0.004 0.01 0.062 0.1 2 4.75 10 19 100 508 mm Back to presentation
Coastal Eng. Manual, 2002 Back to presentation
Timothy Kusky, 2008 Back to presentation
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Back to presentation Wave breaking
Back to presentation Sediment suspension
Back to presentation Longshore current
Sediment transport Swash zone Back to presentation
How sand moves forward? Silvester, 1993 Back to presentation
Lagrangian drift
Back to presentation Lagrangian drift, non-linear wave
Sand ripples
Back to presentation Sand ripples
Back to presentation Non-linearity of wave induced current
Cross-shore sediment transport Undertow effect Stokes drift Back to presentation
Back to presentation Development of sand-bar during storm
Back to presentation Calm wave profile storm wave profile
Back to presentation Littoral drift is concentrated on the outer sand bar
1. 2. 3. Back to presentation
Sedimentation vs. siltation Sand : Silt : & very fine sand Mangor, 2004 Back to presentation
Sand : wave breaking zone Sand deposition Silt : & very fine sand Silt deposition low velocity Back to presentation