Seabed scouring Lecture 26

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Seabed scouringLecture 26

• Scour is the excess removal of bed material by
  sediment transport
• unwanted removal (by definition)
• takes place particle-by-particle
• takes place in both cohesive and sandy materials

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Definitions of scouring

• Scour - the interaction between flow, structure
  and sediment
• Scour types
• clear-water scour (no sediment transport)
• live-bed scour (active bedload transport)
• local scour (around single object)
• dishpan scour (around group of objects)
• Structures suffering scour
• Barrages, tidal inlets, navigation channels
• groynes, seawalls, breakwaters
• seabed pipelines, flowlines, electrical cables
• vertical pipes, piles, piers
• gravity based structures, platforms, offshore
  structures
• moorings and marinas

3
Stability of tidal inlets on US sandy shores

• No absolutely stable tidal inlet exists on a
  littoral drift shore (Bruun, 1978)
• the area (A) of a tidal inlet is controlled by
  the volume of tidal water (P) passing through the
  inlet (OBrien, 1979)
• A 4.69 x 10-4P0.85 (units of feet !!!)
• the AP ratio is valid for natural inlets or
  those with one or two jetties
• one jetty (A) 1.04 x 10-5P1.03
• two jetties (A) 5.74 x 10-5P0.95
• inlet infilling/scour is controlled by longshore
  sand transport rate (M)
• P/M gt 150 good flushing
• 50 lt P/M lt 100 entrance bars form
• 20 lt P/M lt 50 unstable channels
• P/M lt 20 no channel inlets

4
Jet streams and scour under bridges

• There is no net creation of energy
• energy dissipation enhanced
• local scour local sedimentation
• overall X-sectional area constant
• scour due to
• flow acceleration
• divergence in transport vectors
• enhanced turbulence production

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Scouring from sun-illuminated swath bathymetry
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Example of scouring
Bathymetry (swath)
local scouring berm
regional/dishpan scour flood and ebb
Backscatter (swath)
finer sand (featureless)
coarse sand/gravel on 2-D megaripples and
sand ribbons
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scour under bridges

• U water discharge (Q)/area (A)
• Qs ? Un and U ? (1/A)n
• as discharge is constant (for a river/estuary)
  (Qn)/An constant (Bruun, 1978)
• Q1A1 Q2A2 Q3A3
• scour in river is thus possible to derive using
  tidal inlet relationship
• where A is in ft2, P is in ft3 !!!!!!

Q3 A3
Q2 A2
Bridge footings
Q1 A1
A 4.69x10-4Q0.85
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Tidal scouring/deposition over pipeline from
swath bathymetry
Cable crossing (deposition/widening)
ebb current scour
flood current scour (narrowing)
sun-illuminated bathymetry
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Motion around a sphere - laminar flows

• Laminar flow around a sphere - drag viscous, no
  flow separation
• no eddies, no turbulence
• body force dominates at front of sphere ?
  pressure gradient (shear)
• downwelling at front upwelling at back
• accumulation dominates front/back
• symmetrical pattern for oscillatory flows

10
Motion around a sphere - transitional flows

• Boundary layer dominated by sweeps and bursts
• development of attached lee eddies in boundary
  layer
• turbulent transfer to bed significant ? scouring
  downstream
• deposition in downwelling (converging) vortex
• erosion in attached (diverging) lee eddy

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Motion around a sphere - turbulent rough flows

• Turbulence intense flow separation dominant
  downstream
• lee vortex streams with convergence in turbulent
  wake region
• erosion in lee vortexes
• erosion in upstream convergent vortexes

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Predicting scour

• Scour related to
• local disturbance of flow field with associated
  energy dissipation
• local reduction in cross-sectional area of flow
  field with local acceleration
• sediment diameter (d50)
• Flow patterns leading to scour
• surface rollers (in front of pipe or pile of
  diameter b and spacing B)
• downflow (in front of pile in water depth h)
• vortex shedding and flow separation
• horse-shoe vortexes in downstream scour holes
• Equilibrium scour depth (dse)

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Continuity and scouring

• Rate of change in bed level (h) is related to the
  divergence in erosion/deposition vectors in
  transport direction, sediment porosity, and
  discharge rate

14
Jets in entrained channels

• Entrainment of harbour entrances ? coastal jets
  ? scouring
• scour balanced by deposition at flood and ebb
  tidal deltas in regions of flow deceleration

15
Sissiboo River scouring project

• stability F(?/M Qm/M ?o)
• peak velocity to x-sectional area
• Area (A) kQmn
• tidal prism to x-sectional area
• Area(A) k?0.85 (OBrien)
• jet stream calculation (1-D) ? Ux
• Uo f(?/A) constant
• Ux Uoe-??/2
• bed stress calculation (1-D) ? ?o
• ?o Cd?Uo2 (Cd 0.003)
• gs f(?o)

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Rustico Bay inlet study

• Consider Rustico Bay inlet
• Tidal Prism (P) 1.95 x 108 ft3
• x-sectional area (A) 1800 ft2
• longshore sand transport (M) 4.4 x 106
• ft3/year
• Question (1) is the inlet in equilibrium ?
• Question (2) what is the equilibrium area ?
• Question (3) what type of inlet is expected ?
• Question (4) how is channel depth best ensured ?