Sand transport rates tides only

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The study region northern continental margin of
British Columbia

• the issues
• coastal erosion of Graham Island
• shipping hazards from Rose Bar
• source of sand in MacIntyre Bay
• tree barrackades and their uses
• gold mining effects
• the settling
• glaciated terrain (deep valleys)
• outwash soft fine sand
• rapid sealevel rise
• large waves (9 m high, 20 seconds)
• big tides (6 m range)
• strong NE storm winds

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Rose Bar
Dixon Entrance
Submerged ridge
Rose Spit
Hecate Strait
Graham Island
Dogfish Bank
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View of eroding bluff looking northwardsnorth-eas
tern Graham Island
Tree barrackades
Stable beach
Grassed dune ridges
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View of Graham Island shoreface looking northwards
near-shore ridge
destruction of rain forest
Onshore migrating gravel bar
narrow beach heavy mineral rich
Eroding dune belt
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View of Graham Island shoreface looking southwards
Gravel bar
Eroding bluff
Tree barrackades forming
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Tree barrackades (convergence region)(treed
cliff line in distance)
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Eroding gold-bearing cliffs in north-easternGraha
m Island
Cut and fill outwash low in heavy minerals
Erosion surface rich in gold
Beach deposits with glacial dropstones
Modern beach
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MacIntyre Bay looking eastwards
Rose Bar
Rose Spit
Rippled fine sand
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View of MacIntyre Bay looking westwards
bedrock coastline
Sea foam from north Pacific swell
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Grid for tidal model (Foreman and Walters, 1990)
Output of wind-driven current model (Hannah et
al., 1991)
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Phase and amplitude of the semi-diurnal tide -
British Columbia
Eastward tidal wave propagation
Amphidromic point (tidal convergence)
Northward tidal wave propagation
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Grain size of shelf sediments around Rose Spit

• Graham Island
• fine sand, glacial outwash
• unconsolidated
• 20 m above present sealevel
• Dogfish Bank
• fine to medium sand
• MacIntyre Bay
• fine sand
• Hecate Strait
• mixed gravel - sand
• Dixon Entrance
• silt
• Rose Bar/NE coastline
• medium sand

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Peak waves during peak of 26 February, 1984 storm

• Winds blowing to NE
• waves moving to NE
• exposure of eastern Graham Island
• attenuance over Dogfish Bank
• shelter within MacIntyre Bay

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Tidal currents during peak of 26 February, 1984
storm

• Peak ebbing tides
• (0.45 m/s)
• convergence along southern coastline (amphidromic
  point)
• acceleration around Rose Spit
• peak flows over Rose Bar
• weak currents over southern Dogfish Bank
• (0.05 m/s)

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Wind-driven currents during peak of 26 February,
1984 storm

• Stongest in Hecate Strait
• (0.7 m/s)
• westwards in MacIntyre Bay
• (0.15 m/s)
• strong over Dogfish Bank
• (0.45 m/s)
• weakly to west in Dixon Entrance
• (0.05 m/s)
• moderate northward flow along coastline
• (0.2-0.3 m/s)

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Combined flows during peak of 26 February, 1984
storm

• Complex flow patterns
• additive on ebb
• unstable on flood
• veering around Rose Spit
• flow deceleration on lee of Rose Bar
• northward sweeping of Dogfish Bank

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The structure of the sediment transport
model(2-D SEDTRANS)

• Finite difference scheme
• 60 x 60 element matrix (1 nm)
• depth averaged tidal currents
• depth averaged wind-driven currents
• no wave refraction
• 1-hour time steps
• CASES
• no storm (tides)
• storm(wind and waves)
• time-series transport
• net accretion/erosion

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Calibration of SEDTRANS(using benthic lander
RALPH)
Storm events

• Sand transport (bedload) rate over 30 days
• (peak in transport corresponds to peaks in waves)
• wave contribution to friction velocity (cm/s)
• tidal contribution to friction velocity (cm/s)

Oscillatory tidal currents
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A Shields plot of time-series data(used to
calibrate thresholds)

• Sheet flow threshold uncalibrated
• flat bed/scouring
• saltation/suspension threshold
• equilibrium ripples found
• no bioturbation evident
• traction threshold
• flat bed or bioturbation
• relict ripples
• no sand in suspension

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Sand transport rates - tides only (26 February,
1984)

• 1800 hours, transport restricted to Rose Spit -
  neap tides
• maximum transport rates 0.0005 kg/m2/s
• transport to the north
• neap tides do not transport sand

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Sand transport rates - tides only (26 February,
1984)

• 2400 hours, flood tide transport restricted to
  Rose Bar and Rose Spit
• transport to the south
• no transport over Dogfish Bank
• maximum transport rates - 0.0005 kg/m2/s

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Sand transport rates - storm and tides (26
February, 1984)

• 0200 hours, flood tides converging on eastern
  Graham Island
• amphidromic Point at tree barrackades
• transport greatest on Rose Spit
• transport focussed on submerged ridge
• transport to 20 m depth

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Sand transport rates - storm and tides (26
February, 1984)

• 0800 hours, strong transport to the north
• ebb tidal currents and wind-induced flows
  northward
• no sand transport below 50 m depth
• deceleration of vectors along northern margin of
  Rose Spit
• westward veering of vectors in Dixon Entrance

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Sand transport rates - storm and tides (26
February, 1984)
Flood tidal wave

• 1200 hours, start of flood tide
• convergence of transport near Rose Bar
• transport to 30 m depth (all of Dogfish Bank)
• transport northwards along margin of Graham
  Island
• maximum transport rates - 0.005 kg/m2/s

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Sand transport rates - storm and tides (26
February, 1984)

• 1600 hours, end of flood tide immediately after
  storm peak
• transport to 100 m depth (Dogfish Bank)
• transport to the north on Dogfish Bank
• westward transport in MacIntyre Bay
• maximum transport - 0.01 kg/m2/s
• maximum transport on submerged ridge

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Sand transport rates - storm and tides (26
February, 1984)

• 1800 hours storm peak and ebbing tide acting
  together
• strong northward sand transport
• westward transport into MacIntyre Bay
• northward transport along coastline of Graham
  Island
• maximum transport rates - 0.05 kg/m2/s
• maximum transport over submerged ridge

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Sand transport rates - storm and tides (26
February, 1984)

• midnight, 8 hours after storm peak
• flood tide against wind-driven flow
• northward transport in south/southward transport
  in north
• convergence near Rose Bar
• deceleration of transport vectors northwards
  along coastline of Graham Island

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Net effects (erosion/deposition) for storm
duration

• accretion in region of convergence (southern
  Graham Island)
• slight erosion of Dogfish Bank
• high erosion/deposition along ridge
• high deposition on Rose Bar

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