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Sand transport rates tides only

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The study region. northern continental margin of British Columbia. the issues: ... Sea foam from. north Pacific swell. Grid for tidal model (Foreman and Walters, 1990) ... – PowerPoint PPT presentation

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Title: 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

3
Rose Bar
Dixon Entrance
Submerged ridge
Rose Spit
Hecate Strait
Graham Island
Dogfish Bank
4
View of eroding bluff looking northwardsnorth-eas
tern Graham Island
Tree barrackades
Stable beach
Grassed dune ridges
5
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
6
View of Graham Island shoreface looking southwards
Gravel bar
Eroding bluff
Tree barrackades forming
7
Tree barrackades (convergence region)(treed
cliff line in distance)
8
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
9
MacIntyre Bay looking eastwards
Rose Bar
Rose Spit
Rippled fine sand
10
View of MacIntyre Bay looking westwards
bedrock coastline
Sea foam from north Pacific swell
11
Grid for tidal model (Foreman and Walters, 1990)
Output of wind-driven current model (Hannah et
al., 1991)
12
Phase and amplitude of the semi-diurnal tide -
British Columbia
Eastward tidal wave propagation
Amphidromic point (tidal convergence)
Northward tidal wave propagation
13
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

14
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

15
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)

16
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)

17
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

18
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

19
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
20
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

21
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

22
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

23
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

24
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

25
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

26
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

27
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

28
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

29
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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