quantitative description of particle dispersal over irregular coastlines
DESCRIPTION
Quantitative Description of Particle Dispersal over Irregular Coastlines. Tim Chaffey, Satoshi Mitarai, Dave Siegel. BACKGROUND. Topographic eddies may be important in determining habitat connectivity Eddies can retain larvae for time scales comparable with their PLD - PowerPoint PPT PresentationTRANSCRIPT
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Quantitative Description of Particle Dispersal over Irregular Coastlines
Tim Chaffey, Satoshi Mitarai, Dave Siegel
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BACKGROUND• Topographic eddies may be important in determining habitat
connectivity– Eddies can retain larvae for time scales comparable with their PLD
• High local recruitment is observed in island wake eddies (e.g., Swearer et al, 1999)– But, such clear pattern will not be observed in coastal eddies where
currents are less persistent in direction (Graham & Largier, 1997) • Filament formation may be important in offshore transport of larvae
(Haidvogel, 1991)– Offshore filaments present an obstacle to nearshore settlement in
PLD. • Few notable studies (Largier, 2003)
– Geostrophic size & flow time scale considered
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GOAL OF THIS STUDY• Use coastal 3-D physical model to investigate the wind driven circulation around
idealized irregularities in the coastline.• Estimate the role of coastal headland eddies on larval dispersal using idealized
ROMS simulations– Do headlands create a consistent connectivity or a stochastic connectivity? – Are the spatial scales of settlement similar to the straight coastline case (50
km)?– If there is a consistent connectivity, does it remain constant under different
wind regimes?– Is there a critical headland amplitude/width?– How important is headland spacing?– Describe the physics of filament formation and eddy recirculation around
headlands?• Can we develop theoretical relationships between headland size, wind forcing,
bathymetry and release-settlement relationship?– If we can develop theory, we can use this theory to predict release-settlement
relationships over realistic coastlines given wind, headland size, and bathymetry.
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How this all comes together
1 - Collect CalCOFI data (SSH, temperature), bathymetry data, wind data from buoys.
2 - Initialized three-dimensional coastal circulation model (ROMS) with above data.
3 - Use 3D flow, 3D temperature, and SSH from model to compute flow statistics offline (Matlab).
4 - Use 3D flow from model to track particles offline (Matlab). We have ultimate flexibility in how we manipulate particles.
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Model Setup • Wind field from buoy measurements
– Wind field sum of a mean and perturbation component
– July wind field (upwelling period)• Pressure gradient quantified using dynamic
height data from CalCOFI July ship track survey. – Pressure gradient rotated to along the coast
line inside the 500 m isobath Domain: 288 km Cross-shore, 256 km Alongshore
• Bathymetry - 0 m to 500 m offshore• Irregular coastline (headland) created using a
Gaussian function
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HEADLAND DESIGN• Gaussian-shape headland in idealized
simulations– Three parameters
2. Width (w)(twice the std of Gaussian function)
1. Amplitude (a)
3. Domain size (d) = distance between headlands
a = 20 kmw = 20 kmd = 256 km
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Variable Model Parameters
• Wind Field– Uniform - Wind field principal axis has uniform (N-S) direction over
domain– Alongshore - Wind field principal axis rotated to follow land inside
500 m isobath
• Pressure Gradient– Alongshore - Pressure gradient principal axis rotated to follow
land inside 500 m isobath
• Bathymetry – Compression - Distance from land to 500 m isobath compresses
near headland– Alongshore- Distance from land to 500 m isobath same at all
points.
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Wind Field Rotation
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Bathymetry - Compression of Isobaths at Headland
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Bathymetry - Non-Compression of Isobaths at Headland
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Notation for Cases Tested
BA - WUBA - WAAlongshore
BC - WUBC - WACompression
UniformAlongshore
Wind
Bathymetry
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Particle Tracking
• 90,000 particles randomly released 10 km from land and in the upper 10 m, but with a uniform release distribution over 90 days.
• Particles locations updated every three hours.• Particles settle when within competency
window and 10 km of land.• Particles are treated as packets that can settle
multiple times.• Particle bouncing schemes vary
– Reflecting - particles are reflected off boundary – Non-reflecting - particles entering land are returned
to prior time step over water
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Model Parameters
BA - WUBA - WAAlongshore
BC - WUBC - WACompression
UniformAlongshore
Wind
Bathymetry
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Connectivity, Reflecting Condition (∆t = 3 hr)
BA-WA(1) BA-WA(2) BA-WA(3) Mean
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Arrival Diagram - Reflecting Condition
BA-WA(1)
BA-WA(2)
BA-WA(3)
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Settler Dispersal Kernel - Reflecting Condition
BA-WA(1) BA-WA(2) BA-WA(3) Mean
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Connectivity, Non - Reflecting Condition (∆t = 3 hr)
BA-WA(1) BA-WA(2) BA-WA(3) Mean
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Arrival Diagram - Non -Reflecting Condition
BA-WA(1)
BA-WA(2)
BA-WA(3)
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Settler Dispersal Kernel - Non - Reflecting Condition
BA-WA(1) BA-WA(2) BA-WA(3) Mean
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Nearshore Flow Structure BA-WA
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Model Parameters
BA - WUBA - WAAlongshore
BC - WUBC - WACompression
UniformAlongshore
Wind
Bathymetry
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Connectivity, Reflecting Condition (∆t = 3 hr)
BC - WU(1) BC - WU (2)
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Arrival Diagram - Reflecting Condition
BC-WU (1)
BC-WU (2)
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Settler Dispersal Kernel - Reflecting Condition
BC - WU(1) BC - WU (2)
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Connectivity, Non-Reflecting Condition (∆t = 3 hr)
BC - WU(1) BC - WU (2)
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Arrival Diagram - Non -Reflecting Condition
BC-WU (1)
BC-WU (2)
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Settler Dispersal Kernel - Non-Reflecting Condition
BC - WU(1) BC - WU (2)
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Nearshore Flow Structure BC-WU
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Summary• Dispersal for the alongshore wind cases are
realistic while uniform wind cases are artificial.• For uniform wind cases, nearshore irregularities
in the flow create artificial particle accumulation on the wayward side of the headland
• A semi-annual pattern of dispersal is present in the alongshore wind case for both bathymetrys and both bouncing schemes.
• For alongshore wind cases, particle dispersal over 1.5 years has a uniform distribution.
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Future Work• Experiment with alternative bouncing scheme to remove
artificial accumulation of particles• Compare time integrated temperature fields to CALCOFI
data to validate simulations• Investigate vertical velocity at fixed depth and mixed
layer depth to theoretical predications to validate simulations
• Collaborate with Bernardo Broitman (NCEAS) to assess the variation of isobathic distances with coastline
• Bridge the connections between headland size, flow structure, and dispersal. Time scales will be important!
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Model Parameters
BA - WUBA - WAAlongshore
BC - WUBC - WACompression
UniformAlongshore
Wind
Bathymetry
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Connectivity, Reflecting Condition (∆t = 3 hr)
BC-WA(1) BC-WA(2) BC-WA(3) Mean
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Arrival Diagram - Reflecting Condition
BC-WA(1)
BC-WA(2)
BC-WA(3)
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Settler Dispersal Kernel - Reflecting Condition
BC-WA(1) BC-WA(2) BC-WA(3) Mean
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Connectivity, Non-Reflecting Condition (∆t = 3 hr)
BC-WA(1) BC-WA(2) BC-WA(3) Mean
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Arrival Diagram - Non - Reflecting Condition
BC-WA(1)
BC-WA(2)
BC-WA(3)
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Settler Dispersal Kernel - Non - Reflecting Condition
BC-WA(1) BC-WA(2) BC-WA(3) Mean
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Nearshore Flow Structure
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Mean Dispersal Statistics
• Mean alongshore distance - 35.9 km
• Standard deviation of alongshore - 56.1 km
• 11.5 % particles settle at least once
• Mean PLD of settling particles 25.2 days (competency window = 20 - 40 days)