modelling hydrodynamics in the nearshore 21 may 2009 jim gunson, graham symonds, liejun zhong &...
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Modelling Hydrodynamics in the Nearshore
21 May 2009
Jim Gunson, Graham Symonds, Liejun Zhong & Nick Mortimer CMAR
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Setting
Purpose:- Operational prediction of nearshore conditions (Bluelink)- Better understanding of surf-zone circulation.
Outline of talk: - Overview of nearshore hydrodynamic modelling - Marmion lagoon- Secret Harbour beach- Linkages with other models
.
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Background
When incoming waves break in the surf zone, they can drive currents. Need model that couples a wave model to a current model.
Critical elements:- Knowledge of boundary conditions- Knowledge of bathymetry- Computational cost
Wave model: - Conservation of wave action (SWAN, Xbeach) phase-averaging.- Mild-slope equation (Ref-Dif), phase-resolving.
Circulation model: - 2D (shallow-water equations): Storm-surge models, Curvcirc, Xbeach.- 3D (Navier-Stokes equations): Ocean models, PolCOMMS, ROMS, SHOC.
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Marmion Lagoon habitat distribution
Marine habitat types correspond to wave energy experienced
• Algae in high energy
• Seagrass in low energy
Mean wave height Benthic habitat
Bare sand
Seagrass
Low relief reef Algae dominated
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Lagoon Measurement Program
• From July 2007 to May 2008• An area of order 5km x 10km• Reef lines of 1~3m depth• About 10m deep inside lagoon• Measurements of wave,
current, pressure, T, S, Chlorophyll, nitrate, etc
RDIN
AQ2
AQ1
RDIS
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CTD Survey - 6/2/2008
• High nutrient levels over reef• Use the numerical model to
identify the possible sources of nutrient over the reefs
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Wind, wave and currents during July 2007
• Southerly wind component at all times
• Low variation in wave direction
• Low correlation between currents and wind
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Marmion Lagoon Region
• Complex bathymetry with depth change of 5-10m over several hundred meters across a reef. Deep channels exist between reefs in reef lines.
• Weak stratification although there are seasonal variations of temperature and salinity
• Weak tidal currents but tidal sea level may be important in terms of wave breaking over the reefs
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Marmion Lagoon Model
• ROMS Configuration• Physical domain, 10km x 30km
• Grid size, 120 x 240 x 8
• Horizontal resolution, 50~150m
• Tidal forcing from 8 tidal constituents K1, O1, P1, Q1, M2, S2, N2 and K2 whose harmonic constants are derived from TPXO6
• surface momentum, heat and moisture fluxes calculated from bulk parameterizations
• Wave data from AWAC site
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Dynamic feedback between ROMS & SWAN
ubar, vbar,
Dwave, Hwave, Lwave,Pwave_top, Pwave_bot,Ub_swan, Wave_dissip
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Wind, wave and currents during July 2007
• Southerly wind component at all times
• Low variation in wave direction
• Low correlation between currents and wind
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Model-data comparison
OBS
No coupling
Two-way coupling
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Wind, wave and currents during July 2007
• Southerly wind component at all times
• Low variation in wave direction
• Low correlation between currents and wind
![Page 14: Modelling Hydrodynamics in the Nearshore 21 May 2009 Jim Gunson, Graham Symonds, Liejun Zhong & Nick Mortimer CMAR](https://reader033.vdocument.in/reader033/viewer/2022051316/56649e055503460f94af17e5/html5/thumbnails/14.jpg)
Model-data comparison
OBS
No coupling
Two-way coupling
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Nearshore Experiment
BLUElink II Nearshore ExperimentSecret Harbour, WAFeb 9 to Mar 6, 2009
Aim:To measure wave driven, alongshore currents and compare with numerical simulations using selected models.
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VEC1
VEC2 VEC3
VEC4
SBE26-1
SBE26-2
AWAC
Nortek Vector (u,v,w,P,T)
Seabird SBE26 (P,T)
Nortek AWAC (u(z),v(z),
Radar
Surf zone
0
-1
-2
-10
10
0 50 100 1000
Dep
th (
m)
Cross-shore distance (m)
Instrument array (not to scale)
Sh
ore
bre
ak
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Vector current meter deployment
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The Surf Zone
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Wind and wave observations
Sea-breeze cycle
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Secret Harbour – model domain
Choice:- Extends over instruments- Resolve surf-zone- Rotated grid
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Secret Harbour – NearCoM, preliminary run
- Model domain: 1150m x 1450m, Δx=Δy=5m- Forced by swell (Hs=.8m, Tp=10s, Dir=10º) on seaward boundary.
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Linkages with other models
Hydrodynamic variables of interest to BGC:- Wave orbital velocity- Wave-induced vertical mixing- Bottom pressure variations- Bottom velocity- Stokes drift- Tidal signal (local amplitude and phase)
Biogeochemistry
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Linkages with other models
Capabilities:- NearCoM- Xbeach- Scouring- Sand-bar formation- Changing sea-level
Morphodynamics
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Linkages with other models
Challenges:- Rebuilding a beach- 3D circulation- Long time-scales with short time-scales- Knowledge of bathymetry crucial- Data assimiliation / sensitivity studies
Morphodynamics