Modelling hydrodynamics in the nearshore
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Modelling Hydrodynamics in the Nearshore. Jim Gunson, Graham Symonds, Liejun Zhong & Nick Mortimer CMAR. 21 May 2009. Setting.

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Modelling Hydrodynamics in the Nearshore

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Modelling Hydrodynamics in the Nearshore

Jim Gunson, Graham Symonds, Liejun Zhong & Nick Mortimer CMAR

21 May 2009


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.


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 costWave 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.


Marmion Lagoon habitat distribution

Marine habitat types correspond to wave energy experienced

  • Algae in high energy

  • Seagrass in low energy

Bare sand

Seagrass

Low relief reef

Algae dominated

Benthic habitat

Mean wave height


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

AQ1

AQ2

RDIS


CTD Survey - 6/2/2008

  • High nutrient levels over reef

  • Use the numerical model to identify the possible sources of nutrient over the reefs


Wind, wave and currents during July 2007

  • Southerly wind component at all times

  • Low variation in wave direction

  • Low correlation between currents and wind


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


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


ubar, vbar, h

Dwave, Hwave, Lwave,

Pwave_top, Pwave_bot,

Ub_swan, Wave_dissip

Dynamic feedback between ROMS & SWAN


Wind, wave and currents during July 2007

  • Southerly wind component at all times

  • Low variation in wave direction

  • Low correlation between currents and wind


Model-data comparison

OBS

No coupling

Two-way coupling


Wind, wave and currents during July 2007

  • Southerly wind component at all times

  • Low variation in wave direction

  • Low correlation between currents and wind


Model-data comparison

OBS

No coupling

Two-way coupling


Nearshore Experiment

BLUElink II Nearshore Experiment

Secret Harbour, WA

Feb 9 to Mar 6, 2009

Aim:

To measure wave driven, alongshore currents and compare with numerical simulations using selected models.


Instrument array (not to scale)

Radar

10

Shore break

Surf zone

Depth (m)

0

SBE26-1

-1

VEC2

VEC3

VEC1

-2

VEC4

SBE26-2

-10

AWAC

0

50

100

1000

Nortek Vector (u,v,w,P,T)

Cross-shore distance (m)

Seabird SBE26 (P,T)

Nortek AWAC (u(z),v(z),z)


Vector current meter deployment


The Surf Zone


Wind and wave observations

Sea-breeze cycle


Secret Harbour – model domain

Choice:- Extends over instruments- Resolve surf-zone- Rotated grid


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.


Linkages with other models

Biogeochemistry

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)


Linkages with other models

Morphodynamics

Capabilities:- NearCoM- Xbeach- Scouring- Sand-bar formation- Changing sea-level


Linkages with other models

Morphodynamics

Challenges:- Rebuilding a beach- 3D circulation- Long time-scales with short time-scales- Knowledge of bathymetry crucial- Data assimiliation / sensitivity studies


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