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

Modelling Hydrodynamics in the Nearshore

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

21 May 2009


Setting

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

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

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

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

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

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

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

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


Dynamic feedback between roms swan

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 20071

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

Model-data comparison

OBS

No coupling

Two-way coupling


Wind wave and currents during july 20072

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 comparison1

Model-data comparison

OBS

No coupling

Two-way coupling


Nearshore experiment

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.


Modelling hydrodynamics in the nearshore

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

Vector current meter deployment


The surf zone

The Surf Zone


Wind and wave observations

Wind and wave observations

Sea-breeze cycle


Secret harbour model domain

Secret Harbour – model domain

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


Secret harbour nearcom preliminary run

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

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 models1

Linkages with other models

Morphodynamics

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


Linkages with other models2

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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