Modelling Hydrodynamics in the Nearshore

21 May 2009

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

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 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.

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

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

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

Dynamic feedback between ROMS & SWAN

ubar, vbar,

Dwave, Hwave, Lwave,Pwave_top, Pwave_bot,Ub_swan, Wave_dissip

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 ExperimentSecret Harbour, WAFeb 9 to Mar 6, 2009

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

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

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

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

Linkages with other models

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

Morphodynamics

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