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 RDI N 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 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.

VEC1 VEC2VEC3 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 Depth (m) Cross-shore distance (m) Instrument array (not to scale) Shore break

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