LONG WAVE SURGE Understanding infragravity wave energy in ports
Peter McComb Long period waves and harbour surges
§ What is the problem?
§ What is causing it?
§ What can we do about it?
www.metocean.co.nz The problem
6 degrees of freedom
www.metocean.co.nzwww.metocean.co.nz Long period waves (LPW)
Red = inside harbour Blue = outside harbour Water level oscillations with periods of greater than swell but less than tides. Typically 25 – 1200 seconds with small amplitude. seiche
§ Bound long waves – tied to the wave group structure but can be released at the coast
§ Free long waves
§ Surf beat – long wave energy released in the surf zone
FIG IG swell sea long waves
www.metocean.co.nz Discovering LPW Infra-gravity 25-120s
Total
Far Infra-gravity 120-150+s
Tide removed
Wave group modulation (sets)
Sea / Swell removed
IG – often modulated by tide FIG – not modulated by tide www.metocean.co.nz Wave groups
L h 1 1 2 E p = rgz.dz.dx = rgH Energy flux L òò 16 0 0 1 2 Et = E p + Ek = rgH 1 L h 1 1 8 E = r w2 + u 2 .dz.dx = rgH 2 k ò ò ( ) L 0 -h 2 16
www.metocean.co.nz IG and FIG waves
Infra-gravity 25-120s
IG
Far Infra-gravity 120-150+s
FIG
0.1 0.1 0.08 0.09 0.06 0.08 0.04 0.07 FIG waves are created by the modulation of wave energy 0.02 0.06 0 0.05 -0.02 0.04 into ‘sets’, which is beneficial for surfing. FIG waves have -0.04 0.03 G Stdev (m)
FIG water level (m) FIG water -0.06 0.02 -0.08 0.01 the same period as the set frequency. -0.1 0 82 82.05 82.1 82.15 82.2 82.25 82.3 Julian Day 2003
Time-series correlation of FIG to wave groups www.metocean.co.nz Generation of LPW IG and FIG waves are linked to the incident swell
§ Generated by swell wave groupings IG waves: § Highly correlated to swell wave energy flux § Are released in the surf zone § Released as swell waves transform from ~20 misobath § Decay with distance offshore § Propagate freely to refract and reflect § Have a broad range of periods (centre 40-50s) § Not always co-linear with swell wave direction § Often modulated by tide § Broad range of frequencies but typically <150s period § Correlate to Hs (T>8s)a x Tpb FIG waves: § Can resonate inside a harbour § Initially bound to the wave group § Found in long-period wave climates § No decay in ~20 m depth range § No tidal modulation, highly variable periods § Correlate to Hs (T>8s)a x Swb
www.metocean.co.nz LPW and seiche
Red = inside harbour Blue = outside harbour LPW SWELL Port Taranaki
www.metocean.co.nz ConsiderLPW nodes sampling and anti-nodes: strategies node
Consider nodes and anti-nodes:
anti-node
In a complex location one measurement site may not be enough
www.metocean.co.nz ConsiderLPW nodes sampling and anti-nodes: strategies
www.metocean.co.nz Measured spectra inside the harbour
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra
www.metocean.co.nz Measured spectra outside the harbour
Mean spectra
Energetic event
www.metocean.co.nz Summary of our observations
§ Ports open to an adjacent beach / reef are the best collectors of IG.
§ Mean efficiency is 6-9%, but the tidal modulation creates different responses and outcomes
§ IG waves are modulated in the surf zone, non-linear transfer from IG back to swell.
§ Profile of the shallow subtidal zone defines the modulation by tide (Thomson, 2006). Steep beach and narrow surf zone – little modulation.
§ IG waves can be hindcast / forecast with onsite data to calibrate.
§ To model IG waves into the harbour – need to know the IG direction. § Broad spectrum outside the harbour
www.metocean.co.nz Modelling LPW
Boussinesq model (Kirby, 1995)
§ refraction/diffraction Reflection Wave breaking § reflection § wave breaking Refraction § non linear energy transfers § turbulent mixing effects § bottom friction effects Diffraction
Modified version used § wave conditions from directional wave spectrum § run-up scheme added Sea-Swell waves § internal fully reflective boundaries
www.metocean.co.nz Modelling LPW
(IG) (MW)
Open circles: low tide Open circles: low tide Solid circles: high tide Solid circles: high tide
www.metocean.co.nz Modelling LPW
Measured (low tide) LPWhigh tide > LPWlow tide Measured (high tide) Modelled (high tide) Modelled (low tide) Modelled (low tide) Modelled (high tide)
(Eastland Port) (Port Taranaki)
(MW position)
www.metocean.co.nz LPW directionality
Eastland Port Port Taranaki
Two energetic modes in the Large amount of LPW energy south-west and west directions coming from North-West
High LPW energy level One low energetic single mode in the northeast direction Access channel oriented south- west Access channel oriented north- east
Problematic conditions for Solution for improvements improving LPW tranquillity
www.metocean.co.nz LPW are complex ……..
www.metocean.co.nz LPW are complex ……..
www.metocean.co.nz LPW are complex ……..
Low freq LPW (>75 s)
High freq LPW (>75 s)
www.metocean.co.nz 2D LPW spectra along the berth
ABC GHI
Surface elevation Gradient! Surface elevation Gradient!
Cross velocity Cross velocity component component
Along velocity Along velocity component component
à mooring line forces and induced vessel motion?
Strong movement along 3 axis!!!
www.metocean.co.nz Harbour design solutions testing
LPW Sea-Swell
www.metocean.co.nz LPW summary
Total water level (m)
15
14
13 § LPW fields are spatially complex and they change with the tide and swell
Depth(m) 12
conditions. 11 Data processing steps
10 0 2 4 6 8 10 12 14 16 18 § The combination of harbour and adjacent coast geometry is important. Time (hours)
§ LPW usually cant be seen - need to be measured and modelled. Total infragravity water level variations (m) 0.1 § Each berth often has a unique LPW climate, and there may be gradients in energy 0.05
over vessel length scales. 0 Height(m) § Each vessel and mooring configuration has a unique response to LPW -0.05
-0.1 0 2 4 6 8 10 12 14 16 18 § The incident LPW have a broad spectrum.. Time (hours)
What can we do about it? IG w ater level variations (m) 0.05 1) Reduce LPW penetration 0.025 0 2) Dampen the vessel motion -0.025 3) Manage the onset of problematic occasions -0.05 0 2 4 6 8 10 12 14 16 18
Time (hours)
www.metocean.co.nz LPW prediction methods
0.25 B measured IG B modelled 0.20
0.15
0.10 IG wave height (m) § Local water level measurements (1 or 2 Hz) 0.05
0.00 240 250 260 270 280 290 300 § Hindcast the incident wave spectra Julian day 2003
0.5 § Multi-variate analysis to correlate spectral estimates with zero- Hz 3 TG Fig modelled 0.4
crossing IG & FIG data 0.3 FIG
0.2 § Include tide in the MVA 0.1
0 195 205 215 225 235 245 255 265
A 3-stage nested domain is typical
www.metocean.co.nz Correlation to offshore (hindcast) short wave spectra
BIAS (m) 0.01 RMSE 0.31 MAE (m) 0.24 MRAE 0.14 SI 0.16
www.metocean.co.nz Correlation to offshore short wave spectra
www.metocean.co.nz Correlation to offshore short wave spectra
25-70 s 70-120 s
www.metocean.co.nz Spectral wave forecasts used to predict LPW for specific berths
§ 1-3 months berth LPW data required for calibration § Warning levels can be provided § Forecast accuracy is similar to swell (5-7 days) § Semi-empirical technique § Includes tidal modulation
Comparison of measured and forecast LPW
www.metocean.co.nz This works well …..
Generic LPW safety thresholds:
<0.10 m – not usually a problem
0.10 m – first threshold of concern
0.10-0.15 m – management is recommended
0.15-0.20 m – management is required
>0.20 m – safety is compromised
7-day forecast of LPW
www.metocean.co.nz LPW monitoring in real time
Long waves at Port Geraldton
www.metocean.co.nz Gracias por tu tiempo Operational Oceanography
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