Nearshore wave characteristics related to a strong Etesian wind event monitored at the Gouves beach...

NEARSHORE WAVE CHARACTERISTICS RELATED TO A STRONG ETESIAN WIND EVENT MONITORED AT THE GOUVES

BEACH ZONE (HERAKLIO, CRETE ISLAND).

S.E. Poulos, T.A. Plomaritis, G. Ghionis

Faculty of Geosciences – University of Athens Faculty of Marine Sciences – University of Cadiz

10ο Πανελλήνιο Συμπόσιο Ωκεανογραφίας & ΑλιείαςΕΚΠΑ

10ο Πανελλήνιο Συμπόσιο Ωκεανογραφίας & Αλιείας

Nearshore Wave characteristics related to Etesian events

Introduction

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• The wave climate of the essentially tideless Aegean Sea (Tsimplis 1994) is dominated by the wind regime, which varies seasonally and is modified spatially by the coastal morphology (irregular coastline, hundreds of islands) and the topography of the surrounding landmasses of the southern Balkans and W Asia.

• The principal wind system that prevails over the Aegean Sea during the summer period (June to September) is the Etesians.

• This wind system, dictating the offshore wave regime, is also expected to play a significant role in the formation, stability and evolution of all nearshore and beach zones that face to the north.



Objectives

Use in-situ hydrodynamic field measurements to: • Identify the effect of a strong Etesian wind event (≥6 B) on the nearshore wave regime

• Provide an insight into how the wave characteristics evolve in the nearshore zone

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Study Area

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Wind Climate

• The prevailing winds (Etesians) are from NW with an annual frequency of 29.4%, followed by the N winds with an annual frequency of 16.7%

• 60% of the time wind speeds are <4B (≤5.5 m·s-1)

• 10% of the time wind speeds are >5B (>10.7 m·s-1)

• 3% of the time wind speeds are >6B (>13.8 m·s-1)

• <0.5% of the time wind speeds are >8B (>20.7 m·s-1)

Etesian wind speed classification (Carapiperis, 1968):

• Weak (≤3.3 m·s-1)• Moderate (3.4 – 7.9 m·s-1)• Strong (≥8 m·s-1)



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Wave Climate

• The wind-generated offshore waves in the SE Aegean (fetch lengths: NW≈ 187 km and N≈ 85 km) have significant wave heights <1.5 m for 55% of the time

According to the Wind and Wave Atlas (Athanasoulis and Skarsoulis, 1992):

• During the summer, when the Etesians dominate the wind climate, waves approaching from the NW represent more than half (52.2%) of the incoming waves

• 47.4% of the time they have wave heights between 0.5 and 1 m

• 23.2% of the time they have wave heights between 1 and 1.5 m



Methodology

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Station Latitude Longitude Distance from shoreline Water depth (m)

G1 35o 20.157′ 25o 17.294′ 75 m 2.60

G2 35o 20.196′ 25o 17.306′ 122 m 3.95

G3 35o 20.290′ 25o 17.290′ 232 m 5.62

P.B. 35o 36.140′ 25o 34.630′ ~30 km ~35.0

ABR

Burst measurements for 10 minutes every hour at 4 Hz

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DATE AND TIME

BA

RO

MET

RIC

PRES

SUR

E (h

Pa)



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26–30 JULY 2003 ETESIAN WIND STORM EVENT

0

10

20

30

40

WIN

DSP

EED

(kno

ts)

10-min AVERAGE

MAX. GUST in 3 hrs

04590

135180225270315360

WIN

DDI

RECT

ION

(deg

rees

)

Wind speeds up to 11 m·s-1, gusting up to 14 m·s-1 (>8 m·s-1)



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Nearshore Wave Conditions



Tidal and sea level conditions

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G3

G2

G1

SET-UP

4 cm

5 cm

10 cm



Wave conditions

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Maximum waveConditions:

Hs > 1.5 m

Tp = 5-7 s

Peak (Hsmax)sequence:

G2: 1.65 m

G1: 1.55 m

G3: 1.40 m



Wave conditions

Pre-

stor

m

Stor

m b

uild

up

Stor

m d

ecay

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Rapid build-up

Slower decay



Wave breaking HsHmax

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2tan TgHB b

b

Inshore Breaker TypeParameter (Galvin, 1972):

Bb Breaker TypeSurging/ Collapsing

0.003Plunging

0.068Spilling



Spectral Analysis

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Wave energy is concentratedIn two frequency ranges:around 0.2 Hz (~5 s)and 0.26-0.27 Hz (3.7-3.8 s)

Wave energy is focusedIn the frequency range:0.15-0.16 Hz (6.2-6.7 s)

Wave energy focusingindicates that most of theenergy in the nearshore zoneoriginates from wind-generatedgravity waves



Wave Spectra

Infragravity waves probably due to

reflection at the shore

Energy at high frequencies probably

due to breaking

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Conclusions

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• Etesian winds dominate the wind and the wind-induced wave climate in the Aegean Sea during the summer period

• In the South Aegean Sea the Etesians have a distinctive NW (305-315°) direction due to the effect of the Cretan mountains

• Etesian-generated waves approach the northern Cretan coast from N330°

• The barometric pressure changes during an Etesian storm event are strongly correlated to the fluctuation of the wind speed, increasing during the period of strongest winds



Conclusions

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• The monitored strong Etesian winds event (speeds >11 m·s-1) produced rough seas with nearshore significant wave heights exceeding 1.5 m and corresponding periods >6 s at the northern coast of Crete• The build-up phase of the storm event was more rapid than the decay phase which was slower and more variable due to the continued arrival of swell waves• The significant wave height at breaking was <2 m and the breaking zone extended up to 4 m water depth• The combined wind and wave set-up reached 10 cm at a water depth of 2.6 m• The difference between the significant and maximum wave heights ranged between 0.5 and 1.5 m, with higher differences during the decay phase of the storm and the highest differences observed during the peak of the storm at the shallower stations• Differences in Hs and Tz between the three stations were more pronounced when Hs > 1 m (when wave transformation and breaking processes become important)• Wave energy was concentrated in the 0.26-0.27 Hz (3.7-3.8 s) and 0.15-0.16 Hz (6.2-6.7 s) frequency bands• Infragravity wave activity (20-30 s) was detected during the peak of the storm at all stations, with the highest energy concentration at the shallow station (G1)



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Nearshore wave characteristics related to a strong Etesian wind event monitored at the Gouves beach...

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