characteristic features of long wave energy within ports ...€¦ · wave hindcast validation data...

Characteristic features of long wave energy within Ports and Harbours on exposed coasts

Wh k i A W R fl tiWhakarire Ave Wave Reflections

MetOcean Solutions Ltd www.metocean.co.nz

Scope

Evaluate the likely effects on the surf quality due to proposed seawall modifications at Whakarire Ave, Napier.Whakarire Ave, Napier.


Methods

Use numerical wave models to simulate:

- the wave climate in the Napier region - the wave climate in the Westshore region- wave patterns at the existing surf breakp g- wave patterns with the seawall modifications

The wave models applied include the relevant physical process such as:

- Generation and propagation- Seabed friction

Refraction and diffraction0° 15°345° 9

x 10-3

- Refraction and diffraction- Reflection- Wave spectra

30°

45°

60°

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90°270°

285°

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330°

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Direction/Frequency [Hz] for Node: 14454

105°

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150°165°180°195°

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Methods - bathymetry


Methods – wave climateWave heightWave height

Wave period

Wind

Tide /storm surge

Current

SSTSST


Methods – wave climate

W hi d t f 10 i dWaves were hindcast for a 10-year period.

Validated against the Port wavebuoy data


Wave hindcast validation

Data location Mean Hs (m)

Mean Hs2

(m2Mean wave power

(W 1(m) (m2) (W m-1

)

Kupe Field Measured 2.02 5.05 21281

Modelled 2.02 4.88 22448

Maari Field Measured 2 23 6 2 27106Maari Field Measured 2.23 6.2 27106

Modelled 2.19 5.67 27213

Pohokura Field Measured 1.78 3.99 18039

Modelled 1.94 4.38 20690

Baring Head Measured 1.26 2.13 9060

Modelled 1.16 1.88 8380

Steep Head Measured 2.01 4.69 21847

Modelled 1.74 3.71 17468

Steep Head Measured 1.99 6.20 19442

Modelled 1.88 4.11 18900

Bay of Plenty Measured 0 99 1 36 5094Bay of Plenty Measured 0.99 1.36 5094

Modelled 1.14 1.81 5647


Methods – wave patterns


Results – wave climate

Estimates of surfable time for the Reef and Westshore Beach based on swell (T>6s) and total (T>3s) wave heights, plus favourable winds.

The Reef Westshore

Swell Total Swell Total

% time good surf 2 89 4 26 3 16 6 28% time good surf 2.89 4.26 3.16 6.28

% time marginal surf 4.32 9.51 4.53 11.91

% time surfable 7.21 13.77 7.69 18.19


Existing surf conditions

Wave period has a relatively small effect on the wave heights along th b k t ththe break, except on the low-energy inside section.

Wave direction has a significant effect on the wave height gradient at th b k it ithe break, as it is exposed to the northerly waves.

Tide level has an important effect on the wave patterns and the s rfers ride changing thesurfers ride, changing the height gradients along the ride and directly influencing the length. Wave height effects (A=1m, B=2m)


g g g ( )

Seawall effects on surf quality


Seawall effects on surf quality

The original seawall option will reflect wave energy back into the path of the incoming crests – predominantly on the inside half of the ride and during smaller wave surf conditions and over the higher tides.

The actual reflectivity of the seawall structure will directly influence the effects on the surfing wave quality. A low-gradient, dissipative structure will have less impact than a traditional breakwater wall.

Changing the seawall orientation can reduce the reflected wave energy, particularly on the inside section of the surfers ride.


Alternative seawalls

Rotated V-shaped V-shaped alternative


Comparison of the seawalls

1.2

1.4

0.6

0.8

1e

heig

ht (m

)

0.2

0.4

0.6

Wav

e

00 100 200 300 400 500 600

Distance

Existing Proposed breakwater Rotated breakwater V-shaped V-shaped alternative


Wave sheltering effects

1

1.2

0.4

0.6

0.8W

ave

heig

ht (m

)

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Distance (m)


Existing Bathy Proposed breakwater Proposed breakwater and island V-shaped V-shaped Alternative

Wave effects on the surf of 19 June 2007

1.6

1.8

Main section0 8

1

1.2

1.4H

eigh

t (m

)

Main section

Inside section0.2

0.4

0.6

0.8

Wav

e H

00 100 200 300 400 500 600

Distance (m)

Existing Proposed breakwater Rotated breakwater Alternative V-shaped


Summary

The original seawall option would reduce the surf quality at the break. The primary effects would be due to wave reflections likely to be manifest as backwash on the insidereflections, likely to be manifest as backwash on the inside section of the ride, and wave crest interference patterns on the outer section and at the takeoff zone.

The effects would be greatest under high tide / low wave height conditions, but will be partially evident under most of the typical surf conditions.

Rotation of the seawall orientation to increase the angle of incidence to the incident wave crests reduces (but not eliminates) the negative effects on the surf quality. However, rotation reduces the area and utility of the tranquil zone behind the structure.


Summary

The V-shaped seawall alternatives exhibit the least impact on the surf quality, by reducing the amount of reflected wave energysurf quality, by reducing the amount of reflected wave energy

back into the surfing region. Two options have been tested; one that follows the native seabed shape and a design that that is as

close to the shore as possible.

All the seawall options that were tested in this study are effective in creating a tranquil zone on the leeward side, although the

Alternative V-shaped structure was the least effective.Alternative V shaped structure was the least effective.

At present the waves are focussed into the western corner of the beach, exacerbating the local erosion issues. A sheltered zone

ith th t iti t th b h i t d t bwith a smooth transition to the open beach is expected to be conducive to the establishment and maintenance of a low-energy

beach (i.e. directly in the lee and immediately adjacent to it).


characteristic features of long wave energy within ports ...€¦ · wave hindcast validation data...

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