Westshore Reef Surf Conditions

WESTSHORE REEF SURF CONDITIONS

Predicted effects from the proposed Whakarire Ave Breakwater

April 2008

Suite 3, 17 Nobs Line, New Plymouth, New Zealand T: 64-6-7585035 E: [email protected]

Whakarire Ave Breakwater - Surf Effects

Status

Version Date Status Approved By: RevA 15/11/2007 Draft for review Beamsley RevB 27/11/2007 Updated draft McComb RevC 12/03/2008 Updated draft McComb RevO 03/04/2008 Approved for release McComb

It is the responsibility of the reader to verify the currency of the version number of this report.

The information, including the intellectual property, contained in this report is confidential and proprietary to MetOcean Solutions Ltd. It may be used by the persons to whom it is provided for the stated purpose for which it is provided, and must not be imparted to any third person without the prior written approval of MetOcean Solutions Ltd. MetOcean Solutions Ltd reserves all legal rights and remedies in relation to any infringement of its rights in respect of its confidential information.

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TABLE OF CONTENTS

1 INTRODCUTION ...... 1 1.1 BACKGROUND ...... 1 1.2 SCOPE AND STRUCTURE ...... 3 1.3 SUMMARY OF SURFING WAVE PARAMETERS ...... 4 1.3.1 Breaking wave height...... 4 1.3.2 Wave peel angle...... 4 1.3.3 Wave breaking intensity...... 5 1.3.4 Wave section length...... 5 1.3.5 Surfability...... 5 2 STUDY METHODS ...... 7 2.1 BATHYMETRY SOURCES ...... 7 2.2 WAVE HINDCAST MODELLING ...... 8 2.2.1 Numerical model...... 8 2.2.2 Model domains...... 8 2.2.3 Boundary conditions ...... 9 2.2.4 Winds...... 9 2.2.5 Model output...... 9 2.3 NEARSHORE WAVE MODEL ...... 11 2.4 REFLECTIVITY ...... 16 2.5 SURFABILITY ...... 18 3 RESULTS ...... 19 3.1 WAVE HINDCAST VALIDATION ...... 19 3.2 WAVE CLIMATE ...... 22 3.2.1 Surfability...... 26 3.3 EXISTING SURF BREAK FUNCTIONALITY ...... 26 3.4 BREAKWATER EFFECTS ON SURF QUALITY ...... 30 3.5 SHELTERING EFFECTS ...... 48 3.6 SIMULATION OF THE JUNE 2007 SURF EVENT ...... 50 4 SUMMARY ...... 56 5 REFERENCES ...... 59

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TABLE OF FIGURES

Figure 1.1 Coastal protection structures proposed by BECA...... 2 Figure 1.2 Coastal protection structures proposed by BECA, including the detached breakwater (island) offshore of Westshore Beach...... 2 Figure 1.3 Location of the The Reef surf break (also known as Perfume Point). The proposed seawall / breakwater is also shown on this image...... 3 Figure 1.4 Wave-rose’ depicting the effects of wind strength (knots) and direction on surfing waves (modified from Walker, 1997)...... 6 Figure 2.1 Coverage of the nearshore bathymetry survey...... 7 Figure 2.2 Swan numerical model domain extents for New Zealand (A), Hawke’s Bay (B) and Napier (C). Also show is the location of the Port of Napier, Bluff Hill and the hindcast output locations within the Napier domain...... 10 Figure 2.3 Existing bathymetry near Whakarire Ave and Westshore Beach, Napier...... 12 Figure 2.4 Proposed breakwater option...... 13 Figure 2.5 Proposed breakwater plus island option ...... 13 Figure 2.6 Alternate breakwater rotated...... 14 Figure 2.7 V-shaped breakwater...... 14 Figure 2.8 Alternative V-shaped breakwater...... 15 Figure 2.9 CGWAVE model output transect locations...... 15 Figure 2.10 Comparison of data on rock-armoured slopes with reflection formulae. The two rough fit slopes reflect the work of Postma (1989) and the prediction by Seelig and Ahrens (1981), after (CIRIC, CUR and CETMEF, 2007)...... 17 Figure 3.1 Comparison between measured and hindcast significant wave heights at the waverider buoy site (VBWR1 on Fig. 2.1)...... 21 Figure 3.2 The effect of different offshore wave periods (A), directions (B) and tide elevations (C) on the wave height along the surfers path for the existing bathymetry. See Figure 2.9 for transect locations...... 28 Figure 3.3 Wave height and crests patterns for a 1 m (A) and 2 m (B) incident wave at The Reef...... 29 Figure 3.4 Polar plot of spectral wave boundary condition (JONSWAP, with Hs = 1 m, Tp = 12 s, and gamma 3)...... 29 Figure 3.5 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – proposed breakwater case. The lower image is a close up of the nearshore wave crest patterns...... 32 Figure 3.6 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – rotated breakwater case. The lower image is a close up of the nearshore wave crest patterns...... 33 Figure 3.7 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – V-shaped breakwater. The lower image is a close up of the nearshore wave crest patterns...... 34

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Figure 3.8 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – alternative V- shaped breakwater. The lower image is a close up of the nearshore wave crest patterns...... 35 Figure 3.9 Modelled wave heights along the approximate surfer’s path at mid-tide level (Hs=1m, Tp=12s)...... 36 Figure 3.10 Wave surface plot of the energy peak for original breakwater option at mid-tide with zero reflectivity (A), 0.4 reflectivity (B) and 0.8 reflectivity (C)...... 37 Figure 3.11 Spectral output locations along the approximate surfers path; SP1 is offshore and SP4 is inshore...... 38 Figure 3.12 Directional spectra for locations shown on Figure 3.9. Existing case, with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1)...... 39 Figure 3.13 Directional spectra for the proposed breakwater option, 0.4 reflectivity with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1)...... 40 Figure 3.14 Directional spectra for the rotated breakwater option, 0.4 reflectivity with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1)...... 41 Figure 3.15 Directional spectra for site SP4 proposed breakwater option, comparing 0.4 and 0.8 reflectivity (Hs=1m and Tp=12s). The 2D scale is spectral density and the radial scale is frequency (0-1)...... 42 Figure 3.16 Spectral model comparison of the wave height distribution for existing case and the proposed breakwater. Lower imager shows the wave height difference from the modifications...... 43 Figure 3.17 Wave height distribution for existing case and the proposed breakwater plus island for a 1 m spectral wave height with a 12 sec period. Lower imager shows the wave height difference from the modifications...... 44 Figure 3.18 Spectral model comparison of the wave height distribution for existing case and a rotated breakwater. Lower imager shows the wave height difference from the modifications ...... 45 Figure 3.19 Spectral model comparison of the wave height distribution for existing case and V-shaped breakwater. Lower imager shows the wave height difference from the modifications ...... 46 Figure 3.20 Spectral model comparison of the wave height distribution for existing case and alternative V-shaped breakwater. Lower imager shows the wave height difference from the modifications ...... 47 Figure 3.21 Wave heights along the inshore transect shown on Figure 2.9. (spectral simulation, Hs=1m, Tp=12s, mid tide)...... 49 Figure 3.22 Waves at the Reef surf break on the 19 th June 2007 (Photo courtesy of Neil Daykin, Hawke’s Bay Regional Council Engineering Section).. 51 Figure 3.23 Time series of swell wave height ( Hs), peak wave period ( Tp) and swell wave direction for the period 7-June 2007 – 24-June 2007 (sites VBWB2 and CR1)...... 52 Figure 3.24 Wave height distributions for the incident on June 19 th 2007. A – Existing, B – Proposed breakwater, C – Rotated breakwater. Tide = MSL, simulated with a wave spectra...... 53

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Figure 3.25 Wave height distributions for the incident on June 19 th 2007. A – Existing, B – Alternative V-shaped breakwater. Tide = MSL, simulated with a wave spectra...... 54 Figure 3.26 Wave height transects along the surfer’s path for conditions on the 19 th June 2007. The V-shaped breakwater has a similar wave height profile to the Alternative V-shaped shown here...... 55

LIST OF TABLES

Table 3.1 Accuracy measures for hindcast significant swell heights. MAE: mean absolute error, RMSE: RMS error, MRAE: mean relative absolute error, BIAS: bias...... 20 Table 3.2 Significant wave height statistics for representative locations nearshore and offshore, as shown on Figure 2.2c...... 23 Table 3.3 Joint probability distribution (ppt) of the significant wave height and the peak spectral wave period at the Port Waverider location...... 23 Table 3.4 Joint probability distribution (ppt) of the significant wave height and the mean wave direction at the Port Waverider location...... 24 Table 3.5 Joint probability distribution (ppt) of the significant wave height and the peak spectral wave period at site CR1 near Whakarire Ave...... 24 Table 3.6 Joint probability distribution (ppt) of the significant wave height and the mean wave direction at site CR1 near Whakarire Ave...... 25 Table 3.7 Estimates of surfable time for the Reef and Westshore Beach based on swell (T>6s) and total (T>3s) wave heights, plus favourable winds... 26

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1 INTRODCUTION

1.1 Background

The Napier City Council has commissioned BECA to develop a coastal protection strategy for the Whakarire Avenue area. A previous report by BECA in 2003 examined five coastal protection options, in addition to the ‘do nothing’ option. These options included an enhanced seawall, and enhanced seawall and infilling of backshore, a groyne to limit wave focusing, a wave spending beach and an attached breakwater with beach creation. After consultation with the Napier City Council and local residents, BECA has recommended the option shown in Figure 1.1, consisting of a 100 m extension to the existing seawall. Further, the option of a detached breakwater offshore of Westshore Beach to stabilise the southern end of Westshore Beach has also been proposed (Fig. 1.2).

The coastal protection options are focused on land protection and enhancement of coastal amenities. The effects of reef ecology, surfing conditions, landscape and heritage values were not considered by BECA as part of the original consultation. BECA have commissioned MetOcean Solutions Ltd to investigate the implications of the proposed breakwater structure on surfing activities, particularly at the adjacent surf break known as The Reef (Fig. 1.3).

The Reef is predominantly a right-hand break that is usually surfed at the mid-tide level on smaller swells and over most of the tide on the larger swells.

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Figure 1.1 Coastal protection structures proposed by BECA

Figure 1.2 Coastal protection structures proposed by BECA, including the detached breakwater (island) offshore of Westshore Beach.

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Figure 1.3 Location of the The Reef surf break (also known as Perfume Point). The proposed seawall / breakwater is also shown on this image.

1.2 Scope and structure

In this assessment, a numerical wave model has been used to estimate the wave heights and wave energy gradients before and after the proposed coastal modifications. Wave modelling simulates the refraction, reflection, diffraction, shoaling and frictional attenuation of the swells as they propagate from offshore to the beach. By simulating the same wave conditions for the configuration before and after any coastal protection modifications, the magnitude and effect of the changes to the wave climate can be assessed, and in turn these are interpreted in terms of the impacts on the wave quality for surfing.

The scope of the report is as follows: The wave modelling techniques are described in Section 2, including the bathymetry sources and hindcasting methods. Wave model results showing the before and after outcomes for generic swell conditions at

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the surf break are presented in Section 3, along with a simulation from a specific event in June 2007. The results are summarised in the final Section 4, and interpreted in terms of effects on the surf conditions. A summary of the parameters that define surfing conditions is provided in the following Section 1.3.

1.3 Summary of surfing wave parameters

A detailed review of surfing wave parameters has been made by Scarfe (2003), and these parameters are summarised here. The four most important wave parameters for analysis of surfing waves are breaking wave height, wave peel angle, wave breaking intensity, and wave section length.

1.3.1 Breaking wave height

The breaking wave height is considered one of the most important parameters at a surfing break. Typically, the arrival of waves is modulated into groups (sets) and surfers seek to ride the largest waves in these groups. Accordingly, the average of the top 10 % of waves is the best statistic for description of the surfable waves, rather than the significant wave height, which is approximately the average of the highest third of the waves.

1.3.2 Wave peel angle

The wave peel angle is the angle (from 0-90°) between the trail of broken white- water and the crest of the unbroken wave as it propagates shoreward. Low peel angles create fast surfing waves, while angles approaching 0° result in a closeout, whereby the wave face collapses and ends the ride. High peel angles create slow waves which are less challenging to surf. The wave peel rate describes how fast the wave breakpoint advances laterally along the wave crest, and this is closely related to the wave peel angle.

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1.3.3 Wave breaking intensity

The seabed gradient is the main factor that controls the wave breaker intensity, which is categorised as spilling, plunging, surging, or collapsing breakers. The best surfing waves are those with steep or plunging faces. Breaker intensity is influenced by the wind conditions; increasing in offshore winds and decreasing in onshore and cross-shore conditions. Offshore wind conditions act to delay wave breaking, causing the wave to break in shallower water and thereby increasing the breaker intensity.

1.3.4 Wave section length

Variations along the wave crest (i.e. due to unorganized swells, wave focusing and undulating bathymetry) can cause wave face to exhibit a varying character along the length of the ride. The section length describes the surf ride for specific wave height, peel angle and breaker intensities.

1.3.5 Surfability

The ‘threshold of surfability’ depends on the quality of the surfer and the blending of wave height and period and wind strength and direction. As such, parameters such as the incident wave height, period, directional spreading and prevailing wind velocities are usually considered in an assessment of surfing frequency at a given location. In order to quantify the number of surfable days, Mead et al . (2004) used a threshold of H > 0.75, and T > 6 s, while Black et al . (2004) used similar wave height and period limitations while limiting the directional spreading to less than 40.

While the wave height and period are used to quantify the total number of days where conditions are suitable for surfing, the quality of the surf is dependant also on the wind velocity (Fig. 1.4), with offshore-directed winds most favourable. In contrast, strong alongshore directed winds are least favourable and can produce sea waves (i.e. T < 6 s) with crests orientated perpendicular to the swell wave crest,

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adversely effecting surfing conditions. Light onshore directed winds are less likely to affect the surface conditions of the surfable wave (e.g. Fig. 1.4).

Figure 1.4 Wave-rose’ depicting the effects of wind strength (knots) and direction on surfing waves (modified from Walker, 1997)

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2 STUDY METHODS

2.1 Bathymetry sources

Bathymetry data for the numerical domains were derived from several sources. Regional scale data was digitised from the published nautical charts, and supplemented with soundings from the Port of Napier. A high-resolution survey of the nearshore Whakarire Ave area was commissioned by the NCC for this study, and the coverage is shown on Figure 2.1.

Figure 2.1 Coverage of the nearshore bathymetry survey

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2.2 Wave hindcast modelling

A 10-year hindcast of the wave climate at Napier was undertaken for this assessment. The output of the hindcast was a 1-hour time-series of the wave conditions from June 1997 – June 2007. The methods used are detailed in the following sections.

2.2.1 Numerical model

SWAN was used for the wave hindcast modelling. SWAN is a third generation ocean wave propagation model, which solves the spectral action density balance equation for wavenumber-direction spectra. This means that the growth, refraction, and decay of each component of the complete sea state, each with a specific frequency and direction, is solved, giving a complete and realistic description of the wave field as it changes in time and space. Physical processes that are simulated include the generation of waves by the surface wind, dissipation by white-capping, resonant nonlinear interaction between the wave components, bottom friction and depth limited breaking. A detailed description of the model equations, parameterizations, and numerical schemes can be found in the online documentation (The SWAN team, 2007). All 3 rd generation physics are included.

The Collins friction scheme was used for wave dissipation by bottom friction, with a friction factor of 0.015. The solution of the wavefield is found for the non- stationary (time-stepping) mode. Boundary conditions, wind forcing and resulting solutions are all time dependent, allowing the model to capture the growth, development and decay of the wavefield.

2.2.2 Model domains

The hindcast was carried out on a three level nested domain. A coarse grid with resolution of 0.05° longitude by 0.05° latitude (~4 km by 5 km) covered all of the New Zealand (Fig 2.2a). A medium grid consisting of the greater Hawke’s Bay area with resolution of 0.002° longitude by 0.002° latitude (Fig 2.2b). The nested high

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resolution grid of 0.0001° longitude 0.0001° latitude (~10 m) was used for the Napier region (Fig 2.2c).

2.2.3 Boundary conditions

The wave spectra on the open ocean boundaries of the coarse domain were obtained from the NOAA WAVEWATCH III (NWW3) archived nowcasts. NWW3 is a state-of-the-art wave generation, propagation and transformation model for forecasting the evolution of directional wave energy spectra across the global oceans. Along the open boundaries of the SWAN model domain, the primary statistical parameters of the incoming wavefield are interpolated from the NWW3 hindcast solution. Boundary conditions for the nested grid come directly from the solution within their parent domain.

2.2.4 Winds

Generation of waves within the model domains occurs in response to wind forcing. A spatially and temporally varying wind field is specified from the blended NCEP re-analysis/QuikScat winds 1. These data are 10 m wind velocity vectors in a 6- hourly gridded format at a resolution of 0.5° of longitude and latitude. The wind field is a combination of the ~12 hourly QuikScat satellite measurements and the NCEP global re-analysis (Milliff et al ., 2004). The blended data product combines the benefits of measured satellite data with good spatial resolution and the continuous temporal coverage of the modelled re-analysis.

2.2.5 Model output

Directional wave spectra were output at hourly intervals over the 10-year hindcast run, and the standard spectral wave parameters were derived. Figure 2.2c illustrates the site specific outputs available from the Napier hindcast model simulation. For this report only site VBWB2, HRd, CR1, WSSC and GAP have been processed. These sites provide information on the offshore incident waves (site VBWB2) and

1 Data product from National Center for Atmospheric Research (NCAR)

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the wave height gradient alongshore from Harding Road to The Gap, centred on the Whakarire Avenue area.

A B

Port of Napier

Bluff Hill

Figure 2.2 Swan numerical model domain extents for New Zealand (A), Hawke’s Bay (B) and Napier (C). Also show is the location of the Port of Napier, Bluff Hill and the hindcast output locations within the Napier domain.

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2.3 Nearshore Wave model

The CGWAVE model was used to simulate the monochromatic and spectral wave transformation from the offshore to the nearshore regions. This wave model simulates the combined effects of wave refraction-diffraction within the mild-slope equation, and includes the effects of reflection, wave dissipation by friction, breaking, nonlinear amplitude dispersion, and harbour entrance losses (Panchang and Xu, 1995). CGWAVE is a finite-element model, which means that it is ideal for resolving complex localised bathymetry and harbour walls in a numerically- efficient manner. This numerical model is an industry-standard tool for use in harbours and coastal regions with complex bathymetry.

Six numerical domains were established for the CGWAVE simulations;

• Existing bathymetry – this configuration represents the conditions that currently persist at the site (Fig 2.3)

• Proposed breakwater – this configuration represents the option recommended by BECA to address the net loss of sediment from the Westshore and Whakarire Avenue littoral system. (Fig 2.4). The breakwater in this case is 100 m in length.

• Proposed breakwater plus island – this configuration represents the enhanced option recommended by BECA (Fig 2.5).

• Alternative breakwater rotated – this configuration represents a modification to the original breakwater option recommended by BECA with the breakwater orientation altered slightly, with the tip located further south (Fig 2.6).

• V-shaped breakwater – this configuration follows the native seabed shape, minimising construction volumes and reducing the effect of reflected waves on the surf break.

• Alternative V-shaped breakwater – this alternative was recommended by BECA and is positioned closer to shore than the V-shaped structure.

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Wave model data were output along several transects near the surf break, as shown on Figure 2.7. To estimate the directional spectrum at a particular node, a cross power spectrum was calculated between a node and at least 10 ‘wet’ neighbours within a 20 m radius. A Direct Fourier Transform Method (Barber 1961) was then applied.

Figure 2.3 Existing bathymetry near Whakarire Ave and Westshore Beach, Napier. C

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Figure 2.4 Proposed breakwater option

Figure 2.5 Proposed breakwater plus island option

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Figure 2.6 Alternate breakwater rotated.

Figure 2.7 V-shaped breakwater

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Figure 2.8 Alternative V-shaped breakwater

Figure 2.9 CGWAVE model output transect locations .

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2.4 Reflectivity

Coastal structures are often highly reflective to the wave energy, and can reflect up to 80% of the non-breaking incident wave heights. The profile, slope and porosity of the structure has a significant impact on the amount of reflection that occurs, along with other dissipative measures such as wave breaking on a subtidal berm. Wave reflection is described using a coefficient, Cr , defined in terms of the incident and reflected wave heights ( Hi and Hr respectively) or wave energies ( Ei and Er respectively):,

H r Er Cr = = H i Ei

With a random wave field, values of Cr may be defined using the significant incident and reflected wave heights as representative of the incident and reflected wave energy (CIRIC, CUR and CETMEF, 2007).

A range of reflection coefficients for different types of seawall / breakwaters (smooth and rough slopes) is reproduced in Figure 2.8, based on the work of Battjes (1974), Seelig and Ahrens (1981), Van der Meer (1988) and Allsop and Channel (1989). Most predictive equations use surf similarity parameter (ξ) and two or more empirical coefficients to define Cr ; with the empirical coefficients varying depending on the nature of the structure (i.e. slope, construction, material etc). For the proposed breakwater structure at Whakarire Ave, reflectivity values of 0.4 (low – for a rough surface at about 1:3 slope) and 0.8 (high – smooth steep surface) have been tested. The seawall shown in Figure 1.1 has a value of about 0.6, and BECA have advised that a design value of 0.4 is appropriate for the numerical studies presented here.

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Figure 2.10 Comparison of data on rock-armoured slopes with reflection formulae. The two rough fit slopes reflect the work of Postma (1989) and the prediction by Seelig and Ahrens (1981), after (CIRIC, CUR and CETMEF, 2007).

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2.5 Surfability

The wave hindcast data have been used to estimate the surfability of the Westshore region. Surfable conditions were defined for swell conditions were the wave height (H) was greater than 0.75 m. In this definition, H was derived from the swell component of the wave spectra (T > 6 s), using the Longuet-Higgins (1952) th estimate of H1/10 (i.e. the average of the highest 10 of the waves) defined from the significant wave height,

H 1 = 27.1 H s 10

A consideration of the directional spreading was not applied to the surfability statistics, as refraction of the swell crests around the Napier headland and across the shallow nearshore region results in a typically narrow directional spread to the incident swells (as also observed in the ASR (2001) measurements).

In addition to the wave height threshold, the wind speed and direction (as measured at the Napier Airport) was used to further define the surfable periods into ‘good’ and ‘marginal’, as per the criteria in Figure 1.4. Only daylight hours were considered in the surfability analysis.

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3 RESULTS

3.1 Wave hindcast validation

The wave hindcast data were compared with 9 months of hourly wave data measured by the Port of Napier waverider buoy (site VBWR1 on Fig. 2.1; August 2000 – May 2001). These data were made available to the Napier City Council as part of previous studies into the ongoing erosion at Westshore Beach (ASR, 2001).

A time-series validation plot of the measured and hindcast swell wave heights is presented in Figure 3.1, showing that the numerical model is faithfully representing the periods of high and low energy. The measured and hindcast data have similar statistical means (0.76 m and 0.68 m, respectively) and medians (0.85 m and 0.71 m, respectively). Further quantitative measures of the accuracy of the hindcast

are calculated from the measured, xm and hindcast, xh, data. These are defined as:

Mean absolute error: xh − xm

2 RMS error: ()xh − xm x − x Mean relative error: h m xm

Bias: xh − xm

where the line indicates an average over all pairs of measured/hindcast data.

The results for the comparison of the significant swell heights for the year 2000- 2001 are presented in Table 3.1. The mean absolute error (MAE) is the most direct representation of what the typical deviation of the hindcast from the measured value. The RMS error exaggerates large differences in measured and hindcast wave heights and is therefore larger than the MAE. The mean relative absolute errors are an expression in percentage terms of the error compared to actual, and shown that the hindcast wave heights are on, average, within +/- 34% of the measured values. The bias, which represents a constant ‘offset’ in the hindcast significant wave

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heights, indicates that overall the model slightly over predicts the swell heights by ~14 cm. Notably, some of the error in the hindcast wave heights is due to timing of the wave events, as the hindcasting technique has an inherent phase resolution of ~3 hours.

Table 3.1 Accuracy measures for hindcast significant swell heights. MAE: mean absolute error, RMSE: RMS error, MRAE: mean relative absolute error, BIAS: bias

MAE (m) RMSE (m) MRAE (%) BIAS (m)

0.27 0.36 34 0.14

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Figure 3.1 Comparison between measured and hindcast significant wave heights at the waverider buoy site (VBWR1 on Fig. 2.1)

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3.2 Wave climate

Annual, seasonal and monthly significant wave height statistics for an offshore site (VBWB1) and a location near Whakarire Ave (CR1) are presented in Table 3.2, clearly showing the attenuation of wave energy and the shelter afforded by the Napier Headland. Wave heights near the Reef surf break are approximately half the size of the incident heights offshore at the port waverider buoy. These data have been sourced from the 10-year wave hindcast.

The most energetic months are July and August, with mean monthly offshore wave heights of 1.39 m and 1.36 m, respectively. December is the least energetic month, with a mean offshore significant wave height of 0.88 m and a 95 th percentile exceedence value (P95) of 1.81 m.

The wave climate at the offshore location is further summarised in Tables 3.3 and 3.4, providing the joint probability distribution of the significant wave height and peak spectral wave period (Table 3.3) and the joint probability distribution of the significant wave height and mean wave direction (Table 3.4). The nearshore wave climate at Whakarire Ave is described by the joint probability distribution of significant wave heights and peak spectral wave periods (Table 3.5) and the joint probability distribution of significant wave heights and mean wave directions (Table 3.6).

At the port waverider location, the incident waves have two distinct frequency bands; a wind-wave component in the 2-6 second period range, and a swell component in the 8-12 second period range. The largest wave events are swell- dominated and occur in the 10-16 second period range. Near the Whakarire Ave location, there is a similar wave height / period distribution, but the wave directional range is much narrower due to refraction in the intermediate water depths (plus topographic sheltering and diffraction due to the Napier Headland and the Port). Notably, some 54% of all waves come from 40-60 oT, and the largest waves are from 30-60 oT.

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Table 3.2 Significant wave height statistics for representative locations nearshore and offshore, as shown on Figure 2.2c.

Significant wave height Site VBWB2 Site CR1 Hs (m) Mean Med P95 Mean Med P95 Annual 1.08 0.93 2.27 0.49 0.40 1.17 Summer 0.96 0.84 1.98 0.47 0.40 1.08 Spring 0.94 0.81 1.91 0.43 0.35 1.02 Autumn 1.12 1.00 2.22 0.51 0.43 1.15 Winter 1.30 1.12 2.69 0.56 0.42 1.31 January 0.94 0.82 2.05 0.45 0.39 1.07 February 1.08 0.98 2.18 0.53 0.46 1.13 March 1.08 0.95 2.12 0.51 0.44 1.12 April 1.17 1.06 2.24 0.52 0.45 1.16 May 1.13 0.98 2.32 0.49 0.40 1.15 June 1.15 0.98 2.47 0.48 0.38 1.17 July 1.39 1.22 2.83 0.61 0.46 1.38 August 1.36 1.18 2.70 0.58 0.44 1.35 September 0.95 0.85 1.90 0.44 0.36 0.99 October 0.91 0.78 1.98 0.42 0.35 0.99 November 0.95 0.80 1.89 0.44 0.36 1.08 December 0.88 0.75 1.81 0.42 0.35 1.02

Table 3.3 Joint probability distribution (ppt) of the significant wave height and the peak spectral wave period at the Port Waverider location

Peak spectral wave period (s) Hs (m) 0 to 2 2 to 4 4 to 6 6 to 8 8 to 10 10 to 12 12 to 14 14 to 16 16 to 18 18 to 20 SUM > 0 <= 0.25 5.5 4.9 0 0 0.3 0 0.1 0.3 0.1 0 11.2 > 0.25 <= 0.5 2.8 97 6.9 0.5 3.5 1 0.6 1.5 0.1 0 113.9 > 0.5 <= 0.75 0 127.4 68 2.4 14.3 6.9 3 5 1.6 0 228.6 > 0.75 <= 1 0 61.1 88.1 4.5 22.2 8.3 4.5 5.3 1.2 0 195.2 > 1 <= 1.25 0 17.3 67.7 12.4 29.4 11.9 2.8 4 0.3 0 145.8 > 1.25 <= 1.5 0 2.9 42 14.6 21.4 14.8 2.7 2.2 0.2 0 100.8 > 1.5 <= 1.75 0 0.5 24.7 8 15.4 15.3 1.8 0.7 0.2 0 66.6 > 1.75 <= 2 0 0.1 13.7 5.2 10.8 19 2.5 0.4 0.3 0 52 > 2 <= 2.25 0 0 5.2 5.6 5.3 16.3 1.9 0.2 0 0 34.5 > 2.25 <= 2.5 0 0 1.2 3.6 2.8 12.1 1.5 0.2 0.1 0 21.5 > 2.5 <= 2.75 0 0 0.1 1.8 2.9 6.7 1 0 0 0 12.5 > 2.75 <= 3 0 0 0 0.6 1.8 5 0.8 0 0 0 8.2 > 3 <= 3.25 0 0 0 0.2 0.9 2.6 0.6 0 0 0 4.3 > 3.25 <= 3.5 0 0 0 0.1 0.7 0.6 0.6 0 0 0 2 > 3.5 <= 3.75 0 0 0 0.1 0.5 0.3 0.2 0.1 0 0 1.2 > 3.75 <= 4 0 0 0 0 0.2 0.3 0.1 0.1 0 0 0.7 > 4 <= 4.25 0 0 0 0 0.1 0.3 0 0.1 0 0 0.5 > 4.25 <= 4.5 0 0 0 0 0.1 0.1 0 0.1 0 0 0.3 > 4.5 <= 4.75 0 0 0 0 0 0.1 0 0 0 0 0.1 > 4.75 <= 5 0 0 0 0 0 0 0 0.1 0 0 0.1 SUM 8.3 311.2 317.6 59.6 132.6 121.6 24.7 20.3 4.1 0 1000

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Table 3.4 Joint probability distribution (ppt) of the significant wave height and the mean wave direction at the Port Waverider location

Mean wave direction (degT) Hs (m) 337.5 22.5 67.5 112.5 157.5 202.5 247.5 292.5 SUM to 22.5 to 67.5 to 112.5 to 157.5 to 202.5 to 247.5 to 292.5 to 337.5 0 – 0. 25 0.9 0.3 0.7 0.6 0.5 0.8 3.5 3.9 11.2 0.25 - 0.5 10.7 6.8 15 11.9 4.5 6.9 23.9 34.1 113.8 0.5 - 0.75 24.4 23.4 69.1 37.5 9.2 6.5 22.7 35.9 228.7 0.75 - 1 16.3 25.6 81.2 46.3 7.3 3.9 6 8.3 194.9 1 - 1.25 5 21.9 65.9 44.4 5.1 1.5 1.1 1 145.9 1.25 - 1.5 1.2 15.1 42.7 37.3 3.4 0.5 0.4 0.1 100.7 1.5 - 1.75 0.1 9 30 25.4 1.9 0.1 0 0 66.5 1.75 - 2 0.1 5.2 27.5 18 1.1 0 0 0 51.9 2 - 2.25 0 3.5 20.8 10.2 0.2 0 0 0 34.7 2.25 - 2.5 0 1.8 14 5.7 0.1 0 0 0 21.6 2.5 - 2.75 0 0.6 7.8 4.1 0.1 0 0 0 12.6 2.75 - 3 0 0.2 4.7 3.4 0 0 0 0 8.3 3 - 3.25 0 0.1 2.6 1.8 0 0 0 0 4.5 3.25 - 3.5 0 0.1 1 1 0 0 0 0 2.1 3.5 - 3.75 0 0.1 0.5 0.6 0 0 0 0 1.2 3.75 - 4 0 0 0.2 0.5 0 0 0 0 0.7 4 - 4.25 0 0 0.2 0.2 0 0 0 0 0.4 4.25 - 4.5 0 0 0.1 0.1 0 0 0 0 0.2 4.5 -4.75 0 0 0 0.1 0 0 0 0 0.1 4.75 - 5 0 0 0 0.1 0 0 0 0 0.1 SUM 58.7 113.7 384 249.2 33.4 20.2 57.6 83.3 1000

Table 3.5 Joint probability distribution (ppt) of the significant wave height and the peak spectral wave period at site CR1 near Whakarire Ave.

Peak spectral wave period (s) Hs (m) 0 to 2 2 to 4 4 to 6 6 to 8 8 to 10 10 to 12 12 to 14 14 to 16 16 to 18 18 to 20 SUM

> 0 <= 0.25 63.9 72.2 15.8 8.2 30 21.1 12.2 20.9 5.5 0 249.8 > 0.25 <= 0.5 6.8 167.4 30.6 32.1 61.6 40.3 17.4 20.2 4.2 0 380.6 > 0.5 <= 0.75 0 79.9 12.5 11 40.5 33.6 5.2 3.2 0.9 0 186.8 > 0.75 <= 1 0 18.8 22.2 2.9 16.1 29.6 2 0.5 0.1 0 92.2 > 1 <= 1.25 0 0.9 23.1 2.2 4.3 22.2 3.1 0.4 0 0 56.2 > 1.25 <= 1.5 0 0 12.8 1.1 1.6 7 0.9 0 0.1 0 23.5 > 1.5 <= 1.75 0 0 4.7 1.5 0.4 1.9 0.5 0 0 0 9 > 1.75 <= 2 0 0 0.5 0.9 0.1 0.1 0.2 0 0 0 1.8 > 2 <= 2.25 0 0 0 0.2 0.1 0 0 0 0 0 0.3 > 2.25 <= 2.5 0 0 0 0 0 0 0 0 0 0 0 SUM 70.7 339.2 122.2 60.1 154.7 155.8 41.5 45.2 10.8 0 1000

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Table 3.6 Joint probability distribution (ppt) of the significant wave height and the mean wave direction at site CR1 near Whakarire Ave.

Mean wave direction (degT) Hs (m) 0 to 10 10 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 310 310 to 320 320 to 330 330 to 340 340 to 350 350 to 360 SUM 0 – 0. 25 20.1 17.3 20.8 23.8 42.1 57.2 4.4 0.8 4.4 10.8 21.2 27 249.9 0.25 - 0.5 17.8 18.8 27.1 40.5 91.1 130.4 1.2 1.6 4.8 10.2 16.9 19.7 380.1 0.5 - 0.75 9.1 13.5 22.7 29.5 40.2 63.8 0.1 0.1 0.3 0.8 2 4.4 186.5 0.75 - 1 1.4 3.9 12.5 19.2 20.8 34.3 0 0 0 0 0 0.2 92.3 1 - 1.25 0.1 0.6 5.2 13.5 17.3 19.3 0 0 0 0 0 0 56 1.25 - 1.5 0 0.1 0.7 7.6 8 7.1 0 0 0 0 0 0 23.5 1.5 - 1.75 0 0 0.2 3.3 3.9 1.5 0 0 0 0 0 0 8.9 1.75 - 2 0 0 0 0.9 0.8 0.1 0 0 0 0 0 0 1.8 2 - 2.25 0 0 0 0.2 0 0.1 0 0 0 0 0 0 0.3 2.25 - 2.5 0 0 0 0 0 0 0 0 0 0 0 0 0 SUM 48.5 54.2 89.2 138.5 224.2 313.8 5.7 2.5 9.5 21.8 40.1 51.3 1000

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3.2.1 Surfability

The surfability statistics are presented in Table 3.7. Good surfing conditions can be anticipated for around 3% of the time and total surfable conditions are about 14%. Westshore has a similar level of good surf but more total surfable conditions.

Table 3.7 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 marginal surf 4.32 9.51 4.53 11.91 % time surfable 7.21 13.77 7.69 18.19

3.3 Existing surf break functionality

A range of initial monochromatic simulations were undertaken with the CGWAVE model, varying the wave height, wave period, wave direction and tidal water level with the existing shoreline and bathymetry. Monochromatic simulations are a useful method to examine how different wave periods and angles respond to the seabed shape. This type of simulation does not however fully represent the true ocean swell condition, which typically features a range of wave periods, directions and heights. Nonetheless, the monochromatic cases are useful to understand the functional response of wave fields with different periods. The results of the monochromatic tests are presented in Figure 3.2, showing the wave heights along the approximate surfer’s path (Fig. 2.7). In summary:

• Wave period (Fig 3.2A) has a relatively small effect on the wave heights along the surfers path, except on the low-energy inside section.

• Wave direction (Fig. 3.2B) has a significant effect on the wave height gradient at the break, as the break is more exposed to the northerly (i.e. 40 oT) waves.

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• Tide level has an important effect on the wave patterns and the surfers ride (Fig. 3.2C), changing the height gradients along the ride and directly influencing the length.

The effect of wave height is illustrated in Figure 3.3, using a spectral model simulation (Fig. 3.4). The bathymetric structure that forms the Reef surf break actually extends offshore as an elongated spur, and this shape encourages the wave energy to focus into the Reef, leaving a refraction ‘wave shadow’ to the immediate west. During larger wave conditions, the focussing onto the Reef results in wave breaking further offshore (and adjacent to the estuary channel). At the mid – lower range of wave heights, the crests exhibit a tendency to bifurcate, whereby the crests are ‘snapped’ due to refraction along the inside section of the Reef.

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1 0.9 0.8 0.7 A 0.6 0.5 0.4

Wave height (m) height Wave 0.3 0.2 0.1 0 0 100 200 300 400 500 600 Distance

ORIGINAL 60 deg 12 s ORIGINAL 60 deg 14 s ORIGINAL 60 deg 16 s

1.2

0.8 B

0.6

0.4 Wave height (m) height Wave

0.2

0 0 100 200 300 400 500 600 Distance

ORIGINAL 40 deg 12 s ORIGINAL 60 deg 12 s ORIGINAL 80 deg 12 s

1.2

1 C 0.8

0.6

0.4 Wave height (m) height Wave

0.2

0 0 100 200 300 400 500 600 Distance

Original 60 deg 12 s MLWS Original 60 deg 12 s MSL Original 60 deg 12 s MHWS

Figure 3.2 The effect of different offshore wave periods (A), directions (B) and tide elevations (C) on the wave height along the surfers path for the existing bathymetry. See Figure 2.9 for transect locations.

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Refraction shadow

Figure 3.3 Wave height and crests patterns for a 1 m (A) and 2 m (B) incident wave at The Reef

Figure 3.4 Polar plot of spectral wave boundary condition (JONSWAP, with Hs = 1 m, Tp = 12 s, and gamma 3).

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3.4 Breakwater effects on surf quality

The effect of different breakwater orientations and reflectivity’s on the surf conditions at the Reef has been examined by comparing the modelled wave heights and crest patterns along the approximate surfer’s path (with and without the breakwater modifications). For these simulations, breakwater reflectivity coefficients ranging from 0.4 to 0.8 were used, and the six numerical domains are shown on Figures 2.3 – 2.8.

Model results for the wave height distribution and crest patterns for a spectral simulation (JONSWAP, Hs=1m, Tp=12s, direction=60o) on the various breakwater options are given in Figures 3.5 - 3.8. A reflectivity of 0.4 was used in these simulations, at a mid-tide level. Each of the breakwater options can be seen to have an impact on the wave patterns at the surf break, with the highest effect due to the reflections off the original proposed breakwater structure. Increasing the angle of incidence between the wave crest and the breakwater (i.e. the rotated and V-shaped breakwaters) reduces the amount of reflection back into the surfing wave face.

These results are further quantified in the wave height transects along the approximate surfers path (Fig. 3.9). The reflected wave interference can be observed on both the inside and outside sections of the surfers path (~200 and 400 metres along the transect). Notably, the two V-shaped breakwater options have only a negligible effect on the surfing wave heights under these wave conditions.

The effect of the reflectivity coefficients on the wave crest patterns under the proposed breakwater is illustrated on Figure 3.10. At 0.8 reflectivity there is a significant distortion of the wave crests evident on the inside half of the ride, which will have a negative impact on the surfing wave quality. The stage of the tide also influences the amount of wave energy reflected from the breakwater. During high tides (or small waves) there is proportionally more reflection of the incident wave energy off the breakwater, principally because there is less dissipation over the reef platform. Conversely during low tides and larger waves, the proportion reflected is lessened.

Detailed analysis of the directional wave spectra was undertaken to further quantify the effect of reflection from the proposed breakwater structure. Wave spectra were

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extracted from the model outputs at four locations near the surfer’s path (shown on Fig. 3.11, and analysed to identify the reflected wave component.

The results are presented as polar energy plots on Figures 3.12– 3.15 and the annotation on Figure 3.15 shows the incident and reflected energy peaks. These results confirm that:

• The original proposed breakwater 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 breakwater 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 breakwater orientation can reduce the reflected wave energy, particularly on the inside section of the surfers ride.

Wave crest interference patterns can be seen to extend beyond just the inside section of the ride for both breakwater options (i.e. original and rotated Figs, 3.16 and 3.18). At the start of the surf ride, the wave height distribution results show interference patterns that are not present in the existing configuration. The two V-shaped options exhibit the least interference (e.g. Figs. 3.19 and 3.20).

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Interference patterns

Figure 3.5 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – proposed breakwater case. The lower image is a close up of the nearshore wave crest patterns.

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Figure 3.6 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – rotated breakwater case. The lower image is a close up of the nearshore wave crest patterns.

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Figure 3.7 Wave height distributions and wave crest patterns for a 1 m spectral wave simulation, showing the energy peak (Tp=12s) with a 0.4 breakwater reflection coefficient. A – Existing, B – V-shaped breakwater. The lower image is a close up of the nearshore wave crest patterns.

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1.4

1.2

0.8

0.6

Wave height (m) 0.4

0.2

0 0 100 200 300 400 500 600 Distance

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

Figure 3.9 Modelled wave heights along the approximate surfer’s path at mid-tide level (Hs=1m, Tp=12s).

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Figure 3.10 Wave surface plot of the energy peak for original breakwater option at mid-tide with zero reflectivity (A), 0.4 reflectivity (B) and 0.8 reflectivity (C).

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Figure 3.11 Spectral output locations along the approximate surfers path; SP1 is offshore and SP4 is inshore.

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-3 -3 345 0° 15 x 10 345 0° 15 x 10 ° ° 9 ° ° 10 330 ° 30 ° 330 ° 30 ° 315 ° 45 ° 8 315 ° 45 ° 9

300 ° 60 ° 7 300 ° 60 ° 8

285 ° 75 ° 6 285 ° 75 ° 7

270 ° 90 ° 5 270 ° 90 ° 6

255 ° 105 ° 4 255 ° 105 ° 5

3 4 240 ° 120 ° 240 ° 120 °

225 ° 135 ° 2 225 ° 135 ° 3

210 ° 150 ° 210 ° 150 ° 1 2 195 ° 180 ° 165 ° 195 ° 180 ° 165 ° Direction/Frequency [Hz] for Node: 12146 Direction/Frequency [Hz] for Node: 13865 Location SP1 Location SP2

-3 345 0° 15 345 0° 15 x 10 ° ° 0.018 ° ° 8 330 ° 30 ° 330 ° 30 °

315 ° 45 ° 0.016 315 ° 45 ° 7 300 ° 60 ° 0.014 300 ° 60 ° 6

285 ° 75 ° 0.012 285 ° 75 ° 5 270 ° 90 ° 0.01 270 ° 90 ° 4 255 ° 105 ° 0.008 255 ° 105 ° 3 0.006 240 ° 120 ° 240 ° 120 °

2 225 ° 135 ° 0.004 225 ° 135 °

210 ° 150 ° 210 ° 150 ° 0.002 1 195 ° 180 ° 165 ° 195 ° 180 ° 165 ° Direction/Frequency [Hz] for Node: 15464 Direction/Frequency [Hz] for Node: 16994 Location SP3 Location SP4

Figure 3.12 Directional spectra for locations shown on Figure 3.9. Existing case, with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1).

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-5 -4 345 0° 15 x 10 345 0° 15 x 10 ° ° 14 ° ° 330 ° 30 ° 330 ° 30 ° 315 ° 45 ° 315 ° 45 ° 12

300 ° 60 ° 300 ° 60 ° 2

10 285 ° 75 ° 285 ° 75 °

270 ° 90 ° 8 270 ° 90 ° 1.5

255 ° 105 ° 255 ° 105 ° 6

1 240 ° 120 ° 240 ° 120 ° 4 225 ° 135 ° 225 ° 135 °

210 ° 150 ° 210 ° 150 ° 2 0.5 195 ° 180 ° 165 ° 195 ° 180 ° 165 °

Location SP1 Location SP2

-4 -5 345 0° 15 x 10 345 0° 15 x 10 ° ° ° ° 16 330 ° 30 ° 330 ° 30 ° 315 ° 45 ° 315 ° 45 ° 14 5 300 ° 60 ° 300 ° 60 ° 12 285 ° 75 ° 285 ° 75 ° 4 10 270 ° 90 ° 270 ° 90 ° 8 3 255 ° 105 ° 255 ° 105 ° 6 240 ° 120 ° 240 ° 120 ° 2 4 225 ° 135 ° 225 ° 135 ° 210 ° 150 ° 210 ° 150 ° 1 2 195 ° 180 ° 165 ° 195 ° 180 ° 165 °

Location SP3 Location SP4

Figure 3.13 Directional spectra for the proposed breakwater option, 0.4 reflectivity with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1).

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-5 -4 345 0° 15 x 10 345 0° 15 x 10 ° ° 10 ° ° 330 ° 30 ° 330 ° 30 ° 315 ° 45 ° 9 315 ° 45 °

300 ° 60 ° 8 300 ° 60 ° 2

285 ° 75 ° 7 285 ° 75 °

270 ° 90 ° 6 270 ° 90 ° 1.5

255 ° 105 ° 5 255 ° 105 °

4 1 240 ° 120 ° 240 ° 120 °

225 ° 135 ° 3 225 ° 135 °

210 ° 150 ° 210 ° 150 ° 2 0.5 195 ° 180 ° 165 ° 195 ° 180 ° 165 °

Location SP1 Location SP2 -4 345 ° 0° 15 ° x 10 5 -5 330 ° 30 ° 345 0° 15 x 10 ° ° 16 330 ° 30 ° 315 ° 45 ° 315 ° 45 ° 14 300 ° 60 ° 4 300 ° 60 ° 285 ° 75 ° 12 285 ° 75 ° 270 ° 90 ° 3 10 270 ° 90 ° 255 ° 105 ° 8 255 ° 105 ° 2 240 ° 120 ° 6 240 ° 120 ° 225 ° 135 ° 4 225 ° 135 ° 210 ° 150 ° 1 195 ° 180 165 ° 210 ° 150 ° ° 2 195 ° 180 ° 165 °

Location SP3 Location S P4

Figure 3.14 Directional spectra for the rotated breakwater option, 0.4 reflectivity with Hs=1m and Tp=12s. The 2D scale is spectral density and the radial scale is frequency (0-1).

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-5 345 0° 15 x 10 Incident energy ° ° 16 330 ° 30 ° 315 ° 45 ° 14 300 ° 60 ° 12 285 ° 75 ° 0.4 reflectivity 10 270 ° 90 ° 8 255 ° 105 ° 6 240 ° 120 °

4 Reflected energy 225 ° 135 ° 210 ° 150 ° 2 195 ° 180 ° 165 °

-4 345 0° 15 x 10 Incident energy ° ° 1.8 330 ° 30 ° 315 ° 45 ° 1.6

300 ° 60 ° 1.4

285 ° 75 ° 1.2

270 ° 90 ° 1

255 105 0.8 ° ° 0.8 reflectivity 0.6 240 ° 120 °

0.4 Reflected energy 225 ° 135 ° 210 ° 150 ° 0.2 195 ° 180 ° 165 °

Figure 3.15 Directional spectra for site SP4 proposed breakwater option, comparing 0.4 and 0.8 reflectivity (Hs=1m and Tp=12s). The 2D scale is spectral density and the radial scale is frequency (0-1).

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Figure 3.16 Spectral model comparison of the wave height distribution for existing case and the proposed breakwater. Lower imager shows the wave height difference from the modifications.

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Figure 3.17 Wave height distribution for existing case and the proposed breakwater plus island for a 1 m spectral wave height with a 12 sec period. Lower imager shows the wave height difference from the modifications.

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Figure 3.18 Spectral model comparison of the wave height distribution for existing case and a rotated breakwater. Lower imager shows the wave height difference from the modifications

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Figure 3.19 Spectral model comparison of the wave height distribution for existing case and V-shaped breakwater. Lower imager shows the wave height difference from the modifications

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Figure 3.20 Spectral model comparison of the wave height distribution for existing case and alternative V-shaped breakwater. Lower imager shows the wave height difference from the modifications

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3.5 Sheltering effects

All the breakwater orientations that were tested were effective at creating a wave-sheltered region in their lee, as illustrated in Figures 3.16 – 3.20. In the absence of a control structure, the Reef acts to focus wave energy into the eastern corner of the beach, which undoubtedly has an impact on the sediment transport and long-term beach stability. Further, this wave focussing on the Reef, which starts well beyond the surfing zone, gives rise to a refraction shadow just west of the surf break. The shadow zone is annotated on Figure 3.3, and the breakwater options extend almost to the edge of this shadow zone.

The wave refraction shadow zone may be beneficial to the beach stability following modification works - allowing a more gentile transition in wave energy from the tranquil region behind a breakwater structure to the open beach. However, to ensure that the control structures extend to this zone, approximately 25 m extra length would be required. Unfortunately, this would probably truncate the very last part of the surfing wave, especially during small wave conditions.

Wave height gradients along the shoreline are presented in Figure 3.21 for the existing and various breakwater options (including the island). The results show that the island breakwater option provides a reasonable degree of wave shelter in the lee. However, the requirement for an island structure needs to be identified from the long-term sediment transport implications, which is beyond the scope this report. The alternative V-shaped breakwater is the least effective (of those tested) at providing shelter and a gentle wave height gradient, and this occurs because the western side of the structure is positioned too far shoreward.

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1.2

0.8

0.6

Wave Wave height (m) 0.4 Effect of offshore island 0.2 Effect of breakwater

0 0 100 200 300 400 500 600 Distance (m)

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

Figure 3.21 Wave heights along the inshore transect shown on Figure 2.9. (spectral simulation, Hs=1m, Tp=12s, mid tide).

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3.6 Simulation of the June 2007 surf event

During June 2007 there were several days where there were good surfing conditions at The Reef (N. Daykin, pers comm ), and an example photograph is provided on Figure 3.22. The wave hindcast data for this period are graphed on Figure 3.23, showing significant swell heights at the Reef location at the 1 – 1.5 m level.

The hindcast wave spectra from the 19 th of June 2007 was modelled with CGWAVE, and the results for existing, proposed breakwater, and rotated breakwater are presented in Figure 3.24. The results for the alternative V-shaped breakwater are presented in Figure 3.25. The wave height and wave crest patterns clearly indicate interference at the start of the surfers ride for both linear breakwater options, but the effects on the inside section of the ride are minimised for the rotated breakwater case. In these cases the probable effect on surfing would be a deterioration of smooth peeling wave face, with ridges distorting the wave shape and causing irregular breaking. For the alternative V- shaped breakwater, the wave reflections at the break are minor and the model results do not suggest any significant negative effects to the surf quality. The model results are further illustrated on a wave height transect along the approximate surfers path, showing the wave heights for the energy peak for each of the tests (Fig. 3.26).

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Figure 3.22 Waves at the Reef surf break on the 19 th June 2007 (Photo courtesy of Neil Daykin, Hawke’s Bay Regional Council Engineering Section).

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3.5

2.5

1.5

Significant wave height (m) waveheight Significant 0.5

0 07/06/07 09/06/07 11/06/07 13/06/07 15/06/07 17/06/07 19/06/07 21/06/07 23/06/07 Date

VBWB2 CR1

20 18 16 14 12 10 8 6 Peak wave period (s)Peak waveperiod 4 2 0 07/06/07 09/06/07 11/06/07 13/06/07 15/06/07 17/06/07 19/06/07 21/06/07 23/06/07 Date

VBWB2 CR1

140

120

100

20 Swell wave direction (degrees T) wave direction Swell

0 07/06/07 09/06/07 11/06/07 13/06/07 15/06/07 17/06/07 19/06/07 21/06/07 23/06/07 Date

VBWB2 CR1

Figure 3.23 Time series of swell wave height ( Hs), peak wave period ( Tp) and swell wave direction for the period 7-June 2007 – 24-June 2007 (sites VBWB2 and CR1).

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Figure 3.24 Wave height distributions for the incident on June 19 th 2007. A – Existing, B – Proposed breakwater, C – Rotated breakwater. Tide = MSL, simulated with a wave spectra.

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Figure 3.25 Wave height distributions for the incident on June 19 th 2007. A – Existing, B – Alternative V-shaped breakwater. Tide = MSL, simulated with a wave spectra.

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1.8

1.6

1.4

1.2

1 0.8 Main section 0.6 Wave Height (m) Height Wave

0.4 Inside section 0.2

0 0 100 200 300 400 500 600 Distance (m)

Existing Proposed breakwater Rotated breakwater Alternative V-shaped

Figure 3.26 Wave height transects along the surfer’s path for conditions on the 19 th June 2007. The V-shaped breakwater has a similar wave height profile to the Alternative V-shaped shown here.

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4 SUMMARY

Hindcast modelling of the wave climate in the Westshore / Reef area indicates this region has a relatively low-energy climate, and that surfing occasions are often limited by the lack of swell height. On an annual basis, good surfing conditions can be anticipated for approximately 3% of the time, and surfable conditions are likely to exist for less than 20% of the time.

Despite the relative infrequency of surf events, the Reef surf break provides a well-used resource when the conditions are suitable. The seabed shape at the surf break features an elongated reef that extends well seaward of the surfing section, and this shape encourages the focussing of wave energy onto the break point. This focussing also directs the wave energy into the eastern corner of the Westshore Beach, which is likely to be part of the ongoing erosion issue in this specific area.

The influence of the proposed breakwater on the surf conditions at the Reef has been modelled for generic swell conditions, and a specific surf event. The modelling indicates that:

• The proposed breakwater option will reduce the surf quality at the Reef break. The effects are primarily due to wave reflections off the structure, and it is 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 location.

• The reflections are not expected to change the wave gradients, and the primary wave peel angle will not be altered. However, the localised interference from the reflected wave crests will interrupt the continuity of the surfing wave face, potentially reducing the surfing section lengths and altering the local-scale peel angles. Probable effects include ridges on the surfing wave, distorting the shape and causing irregular breaking.

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

MetOcean Solutions Ltd 56 Whakarire Ave Breakwater - Surf Effects

• The effects can be minimised by reducing the reflectivity of the breakwater, although a low-gradient structure with only 0.4 reflectivity is still anticipated to have a noticeable, negative impact on the wave quality.

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

• The V-shaped breakwater options exhibit the least impact on the surf quality. This shape significantly reduces the reflected wave energy into the surfing region, and may facilitate a lower overall wall height due to the increased angle of incidence to the wave crests. Two options have been tested; one that follows the native seabed shape and an alternative design that that is as close to the shore as possible.

• All the breakwater 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. A combination of the western arm of the V-shaped breakwater, with the eastern arm of the Alternative V-shape design would be a worthwhile consideration for planning purposes.

• At present the waves are focussed into the western corner of the beach, exacerbating the local erosion issues. A sheltered zone with 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). Extension of the breakwater option by a further ~25 m would ensure structure blends in with a natural wave refraction shadow zone, which would provide additional benefit to the stability of a maintained beach. However, this would probably lead to the truncation of the very last part of the surfing wave, especially during small wave conditions over the high tides.

MetOcean Solutions Ltd 57 Whakarire Ave Breakwater - Surf Effects

• Numerical tests of the island breakwater option show that this structure would provide a reasonable degree of wave shelter in the lee. However, it should be noted that the ultimate requirement for an island structure needs to be based on the long-term sediment transport implications, and such conclusions are beyond the scope the present report.

MetOcean Solutions Ltd 58 Whakarire Ave Breakwater - Surf Effects

5 REFERENCES

ASR (2001). Westshore Coastal Process Investigation: A Study to Determine the Coastal Processes in the Bay at Westshore and Provide a Long-Term Solution to Erosion Problems. Report prepared for NCC, September 2001.

Allsop, N.W.H., and Channel, A.R. 1989. Wave reflections in harbours: Reflection performance of rock armoured slopes in random waves. Hydraulic Research, Wallingford. Report OD 102.

Battjes, J. A. 1974. Wave runup and overtopping. Report to the Technical Advisory Committee on Protection Against Inundation, Rijkswaterstaat, The Hague, Netherlands.

Barber,N.F. 1961. The directional resolving power of an array of wave detectors, Ocean Wave Spectra. Prentice Hall. Inc. pp.137-150

Black, K. Beamsley, B. J., Johnson, D., Mead, S. and Mathew, J., 2004. Boscombe Surfing Reef Detailed Design. Field data and initial design report. A report prepared for the Bournemouth Borough Council. P 182.

CIRIC, CUR and CETMEF, (2007). The Rock Manual, The use of rock in hydraulic engineering (2nd Edition). C683, CIRIA, London.

Daykin, N (2007) Haumoana Littoral Cell: Coastal Storm Analysis. Hawke’s Bay Regional Council report, Report number ISSN 1174 3085.

Mead, S., Black, K., Scarfe, B. and Blenkinsopp, C. 2004. Feasibility and Preliminary Design Study for an Artificial Surfing Reef at Mahomet’s Beach, Geraldton, Western Australia. Report prepared for the Geraldton Boardriders Club (Inc.). p 96.

Postma, G. M., 1989. Wave reflection from rock slopes under random wave attacks, PhD thesis, Delft University of Technology.

Scarfe, B. M. H.S. Elwany, S. T. Mead, and K. P. Black. The Science of Surfing Waves and Surfing Breaks - A Review. March 7, 2003. Scripps Institution of Oceanography Technical Report. http://repositories.cdlib.org/sio/techreport/17

Seelig, W.N. and J.P. Ahrens, 1981. Estimation of wave reflection and energy dissipation coefficients for beaches, revetments and breakwaters. CERC Technical paper 81-1, Fort Belvoir, U.S.A.C.E., Vicksburg, MS

The SWAN Team, 2007. Technical documentation SWAN Cycle III. Available at: http://vlm089.citg.tudelft.nl/swan/online_doc/swantech/swantech.html

Van der Meer, J. W., 1988. Rock Slopes and gravel beaches under wave attack. Unpublished PhD thesis, Delft University of Technology, The Netherlands.

Walker, J. R., 1997. A Summary of Surfing Reef Parameters. 1st International Surfing Reef Symposium, Sydney, March 1997.

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