Appendix D - Mercury Bay South Residents Ratepayers ... · preferred option. This report will also inform and guide the compilation of the forth coming resource consent application

Appendix D Coastal Processes and Design Report

I

- IIIGIWT C O N S U L T I N C

Cooks Beach

For Thames Coromandel District Council

Coastal Processes and Design Report

April 2019

REPORT INFORMATION AND QUALITY CONTROL

Jan van der Vliet

Coastal Engineer.

Thames-Coromandel District Council

Sam Morgan

Senior Coastal Consultant

Simon Karl

Senior Planning and Policy Consultant

Aaron Andrew

Managing Director

AA4637_Cooks Beach Coastal Processes and Design Report V2.0

V.2 July 2019

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CONTENTS Page

1 INTRODUCTION ........................................................................................................................................1 1.1 Site Description ....................................................................................................................................1

2 GEOMORPHIC SETTING .............................................................................................................................2 2.1 Recent Coastal Change .........................................................................................................................3 2.2 Beach Monitoring Trends .....................................................................................................................5

3 LOCAL COASTAL PROCESSES .....................................................................................................................9 3.1 Wave Climate .......................................................................................................................................9 3.2 Coastal Water Levels ..........................................................................................................................10

4 DESIGN BASIS AND RATIONALE ..............................................................................................................10 4.1.1 Assessment o f Erosion Issue ..............................................................................................................10 4.1.2 Design Concept Response ..................................................................................................................10 4.1.3 Beach Nourishment Design ................................................................................................................11 4.1.4 Backstop Seawall Design ....................................................................................................................11

5 COASTAL PROCESSES IMPACT ASSESSMENT ...........................................................................................12 6 CONCLUSIONS ........................................................................................................................................12

List of Tables

Table 1: Predicted extreme water levels for the subject area converted t o WVD-94 (Source: Table 7-1, Stephens eta l . , 2015) ..................................................................................................................................................10

List of Figures

Figure 1: Location o f the subject site w i th the red box indicating the area o f concern . ......................................... 1 Figure 2: Geomorphic features o f the Cooks Beach system . ................................................................................... 2 Figure 3: Offshore bathymetry and wider shore alignment. Cooks Beach indicated by red dashed square...........3 Figure 4: Channel position and orientation comparison between 1944 and 1984 indicated by the red arrows. The

yel low arrow indicates the location and f low direction o f the flood t ide sub-channel . ................................ 4

Figure 5: Mangrove colonisation in the Purangi Estuary since 1971 indicated by red dashed lines . ...................... 4

Figure 6: Location o f beach monitor ing profiles across Cooks Beach......................................................................6 Figure 7: Beach monitor ing data f rom the western end o f Cooks Beach

. ............................................................... 6 Figure 8: Beach monitor ing data f rom the central portion o f Cooks Beach

. ........................................................... 6 Figure 9: Beach monitor ing data f rom the near-eastern end o f Cooks Beach

. ....................................................... 7 Figure 10: Beach monitoring data f rom the mid-eastern end o f Cooks Beach........................................................7 Figure 11: Beach monitoring data f rom the far-eastern end o f Cooks Beach

. ........................................................ 8 Figure 12:Migration o f dune crest / bank edge f rom datum point at far-eastern end o f Cooks Beach ..................8 Figure 13: Offshore wave rose data for the subject site..........................................................................................9 Figure 14: Extreme offshore wave heights for subject site

. .................................................................................... 9

List of Appendices

Appendix A: Historical Aerial Photographs

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1 INTRODUCTION

Thames Coromandel District Council (TCDC) recently engaged 4Sight Consulting Ltd (4Sight) t o develop options to manage coastal erosion o f the southern reserve area at Cooks Beach on the eastern coast o f the Coromandel Peninsula. It is understood that TCDC's preferred opt ion is t o build a backstop wall t o ensure the integrity of infrastructure, as well understand the viability o f a groyne control structure. This structure will then be subject to a nourishment program which will be supplemented by dune planting as a means o f retaining natural character and amenity values on the beach.

This report presents the findings o f a site-specific coastal processes investigation and assesses the nature o f the erosion hazard at Cooks Beach. This information has also been used to identify the design parameters for the backstop wall and groyne structure, as well the extent o f the beach nourishment programme. Two options for a vertical seawall have been developed and the various pros and cons discussed t o assist TCDC when making a decision over the preferred option. This report wil l also inform and guide the compilation o f the for th coming resource consent application.

Figure 1: Location o f the subject site wi th tne

1.1 Site Description Cooks Beach is situated on the east coast o f the Coromandel Peninsula approximately 3km east o f the Whitianga township. The subject area o f sand loss is situated toward the eastern extent o f the beach and has been exposed to notable erosion over the past f e w years. The erosion has, at least in part, been attr ibuted t o end effects f rom a recently constructed rock revetment seawall immediately t o the west o f the site. In order t o address the end effects issue, re- nourishment works have been undertaken and an emergency rock revetment sea wall constructed.

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2 GEOMORPHIC SETTING

Cooks Beach is approximately 3km in length and orientated roughly east t o west, thus being open to wind and wave events f rom the northerly quarter. Significant protection is afforded t o the site due to it's inset position wi th in Mercury Bay and Shakespeare Cliffs t o the west and Cooks Bluff t o the east. Further details about the wave climate and other coastal processes are discussed Section 3 below.

The wider system is considered to be an example o f a wave dominated estuarine system, characterised by the spit, ebb t ide delta and f lood t ide delta features (Figure 2). It is considered that the central basin estuarine feature is poorly formed due t o the limited accommodation space afforded by the small drowned river valley being occupied by the Purangi Estuary system.

In accordance wi th the NIWA beach classification system' the beach itself is considered t o be 'Reflective' due t o its short beach length, beach cusps and wave climate. The bathymetry offshore gently grades into Mercury Bay t o a depth o f lOm before a sharp drop t o 50m beyond the extent o f the bay. Cooks Bluff t o the east o f the beach appears to provide significant shelter t o the shoreline as the depth contours indicate a build up o f sand material in it lee (Figure 3).

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2.1 Recent Coastal Change Analysis o f the available historic aerial photos f rom the Retrolens website was undertaken as a part o f this investigation. Of note is the orientation and eastward shift o f the ebb t ide delta and Purangi Estuary channel between 1971 and 1984. It is noted tha t the position o f the channel has also moved post 1984 but the dominant trajectory has settled t o a northerly f low as opposed t o the more north-west f low in the aerial imagery prior to 1984. The 1984 image is also the first t ime a flood t ide sub-channel is apparent in the air photo record. This is summarised in Figure 4 below and the complete photo set analysed is provided wi th in Appendix A. Detailed analysis o f the flood t ide delta form has no t been undertaken, but changes in form and position are apparent. Anecdotal evidence suggests that the delta has been expanding and therefore decreasing the size o f the channel adjacent to the main boat ramp over the past 10

years o r so.

It was also noted tha t significant areas o f mangrove colonisation have occurred post 1971 (FigureS below). This is not

uncommon across northern New Zealand and has been often attr ibuted t o land use changes and/or hydrodynamics changes to the system. At Cooks Beach it is mostly likely tha t land use change in the catchment and the limited accommodation space are the primary contr ibut ing factors. Whilst diff icult t o ascertain f rom aerial photography, sediment infilling is often associated wi th mangrove colonisation, which in turn can lead t o a reduction in the tidal prism size. Wi th a reduced tidal prism, the ebb tide f lows and volumes are expected t o have reduced which may in tu rn have contributed t o the change in the form o f the ebb t ide delta noted above.

AA4637_Cooks Beach Coastai Processes A n d Design Report V2.0 3

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icates the location and f low directio lb-channel.

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2.2 Beach Monitoring Trends A high-level analysis o f the beach monitor ing sites along Cooks Beach has been undertaken using the Beach Morphology Analysis Package (BMAP). More detailed analysis here focused on Profile CCS31-2 (Figure 6) as this is situated in the area o f concern. The other remaining profiles were briefly analysed for context o f wider beach behaviour. Figures 7 & 8 demonstrate the western end o f the beach monitoring to have a mild accretionary trend in volumes above Mean Sea Level (MSL). However, due to the sparsity o f early data points the analysis may be slight skewed and i t is assumed over this part o f the beach is a dynamic equilibrium.

Data f rom profile CCS31 illustrates a reasonable strong erosion trend wi th an estimated 30% loss o f material above MSL over the 40-year monitor ing period (Figure 9). However, the position o f the dune has not retreated t o the same extent and this is thought t o be related to the stability afforded by the dune planting cells. The erosion trend is no t as strong in profile CCS31-1 (Figure 10), but this is thought to be related t o the degree o f modification that has occurred and eventual construction o f a revetment seawall. Greater detail about the nature o f the modifications may provide

an insight t o the nature o f the beach behaviour at this monitoring point bu t for the purpose o f this assessment i t has been assumed that

Similarly, the data f rom CCS31-2 shows a weaker erosion trend (Figure 11) bu t this is thought to be the result o f site modifications such as the nourishment and dune reconstruction works in 2016 artificially accreting the profile.

The position o f the RL2.5m contour relative t o the bench mark has been used as a proxy for the dune crest/bank position. Despite modifications and nourishment works at CCS31-2, this part o f the beach system the dune crest has retreated approximately 20m over the past 27 years (Figure 12).

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Figure 6: Location o f beach monitor ing profiles across Cooks Beach.

CCS29 V o l u m e a b o v e M e a n Sea Level

250

200

50

1976 1982 1987 1993 1998 2 0 0 4 2 0 0 9 2 0 1 4 2020

Date

Figure 7: Beach monitor ing data f rom the western end o f Cooks Beach.

CCS30 V o l u m e a b o v e M e a n Sea Level

E -_ 80

E IS E

60

40

20

1976 1982 1987 1993 1998 2094 2 0 9 9 2 0 1 4 2 0 2 0 2025

Date

Figure 8: Beach monitor ing data f rom the central portion o f Cooks Beach.

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CCS31 V o l u m e A b o v e M e a n Sea Level 120

100

80

E 60

E 11) E

40

20

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1976 1982 1987 1993 1998 2 0 0 4 2 0 0 9 2 0 1 4 2020

Date

Figure 9: Beach monitor ing data f rom the near-eastern end o f Cooks Beach.

Profile CCS31-1 Volume Above Mean Sea Level

70

40

1.

1997 1993 1998 2994 0999 2014 2020

Date

Figure 10: Beach monitor ing data f rom the mid-eastern end o f Cooks Beach.

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P r o f i l e C C S 3 1 - 2 V o l u m e A b o v e M e a n 5 8 4 Level

3at0

Figure 11: Beach monitor ing data f rom the far-eastern end o f Cooks Beach.

CCS31-2 Dune c res t ! Bank edge position 45.00

40.00

5.00

0.00

1987 1993 1998 2 0 0 4 20119 2014 2020

Date

Figure 12:Migration o f dune c r e s t ! bank edge f rom datum point at far-eastern end o f Cooks Beach

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3 LOCAL COASTAL PROCESSES

3.1 Wave Climate The site is generally orientated east t o west and therefore exposed t o waves f rom the northwest t o the northeast. Due t o the limited fetch f rom the northwest and the north due t o landmasses, i t is considered that the largest waves approaching the beach are generally f rom the northeast either as wind generated storm events o r open ocean swells refracting into the bay. Large east swell events are also thought t o reach the site after similarly refracting around Cooks Bluff. Figure 13 below shows the wave rose f rom the nearest offshore modelling location, and Figure 14 provides the extreme offshore wave heights. In general, waves operating at the beach during storm and/or swell events wil l be in the range o f 0.5-2m.

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Signif icant W a v e He ight [m] 13 1 7 7 138

W i n d s p e e d [knots] 45 52 65

FIgure i & Extreme offshore wave heights for subject 5it03

2 https://app.metoceanview.com/hindcast/sites/nz/-36.8/175.8

https://app.metoceanview.com/hindcast/sites/nz/-36.8/175.8

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3.2 Coastal Water Levels Table 1 below present a range o f water levels associated wi th storm surge events obtained f rom Stephens et. al., 20151. The value for MHWS was obtained f rom the Waikato Regional Council website and then converted t o the local datum (MVD-94). A nominal value o f l m has been added to al low for future sea-level rise which is based upon guidance provided f rom the Waikato Regional Council5.

Return Period MHWS 5yr API 20yrARI SOyr API 100yr AR!

WVO-94 RL(m) 2 2 7 2 4 9 2 6 0 2 6 8 275

+SL Rise 3 2 7 3.49 3.6 3.68 3.75 (lOOyrs)

4 DESIGN BASIS AND RATIONALE

4.1.1 Assessment o f Erosion Issue

Based on the information presented above, i t is considered the eastern end o f Cooks Beach is subject t o a progressive erosion issue. The cause o f the erosion appears t o be related t o modification o f sediment deposition patterns reducing the volume o f sand material supplied t o the eastern beach. This is thought to be the function o f changes t o the ebb t ide delta tidal prism and infilling the Purangi Estuary, i.e. a shallower estuary has less tidal water capacity and subsequently has less potential energy in the ebb f l ow t o transport sand t o the beach.

This conclusion is consistent w i th estuarine evolutionary models which predict a loss o f material f rom a barrier feature

as a wave dominated estuary reaches its maturity. It is noted that upon reaching maturi ty the barrier feature is expected t o then accrete as marine sediment sources are no longer able t o be accepted wi th in the estuarine system6. At the site specifically, this appears to be punctuated by end effects related t o the construction o f the adjoining revetment seawall t o the west. The susceptibility o f the site is heightened by the proximity o f a f lood-tide sub-channel tha t has erosive capacity and has apparently become a more permanent feature post 1970.

4.1.2 Design Concept Response

It is understood tha t TCDC have undertaken an internal initial options analysis for coastal protection measures at this location. A 'do nothing' is not considered applicable in this location due t o the proximity o f infrastructure. A conventional hard protection structure was not considered appropriate due t o the amenity and natural character values at the site. Therefore, TCDC requested a design fo r a backstop seawall along the length o f the eroded beach east o f the private rock revetment up towards the Purangi River mouth. Design fo r a supplementary beach nourishment and dune restoration regime has also been nominated as a measure t o retain the high natural character values o f the area whilst providing a buffer t o coastal erosion at the site. Investigations have also been requested into assessing the concept o f a groyne design t o help retain sand in the upper portion o f the beach in the subject area.

Stephens, S., B. Robinson, R. Bell (2015) Analysis of Whitianga, Tararu and Kawhia sea-level records to 2014. NIWA report HAM2015-046 for Waikato Regional Council.

https://waikatoregion.govt.nz/assets/PageFiles/41257/3700622-Coastal Inundation Tool Guidance.pdf 6 Dalrymple, R., Zaitlin, B. and Boyd R., 1992. Estuarine Facies Models: Conceptual Basis and Stratigraphic Implications. Journal of Sedimentary Petrology. v.62, n.6,

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4.1.3 Beach Nour ishment Design

The aim o f beach nourishment at the site is to create a sufficient wid th o f sand above MHWS to enable the establishment o f dune plants and push the flood t ide sub-channel seaward and away from the subject area. To enable this, i t is proposed t o shift the shoreline position approximately 40-45m seaward by depositing 15,000m' at the eastern end o f Cooks Beach. This will adjust the position o f MHWS beyond the extent o f the existing emergency rock revetment and infill the f lood t ide sub-channel.

The nourishment wil l be achieved by extracting up t o 15,000m' from the ebb t ide delta immediately offshore from the Purangi River mouth. The intent is t o recreate the flood t ide sub-channel seaward o f the deposition area to encourage the system t o 'accept' the deposited sand.

Should not enough sand be available wi th in the ebb t ide delta, a secondary sand source has been identif ied wi th in the channel straight. This area has been restricted in extent by the shellfish beds below the mid-t ide mark and toward the channel. This habitat has been detailed in an ecological assessment which is contained wi th in a separate report to be included wi th the resource consent application. In order t o avoid impacting on this shellfish community, it is proposed that physical works in the area operate wi th a 5m exclusion buffer around the beds. The extent o f beds and therefore the location o f sand material t o be extracted should be confirmed prior t o physical works due t o the fluctuating morphology o f the sand bank feature. In order t o offset any increased coastal processes activity created by deepening the sand flat, a small area o f nourishment and planting is proposed by the exposed road edge.

It is expected that the nourished material will continue t o erode over t ime as i t is not practical t o eliminate the causes o f erosion. The residence t ime fo r the nourished material is diff icult t o determine as it is dependent upon fluctuations in weather conditions, frequency o f storm events and delivery o f sand to area. Hence, it is considered necessary that the nourishment programme is able t o be repeated as required and planning permission fo r this maintenance activity be obtained.

4.1.4 Backstop Seawall Design

Two designs fo r a vertical 'backstop' wall have been considered wi th in the concept design stage, being (1) a stabilised sand trenched wall; or (2) a tradit ional t imber retaining wall. The wall al ignment has been set back wi th in the existing

reserve area t o avoid fur ther structures being located in the active coastal zone. Final al ignment wil l need t o be determined on site once local services have been located, as the survey provided did not identify these. However, due t o the simplistic and flexible nature o f the proposed structures, avoiding any services is no t seen as problematic. The wall height is to be set at existing ground levels, and i t is recognised that in the future this area may become inundated wi th predicted sea level rise. However, the impact o f potential overtopping is expected t o be mitigated through the nourishment regime and additional drainage capacity wi th in the wall design allowing the water moving over the structure t o move back ou t efficiently. Founding depths are determinant on the retained height above average sand levels w i th and allowance fo r scouring during storm events. This depth wil l require fur ther investigation at the detailed design stage.

Timber Wall

The favoured opt ion proposed is a conventional t imber seawall design w i th additional allowance for boards below the sand to prevent undermining. Drainage behind the wall wil l prevent the build up o f ground wate r and lessen the potential impact o f hydraulic loading through the wall should i t be exposed during a storm event. Because the wall is t o be built in-ground the construction is more complicated than more conventional t imber walls. This is in part due to the additional trenching required t o al low for the rails t o be fixed t o the rear o f the posts and the geotextile cloth placed correctly.

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5 COASTAL PROCESSES IMPACT ASSESSMENT

Backstop Seawall

If existing assets and use o f the subject area are t o be maintained, then an appropriate management response is required. The back-stop wall is expected t o be outside o f the influence o f coastal processes fo r the majority o f time due t o the buffering influence o f the supplementary beach nourishment, which wil l also retain natural character and amenity values. As explained below, this approach significantly reduces the risk o f adverse effects on local coastal

processes as a result o f introducing the proposed structure.

Beach Nourishment

The quant i ty and depth o f sand being extracted f rom the ebb t ide delta is in the order o f wha t is naturally moved during storm/wave events, and hence the system is expected t o adjust fol lowing physical works. Sand f rom initial deposition is expected t o erode during storm events over t ime and some adjustment at the margins as the system adjusts to the sand. Therefore, it is anticipated tha t maintenance nourishments wil l be required on a periodic basis.

The soft packed nature o f sand and relatively deep anoxic layer f rom the secondary extraction area indicates tha t this deposit is mobile and is perhaps a transitional feature between the flood and ebb t ide deltas. Due t o the dynamic nature o f this area, the quant i ty and depth o f the extraction is no t expected t o adversely impact the feature.

There is a recognised risk o f coastal processes and tidal energy acting upon the upper shoreline bank and margins as a result o f slightly deeper water in the area. This is proposed t o be mitigated by depositing a small amount o f sand in f ront o f the bank and stabilising w i th planting.

6 CONCLUSIONS

A description o f the site's geomorphology, coastal processes and coastal hazards are provided above. This information has been used in the understanding o f coastal erosion at the site. In general, there appears t o be a progressive erosion issue at the east end o f Cooks Beach. The degree and rate o f erosion appears to have been exacerbated by the installation o f the adjoining private revetment seawall to the west o f the subject area.

The report and associated drawing set presents t w o options for a backstop seawall t o preserve the integrity o f roading and other infrastructure in the area. The effects o f this structure are proposed to be offset and mitigated by its buried nature and proposed nourishment regime. It is proposed to plant dune species as a means o f stabilising and retaining the dune sands.

In general, this approach and design has been undertaken t o minimise the potential for impacts on local coastal

processes. Further, investigation work is required to establish what effects a potential groyne structure wil l have on the local coastal processes.


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Appendix A:

Historical Aerial Photographs


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Documents

Appendix D - Mercury Bay South Residents Ratepayers ... · preferred option. This report will also inform and guide the compilation of the forth coming resource consent application