CO AASSTTALL E SS PP T DD FF INN FTTHHE E T B PP ,, …co aassttall esseeddiimmeenntt pprroovvennaannccee anndd ptrraannsspoortt dduuee ttoo ffaauullttiinngg inn ftthhe e tbaayy oof

CCOOAASSTTAALL SSEEDDIIMMEENNTT PPRROOVVEENNAANNCCEE

AANNDD TTRRAANNSSPPOORRTT DDUUEE TTOO FFAAUULLTTIINNGG

IINN TTHHEE BBAAYY OOFF PPLLEENNTTYY,, NNZZ UUSSIINNGG GGPPRR AANNDD AASSSSEESSSSIINNGG AANNTTHHRROOPPRROOGGEENNIICC SSEEDDIIMMEENNTT TTRRAANNSSPPOORRTT WWIITTHHIINN TTHHEE NNZZ CCUULLTTUURRAALL LLAANNDDSSCCAAPPEE

MMaatttthheeww ZZwwiinnggrraaff

UUnniivveerrssiittyy ooff AAuucckkllaanndd GGeeooggrraapphhyy 333333

1188 JJuunnee 22001100

M. ZWINGRAF Using GPR to Investigate Coastal Sediments and Faulting 2

TABLE OF CONTENTS

ABSTRACT .................................................................................................................................................................................. 4

INTRODUCTION ......................................................................................................................................................................... 5

RANGITAIKI PLAINS – BAY OF PLENTY – NZ ........................................................................................................................ 6

Geologic setting ..................................................................................................................................................... 6

Coastal status and progradation ..................................................................................................................... 6

Sedimentary provenance and accommodation space ............................................................................ 7

Geomorphology ..................................................................................................................................................... 8

Tectonics .................................................................................................................................................................. 9

Anthropogenic sediment transport ............................................................................................................... 9

GROUND PENETRATING RADAR (GPR) .............................................................................................................................. 10

MAURI MODEL ASSESSMENT ................................................................................................................................................ 10

METHODS ............................................................................................................................................................................... 12

RESULTS.................................................................................................................................................................................. 14

DISCUSSION ............................................................................................................................................................................ 17

CONCLUSION ........................................................................................................................................................................... 20

REFERENCES........................................................................................................................................................................... 21

ATTACHMENTS ....................................................................................................................................................................... 22


ACKNOWLEDGMENTS

I would like to thank the efforts of all who helped and guided me in the completion of this project. This

research opportunity was made possible through my Frontiers Abroad field camp semester of study at the

University of Auckland. I appreciate the work of Dan Hikuroa, Amy Dougherty and Darren Gravley in

helping me develop this project and for their continued advising throughout the duration of my research. I

also would like to acknowledge the technical assistance of Dave Wackrow, Brendan Hall, Dave Jenkinson

and Simon Bloomberg in processing sediment samples, RTK data and conducting GPR work in the Bay of

Plenty.


ABSTRACT

The Rangitaiki Plains (RP) in the Bay of Plenty, New Zealand are home to the most rapidly expanding

rift of the Taupo Volcanic Zone (TVZ) as well as a complex barrier system which has prograded over 8 km

in the past 6000-7000 years. With 65% of New Zealanders living within 5 km of the coast, it is important to

understand how these dynamic systems evolve and function. High-resolution LiDAR imagery data and

ground penetrating radar (GPR) were used to investigate faults which have cut across stranded inland

dune and beach ridge features. This project has set out to determine if these faults are the reason for the

incision and erosion of these features via subsequent fluvial action through the fault scarps. GPR data

revealed that the hypothesized faults were present at the study locations and associated with fluvial

sediments and paleochannels. How much effect the erosion of past dunes and beach ridges would have on

the overall coastal sediment budget is unknown. It is thought they have played a past role in delivering

sediment to the coast, as high-relief stranded dune ridges no longer cover a majority of the modern day

plains.

Anthropogenic sediment transport was also investigated to determine the environmental, cultural,

social and economic sustainability of such operations using the Mauri Model. The Mauri Model is a holistic

decision making framework which allows indigenous Maori ideals to be taken into account. This tool was

used to compare sand extraction operations at Pakiri, Northland with a hypothetical one in the Bay of

Plenty. Analysis determined that beach nourishment is important to restoring beaches the Bay of Plenty

would be a preferable source to Pakiri.


INTRODUCTION

Coastlines are among the most vulnerable and rapidly changing environments in New Zealand and the

world. It is estimated that at least 70% of the world’s sandy beaches are currently eroding (Dickson, 2010).

Despite a long Holocene progradational history, the coastline of the RP seems to be trending more towards

a dynamic equilibrium (EBOP, 2007). Focused on the edge of the TVZ where the Rangitaiki, Whakatane and

Tarawera Rivers flow together, it is the world’s most productive rhyolitic volcanic center (Figure 1).

Several factors have gone into the building the Holocene history of the RP. Previous work by Pullar and

Selby (1971), Beanland and Berryman (1992), Begg and Mouslopoulou (2009) and Summers (2009) have

investigated the evolution of the RP and principle controls on its evolution. Coastal evolution of the Bay of

Plenty has been shaped by wave processes, sea level rise, eruptive events, sediment supply and transport,

accommodation space and faulting associated with active tectonics (Dickson, 2010). All of these factors

have combined to alter the shape of today’s coastline in the eastern Bay of Plenty.

Recently developed technologies have been used in unison and allowed researchers to attain high

resolution topographic data and look into the shallow subsurface and therefore the recent past. High-

resolution LiDAR (light detection and ranging) has allowed unparalleled imaging of surface topography.

Ground penetrating radar (GPR) has become a powerful tool for studying the evolution of coastal

environments by non-invasively revealing subsurface stratigraphy. This project used LiDAR to identify

areas of topographic interest to search for currently unidentified faults and evidence of paleoseismicity.

The region’s prograding barrier system has great preservation potential for recording evolution which GPR

will help to reveal. GPR analysis will also help to identify perturbations to this record in the forms or

faulting, rivers and other sediment inputs into the system.

These natural modes of sediment transport are paralleled in anthropogenic efforts to restore degraded

coastlines. Effects of sand extraction on the cultural landscape of New Zealand have not been widely

examined, namely the effects on the mauri* of extraction and nourishment sites respectively. This analysis

will take into account social, environmental, economic and cultural effects using the Mauri Model decision

making framework (Morgan, 2006). The issue of anthropogenic change will be looked at from a holistic

earth systems perspective to determine the most appropriate location for such action. This will include a

hypothetical sand extraction operation off the coast of the Bay of Plenty for nourishment projects in

eroding areas.

*Mauri is the pervasive Maori idea of a binding force that bridges the physical and spiritual world or that which allows the capacity to support life. It was chosen as the conceptual backbone for the Mauri Model decision making framework developed by Morgan (2006). It will be discussed in detail further on (see page 10).


RANGITAIKI PLAINS – BAY OF PLENTY – NZ

Geologic setting

The RP are a central coastal

depositional area in the Bay of

Plenty region of New Zealand’s

North Island (Figures 1 and 2). The

history and geomorphology of this

area has been previously described

in detail by Pullar and Selby

(1971), Beanland and Berryman

(1992) and Begg and

Mouslopoulou (2009). These

lowlands represent a unique case

of a prograding coastal barrier

system despite being located in an

extensional zone which is opening

at rates of up to 15-20mm per year

(Beanland and Berryman, 1992).

The RP’s northwestern and

southeastern edges are bordered

by prominent northeast-southwest

aligned fault scarps. These scarps

mark the northern landward extent

of the TVZ. The RP are also an area

of active subsidence, which have

been filled with several kilometers of pyroclastic and volcaniclastic units from TVZ rhyolitic caldera

eruption events (Manville et al., 2004).

Coastal status and progradation

The coastline of the Bay of Plenty is defined as a low tide terrace according to the Wright and Short

model of beach morphologic state. This type of beach profile acts to reduce the wave height that is capable

of reaching the steep beach, thereby protecting it to some degree from erosion (EBOP, 2007). An active

dune system is approximately 50 m wide with past dune ridges stretching inland up to 9 km. Longshore

current in the Bay of Plenty moves west-east, moving sediments transported to the coast by the Tarawera,

Rangitaiki and Whakatane rivers (Pullar and Selby, 1971). The progradational nature of this barrier system

Figure 1 Map showing the location of study area with respect to the Taupo Volcanic Zone. Location of sand extraction site at Pakiri also included (modified from Summers, 2009).

Littoral drift


has led a depositional system with a well

preserved, detailed stratigraphic history.

The preservation creates the potential to

record hazards which have influenced

coastal evolution (Dougherty et al, 2009).

Facies present include Pleistocene marine,

estuarine, and fluvial sediments which are

interbedded with primary airfall tephra,

pyroclastic flows, and redeposited tephra

(Begg and Mouslopoulou, 2009). Holocene-

age materials, including tephra, greywacke

alluvium, pumice alluvium, and beach and

dune sediments are also present (Beanland

and Berryman, 1992). Reworking by rivers,

dunes and neotectonics have all played

roles in altering the surface and subsurface

of the RP.

At the maximum of the post-glacial

marine transgression (~6.5 ka), the RP

were a deep embayment backed by greywacke cliffs at Awakeri in the southeast of the present day plains.

The headland at Matata was most likely built out with volcaniclastics several kilometers further than

present but backed by the same cliffs as today (Beanland and Berryman, 1992). Pullar and Selby, (1971)

have determined paleoshoreline sequences using the seaward extent of volcanic airfall deposits in dunes,

beach ridges and alluvial terraces. Extensive dune fields with swampy intervening swales have provided an

excellent preservation environment for such tephras. Using such information, the rate of progradation has

been about 8 km over the past 6000-7000 years. However, this has not been a constant rate of shoreline

accretion. Summers (2009) has suggested that barrier evolution has been punctuated by eruptive events

which have resulted in an abundance of sediment being transported to the coast via rivers before the

shoreline returns to a state of equilibrium.

Sedimentary provenance and accommodation space

The rapid spreading rate and frequent volcanic activity of the TVZ have led to a subsidence and massive

infilling of RP, respectively during the Holocene, primarily within the last 5000 to 7000 years before

present. The Rangitaiki River catchment is the most topographically and geologically diverse of the

drainage systems flowing forth from central TVZ (See Figure 1). From the headwaters to the east of Lake

Figure 2 Coastal evolution in the RP region subsequent to the culmination of the post-glacial marine

transgression at ca. 6.5 ka (Beanland and Berryman, 1992),


Taupo, the river flows through a series of rhyolitic, welded ignimbrite layers known collectively as the

Kaingaroa Plateau. Areas of this catchment can reach over 700 m in altitude in the mountains and allow for

a steady supply of ignimbrite resulting in volcaniclastic deposition in the RP (Manville et al., 2004). The

Tarawera River is also a prominent and active conveyor of sediment to the RP as it drains the Okataina

Volcanic Center. The RP are additionally crossed by the Whakatane River on the eastern portion of the

lowlands. Due to its location it is not a major contributor of sediment to the system (Pullar and Selby,

1971).

Accommodation space is also a consideration of any depositional system. In order for sediment to be

deposited, it must have space available for it to do so. Since depositional areas are attempting to move

toward an equilibrium state, areas of negative accommodation space will be eroded, those with positive

space will be filled and those with zero space will be bypassed. Amounts of accommodation space can vary

with relative sea level rise as well substrate slope, subsidence and uplift (Coe, 2003). During the

deglaciation from 10 ka sea level rose steadily followed by a slight relative (~2 m) sea level fall beginning

ca. 4 ka (Masselink and Hughes, 2003). Relative sea level falls usually correspond to a decrease in

accommodation space below the waterline. Despite earthquake-induced subsidence occurring more

frequently than eruption-related sedimentation, net aggradation has occurred. This suggests that the

volume of sediment supplied by each eruptive episode is greater than the space created by tectonic

subsidence. Although future eruptions could lead to further progradation, it is more likely that longshore

drift will be capable of moving

these sediments eastward since

the coastline is now essentially in

equilibrium (Beanland and

Berryman, 1992).

Geomorphology

Using LiDAR data, Begg and

Mouslopoulou (2009) have

specified five distinct geomorphic

settings in the RP which are

presented in Figure 3; (1) a coastal

strip characterized by a series of

recent linear beach ridges and

swales, (2) a belt characterized by

swamp with finely vein-like

Figure 3 Quaternary geology of the RP, highlighting the five main geomorphic features. Stars denote GPR locations (Begg and Mouslopoulou, 2009).


alluvial channels; (3) a zone of low elevation alluvium with braided channels and with abandoned

meandering channels; (4) an elevated zone of alluvium with braided channels and (5) a belt of stranded

beach ridges, sub-parallel to the coastline, with intervening areas of low relief. Vibracores taken at the

same time of this research yielded a mean grain size of these ridges of 0.63 mm, indicating coarse sand.

Using GPR, Summers (2009) noted two modes of geomorphology in the Bay of Plenty’s 8000 year history.

These were identified as an accretionary barrier system which was dominant until ca 1500-2700BP when

it changed to a continuously prograding barrier. It appears that the coast would return to a state of

equilibrium between major perturbations including eruptive events.

Tectonics

The active tectonics of the TVZ have contributed to altering the stratigraphic record held in the RP’s

barrier dunes. Recent examples of this include the 1987 Edgecumbe earthquake, which resulting in a

rupture of the 7 km Edgecumbe fault and a 2 m downdrop of a portion of RP. This further illustrates the

tectonic subsidence within the graben (Beanland and Berryman, 1992). The dominant structure of RP

trends in a northeast-southwest direction with faults following a similar strike. Based on LiDAR, Begg and

Mouslopoulou (2009) have noted 122 active faults in the RP with a median length of 1.4km and median

displacement of 0.4m. Over 90 of these faults have ruptured within the last 640 years. This project has set

out to determine if the two study areas can attribute their surface topography to a faulted subsurface. All

fault traces are elements of the active Taupo Rift and are assumed to represent the surface expression of

normal faults. Numerous fault traces have been partially modified by subsequent active landscape

processes and geomorphology.

Anthropogenic sediment transport

Human influence has had a major impact on coastal systems throughout New Zealand. Whether in the

form of development or shoreline alteration, anthropogenic impact is an input that often negatively

impacts the shoreline. Sand is a resource in high demand for aggregate and nourishment needs. While sand

was once extracted for commercial purposes from dunes at Matata in the Bay of Plenty, it is no longer

mined here (EBOP, 2007). The Mangawhai-Pakiri embayment in Northland is currently an active center of

sand extraction. NIWA (1998) has determined that this coastline is effectively a closed system with a deficit

in its sediment budget brought on by extraction at rates of 76000 m3/yr. Although NIWA (1998) was

unable to establish direct correlations between sand extraction and significant coastline change, it has

predicted that continued removal will result in a recession of the shoreline. Mean grain size in Pakiri Bay

ranges from 0.23-0.27 mm and is preferable for commercial aggregate use. However, reserves of coarse

sand used for nourishment projects are not as plentiful here. Pakiri sands have also been used for

nourishment projects around Auckland during the 1990s to present. Other areas have been explored for

extraction, including the Kaipara Harbour, as controversy has been raised over the Pakiri site.


GROUND PENETRATING RADAR (GPR)

Ground penetrating radar is a powerful, noninvasive geophysical tool that gives insight into shallow

subsurface sediments and facies. The system operates by beaming short pulse electromagnetic waves into

the subsurface using an antennae and the resulting signal recorded in a receiver, both contained in the

same unit. This is housed in a combined unit which is dragged in a straight transect over the study area at a

steady walking pace. The signals are sent to a computer worn by the user where the data is displayed and

logged. Differing dielectric properties of materials due to grain size or water content alter the speed of this

signal which is received in the antenna (Wyatt and Temples, 1996).

The penetration depth of the signal is controlled by frequency, measured in nanoseconds (Summers,

2009; Wyatt and Temples, 1996). In the proceeding methods, lower frequency runs (150ns) allowed for

greater penetration depth with less resolution. Higher frequency runs (300ns) allowed for higher

resolution at lower depths. The 200MHz antenna used in this research achieved an average penetration

depth of 8-10m at 300ns and about 20m at 150ns. Differing facies or sedimentary structures have different

effects on how the signal is propagated and received. These properties allow for facies to be separated out

during processing and interpretation. For example, finer and well-sorted sediments (i.e. dune sands) will

have a weak, homogenous reflector. Conversely, coarser grain and less well-sorted facies such as fluvial

deposits and beach sands exhibit more clearly defined reflectors (Dougherty, 2009).

MAURI MODEL ASSESSMENT

New Zealand seeks to maintain a balance between economic, environmental, social and cultural well-

being. Decision-making tools in New Zealand have traditionally been influenced by English models which

focus on analyzing costs and benefits. Mauri was chosen as the conceptual backbone to sustainability

decisions within a new holistic decision making framework. Mauri is the pervasive Maori idea of a binding

force that bridges the physical and spiritual world or that which allows the capacity to support life. It is

possessed by all things, living and inanimate. This decision support model, the Mauri Model, uses this life

force as a measure to assess the four dimensions of sustainability. These parallel the current RMA decision

making dimensions. The Mauri model gives equal weighting to the sustainability of the community (social),

family (economic), ecosystem (environment) and hapu, or clan group (cultural). The sustainability

assessment of anthropogenic sediment transport undertaken in this project is made using the mauri

sustainability barometer (see Figure 4) designed by Morgan (2006). This helps to decide whether the issue

is seen as enhancing (+2), maintaining (+1), neutral (0), diminishing (-1) or destroying (-2) the mauri of a

certain dimension. The associated numbers are used to come up with an overall sustainability rating. This

allows indigenous values to be placed right alongside traditional Western systems-thinking in a decision

making framework.


Fig 4: Indicators of mauri are scored in four spatial and temporal categories. Scores are determined using the "Mauri-o-meter" developed by Morgan (2006).


METHODS

Based upon high-resolution LiDAR imagery of the RP, GPR transects were conducted in two locations

within the township of Thornton in the Bay of Plenty during 16-19 February 2010. The data was collected

using a GSSI SIR-2000 system using a 200MHz antenna at 150- and 300 ns frequencies for varying depths

penetration and resolution. LiDAR data collected by Environment Bay of Plenty in 2006 was used to locate

subtle features such as beach ridges, streambeds and fault traces. These sites included a paddock and

roadside on Huna Road and a paddock off of Powdrell Road (Refer to Attachment 1). The features under

investigation appeared as hummocky ridges in the terrain in LiDAR images. They were hypothesized to be

either Holocene dunes of the prograding system or fluvial deposits of historic rivers. Methods of data

collection and analysis will be modeled after those described after Dougherty et al. (2009) and Summers

(2009).

A real-time kinematics global positioning system (RTK) was employed to survey and record the relative

altitude and topography of the GPR transects. Points were recorded along transects at all topographic

features and marker intervals. This data was relayed to a base station located at a local geodetic marker

and then tied into an absolute geospatial datum. This information was used to normalize the GPR transect

data both laterally and vertically. Hand-auger cores are taken to ground-truth the GPR data to better

visualize subsurface facies.

Huna Road

The presence of a shore parallel beach ridge-like feature was obvious among the relatively flat

surrounding farmlands. Beanland and Berryman (1992) and Begg and Mouslopoulou (2009) have

identified this feature as beach and dune sediments deposited between 1.7-2.7 ka BP. GPR transects were

established to determine the composition and near-surface stratigraphic features that were present. A c.

0.5 kilometer long N-S transect was initially scanned to log features of the ridge (see Attachment 2). This

was followed by more parallel transects in an adjacent paddock to the northeast. Six 60 m perpendicular E-

W transects (see Attachment 3) were run across Huna Road in order to identify the presence of faulting

that may be related to the sudden drop in surface elevation on the eastern end of the feature. All transects

were scanned at both 150- and 300ns time windows in order to provide proper depth recordings and

higher resolution scans of the near surface. All marks were recorded using real-time kinematics (RTK) and

two hand-auger cores were taken to ground truth the GPR data.

Powdrell Road

Located off of Powdrell Road in Thornton, a paddock was identified as having a similar beach ridge

feature dated by Begg and Mouslopoulou (2009) to 2.7 ka BP. This was further investigated with the use of

GPR (see Attachment 4). Two 75 m transects were scanned parallel to the features and two 50 m transects

were established at an oblique angle to the feature. All transects were marked at 5 meter intervals and


scanned at both 150- and 300 ns. All marks were recorded using RTK and two hand-auger cores were taken

to ground truth the GPR data collected.

Once out of the field, the data collected was loaded in Radan 6.5 software for distance and surface

normalization using the RTK points collected during field work. The data files were examined and

interpreted by located and differentiating facies and subsurface features based on standards set forth by

Wyatt and Temples (1996) and Dougherty (2009).

Mauri Model Assessment

Three indicators were identified for each of the four dimensions (environmental, economic, cultural

and social). These are chosen to be encompassing and unbiased indicators which would not skew results.

Four separate spatial and temporal analyses were conducted to determine the sustainability of sand

extraction from source to sink in two environments: Pakiri and off the coast of the RP. The mauri of a

hypothetical sink location was also examined immediately after and fifty years after nourishment to

analyze the effects of sand extraction and nourishment.

The environmental dimension focuses on the stability of sediment budget of the source and sink, as

well as the biological resilience of the respective areas. The cultural aspects examined the likelihood of

altering the state of sacred places (waahi tapu) by the removal of ancestral remains (koiwi). Also

considered was marring a sink site with the deposition of human wastes onto sacred places. Social

concerns include values relevant to community resilience and take into account affects that sand extraction

or nourishment would have recreational uses and overall aesthetics of the source or sink. Finally, economic

indicators look at effects on property value, infrastructure protection and costs involved with the physical

transportation of sediment from the source to the sink. All of these indicators were carefully considered

and scored on the -2 to +2 scale. The scores for each of the four analyses were averaged to determine the

overall score for each situation. (The rubric for this analysis can be found in Attachment 5).


RESULTS

After the GPR data was compiled and processed, three transects were chosen from a total of twenty five

that best represented each location. Two transects are presented which display the shallow subsurface

characteristics of Huna Road (Transects A and B) and one for the paddock off of Powdrell Road (Transect

C). Several facies types were encountered during the processing and interpretation of the GPR data.

Dune facies

Dune and upper shoreface facies are present in all of the proceeding GPR data. They appear as medium

strength reflectors in the upper several meters of each transect. This facies exhibits a slight 15°-20°

seaward dipping angle. Moderate reflection of this facies is due to the well-sorted nature and finer grain

size of the sediments present.

Water table

The water table is visible in the Powdrell Road transect as a strong, continuous horizontal reflector.

This reflector occurs within a dune facies at 2.5m depth. This presence of the water table was confirmed in

a hand auger core taken at transect C as all sediments at this depth were saturated. The strong reflectivity

is due to the sudden difference between the dielectric properties dry and water-soaked sediments. Because

the water table is relatively close to the surface, attenuation of the signal does not lead to completely

indecipherable readings below this point (Summers, 2009).

Beach facies

Beach facies were encountered in all transects which were studied. These were marked by strong,

continuous reflectors dipping between 25° to 15° in a seaward direction. The shape of these reflectors seen

in Transect A corresponds to the general form of a prograding dune system as set forth by Dickson (2010).

Fluvial facies

Transects B and C both show shallow, concave channel-like structures present from the surface

continuing down to 3-4m extending over 25m on the surface. Strong reflections in this upper layer, as well

as the general shape, suggest this is likely a paleochannel. The strong reflectors along the base of the bed

may be due to coarse alluvial sediments. In the case of the Huna Road transects, LiDAR imagery shows a

large dried oxbow of the Whakatane River several hundred meters to the north and west of the transect.

Faults

Both Transects B and C show evidence of faults and paleoseismicity. These can usually be noticed in

surface topography, but as the median displacement in the RP is only 0.4m, proper detection is best

determined through GPR analysis. Faults are associated with sudden displacements in GPR records and

strong reflectors and ringing from rubble that may be left along the edge of the scarp (Dougherty, 2009).


Figure 1 (Transect A) GPR transect showing dune feature on Huna Road collected at 150ns frequency.

Photos of this feature can be seen in Attachment 2. Strong reflectors are evident around 8-9m depth. These

reflectors form a contact layer (dotted line) between a less reflective layer in the upper 6-7m. This weakly

reflective facies is likely dunes formed of finer grained, well sorted, wind-blown materials. The strong

reflectors beneath the

contact layer have an

associated sloping of

facies. Moving south-

north (landward-

seaward) the angle of

the dip decreases

from 25° to 15°. This

group of reflectors

indicates coarser

grained beachface

sediments.

Figure 2 (Transect B) GPR data indicates the presence of a fault running parallel to Huna Road

(Attachment 3). This transect was conducted perpendicular to the road in an alongshore direction to the

dune feature. The depth of the upper layer of low-reflection dune facies forms a contact layer between

deeper, more reflective beach facies. As the antenna was pulled over the road surface, the signal was

attenuated. This portion of the transect and stratigraphy cannot be accurately determined. Evidence of a

fault occurs in

strong ringing

reflections 30m

into the transect

and starting at

approximately 6m

depth. Moving to

the west, evidence

of alluvial deposits

exists from 35-60m

at depths of up to

3m.


Figure 3 (Transect C) The transect off of Powdrell Road over the dune/beach ridge feature is shown

above. The true angles of subsurface features such as dunes and beach ridges are understated since this

transect was conducted obliquely. Photos of the study site are included in Attachment 4. The data shows a

strong layer of reflectivity is a relatively horizontal line across the length of the transect. Ground truthing

with a hand auger showed highly saturated sandy sediments at a depth of about 2.5m. A beach facies is

present below 5m depth and dips down at approximately 10° in a landward direction. Evidence of faulting

begins in the data around 27 m, correlating with the presence of a channel-like concave facies that

continues to the end of the transect. This channel is of similar dimensions to the one identified in Transect

B and is infilled with poor-moderately reflective sediment found in low-energy environments. It appears

that the water table conforms to the bottom of this channel as well as maintaining an almost constant offset

from the surface topography. Facies below this paleochannel are strong reflectors and show a similar

signature as the adjacent beach facies.

Mauri Model Results

The results for the Mauri Model were calculated and are included in Attachment 5. Comparing sand

extraction from the current location at Pakiri to a hypothetical source off the coast of the RP showed that

current efforts are scoring -0.67, effectively diminishing the mauri of Pakiri. Sourcing sands from the Bay of

Plenty yielded a score of +0.33, which indicates that mauri would be neutrally to slightly enhanced. A

higher score here is due in part that beach facies have in the Bay of Plenty are coarser grained than sand

extracted from Pakiri (~25 vs. ~60 micron) (NIWA, 1998); coarse grains are preferred for long term

stability of nourishment sites. Effects on the sink (nourishment) locations showed promising

improvements in the long run. Although mauri was neutrally affected (+0.08) immediately after

nourishment, fifty years after nourishment led to a maintaining of mauri with a score of +0.88.


DISCUSSION

GPR analyses of select past dune ridges in the Bay of Plenty have revealed that faulting is contributing

to altering preservation of the prograding barrier system present in the RP. This study has been able to cite

two new additional faults to add to the paleoseismic history of the region. This has opened up the

possibility for relict beach and dune ridges to be potential sediment sources for the Bay of Plenty’s

coastline. This means that occasional tectonic events are able to erode away barriers built some thousands

of years before present and redeposit this sediment to the coast. Such processes can be viewed as a sort of

self-preservation of the coastal system as it may have the ability to use its own abandoned inland beach

ridges and dunes as a source of sediments to nourish the shoreface.

Transects B and C both show similar morphologic processes occurring within them as seen in

subsurface stratigraphy. Both study areas, Huna Road and Powdrell Road, exhibit dune ridges underlain by

beach facies as identified by Begg and Mouslopoulou (2009). Both features also share a sharply truncated

western edge scarp which aligns in a similar direction to previously identified faults. Both of these features

show evidence of paleochannels immediately adjacent to the faulting on their western edges. After the

rupture of the fault and the associated downdrop of the land, it is plausible that the discontinuity in

topography would allow the flow of water along the fault line. This stream or river would then have the

potential to erode away the edge of the dune ridge, thus releasing its sediment back into the coastal system.

Such an addition in the coastal sediment budget would aid in allowing sediment supply to outweigh sea

level rise and support coastline accretion.

Another unknown in this research is to what extent rivers, including those incising fault scarps, have

transported sediment trapped in past dune and beach ridges to the coastline. These landforms, whose

distribution is shown by Begg and Mouslopoulou (2009) in Figure 3, currently only make up a small

fraction of the surface topography of the plain behind the active dune systems. Given that this has been a

prograding barrier system for over 6000+ years; all areas of RP have been covered by beach and dune

ridges at one point in time. Also, given the active tectonics of the area and hundreds of faults present,

sediment transport from backbarrier locations could have been spurred by faulting. Not only would

faulting result in subsidence of the land but also provide a pathway for water to flow from the area’s main

rivers and erode away sediment. How much sediment was moved in the past in not known from this

research. Judging from the size of possible paleochannels identified (~25-30m width); the transport

potential may have been great. Although unknowns exist, GPR transects conducted with the aid of LiDAR

for choosing locations are effective and proven tools for conducting research in this field.

Accommodation space may be the larger factor at play in the prograding saga in the Bay of Plenty.

Progradation has occurred as sediment supply has been sufficient to fill the accommodation space created

by rapidly displacing faults. Beanland and Berryman (1992) have determined that the infilling RP seaward


has not occurred at a constant rate (See Figure 2). A large portion of progradation, on the order of 3 – 5 km,

is likely to have occurred between 6.5 ka and 4.8 ka. After this initial progradation, rates have slowed over

time. This would indicate that through time, accommodation space has decreased to the present day as the

coastline has prograded at slower rates. Recent studies by Environment Bay of Plenty may not be acting on

large-enough of a time scale to detect long-term rates of accretion or erosion on this coastline. Therefore

the state of dynamic equilibrium seen on this coastline may be a function of waning space for sediment to

be deposited. If the increase in accommodation space is less than the increase in sediment supply, the shore

will move basinward and result in progradation.

Currently, the status of the Bay of Plenty coastline between Matata and Thornton is classified by

Environment Bay of Plenty as in a state of dynamic equilibrium. Rates of accretion >1m/yr are reported

around the mouths of the Rangitaiki and Tarawera rivers, which are more significant contributors of sand

to the coastal system than the Whakatane River (Pullar and Selby, 1971). This suggests a neutral or positive

sediment supply available to surrounding beach systems downdrift of the river mouths (EBOP, 2007).

From the data available, it is unclear whether accommodation space is playing a leading role in the

morphodynamic state of the coastline in the Bay of Plenty. It is however plausible to suggest that tectonic

events can hold the key for unlocking previously abandoned dune remnants back into the coastal sediment

budget in the form of alluvial flow. This would also depend on the type of material being eroded and

redeposited and the efficiency of coastal processes at redistributing it. Nevertheless, if accommodation

space is decreasing or approaching zero, any of this sediment moved into the marine environment in the

future will be lost to the Bay of Plenty coastal system in favor of open areas of deposition. A more in-depth

understanding of the role that accommodation space plays in the Bay of Plenty would allow for a better

perspective of how future sedimentation for fluvial sources would alter coastline position and resilience.

As the Bay of Plenty has been resolved to be an open system in a current state of dynamic equilibrium,

it would be worthwhile to explore the possibility of sourcing excess sediments for nourishment projects.

This has been conducted in the form of a Mauri Model assessment. As seen in Attachment 5, sourcing

sediments for nourishment projects from Pakiri is not a sustainable operation in the long term. The Bay of

Plenty affords coarse grains, preferred for nourishment projects. This dynamic coastal system is

replenished from major rivers sourcing sediments from volcanic headlands, unlike Pakiri. Culturally,

important considerations in moving large quantities of sand are present. Operations should not risk

disturbing Maori ancestral remains, koiwi, which is actually a concern at Pakiri (Collins, 2009). Also, source

areas should not be areas where human waste has been deposited, as moving remnants of this with sand to

a new location would effectively degrade the mauri of the sink beach. While the Bay of Plenty only scores

+0.33 on the mauri-o-meter, its relative score to that of Pakiri (-0.67) is what makes it a preferable location.


While Pakiri may be the most economic solution to sand extraction because of its location with respect to

Auckland, the environmental and cultural factors must be equally weighed.

It would be worthwhile to analyze other open coastal compartments with the hope of finding other

viable areas will mauri will not be impacted. It is clear, however from the analysis that beach nourishment,

if done properly and maintained through auxiliary efforts such as dune planting, has the power to enhance

a beach’s mauri. While beaches immediately post-nourishment show little more than a neutral shift in

mauri, on a decadal timescale, mauri can be maintained (score = +0.88) and benefits to the local community

can be seen in the form of economic profits, infrastructure protection and increased community resilience.

Time is required for a beach to reach a state of equilibrium with its new supply of sediment, as well as for

biologic communities to return and flourish on improved beaches.

Future Mauri Model assessments may wish to focus on the sustainability of individual beach

nourishment projects into consideration. This would include detailed analyses of sediment budgets, local

coastal processes and geomorphology. Economic and social factors would also have to examine localized

costs and benefits lost or gained from such projects. It should also be taken into account that this

assessment was done only taking into consideration sand extraction for beneficial nourishment projects. In

reality, the vast majority of mined sand at Pakiri and the Kaipara Harbour are for use as commercial

aggregate for construction and manufacturing.

The RP are an exceptional locality to study a prograding barrier system in a world where many beaches

are undergoing active erosion. Studying the natural and anthropogenic sediment sourcing and transporting

within such a system is crucial to the understanding of these dynamic systems. GPR analysis combined with

LiDAR has led to discovery of faults that have influenced paleochannels. In future work, these techniques

should be carried out to study other beach and dune ridges perched in the Bay of Plenty to determine

erosional tendencies and morphology.


CONCLUSION

Ground penetrating radar was used in concert with LiDAR imagery to locate and investigate areas of

possible ground faulting in the RP. This work led to the discovery of two new faults which would have had

the potential to erode vast quantities of sediment from dune ridges which were – for all intents and

purposes – locked out of the sediment budget for this coastline. This is a case of a coastline’s own

“ancestral” sediments being used to replenish itself through interaction with tectonics. The exact amount of

how much sediment has been transported by such occurrences is unknown. However, it may be a possible

explanation to the lack of high-relief beach ridges and dunes present several kilometers behind the active

dune system on the shoreline. This process is just one of many of the modes by which sediment can be

transferred within backbarrier systems and transported to the coastline to be reworked by waves and

longshore currents. Tectonics, sea level rise, accommodation space and human interference are all equally

important factors in the evolution of the RP.

Additionally, the Mauri Model means of assessing resource use has shed light on the feasibility of sand

extraction operations for beach nourishment projects. Beach nourishment operations have proven

beneficial on a decadal timescale if conducted properly. Conversely, the source of this sediment should also

have a similar effect – or none at all – on the mauri of the source. The interplay between natural and

anthropogenic change in the coastal system sheds light on the importance that humans place on this

interface between the land and the sea for economic, cultural, social and environmental reasons.


REFERENCES

1. Beanland, S. and Berryman, K.R (1992). “Holocene Coastal Evolution in a Continental Rift Setting;

Bay of Plenty, New Zealand,” Quaternary International, Vol. 15/16: 151-158.

2. Begg, J.G. and Mouslopoulou, V. (2009). “Analysis of late Holocene faulting within an active rift using

lidar, Taupo Rift, New Zealand,” Journal of Volcanology and Geothermal Research.

3. Coe, A.L. (2003). “Sediment accommodation space – principles and controls,” Cambridge University

Press. Cambridge, UK.

4. Collins, S. (2009). “Ancestors' Bones Halt Sand Mining,” Beachcare News.

http://beachcare.org/news/entry/17. Accessed 11 March 2010.

5. Dickson, M. (2010). “Coastal Evolution,” GEOG351 Lecture Notes, University of Auckland.

6. Dougherty, A., Gravley, D., and Hikuroa, D. (2009). “Surf and Turf: A study of a Holocene prograding

dune system using ground penetrating radar, vibra-core, and total station surveying techniques,”

Frontiers Abroad Earth Systems Study of Science Curriculum.

7. Environment Bay of Plenty (2007). NERMN Ch. 5: Beach Profile Monitoring: Environmental

Publication.

8. Dougherty, A., Summers, M., Gravley D., and Hikuroa, D. (2009). “Paleoseismic insight gained from

augmenting LiDAR with GPR,” Institute of Earth Sciences and Engineering, University of Auckland.

9. Masselink, G. and Hughes, M.G. (2003). Introduction to Coastal Processes & Geomorphology.

Hodder Arnold: London.

10. Morgan, T.K.K.B. (2006). “Decision-support tools and the indigenous paradigm,” Engineering

Sustainability 159 Issue ES4: 169-177.

11. NIWA (1998). “Mangawhai-Pakiri Sand Study Module 6 Final Report Sand Movement and Storage

and Nearshore Sand Extraction in the Mangawhai-Pakiri Embayment,” Hamilton, New Zealand.

12. Pullar, W.A. and Selby, M.J. (1971) “Coastal Progradation in the RP, New Zealand,” New Zealand

Journal of Science.

13. Summers, M. (2009). “Insights into the Evolution of a Coastal Barrier System Using Ground

Penetrating Radar in the Bay of Plenty, New Zealand,” Stanford University.

14. Wyatt, D.E. and Temples, T.J. (1996). “Ground-penetrating radar detection of small-scale channels,

joints and faults in the unconsolidated sediments of the Atlantic Coastal Plain,” Environmental

Geology Vol. 27: 219-225.


Attachment 1: EBOP (2006) LiDAR imagery of RP w/ site locations


Attachment 2: Huna Road – Transect A Raw data, interpretation and site photo

GPR transect A being conducted on Huna Rd


Attachment 3: Huna Road - Transect B Raw data, interpretation and site photos


Attachment 4: Powdrell Road – Transect C Raw data, interpretation and site photo


Sediment available 2 2 0 1

Flux of sediment into system -2 2 -1 0

Resilience of ecosystem -1 2 -1 1

Waahi tapu (sacred place) -1 0 1 1

Potential presence of koiwi (ancestral remains) 0 0 -1 -1

Potential presence of human waste 0 0 -1 -1

Aesthetics -1 0 1 2

Recreational use -1 0 1 2

Community resilience -1 0 1 2

Property value -1 0 1 2

Infrastructure -1 0 1 2

Sediment transport costs -1 -2 -1 -1

-0.67 0.33 0.08 0.83

Overall mauri levels

Auckland (Sink)

+50 years

Environmental

Ecosystem

Cultural

Hapu

Social

Community

Economic

Family

Indicator of well-beingPakiri Beach

(Source)

Rangitaiki Coast

(Source)

Auckland (Sink)

+0 years

Attachment 4: Powdrell Road - Transect C Raw data, interpretation and site photos

SE

Attachment 5: Mauri Model Assessment

Documents

CO AASSTTALL E SS PP T DD FF INN FTTHHE E T B PP ,, …co aassttall esseeddiimmeenntt pprroovvennaannccee anndd ptrraannsspoortt dduuee ttoo ffaauullttiinngg inn ftthhe e tbaayy oof