Coastal Processes Assessment Report

| Coastal Processes Assessment Report|

Coastal Processes Assessment Report | 3233979 | NZ1-16452818-60 0.60 | 25 October 2019 | 69

Appendix F – Sediment processes reports (Coastal Consultants Ltd)

Rangitoto Channel Sediment Study

Coastal Consultants NZ Ltd 1

RANGITOTO CHANNEL – PROPOSED DEEPENING OF

COMMERCIAL SHIPPING LANE

THE EFFECTS OF PROPOSED DREDGING ON

SEDIMENTARY PROCESSES

Prepared for

Beca Carter Hollings and Ferner Ltd.

April 2001



CONTENTS 1.0 INTRODUCTION ............................................................................................................................................ 3

2.0 PREVIOUS STUDIES ...................................................................................................................................... 3

3.0 METHOD ........................................................................................................................................................ 4

3.1 Determining Sediment Transport Pathways .................................................................................................. 4

3.1.1 Analysis of the proportion of gravel, sand and fines in deposits ............................................................ 4

3.1.2 Carbonate content of deposits ................................................................................................................ 6

3.1.3 Grain size analysis ................................................................................................................................. 6

3.1.4 Grain size trend analysis ........................................................................................................................ 6

3.2 Assessing Sediment Mobility in Channels .................................................................................................... 6

3.3 Finer Resolution Study of Beach - Channel Sedimentary Linkages ............................................................. 6

4.0 RESULTS ......................................................................................................................................................... 7

4.1 Observations of the Sediment Surface .......................................................................................................... 7

4.1.1 Channel bedforms .................................................................................................................................. 7

4.1.2 Spatial separation of sediment types around bedforms .......................................................................... 7

4.1.3 Combined sand and shell where bedforms do not exist ......................................................................... 8

4.2 Spatial Distribution of Mud (silt and clay-size material) .............................................................................. 8

4.3 Spatial Distribution of Gravel in Surficial Deposits ................................................................................... 10

4.3.1 Composition of gravel material ............................................................................................................ 10

4.5 Acid Digestion of Samples ......................................................................................................................... 15

4.6 Nested study of surficial samples of Cheltenham Beach ............................................................................ 15

4.6.1 Percentage of shell and mud ................................................................................................................ 16

4.6.2 Hydraulic grain-size of sand fraction ................................................................................................... 17

4.7 Sediment Trend Analysis ............................................................................................................................ 18

4.7.1 Cross-shore sediment trends ......................................................................................................... 18

4.7.2 Along channel sediment trends ............................................................................................................ 18

4.7.3 Cheltenham beach trend analysis ......................................................................................................... 18

4.7.4 Interpretation of trend analyses ............................................................................................................ 19

5.0 INTERPRETATION OF CONTEMPORARY SEDIMENTARY PROCESSES .......................................... 22

5.1 Major Depositional Zones ........................................................................................................................... 22

5.2 Discrete Shell Deposits ............................................................................................................................... 22

5.3 Mobility of Gravel-Size Shell Material ....................................................................................................... 24

5.4 Transport of Sand-size Material in the Channel .......................................................................................... 25

5.5 Transport Linkage Between Rangitoto Channel and Beaches .................................................................... 26

5. 6 Sediment Mobility ..................................................................................................................................... 29

6.0 EFFECTS OF PROPOSED DREDGING ON SEDIMENTARY PROCESSES ............................................ 29

6.1 Effects of Dredging the Rangitoto Channel on Sedimentary Processes ...................................................... 30

6.1.1 Direct effect of dredging the Rangitoto channel: the post-dredge channel surface. ............................. 30

6.1.2 Effect of dredging Rangitoto channel on sediment transport ............................................................... 31

6.1.3 The fate of fine suspended material .................................................................................................... 33

6.2 Effects of Dredging the Approaches to the Rangitoto Channel on Sedimentary Processes ........................ 33

6.2.1 Direct effect of dredging the approach channel: the post-dredge channel surface. .............................. 33

6.2.2 Effect of dredging on sediment entrainment ........................................................................................ 33

6.2.3 Effect of dredging on sediment transport and fate suspended material ................................................ 34

6.3 Avoidance and Mitigation ........................................................................................................................... 34

6.3.1 Monitoring of bed levels and sediment texture in the dredged channels ............................................. 34

6.3.2 Monitoring sedimentation of fine materials ......................................................................................... 35

6.3.3 Beach monitoring ................................................................................................................................. 35

7.0 REFERENCES ............................................................................................................................................... 36

8.0 APPENDICES ................................................................................................................................................ 37



1.0 INTRODUCTION

This study was commissioned by Beca Carter Hollings and Ferner Ltd. (BCHF) as part of investigations for preparation of an application for Resource Consent to dredge the Rangitoto channel and channel approaches. Ports of Auckland Ltd (POAL) are investigating the option of dredging the Rangitoto channel and the approaches to accommodate larger vessels. Removal of the seabed material, including surface shell, would expose the underlying silty clay and silty sand material. It is proposed that the channel be dredged to a depth of 13 m below Chart Datum, requiring the removal of up to 1.2 x 106 m3 of material. The approaches to the channel (in front of the port) are also to be dredged to a depth of 12.5 m below Chart Datum, requiring removal of up to 0.275 x 106 m3 of material. Therefore, a total of 1.5 x 106 m3 may potentially be removed from the channel and its approaches. This study assesses the effects of proposed dredging on the sedimentary system of the channel and its approaches. In order to assess the effects of dredging this study investigates the sediment transport pathways in the Rangitoto channel and approaches, and the sedimentary linkages to adjacent beaches on the North Shore and the lower harbour. The specific objectives of the study are:

1. To establish the sediment transport pathways in the Rangitoto channel and channel approaches.

2. To determine sediment transport linkages between the channel and beaches of the North Shore and the lower harbour.

3. To determine the mobility of sediments in the Rangitoto channel.

4. To assess the effect of proposed dredging on the sedimentary system.

2.0 PREVIOUS STUDIES Few studies have examined sedimentary processes in the Rangitoto channel. A notable exception is the study of Gregory and Thompson (1973a, b) who reviewed the geology of the Waitemata Harbour and undertook a harbour-wide analysis of the surficial sediments. This study only included a handful of samples from the Rangitoto channel and its approaches. Despite the paucity of samples from this area Gregory and Thompson concluded that:

The channel is carpeted by coarse sand, gravel and shell debris mixed with high proportions of silt and clay.

Shell material originated from within the harbour with addition locally produced shell.

Gravel material is derived from Parnell Grits that outcrop within the harbour.

Beaches contain significant shell content thought to be derived from Pleistocene shell bearing deposits in the nearshore.

Coarse sand and shell is transported seaward through the channel and reworked toward the North Shore during storm events.

The southeastern corner of the channel (between Rangitoto and Orakei) contains fine material and is likely to a depositional zone.

Schofield’s (1970) investigation of the coastal sands of Northland and Auckland also included several samples from beaches of the North Shore. These samples assisted in distinguishing these beach samples as part of the ‘Orewa facies’, thought to be a derivative of



the local Waitemata sandstones. However, this study was at too broad a scale to allow fine-scale interpretation of differences in facies and transport patterns within the Rangitoto channel and its approaches. Investigations of the sediment system of the North Shore and Eastern Beaches have also been scarce. Summary conclusions of the few studies that have been undertaken indicate that:

Alongshore transport is toward the southeast on beaches of the North Shore (Schofield, 1970; Gregory and Thompson, 1973b). No quantitative estimates exist.

Flood currents contribute to transport of sediment around North Head to the ‘Devonport Shoal’ west of Victoria Wharf.

Net alongshore transport is westerly on the Eastern Beaches. Estimated gross rates of alongshore transport range between 3,000 and 21,000 m3.y-1 for westbound transport and between 1,500 and 12,000 m3.y-1 for eastbound sediment transport.

3.0 METHOD

3.1 Determining Sediment Transport Pathways

Interpretation of sediment transport pathways is based on examination of surficial deposits collected from the sea floor. Approximately 120 surficial samples were collected along 11 transects connecting Beaches from Takapuna to Devenport, through the shipping channel toward Rangitoto and the Eastern Bays (Fig. 1). Sample collection was performed by scuba with each sample being taken from the top 10 cm of the sediment surface. Approximately 0.5 kg samples (dry weight) were collected at each site. Scuba retrieval allowed visual observations and descriptions of the sediment surface and evaluation of physical evidence of transport (e.g. bedforms). A further 90 samples were previously retrieved in the approaches to the Rangitoto channel (BCHF, 1997; Fig. 1). Results from this study were added to the collected sample suite to interpret sedimentary process in the approaches to the channel fronting the main port area. In order to determine transport pathways and whether linkages exist between beaches and the channel the following analyses of sediment samples were undertaken:

3.1.1 Analysis of the proportion of gravel, sand and fines in deposits

Determination of the proportion of deposits that are gravel (> 2mm), sand (< 2 mm and > 0.063 mm) and fines (< 0.063 mm) was undertaken using the standard analytical procedures of Folk (1974), which are outlined in Appendix 1. The spatial patterns of proportion of gravel, sand and fines in surface deposits were examined as indicators of source, transport and depositional zones throughout the study area.



Figure 1. Location of surficial sediment samples (black dots) collected for analysis of sedimentation processes, Rangitoto channel, Waitemata Harbour. Triangles denote samples collected for the Lower Waitemata harbour Sediment transport Study (BCHF, 1997). Numbers refer to sediment sample transects. Lines off Cheltenham Beach indicate samples transects along which samples were retrieved at 150m spacing.

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3.1.2 Carbonate content of deposits

Samples from selected transects through the shipping channel were analysed to determine the total carbonate content of deposits. This was performed through acid digestion of carbonate material. Visual inspection of the carbonate fraction was also undertaken to determine whether marked differences in species composition could be identified in the study area. A detailed analysis of shell species composition and abrasion was undertaken by Kingett Mitchell and Associates using some samples provided from this study. 3.1.3 Grain size analysis

Hydraulic grain-size analysis of the sand fraction was undertaken using a Rapid Sediment Analyser (RSA). The procedure for analysis using the RSA is outlined in Appendix 1. Results were mapped and analysed to determine whether broad grain-size trends exist, which are indicative of sediment transport pathways. At this level of analysis it is assumed that as sediment transport occurs from a source area samples should become smaller in size (finer) and better sorted in the transport direction as the competency of the flow mechanism diminishes.

3.1.4 Grain size trend analysis

To test interpretations of sediment transport patterns sediment trend analysis was performed. The trend analysis was based on the approach developed by McLaren (1981) and McLaren and Bowles (1985) (see Appendix 2 for details). The premise of the approach is that the sediment size distribution of deposits should undergo predictable changes along a transport pathway. These changes are reflected in changes in the summary statistics of the grain-size distribution (mean, variance, skeweness). Therefore, identification of these trends should assist in reconstructing sediment transport pathways.

3.2 Assessing Sediment Mobility in Channels

During field collection, bedload traps were deployed on the margins of the channel. Due to relocation issues it was not possible to place a trap in the centre of the channel – although this was attempted for short-period deployments. The aim of these deployments is to provide an indication of the relative mobility of sediments in the channel. At each deployment location two Helley-Smith bedload traps was deployed and oriented in opposite directions along the main axis of the channel.

3.3 Finer Resolution Study of Beach - Channel Sedimentary Linkages

In order to further evaluate whether sedimentary linkages exist between beaches of the East Coast Bays and the Rangitoto channel a finer-scale sampling programme was undertaken off Cheltenham Beach. A total of 40 samples were collected along 4 transects at 150 m spacing from the beach to water depths of approximately 8m (Fig. 1). Samples were analysed to determine the proportion of gravel, sand and fine material and the grain-size distribution of the sand fraction (see above and Appendix 1 for details). Trend analysis was also performed on the samples to interpret sand transport pathways.



4.0 RESULTS

4.1 Observations of the Sediment Surface

Several notable features of the sedimentology of the channel floor were evident from scuba observations, which have direct relevance for proposed dredging works.

4.1.1 Channel bedforms

Dive observations in the channel between sample transects 3 and 6 (Fig. 1) identified large scale bedforms on the channel floor. Bedforms were encountered at 6 locations and varied in height between 0.7 and 1.2 m with wavelengths of 10-30 metres. The bedform crests ran perpendicular to the channel axis in a northeast to southwest direction. The stoss slopes were considered to face northwest on the majority of bedforms examined indicating northwest movement of the bedforms. However, it must be noted there was a high degree of variation in morphology of bedforms with the stoss slope being difficult to determine at a number of sites. The bedforms tend to be primarily composed of shell material (particularly the crests). The presence of silt and algae coating the shell is indicative of relative stability of the shell. Based on these observations the bedforms do not appear to be actively migrating. Following diver inspection the extent and magnitude of the bedforms were confirmed using bathymetric survey techniques aboard the Ports of Auckland survey vessel (Fig. 2). The bedforms were best developed on the northern lateral margins (north and southern edges) of the channel. The bathymetric traces show the varied nature of the bedforms with larger features interspersed with smaller bedforms. This variability is highlighted in summary statistics of dimensions of the 19 bedforms identified in Figure 2. The wavelength of these features ranges from 35 m – 134 m (average of 74.75 and SD of 29 m). The height of the bedforms ranges from 0.36 m to 3.24 m (average of 1.37 m, SD 0.74m). Of note, the maximum depth approaches 15 m below RNZN Chart Datum (CD) with the approximate mean depth for the line presented at 12.8m below CD. Additional bathymetric surveys (60 m apart across the channel) were unable to identify continuation of individual bedforms across the channel indicating the bedforms have limited lateral extent. Bedform wave length and height was found to increase toward the southwest. Bedforms along the central section of the channel were much smaller in height and magnitude and were discontinuous. As ship traffic is concentrated along the central axis it is considered that frequent ship movements may have destroyed the integrity of the bedforms in the centre of the channel.

4.1.2 Spatial separation of sediment types around bedforms

The second notable feature of the channel floor is the spatial desegregation of material comprising the seabed. Bedforms are dominated by shell material (>85% whole tests and shell hash). However, the bedforms are situated on a cohesive fine sand\silt surface, which has little shell material. This sediment surface is exposed in the areas between bedform crests. Therefore, the channel floor is not ubiquitously covered with a shell hash that armours the underlying sediment. Rather there are discrete areas of concentrated shell (crest of bedforms) interspersed with discrete areas of cohesive fine sand and mud (with less shell).



Figure 2. Sample of bathymetric survey from 2 buoy towards 4 buoy (northwest to southeast) on northern edge of Rangitoto channel. Survey date: February 2001. Source Ports of Auckland hydrographic office.

4.1.3 Combined sand and shell where bedforms do not exist

In areas in the channel and channel flanks where bedforms are not present the seabed is composed of mixed sand, shell and silt. The surface does not solely consist of several centimeters of shell hash that armours finer sediments beneath. In general the sediment surface in the channel was covered with a fine silt layer and in many places an algal coating. The presence of this material would indicate that the sediment is not frequently entrained and transported.

4.2 Spatial Distribution of Mud (silt and clay-size material)

The proportion of fine particles in surficial deposits of the Rangitoto channel and channel approaches ranges from a maximum of 81% (at the commercial port) to a minimum of < 1% (on beaches). Within the study area the proportion of fine material shows marked spatial variation with three major zones of high mud content (Fig. 3). At the northern edge of the study area (northern end of channel) surface deposits contain more than 30% mud (maximum of 50% in one sample). The largest area of high mud content (> 30% and peaking at 69%) is found in the southeast corner of the channel (between Rangitoto and Orakei Wharf). Apart from a hiatus off Orakei wharf this zone extends along the southern flank of the channel to the commercial port (including the nearshore of Okahu Bay and the wave break). The third area with significant mud content is found in a thin zone confined to the commercial harbour. While only four samples delimit this zone they have an average mud content of 61% (Fig. 3). Previous sampling (BCHF, 1997) also identified areas of high mud content in the approaches to the channel (port area) with discrete samples consisting of 80% mud (Fig. 3). Sample results are generally consistent with results from most recent samples collected (apart from a small zone in the southern side of North Head). Previously these samples were considered to have an average mud content of 40%. However, this study identified mud content of this zone to be 10% or less. This discrepancy cannot be readily explained. It is possible that sampling techniques could explain the differences or that a change in mud content has occurred due to pre-sampling environmental conditions.

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Figure 3. Spatial distribution of the proportion of silt and clay-size sediment in surficial deposits, Rangitoto channel, Waitemata Harbour.

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However, Gregory and Thompson (1973) also show this area to possess a mud content less than 30%, with the area immediately southwest of North Head having mud content less than 10%. The central section of the channel, its approaches and large areas on the lateral margins of the channel between Rangitoto and the East Coast Bays (ECB) have mud concentrations less than 10%. Closer to the shoreline of the North Shore and Eastern Bays surficial deposits have proportions of mud less than 1%. Of note, is the rapid transition from high to low proportions of mud content (>40% - <5%) south to north across the southern sections of the channel and its approaches. Similar findings were presented by Gregory and Thompson (1973). 4.3 Spatial Distribution of Gravel in Surficial Deposits

The highest concentrations of gravel size material (maximum of 61% and mean of > 40%) occur along the Rangitoto Shipping channel between sediment sampling transects 1 and 6 (Fig. 4). High concentrations (greater than 50%) are also found in the approaches to the channel (between Devonport and Okahu Bay, sample transect 11) and is shown to continue toward the west with inclusion of results of previous sediment study (BCHF, 1997; Fig. 4). A striking feature of distribution of gravel is the significant hiatus in gravel content in surface deposits between sample transects 7, 8 and 9 (between the channel approaches and the Rangitoto channel). In the central sections of the channel, off North Head, gravel content reduces to 0% for individual samples, with a mean value of 20%, 4% and 11 % along sample transects 7, 8 and 9 respectively. The net effect of this zone is to separate the two areas of high gravel content. Of further note, is a marked reduction in proportion of gravel in a zone that runs parallel to the beaches. While beach deposits exhibit gravel content of approximately 20% (East Coast Bays) there is a distinct zone below water depths of approximately 5 m where the gravel fraction is almost absent from deposits (0% - 3% of samples). This zone represents a shore parallel hiatus in presence of gravel.

4.3.1 Composition of gravel material

Without exception all gravel analysed consisted of shell material. In order to determine the proportion of sand-size material that was also composed of carbonate material (broken down shell), acid digestion of shell within the sand-size fraction was undertaken along three profiles. Results provide the opportunity to determine the proportion of carbonate material in the sand and gravel fraction of deposits (Fig. 5).

In general the total carbonate content closely follows the proportion of gravel in deposits (Fig. 5). In channel samples, which have highest shell (gravel) content, the sand-size fraction of deposits also has a significant proportion of carbonate material (Fig. 5). Derived from broken down shell this sand-size carbonate material significantly increases the total carbonate content to a maximum of 90% and 75% in mid-channel deposits along sample transects 3 and 5 respectively (Fig. 5). Results indicate the proportion of total carbonate in the sand and gravel fraction decreases



Figure 4. Spatial distribution of the proportion of gravel-size sediment in surficial deposits, Rangitoto channel, Waitemata Harbour.

markedly to values approaching 0% in the nearshore of the ECB beaches. Therefore, there is a relative dilution of siliclastic material offshore toward the centre of the channel.

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Figure 5. Cross-channel changes in proportion of sand (dashed line) gravel (thin black line) and total carbonate (bold black line) for three sample transects, Rangitoto channel. A) Transect 3, B) transect 5, C) Transect 7. Samples start from the North Shore on the left of the figures.

The non-carbonate fraction of sand-size material is thought to be derived from the sandstones and siltstones of the Waitemata beds, which are prominently exposed in cliffs around the harbour and underlie catchments draining into the harbour (Gregory and Thompson, 1973b). Schofield (1970) identified the hydraulically sorted products of the Waitemata sandstones as distinct facies (‘Orewa Facies’). Gregory and Thompson (1973) note that heavy minerals comprise less than 2% of samples in the

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Rangitoto channel with the light mineral component comprising quartz, feldspar and zircon. However, it is also apparent that minerals from the ‘Hauraki A’ facies are present in beach materials (bi-pyramids of quartz) indicating a mixing of sediment within the system. However, the timeframe for such mixing has not been determined (discussed further in section 5). Along sample transect 7 the trend in total carbonate differs from the main channel. Maximum carbonate content occurs to the north side of the channel and corresponds with the zone of highest gravel shell content.

4.4 Grain Size Characteristics of Sand-Size Fraction of Surficial Deposits

Results from RSA analysis of the sand fraction of deposits (Appendix 3) show that the coarsest samples are found in the mid-channel between the ECB and Rangitoto (0.6 – 1.5 or 0.8 – 0.5 mm, Fig. 6). These samples also have the poorest sorting due to the significant contribution of shell to the sand-size fraction. Such poor sorting can either be interpreted as an indicator of a relatively immature deposit that is close to a sediment source or that a deposit has received additional material from a separate source. Both interpretations may be valid in the Rangitoto channel where shell material is broken down and contributes to the sand fraction with the possibility of producing polymodal grain-size distributions. Areas with smallest mean grain-size are found in the southeast corner of the channel (mean size 3.0 or 0.125 mm) and northeast of the study area. These areas correspond to the zones of greatest mud content (Fig. 3). Of note, a band of fine-grained sediment also parallels the beaches of the ECB (Fig. 6). The mean grain size of these deposits ranges between 2.5 and 2.9 (approx. 0.2 – 0.13 mm) and is significantly finer than adjacent beach samples (0.4 – 0.35 mm). Of further significance, these samples possess the smallest sorting values (< 0.5 of all samples analysed. Size determinations on samples previously collected from the approach channel (BCHF, 1997) have not been added to Figure 6. These size determinations were made on the greater then 3 phi (0.125 mm) size fraction and are therefore not directly comparable with samples analysed in this study. However, the grain-size pattern is consistent with the pattern of gravel distribution with coarsest sands found in the centre of the approach channel and finer deposits located on the flanks. Cross-channel plots of grain size and sorting (Fig. 7, Appendix 4) highlight the above trends in grain size and sorting. Deposits with largest mean grain size and poorest sorting (reflecting the multiple sediment source of deposits) occur in the centre of the channel (on sample transects 1 to 6). Moving shorewards along the profile (Fig. 7) the mean grain size of deposits becomes smaller and samples are better sorted. Finest samples are found in the nearshore of beaches (along sample transects 1 to 6, Appendix 4). Beach samples are coarser and more poorly sorted than the nearshore (shallow water) environments (Appendix 4).



Figure 6. Spatial distribution of hydraulic mean grain-size values of surficial deposits, Rangitoto channel, Waitemata Harbour.

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Figure 7. Changes in cross channel mean grain-size and sorting values of surficial deposits, sample transect 2, Rangitoto channel. Bold line = mean grain-size, light line = sorting value.

4.5 Acid Digestion of Samples

Cross channel differences in mean grain size and sorting for two sample transects (Fig.8) show the significant effect shell has on mean grain-size determinations. Whereas, the mean-grain size of the total sand-size fraction increased toward the channel the non-carbonate fraction shows a decrease in size with distance from the shoreline. Greatest differences are found in the channel where carbonate material increases the grain-size estimate by up to 2 phi units. Removal of the carbonate fraction improves the sorting of samples. Of note, grain-size estimates of samples with low carbonate content have very similar grain-size estimates (Fig. 8). However, these samples are no longer the finest along transect 5 (off the ECB). Along transect 7 the effect of carbonate increasing the mean grain-size is limited to the zone closest to Cheltenham Beach in water depths of 8m.

4.6 Nested study of surficial samples of Cheltenham Beach

As outlined in section 2.0 a finer resolution set of samples were collected in four transects seaward of Cheltenham Beach (Fig. 1). Analysis of these samples was undertaken (Appendix 5) to determine whether interpretations of nearshore trends in sediment from the coarse scale sampling programme were valid.

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Figure 8. Comparison of mean grain size of whole sand fraction (all size and sorting) and shell digested sand fraction (digested size and sorting) across sample transects 5 and 7, Rangitoto channel. Samples on left of diagrams begin at the North Shore. Sample locations shown in Figure 1.

4.6.1 Percentage of shell and mud

As shown along transect 2 (Fig. 9) results from Cheltenham beach show a typical offshore trend in textural characteristics of deposits. Gravel-size (shell) material is a conspicuous component of the beach samples, whereas the mud contribution is negligible (Fig. 9). Gravel content diminishes to negligible proportions between the nearshore (i.e. toward the terminal end of the broad and shallow subtidal flats) and vicinity of the 5 m bathymetric contour. In this zone the proportion of mud peaks at approximately 30% (Fig. 9). Beyond the 5 m depth contour shell content increases toward the channel and comprises more than 50% of surficial samples collected at the end of each sample transect (water depths of approximately 8 m). The mud contribution decreases in this outer zone.

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4.6.2 Hydraulic grain-size of sand fraction

There is a consistent offshore trend among the four sample transects in grain-size characteristics of deposits (Fig. 10). The coarsest samples are found on the beach (Fig. 10) and there is a rapid decrease in mean size in the nearshore. The finest mean grain-sizes are found between sample 3 and 7, which corresponds to the outer edge of the subtidal sand flats and the 5 m bathymetric contour. Mean grain-size values subsequently increase beyond the 5 m depth contour toward the channel.

Figure 9. Offshore changes in presence of gravel (shell) and mud material, transect 2, Cheltenham Beach. Transect extends from the beach (sample 1) to a water depth of 8 m. Samples are approximately 150 m apart.

Figure 10. Offshore changes in hydraulic grain size, all sample transects, Cheltenham Beach. Transects extend from the beach (sample 1) to a water depth of 8 m. Samples are approximately 150 m apart.

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4.7 Sediment Trend Analysis

A number of trend analyses were undertaken to explore possible patterns of sediment movement within the channel.

4.7.1 Cross-shore sediment trends

These experiments tested to see whether statistically significant trends in grain size statistics were evident running across the channel and linking to the beaches either of the North Shore, Rangitoto or Eastern Bays. The results of each run are summarised in Table 1. Along all sample transects in the confined channel (transects 2 – 6) there is a tendency for deposits to become finer and better sorted toward the beaches of the North Shore. This is indicative of a transport potential from the channel towards the beaches (Fig. 11). 4.7.2 Along channel sediment trends

Analysis of along channel deposits (Table 2, Fig. 11) all identified an up-harbour transport trend, as deposits become finer, better sorted and more negatively skewed. Trend analysis of samples previously collected in the approach channel also identified up-harbour transport of sediment in all cases tested (BCHF, 1997). 4.7.3 Cheltenham beach trend analysis

Analysis of samples at Cheltenham beach showed contrasting results between transects. A statistical significant relationship was found in all analyses however, the trend varied from FB- to CB+ and changed direction between transects.

Table 1. Summary results of trend analysis in the across channel direction. Onshore refers to movement of sediment toward beaches of the North Shore.

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1 17 FB- 27/136 2.59 Tendency for offshore deposition of fines

1b 10 FB- 12/45 2.87 As above

2 13 FB- 22/78 4.19 As above

2b 9 FB- 12/36 3.78 Onshore deposition of fine material

3 13 FB- 17/78 2.48 Onshore deposition of fine material

3b 10 FB- 14/45 3.78 Onshore deposition of fine material



5 acid 9 CB+ 10/36 2.77 Erosion of fine material out of deposits closer to shore

5 comp 9 FB- 9/36 2.27 Onshore deposition of fine material

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Onshore deposition of fine material

Fine material winnowed to form a lag

7 9 - - - - no significant trend identified. 7acid 9 - - - - no significant trend identified. 8 9 FB- 11/36 3.28 Deposition of fine material toward

southeast 9 10 CB+ 11/45 2.42 As move northwest fine material is

winnowed to leave a lag 10 11 - - - -

NB: when Z-score > 2.33 trend is statistically significant at the 1% level of uncertainty.



4.7.4 Interpretation of trend analyses

There are a number of assumptions underlying trend analysis that should be re-evaluated in the context of the surficial sediments of the field site. The technique has been shown to operate best where there is a single and discrete source of sediment. The addition of shell material to many of the central channel deposits confounds this assumption. In practical terms the shell material increases the mean size and sorting values of grain size distributions. Shoreward of these deposits the proportion of shell decreases and samples become finer and better sorted.

The analysis was repeated along sample transects 5 and 7 on the acid digested samples (Table 1). Of note, the trend identified in the analysis changed from one of deposition of fine material to a trend in which fines are winnowed out of deposits across the channel. Critical to interpretation of the McLaren trend analysis and application to this study is whether the carbonate and non-carbonate fractions are likely to behave in hydraulically distinct ways and therefore, have different transport characteristics. While the sand fraction is composed of both carbonate and non-carbonate material it is important to note that grain-size estimates are hydraulic determinations, which account for differences in mineral density, shape and size that influence hydraulic behaviour. Consequently, most samples did not exhibit marked bimodal or polymodal size distributions. This indicates that the sand fraction is hydraulically sorted and that sand-size material with similar hydraulic properties (whether carbonate or non-carbonate) will behave uniformly. Therefore, it is concluded that interpretations using the entire sand fraction are valid. The trend analysis technique also delivers best results when the transport system is discrete and linear (i.e. sediment moves from a discrete source along a transport path). However, grain-size results suggest that sediment is transported both toward the shore from the channel flanks and offshore from the beach. Furthermore, there is some doubt as to the contemporary source of sediment to the system. In the closely spaced Cheltenham transects finest samples are found in the nearshore zone and are bounded by coarser and poorer sorted samples on the beach and offshore. Depending on the position of the finer samples along each transect the result of the trend analysis is altered. Therefore, results of the trend analysis are considered misleading with respect to the Cheltenham beach samples and are discounted.

Table 2. Summary results of trend analysis in the along channel direction.

Test samples No. Samples

in line


trend

No Observations/

No. pairs examined


1: North – south central channel

11 FB- 15/55 3.31 Deposition of fine material up harbour

2: Zig zag pattern up central channel


3: T3 – T11 central sample only


4: T2-T7 dual samples central channel





Table 3. Summary results of trend analysis on Cheltenham Beach nested samples.

Test No. Samples

in line


trend

No Observations/

No. pairs examined


1: Transect 1, northern beach

10 FB- 11/45 2.42 Deposition of fine material offshore

2: Transect 2 central beach

10 CB+ 11/45 2.42 Winnowing of fine material in both directions

3: Transect 3 central beach

10 FB- 17/45 5.13 Deposition of fine material shoreward

4: Transect 4, southern beach

9 CB+ 10/36 2.77 Winnowing of fine material offshore




Figure 11. Summary of potential sediment transport pathways identified from Trend Analysis for the whole sand fraction of deposits. (See Tables 1 and 2 for results).



5.0 INTERPRETATION OF CONTEMPORARY SEDIMENTARY PROCESSES

Based on observations of the sediment surface, results of laboratory analysis, spatial patterns of the textural, compositional characteristics of samples, and trends analysis (McLaren, 1981) of grain-size statistics a number of conclusions can be made about the nature of the current sedimentation system of the Rangitoto channel and its approaches.

5.1 Major Depositional Zones

Any fine material transported in the Rangitoto channel is deposited at the either end of the channel between the North Shore and Rangitoto (where grain sizes are lowest and the proportion of fine material is highest, Fig. 12, Fig. 2). This is likely to be driven by the oscillation of flood and ebb flows and deposition of material as the flow leaves the constriction of the channel between the North Shore and Rangitoto and settles from suspension. Similarly, as flow leaves the confined channel between Devenport and Okahu Bay it may be deposited in the southeast of the study area (Fig. 12). In the approaches to the channel highest concentrations of fine material are located along the southern shoreline (near the Port and Okahu Bay) and in isolated patches on the northern shoreline (e.g. Stanley Bay).

5.2 Discrete Shell Deposits

The concentration of shell material in the Rangitoto and approach channels appears to be discrete and separate deposits with regard to contemporary sediment transport dynamics. Based on the concentration of shell material in surficial deposits no obvious sediment transport connection exists between the two main areas of shell. Shell concentrations drop to below 5% in the bend of the channel indicating there is little active transport of shell through this zone. Only one sample had shell content above 20% and this was located immediately offshore of North Head. Although considered unlikely, this narrow zone would appear to be the only possible connection between the two shelly deposits. If this zone were to act as a sediment transport pathway it would need to transfer vast quantities of shell material for this to be an effective conduit feeding the central channel area. This finding casts doubt on previous assertions that coarse material is transported from the upper harbour to the channel (Gregory and Thompson, 1973b). To partly test whether such a transport mechanism exists around North Head, two Helley-Smith sediment traps were deployed on March 6th. Retrieval of the sediment traps after one complete tidal cycle found no sediment had been captured. While this was an extremely short field experiment, results indicate that active shell transport does not occur under the neap tide conditions. This does not preclude transport of material under higher energy conditions, particularly during storms. While the two areas of concentrated shell suggest the deposits are part of two distinct sediment circulation systems the major shell constituents of the two deposits appear similar. Detailed species examination of the shell material contained within the two areas is being undertaken by Kingett Mitchell and Associates. If similar in species composition these deposits are likely to have the same source and may, therefore, represent relict deposits in the channel floor.



Figure 12. Inferred transport pathways and depositional zones of fine material (< 0.062mm) in Rangitoto channel. In order to further evaluate whether a connection exists between the two shell deposits, samples of shell hash (broken down shell tests larger than 2mm) were submitted for radiometric dating (Table 4). The samples submitted were retrieved from the middle of the channel on sample transect 5, from the shell rich deposit in the approach channel and Cheltenham Beach. To date only one sample has been returned, which has yielded a date of 1430 +/- 60 yrs BP. As the age was determined on the shell hash it is likely to represent a broad cross-section of ages (both older and younger shell). However, the age

3

4

5

6

8

Transport path of fines on flood & ebb tide

Depositional zones



does indicate that shell in the Rangitoto channel is not dominated by modern material and that modern shell delivery to the channel is not significant. This finding raises the issue as to the source of the shell material. Gregory and Thompson (1973) note that the shell is likely to have come from the upper harbour. However, as noted above a transport mechanism between the approach channel and Rangitoto channel has not been identified. Gregory and Thompson (1973) also not that shell is likely to have been derived from weathered outcrops of Parnell Grits that occur as submarine outcrops in the channel floor. Such a source is consistent with the age determination of the shell hash and supports the conclusion that the shell material is a relict and lag deposits within the channel floor. The shell material is also likely to have undergone minor mixing with shell material from the channel flanks and nearby rock outcrops of Rangitoto (explaining the presence of mussel shells in some samples).

Table 4. Radiocarbon dating results of shell hash deposits.

Sample No Location Radiocarbon Age (yrs)

P5-7 Mid channel off Narrowneck 1 430 60 BP

P6-14 Cheltenham Beach Pending

L18990-A3 Approach Channel Pending

Radiocarbon dating undertaken at the University of Waikato Radiocarbon Dating Laboratory.

5.3 Mobility of Gravel-Size Shell Material

As noted earlier sediment trap deployments failed to trap sediment of any size. Coupled with observations of the sediment surface where shell was dominant (covered in algae and silt) it is concluded that the gravel-size shell material is not actively transported in the channel and approaches. Rather this material acts as a lag, which can afford finer material trapped within the shell matrix protection from strong tidal currents (where shell occurs in high concentrations). Field observations also show that the shell material occurs discontinuously on the channel floor and is concentrated into bedform features. Therefore, the armouring effect is confined to the areas where shell bedforms exist. Bedforms can be used as indicators of transport direction. Orientation of the stoss slopes of bedforms in the channel suggests the features migrate up the channel in a northwest direction. This interpretation conforms to the conclusion of Gregory and Thompson (1973) that coarse material in the channel is transported seaward and is consistent with the ebb dominated tidal regime. However, there is great irregularity to the bedforms and their formative mechanism is unknown. Possibilities include: bow-wave pressure of passing vessels (this would partly explain there prominence toward the edges of the channel); hydraulic response to tidal regime of the channel (although bedforms appear too irregular for such a response); extreme events (e.g. tsunami); anthropogenic activities such as previous bucket dredging (although this does seem to be able to account for the consistent northwest orientation of bedform asymmetry). Although the origin of the bedforms is unknown it is clear that they are not actively migrating along the channel in response to the contemporary hydraulic regime. The shell surface of these features is silted and in many locations the shell has an algal coating. At



a number of locations the bedforms appear to have undergone erosion of the stoss slope, where there are near vertical faces of shell. Such erosion is likely to have been episodic and potentially caused by scour of ship propellers. These erosional scars also have an algal cover indicative of relative stability in the recent past. If these features do migrate in the contemporary system migration would only occur under extreme energy conditions. It is concluded that the bedforms are immobile and may only undergo movement under extreme energy events. A video of the channel floor (obtained by BCHF) provides evidence of entrainment of shell material caused by vessels. It is likely that the episodic passage of vessels is one of the major entrainment mechanisms of coarse material within the channel. Once entrained the net transport direction will be determined by the phase of the tide and settling properties of entrained material. In summary, gravel-size shell material behaves as a lag deposit within the channel. Tentative evidence (bedforms) suggests a northward movement of material over the long-term (inter-decadal). In the short-term shell material may be periodically entrained by the passage of vessels or storms generated from the north to northeast sectors. Such storm events generate significant wave energy in the channel, which is likely to have the capacity to entrain shell material and disperse it shoreward across the broad channel flanks toward the 5 m bathymetric contour. Evidence suggests there is limited transfer of shell material between the beach and area beyond the 5 m bathymetric contour as shell material is absent at this water depth off the North Shore and at similar water depths off the Eastern Bays (Figs. 5, 6, 9, 10, 13). 5.4 Transport of Sand-size Material in the Channel

Sand comprises less than 50% of most channel deposits and is found either bound within rich shell mixtures (on elevated sections of bedforms) or directly on the sediment surface among shell and silt in troughs between bedforms. As previously reported no sand was captured in sediment trap deployments indicating that sand transport is not an active process under ‘mean’ energy conditions. This is curious given the strong tidal currents that exist in the channel (BCHF, 2001). However, it is likely that sand in the system has achieved an equilibrium distribution with regard to the prevailing hydrodynamics and configuration of the sediment surface. Diver observations indicate that the maximum currents occur at the crest of bedforms. Therefore, it is expected that these zones would experience strongest shear stress and the potential for entrainment of sand-size material by tidal fluxes is greatest. These locations correspond to the shell rich deposits where sand has been winnowed away. Sand-size material is only found within the shell matrix which provides an armouring effect to sand-size sediment. In contrast, currents in the troughs between bedforms are lower in magnitude (diver observation), thought to be caused by flow separation from bed immediately downstream of the bedform crest. In these zones shear stress at the boundary is likely to be lower with less likelihood of sand-size material being entrained. Consequently, the trough areas between bedforms have a higher proportion of sand-size material in the surface sediment and shell is less effective at armouring the fine material. In summary, sand-size material within the channel is considered to have achieved a relatively stable equilibrium with regard entrainment and deposition under ‘mean’ tidal energy conditions resulting in low levels of sand transport within the channel. However, there is no doubt that sand introduced to the system or entrained during high energy



events (including the passage of ships) will be transported along the channel axis by the strong tidal currents. The spatial pattern of mean grain-sizes indicates sand in transport is most likely to be deposited in the southeast corner or north of the study area at the terminal ends of the Rangitoto channel. Evidence also suggests that fine sand-size material is deposited toward the southern shoreline of the approach channel. Trend analysis (Fig. 11) indicates that the net movement of sand-size material is up the harbour. Previous trend analysis undertaken on the sample suite in the approach channel (BCHF, 1997) also identified an up harbour transport pathway for sand-size material. This would seem to contradict findings that the channel and approach channel is the marginally ebb-dominated. However, consideration of source areas for sand transport supports the up-harbour transport direction. On the flood tide a much larger source area of sand-size material exits (flanks of the channel between the North Shore and Rangitoto, and inner Hauraki Gulf). In contrast, on the ebb-tide large supplies of sand-size material are not available in the upper harbour and flow is concentrated for greater periods of time in the channel. Furthermore, wave energy as a major entraining mechanism (particularly during northerly storms) is more effective in the outer areas of the harbour and it is likely that greater amounts of material are introduced to the transport system, which can be carried, into the harbour. Independent investigations have also identified up-harbour transport of sediment with accretion of the ‘Devonport Shoal’ (immediately offshore and west of Victoria wharf, Gysberts, 1992). Anecdotal evidence also suggests that this shoal undergoes significant changes in volume and it is probable that sand material may be transferred out of the harbour under extreme ebb discharges. However, in the long-term it is considered that this area is likely to be accreting. Such up-harbour transport of material on channel flanks and marginal shoals is a common mechanism contributing to net infill in ebb-dominated estuaries (Perillo, 1997). Net up-harbour transport is also consistent with broad-scale assertions that drowned river vallies (such as the Waitemata) are in net receipt of sediment and infilling over geological timescales (Gregory and Thompson, 1973b; Perillo, 1996). 5.5 Transport Linkage Between Rangitoto channel and Beaches

The fining and improved sorting trend of sand-size material and diminishing proportion of gravel-size shell in deposits between the channel and nearshore (5 m bathymetric contour) indicate that this material is able to be entrained and dispersed shoreward across this area. Episodic north-northeasterly swell conditions are the most likely driving mechanism for this dispersal. However, the smallest grain sizes on each transect were found in water depths of approximately 5 m while beach samples were found to have significantly larger grain sizes. Using the assumption that sediment is transported along gradients of coarse to fine sediment the results indicate that while material might be transported out of the channel toward the shoreline it cannot be transported across the zone of finest grain size (5m water depth). Similarly, material may be transported offshore of beaches but the bulk of the size distribution cannot be transported past the zone of smallest grain-sizes (Fig. 13). Coincidentally, Schofield (1970) also identified the 5 m depth contour as the wave base for contemporary sedimentation on beaches of the North Shore.



Similar changes in offshore sediment texture and composition are found off the Eastern Bays. Indeed these beaches are fronted by the extensive area of fine material in the southeast of the study area (Fig. 3) and active exchange of sediment between the coarser beaches and channel is unlikely. This study concludes that active sediment exchange does not take place between the beaches of the North Shore, Eastern Bays and the Rangitoto channel (Fig. 13). Analysis of the shell content of deposits supports this interpretation. The proportion of carbonate material in deposits drops from more than 80% maximum in the central channel to below 5% (and 0% in places) in water depths of 5-8 m and increases again on the beaches. Thus, shell material is unlikely to be transported across this zone which is depauperate in shell and is indicative of low mixing between the beach and channel. These conclusions are also supported by results of the detailed study of sediments from Cheltenham Beach (section 3.5). This does not preclude the possibility of minor exchange of sediment in the process of beach erosion and recovery. The above interpretation is based on the assumption that the sand-size fraction behaves as a homogenous deposit. As noted earlier, the sand-size fraction of deposits consists of varying proportions of carbonate material, which influences the mean grain-size of deposits (Fig. 8). Trend analysis of the acid digested samples indicates that grain size decreases from the beaches of the ECB to Rangitoto thus inferring a direct transport pathway from the beaches to the channel. Such results contradict the earlier interpretation using the entire sand fraction. Critical to interpretation of transport pathways and links to the beaches is whether the different sub-populations (carbonate and non carbonate) of the sand-size fraction behave differently in the transport system. As noted earlier hydraulic grain-size determinations were undertaken. These determinations account for the heterogeneous properties of particles within deposits (Kench and McLean, 1996, 1997) and indicate that the sand-size carbonate material is hydraulically sorted in the transport system as opposed to locally produced and deposited in situ. This is confirmed through examination of the grain size distributions of the sand-size fraction which do not exhibit a marked bimodal or polymodal character (diagnostic of local addition of material). Therefore, it is concluded that it is appropriate to base interpretations of transport patterns on the entire sand fraction and the interpretations depicted in Figure 13 are valid. To further support this conclusion, the fining trend (of the non carbonate fraction) toward the channel may be as much a reflection of the preferential removal of particles of larger sand-size from the gravel rich shell deposits by strong shore parallel currents (in the channel), than a real offshore transport of fine material. While fine particles may be afforded protection to entrainment by shell material coarser sand-size material may be unable to take advantage of the same level of protection. Little sedimentary connection is considered to exist between the beaches of the lower harbour and the approach channel. Strong along channel tidal currents and the sheltered



Figure 13. Inferred patterns of sand and gravel sediment transport based on results of textural and trend analysis. nature of lower harbour implies that a mechanism to disperse channel material north and south across the channel margins does not exist. While major sedimentary exchange between the beaches and channel is not considered likely this does not preclude the possibility that minor exchange and mixing occurs during cycles of beach erosion and recovery. It has been noted that beach sediments of the North Shore contain bi-pyramids of quartz, which do not originate from the Waitemata sand and siltstones (Orewa facies – Schofield, 1970). Rather these minerals are characteristic of the Hauraki-A facies introduced to the Hauraki gulf by the Waikato river during the last interglacial period. This material is likely to have been transported shoreward during the last marine transgression (culminating 3 – 6000 years ago, Searle, 1956). The presence of these grains is therefore, likely to result from large scale

Finest sizes0% shell

Sand transport

Gravel lag & transport



sediment transport associated with gross sea level change rather than dispersal under the contemporary sediment transport system. Subsequently, the eroding cliffs of the Waitemata group together with shell material have been added to the beach systems and are likely to be the most significant contemporary sediment sources. Therefore, the beach sediments are considered to be a composite of material laid down during the Holocene sea level transgression and contemporary addition of sediment from the Waitemata group and shell.

5. 6 Sediment Mobility

Sediment trap deployments failed to detect sediment transport during the deployment periods. This indicates that transport of sand and gravel-size material in the main channel is not active during mean energy conditions. This conclusion is supported by visual observations of the sediment surface, which found shell material to be covered with a film of silt and in many places to have an algal coating. Such characteristics are indicative of sediment stability as opposed to frequent transport in which the shell material would be expected to have a cleaner appearance (no deposition or algal growth). Within the channel it is probable that ships provide the most intense entrainment force suspending bed material into the water column. During the settling process sediment will be transported by swift tidal currents along the channel axis. This implies that sediment transport in the channel is episodic and controlled by the frequency of ship traffic. Visual examination of the shell material from the channel shows that the bulk of the shell is a heavily fragmented shell hash. This material has obviously undergone physical breakdown and the episodic entrainment by ships is likely to provide a mechanism for this physical breakdown of shell. It is concluded that rates of sand and gravel transport in the study area are not large. In general, transport of this material is considered to be activated episodically by higher energy events and the passage of vessels (having a localised effect). Shell material in the channels is considered to be a lag deposit.

6.0 EFFECTS OF PROPOSED DREDGING ON SEDIMENTARY

PROCESSES

The assessment of effects of dredging on sedimentary processes is presented in two parts; those effects associated with dredging of the Rangitoto channel and those associated with dredging of the approaches to the channel. It is important to note that hydrodynamic modelling indicates there will be negligible change in channel hydrodynamics following proposed dredge activities (BCHF, 2001).



6.1 Effects of Dredging the Rangitoto Channel on Sedimentary Processes

6.1.1 Direct effect of dredging the Rangitoto channel: the post-dredge channel

surface.

It is proposed that up to 1.2 x 106 m3 of material is dredged from the Rangitoto channel. However, it is important to note that dredging will not target all areas of the channel. Dredge plans indicate that dredging will be concentrated north of ‘A Buoy’ toward the northern end of the study area and at the southern end of the channel between sample transects 6 and 8. Between these areas proposed dredging will be intermittent indicating that for significant sections (50%) of the confined channel (between sample transects 3 and 6) the sediment surface will not be directly disturbed. It is along this section that greatest concentrations of shell were identified together with bedforms. Therefore, the post-dredge channel surface will still contain a significant amount of shell in the mid-sections of the channel and the integrity of the bedform structure will not be entirely destroyed. In contrast, the terminal ends of the channel will be completely dredged to 13 m below CD, which is likely to remove shell from the sediment surface. This represents a significant reduction in shell material at the northern end of the channel where shell material comprises more then 50% of surface samples. However, at the southeastern end of the channel shell content is presently less than 20% so that dredging will not make such a significant change to the texture of surface deposits. Of note, the textural characteristics of deposits on the beaches and broad zone between the beaches and Rangitoto channel (1.6 – 2.0 km wide) will not be directly affected by dredging. This is illustrated in a projected post-dredge sample transect between the ECB and Rangitoto showing the mean grain size of surface deposits (Fig. 14). As shell will be removed from the surface layer dredged areas will be left with decreased mean grain size.

Figure 14. Comparison of mean grain size of whole sand fraction along transect 5 against a post-dredge grain size pattern (composite of whole sand fraction size and acid digested sizes in channel). Samples start at North Shore (left of diagram). Sample locations shown on Fig. 1.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12345678910

Sample number

Ph

i un

its

Current size

Post-dredge size

Post-dredge sorting

Current sorting



6.1.2 Effect of dredging Rangitoto channel on sediment transport

a) Effect of loss of shell on entrainment of channel material

At the terminal ends of the channel where shell material is removed from the sediment surface fine sand (Fig. 15) and silts will be exposed to strong tidal currents. However, this is unlikely to promote enhanced entrainment and transport of fine and sand-size material. Contemporary analogues to this shell depleted post-dredge surface can be found in the channel. Presently there are large areas of the channel floor that are not covered with a shell hash and have exposed sand/silt sediment layers. Observations indicate these areas are not subject to active entrainment and erosion. The relative stability of these zones is attributed to the cohesive nature of the silt and sand deposit that effectively elevates the threshold condition for entrainment. Flume tests undertaken on this material support this assertion (Auckland University, 2001). Furthermore, while shell material does provide an armouring function in parts of the channel (crests of bedforms), where bedforms do not exist (northern and southern ends of the channel) fine sand is mixed with shell and silt on the sediment surface and is not afforded a measure of protection from shell. At these locations no sand transport was observed or measured.

Immediately following channel dredging it is expected that some reorganisation of sand-size material will take place in order that the surface sediment achieve an equilibrium distribution with respect to hydrodynamic conditions. As outlined in section 5.4 the distribution of shell and sand in the channel is likely to reflect spatial variations in bottom shear stress. Immediately following dredging this pattern of shear stress will change producing a consequent localised movement of bottom sediments. It is not envisaged that such changes will promote significant and large-scale destabilisation of the channel sediment. Rather, such adjustments will be localised involve reorganisation of sediment within the channel and take place incrementally associated with the dredging process that will extend for more than a year.

The role of vessels in entraining channel sediments is not considered to change significantly. Larger vessels anticipated to transit the channel will presumably exert maximum stress on the sediment surface and promote sediment entrainment. However, this will be partially offset by the reduced effect of smaller vessels on the sediment surface due to channel deepening. The net effect on frequency of bed suspension due to vessels will be dependent on changes in the frequency of ship traffic. In conclusion direct dredging of the channel is not expected to promote significant and adverse effects on entrainment and transport of the post-dredge sediment surface.

b) Effect of dredging on sediment linkages to beaches

Sediment transport linkages between the channel and shallow water depths toward the shoreline will not be significantly effected following channel dredging. Interpretation of the current sediment transport system indicates that gravel (shell) and sand-size material may be dispersed from the channel and toward the beaches of the North Shore and Eastern Bays under higher energy north-northeasterly conditions. The hydraulic mechanisms driving this transport will remain unchanged. Due to the cohesive nature of the exposed fine sands and silts it is not anticipated that increased fine sand will be transported toward the shoreline. In the unlikely scenario that greater quantities of fine sand are transported toward the shoreline, this sediment would not



be transported onto the beach. The fine sand-size of material at water depths of 5 m imply that energy levels are low along this zone precluding the transfer of a significant amount of sediment shoreward past this point and onto the beaches. Removal of shell material from parts of the channel will reduce the available source of shell material to the area between the channel and shoreline (at water depths below 5m). The net effect of this reduced sediment supply is considered to be small as sufficient shell material exists outside the confines of the proposed dredge area (see Fig. 4). Furthermore, a significant amount of shell material will remain in the central sections of the channel and be available for dispersal toward the shoreline. Of further significance the relative magnitude of transport from the channel toward the shoreline is low and most likely driven by infrequent and episodic storm events generated from the northerly sectors. This will further offset any effect of reduced shell supply from the channel. The beaches do not receive shell material from the channel (section 5.5) and therefore, any reduction in shell material to the offshore system will not effect the beach system. The current beach system is also buffered from the channel by a relatively broad (1.0 – 2.0 km) zone between the channel margins and North Shore beaches. While the effect of shell reduction on this zone is considered minimal any effects that do occur are unlikely to impact on the beaches. As outlined in section 5.5 the zone of active beach sediment exchange is considered to be from the beach to water depths of 5 m. Exchange of sediment from this zone to the offshore system (below 5 m) is minimal. Consequently, dredging of shell material in the channel is also not considered likely to have a ‘draw-down’ effect on beaches. Draw-down refers to a process whereby dredging produces a sink of sediment that ‘draws’ sediment from the surrounding environment. Where this sink is within the active beach profile depth the draw-down effect can potentially remove material form beaches enhancing beach erosion. Within the study area sediment is not actively exchanged between the beaches and channel (as most the grain size distribution of beach sediments is unable to be transported beyond the 5 m contour) and consequently draw-down is not likely to be a significant cause of erosion on the beaches. A similar conclusion was made in an independent study of the Eastern bays based on the low energy wave climate of the study area (Tonkin & Taylor Ltd, 1994).

c) Likelihood of increased sedimentation in the channel

The Rangitoto channel is dominated by strong tidal currents. The Channel effectively acts as a conduit for sediment transport (fine material) with depositional zones existing at the terminal ends of the channel. As the dredging operation will not significantly effect the hydrodynamics of the channel it is not likely to experience rapid sedimentation following dredge operations. This conclusion is supported by the fact that in the past (decades) the current channel has not required maintenance dredging for the purposes of navigation. Therefore, the history of channel operations indicates it is not in net receipt of sediment in sufficient quantities to pose a dredge maintenance issue. Furthermore, previous dredge operations in the channel have not promoted increased deposition (as recorded in repeat bathymetric surveys by POAL).



6.1.3 The fate of fine suspended material

Where fine material is introduced to the water column, through the dredge process, the passage of vessels or episodic increased entrainment from fine sand/silt areas, it is likely to be deposited at the terminal ends of the channel (toward the northeast or southeast of the channel - the zones currently with greatest proportion of silt\clay material, Fig. 12). Consequently, small increases in the rate of deposition of this fine material can be expected in these depositional zones. Maximum elevated rates of deposition are expected to occur during and immediately preceding channel dredging and are expected to decrease toward current rates of deposition following channel dredging. Increased deposition is not expected to cause any significant adverse effects in these depositional areas. Beaches of the North Shore and Eastern Bays will not experience increased deposition of mud material. The beaches exhibit less than 1% mud-size material indicating this size-range of material is unable to be deposited on beaches (due to the action of waves on the beaches). Furthermore, there is no significant transport linkage between the channel and beaches. Evidence suggests that fine material is transported offshore of beaches to water depths around 5 m. Fine material transported shoreward from the channel will also be trapped in this nearshore zone. It must also be remembered that expanses of this cohesive sand/silt material are currently exposed in the channel and are available to the present transport system. There is no evidence to indicate material suspended from these areas has been transported toward the beaches, rather it is likely to have been transported by strong tidal currents and deposited at the terminal ends of the channel.

6.2 Effects of Dredging the Approaches to the Rangitoto Channel on Sedimentary Processes 6.2.1 Direct effect of dredging the approach channel: the post-dredge channel surface.

It is proposed that up to 0.275 x 106 m3 of material is dredged from the approach channel to the Rangitoto channel resulting in a lowering of the present approach channel by approximately 1 metre. While the proposed dredge plans have not been viewed by the consultant it is believed this material will remove shell material from the central area of the channel between Devonport and commercial harbour.

6.2.2 Effect of dredging on sediment entrainment

Hydrodynamic modelling of the channel indicates that dredging will not cause significant change in tidal fluxes in the approach channel. Consequently the range of stresses exerted at the sediment boundary will be the same as present. In contrast, the composition of channel sediments is likely to change particularly in the centre of the channel where the shell lag material is removed to expose finer sand/silt deposits. However, the cohesive properties afforded these deposits by the presence of silt particles means they are unlikely to undergo enhanced erosion leading to further scour of the channel floor. This conclusion is supported by the fact that significant areas of the channel already have shell content less than 30% and in these areas the shell does not armour the finer material and there is no evidence of channel scour.



6.2.3 Effect of dredging on sediment transport and fate suspended material Fine material entrained and or transported into the approach channel on the flood and ebb-tide will not settle in the centre of the channel due to the swift tidal currents on both the flood and ebb tides. Rather it will be deposited toward the southeastern zone of the channel, along the southern border of the channel or within Stanley Bay (Fig. 12). These areas have been identified as depositional zones under the current transport system and there is no reason to expect that these zones will cease to trap material following dredging. Consequently, the approach channel is not expected to experience increased sedimentation. Fine material will also not be deposited on the beaches of the eastern bays or lower harbour. The energetics of these systems precludes deposition of mud, which is generally deposited beyond the 5 m bathymetric contour. As shell material acts as a lag deposit in the approach channel and the hydrodynamics are dominated by strong along channel fluxes little shell is dispersed toward the flanks of the channel. Unlike the Rangitoto channel, no transverse channel hydrodynamic mechanism is likely to overcome the strong tidal currents. Therefore, removal of shell material will have no impact on the channel flanks and beaches. Fine sand transported into the harbour will continue to accumulate on the Devonport Shoal. As increased entrainment of sediment is not envisaged in the system the shoal is unlikely to experience a significant increase in sedimentation. While minor recirculation of this material may occur the shoal is considered to be a medium term depositional zone.

6.3 Avoidance and Mitigation

As outlined above no significant adverse effects of dredging on sedimentary processes in the Rangitoto channel and its approaches are envisaged. It is expected that small increase in deposition of fine material will occur but this deposition will occur in areas already subject to deposition of fine material (Fig. 12) and is not considered to pose a nuisance to commercial or other users of the harbour. Importantly, in respect to the sedimentary processes, channel dredging is not expected to produce significant adverse effects on the beach systems of the North Shore and Eastern Bays as expressed either as erosion or deposition of mud. While no significant adverse effects are envisaged it is recommended that monitoring of the pre and post-dredge system is undertaken in order to verify the findings of this report and ensure any unforeseen effects are detected at the earliest possible moment. A number of monitoring programmes are recommended.

6.3.1 Monitoring of bed levels and sediment texture in the dredged channels

Insertion of a select number of erosion pins along the channel will allow changes in the bed level to be determined. Monitored at 3 month intervals immediately following dredge operations, the sites can be used to assess whether increased scour or deposition occurs in the channel. It is envisaged that the frequency of monitoring would increase (6 monthly to annually) over the three years following dredging.



Rather than have a network of erosion pins along the entire channel a number of stations should be set up with a grid of pins (e.g. over 30 x 30 m area). This would allow bed level changes over a larger area and using replicate measurements (e.g. > 100 pins). Associated with monitoring stations long-term bulk bedload traps can be inserted into the sediment surface in order to determine rates of sediment transport. Repeat sampling and analysis of the texture and composition of surface samples along selected cross-channel transects would also allow changes in sediment characteristics to be evaluated and determine whether shell material is being replenished following dredging.

6.3.2 Monitoring sedimentation of fine materials

A network of erosion pins inserted at selected locations in identified depositional zones (Fig. 12) will also allow rates of deposition to be quantified. Results can be used to trigger management options in the unlikely event of rapid rates of sedimentation being detected.

6.3.3 Beach monitoring

While it is the conclusion of this report that there is no active sediment exchange between the beaches and the channel and that beaches will be unaffected by the proposed dredging scheme, it is recommended that the beach systems of the North Shore undergo monitoring. Such monitoring can be used to generate factual information that dredge operations have not affected the channel and should be undertaken before dredging and for a minimum of 5 years following channel dredging. Pre-dredge monitoring will allow the current variability in beach systems to be identified. In the unlikely event that a impact is detectable on the beaches, monitoring will provide a quantitative basis on which to develop appropriate management strategies. The opportunity exists to build on current beach monitoring in the study area. However, it is suggested that planimetric monitoring using DGPS techniques (on all beaches) coupled with 2-D profiling on selected beaches be undertaken. Such monitoring would take no more than 2 days per survey and should be repeated quarterly.



7.0 REFERENCES

Beca Carter Hollings & Ferner Ltd. (BCHF) 1997 Lower Waitemata Harbour Sediment Study. Report to Ports of Auckland.

Beca Carter Hollings & Ferner Ltd. (BCHF) 2001 Draft Rangitoto Channel Deepening Study.

Folk R.L. 1974 Petrology of Sedimentary Rocks. Hemphill Publishing Co. Austen, Texas.

Gregory M.R. and Thompson S.A. 1973a Waitemata Harbour Study: Geology. Report to Auckland Harbour Board and Auckland Regional Authority. Unpublished report of the Department of Geology, University of Auckland.

Gregory M.R. and Thompson S.A. 1973b Recent Sediments of Waitemata Harbour. Report to Auckland Harbour Board and Auckland Regional Authority. Unpublished report of the Department of Geology, University of Auckland.

Gysberts Burn Ltd. 1992 Windsor Reserve Conservation and Protection Works; Environmental Impact Study. Prepared for North Shore City Council.

Kench P.S. and McLean R.F. 1996 Hydraulic characteristics of heterogeneous bioclastic deposits: new possibilities for interpreting environmental processes. Sedimentology, 43:531-540.

Kench P.S. and McLean R.F. 1997 A comparison of sieve and sedimentation techniques for the analysis of heterogeneous bioclastic sediment mixtures: implications for environmental interpretation. Sedimentary Geology 109:111-119.

McLaren P. 1981 An interpretation of trends in grain size measures. Journal of Sedimentary Petrology, 51:611-624.

McLaren P. and Bowles D. 1985 The effects of sediment transport on grain-size distributions. Journal of Sedimentary Petrology, 55:457-470.

Perillo G.M.E. 1996 Geomorphology and Sedimentology of Estuaries. Developments in Sedimentology 53. Elsevier Sceince B.V. Washington.

Schofield J.C. 1970 Coastal sands of Northland and Auckland. New Zealand Journal of Geology and Geophysics, 13:767-824.

Searle E.J. 1956 Aspects of the Pleistocene and recent history of the Auckland Isthmus. New Zealand Geographer, 12:75-88.



8.0 APPENDICES



APPENDIX 1. SEDIMENT ANALYSIS PROCEDURES The following series of laboratory procedures were undertaken in order to establish the proportion of gravel, sand and silt/clay (less than 63 microns in size) within each sample, and to determine the settling velocity of sand and silt/clay-size sediments. 1. Samples were dried in an oven at 80oC. 2. Samples were split into 100 gram sub-samples. 3. Wet sieve analysis was performed on each sub-sample following the standard techniques of Folk (1974) to determine the proportion of silt and clay size (> 63 microns) particles in deposits. 3. Following wet sieving, the proportion of deposits greater than 63 microns was dried and dry sieved using a 2 mm sieve to separate the sand and gravel fraction of deposits. 4. The separated sand fraction was split into 10 g sub-fractions and analysed using a MacArthur Rapid Sediment Analyser (RSA). The RSA is a sedimentation column with a vertical fall of 1.85 m from release mechanism to balance pan, with a diameter of 0.25 m. A computer linked to the sedimentation column records the time it takes particles to settle through the 1.85 m water column. The resultant settling velocity distribution of a sample is then corrected to a settling rate at a water temperature of 20oC. The settling distribution is subsequently converted to a grain-size distribution using the Gibbs et al. (1977) equation. In using the Gibbs et al. (1977) equation a sediment density value approximating the sediment being analysed is required, as this affects the settling time and subsequent size estimate. In this analysis a sediment density value of 2.65 g cm3 was used. Results are presented in terms of an equivalent sedimentation diameter, which is the diameter of a sphere having the same settling rate as the irregularly shaped settled grain. Results are reported in terms of the phi scale in which phi () = -log2d, where d = particle diameter in millimetres. Statistical parameters of the settled-size distributions were determined using both the moment and graphic methods. Results were similar and only the moment values were further analysed in the report.



APPENDIX 2: MCLAREN SEDIMENT TREND ANALYSIS

McLaren and Bowles (1985) have shown from theory and observation that particular patterns of changes in grain size statistics can be used to determine the net direction of sediment transport and to indicate whether deposItion or erosion is occurring. The theory can be illustrated by considering two bottom samples taken sequentially along a sediment transport pathway; an upstream sample d1(s), and a downstream sample d2(s). When the parameters (mean size, variance, and skewness) of the two distributions are compared there are eight possible trends. The interpretations given to each trend by McLaren et al. (1993) are as follows: A] Dynamic Equilibrium: d1(s) and d2(s) have similar shape but sorting is better downstream. There is grain-by-grain replacement along the transport path and a state of dynamic equilibrium. B] Net Accretion: d2(s) is finer, better sorted and more negatively skewed than d1(s) (FB-).

More fine grains are deposited along the transport path than eroded. The bed is mobile but there is a state of accretion.

C] Net Erosion: d2(s) is coarser, better sorted and more positively skewed than d1(s) (CB+).

More fine grains are eroded than deposited and there is a state of net erosion. A further case is observed where a sediment source is winnowed to form a lag. The lag

will be coarser, better sorted and more positively skewed than the source and the direction of transport is not explicitly known.

D] Total Deposition: d2(s) is finer, better sorted and more negatively skewed than d1(s) and

the X-function increases over the entire size range (FB-). The bed is not mobile and is accreting.

A further case was proposed by McLaren (1981) where sediments become finer, better sorted and more positively skewed (FB+). However, this case was later refuted in the emopirical model of McLaren and Bowles (1985). The trend anlysis relies on detecting specific trends in mean size, sorting and skewness between inter-related sediment deposits. In reality these trends are obscured by local variations and measurement inaccuracies and a statistical approach is adopted to identify significant trends. Typically samples are collected along suspected sediment transport pathways or in a grid pattern over the study area. Grain-size distributions are determined to a high degree of accuracy and all possible pairs of samples along a possible transport path are examined for statistically significant trends corresponding to the four transport regimes. A minimum of eight samples is required to achieve a result with any level of statistical significance. The statistical significance of each trend is determined using the Z-score test (McLaren and Bowels, 1985). The trend is accepted if Z>1.645 (5% level of uncertainty). Most trends had Z-scores of Z>2.33, which corresponds to the 1% level of uncertainty.



APPENDIX 3: TEXTURAL RESULTS OF SEDIMENT SAMPLES FROM RANGITOTO CHANNEL

Sample % Gravel % Sand % Mud Set Vel

(cm/s) Size (mm

)

Mean Std Dev Skew Kurt

X-7B 5.2 62.5 32.3 1.38 0.14 2.77 0.51 -5.34 32.09

X-7 1.6 84.7 13.7 1.44 0.16 2.43 1.04 -1.44 4.00

X-6 42.1 46.1 11.8 1.44 0.32 1.99 1.37 -0.43 1.13

X-5 48.7 31.4 19.8 1.38 0.31 2.08 1.21 -0.59 1.77

P1-2 7.2 67.0 25.9 0.94 0.17 2.59 0.34 -2.24 45.35

P1-4 10.7 68.2 21.1 0.85 0.14 2.75 0.22 -10.67 174.81

P1-6 2.6 75.9 21.6 1.09 0.17 2.53 0.44 -3.95 26.04

P1-8 0.0 86.7 13.3 1.30 0.14 2.84 0.36 -12.89 108.65

P1-10 0.4 85.7 13.9 1.43 0.16 2.34 1.12 -1.20 3.02

P1-11 4.4 80.8 14.8 1.29 0.14 2.81 0.33 -11.25 117.88

P1-12 41.3 47.2 11.5 1.29 0.23 2.11 1.49 -0.16 0.76

P1-13 56.2 33.3 10.5 1.43 0.31 1.69 1.92 0.09 0.42

P1-14 43.9 47.4 8.7 1.33 0.37 1.48 1.57 0.22 0.69

P1-15 23.1 64.5 12.4 1.43 0.21 2.22 1.47 -0.28 0.89

P1-16 15.3 82.8 1.9 1.05 0.13 2.83 0.66 -3.11 14.74

P1-17 4.4 95.6 1.0 1.07 0.18 2.46 0.33 0.30 36.61

P1-18 17.8 82.2 1.0 1.42 0.36 1.54 0.93 -0.25 2.46

X-4B 0.5 60.3 39.2 1.44 0.15 2.65 0.60 -3.14 18.58

X-4 0.7 48.4 50.9 1.44 0.12 3.0553 0.32 -10.39 100.56

X-3 32.7 50.4 17.0 1.44 0.23 2.08 1.03 -0.31 1.94

X-2 44.5 35.0 20.5 1.08 0.17 2.56 0.61 -1.11 7.01

P2-1 39.4 41.2 19.3 1.33 0.22 2.24 1.01 -0.49 2.20

P2-3 50.3 37.3 12.4 1.43 0.26 2.01 1.08 -0.16 1.78

P2-5 41.7 44.1 14.2 1.43 0.30 1.78 1.22 0.02 1.24

P2-7 57.7 32.0 10.3 1.28 0.31 1.71 1.16 0.13 1.28

P2-9 12.4 72.1 15.5 0.90 0.18 2.54 0.25 -0.27 110.44

P2-11 0.3 86.0 13.7 1.42 0.15 2.67 0.52 -6.10 37.02

P2-13 3.6 78.0 18.3 1.43 0.15 2.59 0.66 -3.70 17.67

P2-15 22.6 69.4 8.0 1.43 0.19 2.30 0.73 -1.58 8.47

P2-17 3.7 96.3 0.0 1.43 0.20 2.22 0.53 -3.02 17.01

X-8 35.2 58.0 6.8 1.49 0.33 1.63 0.66 0.77 6.56

X-9 24.8 45.8 29.4 0.88 0.14 2.83 0.31 -5.96 88.44

X-10 31.6 61.6 6.8 1.31 0.20 2.23 0.93 -1.03 4.01

P3-2 28.9 59.0 12.1 1.43 0.34 1.56 1.01 0.47 2.29

P3-4 49.3 43.4 7.3 1.33 0.35 1.52 1.04 0.39 2.15

P3-6 53.4 44.0 2.6 1.43 0.57 0.89 1.38 0.98 2.27

P3-6B 17.1 31.7 51.2 1.15 0.23 2.10 0.99 0.46 1.99

P3-7 36.1 51.1 12.7 1.23 0.19 2.33 0.83 -0.60 3.56

P3-8 45.5 36.1 18.4 1.42 0.18 2.37 0.84 -1.16 5.22

P3-9 28.4 63.7 7.9 1.43 0.17 2.52 0.32 -5.35 73.49

P3-10 10.6 79.7 9.7 1.43 0.17 2.38 0.72 -2.76 11.35

P3-12 43.8 43.5 12.6 1.43 0.29 1.78 1.46 -0.05 0.79

P3-14 0.0 100.0 0.0 1.42 0.19 2.42 0.26 -8.33 132.50

P3-15 27.4 72.6 0.0 1.42 0.44 1.18 0.35 2.98 37.04

P4-2 31.3 60.4 8.3 1.43 0.26 1.93 1.13 -0.07 1.44

P4-4 38.2 52.8 9.0 1.43 0.30 1.78 1.03 -0.03 1.98

P4-5 31.9 54.5 13.7 1.43 0.33 1.59 1.38 0.14 0.87

P4-6 60.9 37.7 1.4 1.42 0.70 0.66 0.78 2.68 9.62

P4-7 31.4 55.2 13.3 1.43 0.24 2.12 0.89 -0.45 2.90

P4-8 35.6 53.7 10.8 1.27 0.21 2.22 0.73 -0.76 5.11



P4-9 45.9 48.7 5.4 1.43 0.25 2.03 0.92 -0.90 3.46

P4-11 26.7 61.0 12.3 1.43 0.16 2.53 0.70 -2.34 10.43

P4-13 3.3 82.9 13.9 1.43 0.13 2.88 0.40 -9.97 80.51

P5-2 3.4 56.1 40.5 1.26 0.14 2.81 0.35 -7.50 65.19

P5-4 26.5 61.8 11.6 1.31 0.26 1.98 0.96 -0.37 2.32

P5-5 41.5 41.3 17.2 1.28 0.23 2.22 0.88 -0.80 3.40

P5-6 39.4 51.5 9.1 1.35 0.36 1.49 1.27 0.28 1.14

P5-7 38.7 50.4 10.9 1.32 0.32 1.63 1.24 0.09 1.10

P5-8 44.7 45.9 9.4 1.31 0.26 1.98 0.99 -0.36 2.10

P5-9 30.2 59.4 10.3 1.31 0.25 2.03 0.84 -0.75 3.55

P5-11 30.7 61.6 7.7 1.26 0.19 2.34 0.60 -2.01 10.27

P5-13 3.0 97.0 1.0 0.84 0.15 2.69 0.18 -18.21 369.78

P5-14 5.8 94.2 1.0 1.30 0.25 1.94 0.38 -3.60 28.43

P6-2 40.3 46.8 12.9 1.34 0.36 1.46 1.16 0.40 1.52

P6-4 40.1 43.1 16.9 1.29 0.27 1.85 1.02 -0.15 1.84

P6-5 61.1 38.8 0.2 1.40 0.69 0.62 0.67 2.97 11.90

P6-6 33.8 56.5 9.6 1.29 0.19 2.35 0.56 -2.37 12.62

P6-7 40.0 49.6 10.4 1.32 0.25 2.03 1.04 -0.44 1.96

P6-8 48.1 41.6 10.3 1.37 0.29 1.85 0.96 -0.24 2.30

P6-9 37.8 57.3 4.9 1.33 0.23 2.07 0.51 -2.39 15.76

P6-11 22.4 77.6 0.5 1.35 0.24 1.96 0.35 -4.46 40.17

P6-13 0.0 99.0 1.0 0.91 0.16 2.67 0.16 -22.10 468.65

P6-14 19.5 80.5 0.5 1.39 0.63 0.76 0.51 3.32 21.39

P7-1 9.2 48.6 42.2 1.43 0.12 2.96 0.42 -8.45 66.41

P7-2 13.5 51.0 35.5 1.43 0.13 2.62 1.22 -1.19 2.98

P7-3 13.3 46.2 40.5 1.43 0.13 2.87 0.37 -8.58 76.88

P7-4 14.7 48.5 36.8 1.42 0.14 2.79 0.42 -7.11 58.43

P7-5 3.0 69.6 27.5 1.43 0.16 2.60 0.54 -4.66 28.23

P7-6 12.1 78.6 9.3 1.48 0.27 1.92 1.01 -0.80 2.48

P7-6 shell 1.30 0.65 0.74 0.35 16.08 131.68

P7-7 40.2 52.9 7.0 1.43 0.39 1.56 1.45 -0.02 0.72

P7-8 34.5 56.8 8.7 1.87 0.62 0.55 6.83

P7-9 27.5 65.6 7.0 2.11 0.52 -0.45 17.54

P8-1 2.9 55.1 41.9 1.43 0.13 2.94 0.32 -12.28 123.36

P8-2 2.8 53.4 43.8 0.82 0.12 3.05 0.22 -17.56 288.01

P8-3 3.0 59.6 37.4 1.43 0.13 2.88 0.30 -11.41 130.57

P8-4 3.0 28.1 68.9 1.12 0.13 2.91 0.28 -6.79 94.23

P8-5 4.1 56.9 39.0 1.23 0.16 2.66 0.31 -5.70 66.86

P8-6 0.0 58.1 41.9 1.42 0.15 2.72 0.23 -13.89 247.91

P8-7 3.5 86.9 9.6 1.07 0.22 2.22 0.28 0.25 53.42

P8-8 4.0 88.3 7.7 1.22 0.22 2.18 0.38 0.38 27.05

P8-10 15.1 84.9 0.5 1.43 0.28 1.83 0.45 -1.20 28.76

P9-A 0.1 56.5 43.4 1.36 0.13 2.85 0.45 -7.34 52.09

P9-B 20.6 76.3 3.1 0.88 0.18 2.45 0.18 -11.39 360.00

P9-C 30.6 68.8 0.5 1.34 0.35 1.49 0.20 4.62 234.91

P9-1 6.7 54.9 38.4 1.43 0.17 2.46 1.05 -1.08 3.27

P9-2 4.1 63.9 32.0 1.43 0.13 2.81 0.57 -4.54 26.02

P9-3 15.9 47.3 36.8 1.43 0.21 2.22 1.14 -0.38 1.68

P9-4/5 22.3 43.4 34.3 1.26 0.18 2.47 0.44 -1.92 24.27

P9-6 5.7 86.2 8.2 1.43 0.19 2.40 0.25 -16.04 211.12

P9-7 5.0 84.3 10.7 1.38 0.19 2.39 0.26 -3.97 124.18

P9-8 28.9 70.3 0.8 1.43 0.33 1.66 1.08 -0.16 1.79



P10-A 41.3 54.9 3.8 1.35 0.19 1.78 0.49 -0.33 17.34

P10-B 1.6 97.9 0.5 1.04 0.26 1.90 0.22 -13.95 235.03

P10-1 0.1 86.5 13.4 1.40 0.14 2.65 0.73 -3.60 14.32

P10-2 3.2 77.0 19.9 0.96 0.16 2.63 0.21 -4.56 176.65

P10-3 28.2 23.0 48.8 1.34 0.24 2.09 1.05 -0.52 2.10

P10-4 36.0 42.0 22.0 1.26 0.21 2.24 0.65 -1.49 7.73

P10-5 23.1 65.2 11.6 1.22 0.22 2.16 0.27 -4.98 79.40

P10-6 35.7 58.7 5.5 1.27 0.22 2.16 0.40 -2.43 27.49

P10-7 3.1 79.6 17.3 0.96 0.17 2.54 0.23 -3.21 121.56

P10-8 3.1 86.1 10.8 0.97 0.17 2.63 0.36 0.67 32.88

P10-9 54.6 36.2 9.2 1.35 0.28 1.83 0.49 -0.38 16.22

P11-1 28.8 65.2 6.0 2.41 1.18 -1.14 2.79

P11-2 4.0 66.8 29.2 2.66 0.68 -3.56 16.26

P11-3 4.8 55.0 40.2 2.65 0.43 -6.35 48.62

P11-4 40.1 43.6 16.3 1.91 0.78 -0.28 4.55

P11-5 58.5 32.1 9.4 2.07 0.53 -0.87 14.79

P11-6 51.2 39.3 9.5 2.00 0.8 -0.24 3.69

P11-7 5.9 90.7 3.3 2.36 0.26 3.85 107.83

P11-8 4.2 95.8 0.5 2.37 0.24 -4.78 152.84



APPENDIX 4:

CROSS CHANNEL CHANGES IN MEAN GRAIN SIZE (dark line, triangle symbol) AND SORTING VALUES (light line, square symbol) FOR SAMPLE TRANSECTS

Profile 1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

02468101214

Sample No.

Ph

i Un

its

Profile 2

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0246810

Sample No.

Ph

i Un

its

Profile 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1234567891011

Sample No.

Ph

i Un

its

Profile 4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

123456789

Sample No.

Ph

i Un

its

Profile 5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12345678910

Sample No.

Ph

i Un

its

Profile 6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12345678910

Sample No.

Ph

i Un

its



Profile 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12345678910

Sample No.

Ph

i Un

its

Profile 8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

123456789

Sample No.

Ph

i Un

its

Profile 9

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1234567

Sample No.

Ph

i Un

its

Profile 10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12345678

Sample No.

Ph

i Un

its

Profile 11

0

1

1

2

2

3

3

12345678

Sample No.

Ph

i Un

its



APPENDIX 5: TEXTURAL RESULTS OF SEDIMENT SAMPLES FROM CHELTENHAM BEACH

Size estimates are moment statistics only.

Sample % Gravel % Sand % Mud Set Vel (cm/s) Size (mm) Mean Std Dev Skew Kurt

C1-1 10.92 88.20 0.88 1.43 0.42 1.40 0.87 1.66 5.46

C1-1A 0.10 97.16 2.74 1.43 0.29 1.76 0.37 0.26 54.97

C1-2 0.00 95.65 4.35 1.43 0.25 2.02 0.38 2.92 34.47

C1-3 0.00 96.05 3.95 0.97 0.22 2.16 0.22 -3.72 149.66

C1-4 0.10 95.54 4.36 1.01 0.28 1.86 0.29 4.09 56.13

C1-5 3.24 76.41 20.35 1.43 0.15 2.64 0.65 -4.12 18.76

C1-6 3.63 83.75 12.61 0.90 0.17 2.54 0.28 -3.37 96.42

C1-7 36.31 54.66 9.03 1.08 0.22 2.18 0.36 1.37 28.72

C1-8 23.08 68.53 8.39 1.43 0.22 2.10 0.88 -0.33 4.69

C1-9 48.21 46.13 5.65 1.43 0.34 1.62 1.10 -0.22 1.53

C2-1 16.02 83.06 0.93 1.13 0.31 1.65 0.55 1.55 12.93

C2-1A 3.78 93.07 3.15 1.43 0.33 1.55 0.49 0.04 18.78

C2-2 0.18 93.11 6.71 1.43 0.17 2.49 0.46 -4.15 32.44

C2-3 0.00 95.79 4.21 1.42 0.20 2.17 0.77 -2.18 7.77

C2-4 0.41 96.44 3.15 1.43 0.35 1.50 0.26 2.09 131.82

C2-5 1.03 68.14 30.83 0.88 0.17 2.54 0.19 -10.31 272.33

C2-6 0.28 92.99 6.73 1.43 0.20 2.11 0.71 -2.16 9.70

C2-7 19.54 72.49 7.97 1.43 0.24 2.00 0.63 -1.09 8.63

C2-8 20.07 68.96 10.97 1.11 0.23 2.14 0.35 0.82 30.02

C2-9 41.77 50.26 7.97 1.43 0.24 2.08 0.55 -1.30 10.57

C3-1 6.13 93.65 0.22 1.43 0.62 0.79 0.59 4.36 19.43

C3-1A 0.45 95.43 4.13 1.08 0.25 1.95 0.36 -4.35 62.38

C3-2 0.09 95.58 4.33 1.42 0.16 2.48 0.76 -2.97 11.16

C3-3 0.00 96.14 3.86 1.41 0.17 2.40 0.71 -3.87 16.40

C3-4 1.07 89.58 9.35 1.39 0.17 2.50 0.42 -6.02 51.72

C3-5 7.81 87.01 5.18 0.92 0.21 2.11 0.55 -3.76 19.31

C3-6 0.32 96.02 3.66 0.90 0.22 2.09 0.39 -7.43 57.71

C3-7 17.87 71.60 10.53 1.43 0.24 1.96 0.72 -1.30 7.22

C3-8 16.94 78.21 4.85 1.43 0.35 1.55 0.78 -0.73 5.13

C3-9 32.92 59.40 7.68 1.43 0.25 2.00 0.79 -0.40 5.08

C4-1 1.24 96.45 2.31 1.17 0.39 1.41 0.37 7.91 56.25

C4-2 0.00 97.32 2.68 1.43 0.23 2.07 0.67 -0.86 8.21

C4-3 0.00 95.64 4.36 1.42 0.18 2.42 0.41 -7.53 55.86

C4-4 7.98 87.35 4.67 0.92 0.18 2.46 0.49 -2.71 27.11

C4-5 0.52 95.52 3.96 1.43 0.28 1.78 0.28 -2.65 108.21

C4-6 0.87 96.70 2.43 1.43 0.26 1.90 0.44 -3.09 28.35

C4-7 0.37 85.89 13.74 1.43 0.18 2.33 0.77 -2.38 9.07

C4-8 9.09 86.52 4.39 1.43 0.26 1.94 0.39 -3.23 40.13

C4-9 40.72 52.94 6.33 1.43 0.27 1.88 0.50 -0.22 13.49

Documents

Coastal Processes Assessment Report