EIS 990

AB020290

Review of Marine aggregate proposal - coastal processes

working paper, September 1992.

NSW DEPT PR1MAR INUST1ES

LIl BO2O29O

R2066/240

REVIEW OF:

MARINE AGGREGATE PROPOSAL - COASTAL PROCESSES WORKING PAPER

SEPTEMBER, 1992 4

/ it

MARINE AGGREGATE PROJECT: COASTAL PROCESSES WORKING PAPER

The Public Works Department's involvement in the review of this

document stems from two distinct roles:

As principal advisor to the State on matters related to

physical coastal process; and

As advisor to the Minister for Public Works in relation to

his concurrence role under Regulation 1 of the Coastal

Protection Act, 1979.

I In exercising its prudential role in this matter the Department

must be satisfied that the proposed activities will not

I adversely affect the stability or behaviour of the shoreline or

its associated sediment bodies. To demonstrate this, was, in

effect, the consultant's task.

I The consultant's report, which will ultimately form part of the

1 EIS, consist primarily of ten appendices (in Volumes 2 and 3)

and a summary report (Volume 1) . The review which follows is an

assessment of each of these appendices, highlighting were it is

considered that deficiencies exist. The exceptions are the

following three appendices which were not reviewed:

Appendix 1 Government Consultation

Appendix III Physiography: largely descriptive

Appendix X Sediment Plumes:not considered relevant to the

Department 's concerns.

General

In addition to the specific comments made in relation to each

appendix the following general comments are made:

The tests applied in the constraint assessment, namely that

the effects of the extraction be an order of magnitude less

than the average annual variation and that they be not

measurable, do not guarantee that they will have no impact.

Such effects could still have an impact in the longer term.

Management practices and the monitoring program must be

defined in detail now. They will need to form part of the

conditions of consent.

How is it proposed that any adverse impacts due to the

extraction be distinguished from those due to other factors

such as, for example, sea level rise?

Conclusion

The matters raised in this review would have to be adequately

addressed in the EIS before the Department could recommend to

the Minister that concurrence under the Coastal Protection Act

be granted.

APPENDIX II - CASE STUDIES

Examples of dredging for marine aggregate and other purposes in

I various parts of the world have been examined for their effects

on wave climate and on the stability of adjacent beaches.

However, wave climate and morphological conditions were

I generally so distinctly different from those pertaining to the

NSW coast that their applicability to the current proposal is

I likely to be limited.

Botany Bay

I Dredging was carried out in quite shallow depths. Significant

changes to wave height and directions occurred with concomitant

damage to some sandy beaches. The fact that no beach drawdown

I was detected within the accuracy of the surveys should not give

too much comfort. Wave energy within the Bay is very much less

I than that offshore and the extent of drawdown must be expected

to be related to wave climate.

I The authors have concluded that there was little evidence of

inf ill or beach drawdown in the case of the entrance dredging.

I However the survey records show up to 0.5m accretion between

1981 and 1986 and bedforms to lm, indicating significant

sediment transport (Transect 6-6 1 , Figure 2.6).

- Kirra Beach

Only analytical work on the behaviour of the borrow area is

reported. There is no survey data. The conclusion by GCCC that

dredging can be conducted in water deeper than 20m cannot,

therefore, be supported.

Japan

The Kochi coast is significantly different from the NSW coast

I both morphologically and oceanographically. The wave climate is

considerably lower (note Figure 4.3 ordinates are logarithmic

I scale) . The natural slopes in the area are extremely steep

(1:10) . Onshore/offshore transport is swamped by alongshore

I

transport. Experience in this area is not likely to be

applicable to the current proposal.

The wave climate in the Genkai Sea is closer to that of Sydney,

though less extreme at the high end. Erosion attributable to

the dredging was experienced. Considerable infilling of the

dredged depression was recorded in 30m water depth.

United Kingdom

The reported UK experience cannot be applied to the NSW coast:

No wave data is presented; no sediment size information is

presented; and sediment response is complicated by tidal

currents.

USA

On Revere Beach dredging was carried out for beach nourishment.

Any adverse effects would likely be masked by the nourishment.

Despite the fact that Revere Beach experiences a considerably

milder wave climate than Sydney, it was found that the borrow

area in 20m of water filled rapidly. It was not clear however

from where the sediment came.

At Rendolo Beach, material was dredged from between 9m and 20m

water depth in 1968. By 1987 a 430m section of the borrow area

was half filled "with sand moving in from deep water". This

aspect was ignored in the discussion.

Overseas Studies

The Japanese field studies reported concluded that dredging was

safe if conducted in more than 35m of water. The wave climate

was marginally to significantly lower than that of the NSW

coast. The analytical studies showed that wave refraction was

significantly affected by dredging in 30m. They concluded that

dredging in 40m would have little effect on the shoreline.

I Field studies in the UK involved a shingle/pebble sized

I sediments and so have very limited relevance to the current

proposal. Even those experiments which involved sand sized

I grains were affected by the presence of shingle which had the

effect of greatly enhancing the sand's stability. The

analytical work reported was based on mild nearshore slopes.

I Wave climate and sediment sizes were not specified.

I Combined analytical and field studies undertaken in France

concluded that excavations could be safely carried out in

I depths greater than 21m provided that H remained below 5.5m.

The wave period however was not specified nor was it clear what sediment size this conclusion related to.

I The reported Dutch work is not considered to be comparable to

I the current proposal. Their's is a low energy coast with

extremely flat nearshore slopes.

SUMMARY

I A number of case studies and analytical studies of the effects

of dredging have been reviewed. However in the majority of

I cases they relate to wave climates and coastal morphologies

distinctly different from those off Sydney. Their relevance to

I the current proposal is therefore likely to be limited. In

others, any adverse effects would have been masked by the

nourishment for which the dredging was carried out.

I The wording of §8.2 is potentially misleading in a number of

I areas;

I The fact that of all overseas regulations studied none

prohibited dredging beyond 30m does not mean that dredging to

the 30m contour is free of any adverse impacts. Overseas case

I studies have recommended safe depths of 35m (field) and 40m (analytical)

The third last paragraph (p84) could be construed to imply that

I dredging in depths greater that 15m would not result in changes

to the propagation of waves. Analytical work suggests that

I

- dredging in water as deep as 30m can have significant effects

I on adjacent shorelines (Horikawa et al, 1977)

I Similarly the second last paragraph could be construed to imply

that beach drawdown would not occur where the dredge hole is in

more than 22m of water. In fact the referenced study (Genkai

I Sea, Japan) found that drawdown occurred in 22m but not at 35m.

The only conclusion that can be drawn from this is that the

I "safe depth" is somewhere between 22m and 35m at this particular location.

I

APPENDIX IV - HYDROGRAPHIC SURVEYS

Why were predicted tides used for tidal correction in lieu

of recorded ones? What were the residuals for these periods?

Sounder calibration was only carried out to 15m. No colrffnent

is made on possible errors in deeper water. Density

differences could be expected to occur in depths greater

than 15m in the summertime.

How was survey data corrected for swell? The swell/sea

states on the survey dates are not recorded.

The claim of a range accuracy of +lm is potentially

misleading as the described position fixing system is

subject to considerably larger errors.

There is no discussion of the overall accuracy of the survey

data.

There are a number of references throughout the report to

sidescan sonar surveys and interpretation but nowhere are

any results presented. Was such a survey conducted or not?

APPENDIX V - PHOTOGR?flIETRIC SURVEY

The objective of the photogrammetric analysis was to establish

the baseline condition of the sandy beaches and any existing

trends in their behaviour. However, as we will see, a number of

factors in combination make it difficult to have confidence in

the final results, particularly in the case of the Providential

Head beaches;

Volumetric analysis is of little use on a beach undergoing

little long term change as day to day and seasonal

variations swamp any long term trends.

Control for the Providential Head beaches was derived from

aerial triangulation of the 1:40000 scale 1977 photography.

This means that any observations in this area are subject to

systematic errors of ±0.8m in elevation. The potential

effect on the volumetric analysis is however likely to be

limited. It is not common practice to rely on aerial

triangulation where height information is required.

There is no discussion of the significance of antecedent

conditions in terms of their affect on this type of

analysis. There appears to have been no attempt to select

photography with similar antecedent conditions.

In §3.5.1 of Appendix VI it was stated that beaches undergo

I seasonal rotation in response to changes in wave energy

direction. This has the potential to introduce systematic

I errors into a volumetric comparison if based on photography

taken at different times of the year. Table 33 reveals that

I in the majority of cases, the earliest and latest

photography used in the volumetric analysis was taken at

different times of the year. Discussion on to what extent

I this may have affected the results is warranted.

I . That the seaward boundaries of the volume calculations was

the 2m contour in each year means that the volume

I differences are effectively calculated to the datum RL, (in

this case Om AHD) if it can be assumed that the profiles are

parallel between Om and 2m AHD. This fact does not appear to

have been recognised nor has the validity of the implicit

assumption been discussed.

It was reported that profiles within Botany Bay were plotted

to Om AHD. Was all photography taken at low tide or were

some observations made underwater? If so, were these

observations corrected for the effects of refraction?

With as few as three years analysed, it would be impossible

to distinguish between long term trend and short term

fluctuations.

There are obvious datum shift problems in some areas. See,

for example, Little Marley, 1947 and Garie, 1961 (Sheet 3)

APPENDIX VI - WAVE REGIME

Model Schematisation

- At the finest grid spacing of 125m the steepest batter slopes

I which can be modelled for a 5m extraction are 1:25. It is very

likely that steeper batters would be produced by the dredging

and that they would take some time to adjust to slopes of 1:25

I or less. Refraction at discontinuities can be very significant

especially where wave fronts are close to normal to the

I discontinuity (see Forster, 1980) . A finer grid at the

extraction boundaries would ensure that the effect of the

discontinuities was correctly modelled.

'Calibration' of Refraction Model

I The refraction model was not calibrated as claimed. The work

I reported at §2.5 is simply a verification exercise consisting

of a comparison of wave heights measured at the Botany Bay

I Waverider with those calculated at the Providential Head site

by linear transfer function. Results of the comparison are

presented only for a relatively long period wave train

I (Tz=lOs.) Even the extreme synthesised storm had a maximum zero

crossing period of less than lOs. The median Tz for Sydney is

Iabout 5.8s.

I The procedure adopted is a check only on wave heights and

implicitly assumes that wave directions are modelled correctly.

Further, the verification is good only to 35m (where the

I current meter was deployed) . Refraction and shoaling from

deepwater to 35m is limited compared to 35m to the shoreline.

I That only small errors were found is not surprising, given that

only relatively small changes are occurring. Note also that the

reported errors are of a similar magnitude to the changes which

I the model predicts will be produced by the proposed extraction.

I

ISchematic Wave Refraction

This work was based on a straight coast with a constant

I

offshore profile. There is no comparison with the

subsequently surveyed profiles.

The basis for the selection of standard deviations of wave direction of 100 and 300 is not given.

The Iloptionsul tested in this section bear little resemblance

to the proposed extraction and even less to the real

bathymetry. Whilst they may have served some purpose in the

conceptual planning of extraction patterns, their inclusion

in this document is an unnecessary distraction.

To say that beaches will undergo rotation equal to the

average change in wave direction is simplistic and ignores,

among other things, the relationship between incident wave

energy and wave direction.

The results of the schematic wave refraction work are

reported in terms of wave direction only. Wave coefficients

are reported for only one option (Option 2) and at only two

locations. (It is worth noting that these two locations do

not correspond to the areas of maximum wave direction

changes; Figs. 3.6 and 3.7). From tables 3.1 - 3.4 it is

possible to deduce that wave heights will be increased by up

to 4% for some wave period/offshore direction combinations,

even for these two locations.

Assessment of the Care Banks and Providential Head Sites

It is not clear how the average "annual wave climate

statistics" used in the assessment were calculated. In any case

the use of average values, no matter how carefully weighted, to

describe a highly variable and non-linear system is potentially

erroneous. It would be preferable to test the sensitivity of

the system over a full range of parameters.

I Tables 4.3 and 5.3 seem to be based on a wave height equal to

I the 24h/year wave and an offshore direction taken from the

average annual wave climate. These are not necessarily

I associated.

The storm assessment at §4.3.2 and §5.3.2 appears reasonable.

I I I I I I

APPENDIX VII - CURRENT REGIME

I I

Tidal (Harmonic Analysis)

Residuals from the harmonic analysis are not reported.

It is notable that Zo is virtually identical for shore-parallel and shore-normal currents.

Amplitudes are ms 1 not m (Tables 1, 2)

Preliminary Wind Current Analysis

The work by Bretschneider is not referenced. There is no

discussion of alternative models nor of the applicability of

Bretschneider's model to the Sydney shelf.

Bretschneider's model is based on depth averaged currents.

The assumption of a logarithmic velocity profile is unlikely

to be valid where there is a significant shore-normal

component in the wind field nor within the generation zone

where inertial effects are likely to give rise to much

greater surface velocities and smaller velocities at depth.

Why was no attempt made to calibrate the wind driven current

model on the surface wind stress parameter and on Manning's

n?

The last paragraph on page 11 (2.3) implies that both the

wind and tidal currents were adjusted to a depth of 1.5m. It

would not have been necessary to adjust the tidal component

as it was computed from readings taken at h = 1.5m

The conclusion that the system displays a 12 hour lag for

transient events as short as 4 hours is surprising. A period

of 12 hours might be a reasonable luacceleration time" for

current speeds to become fully developed in response to a

step function wind speed.

L I I I I I I I

I I

I The analysis of EAC and CTW currents would have been

I assisted by plots of current residuals (i.e. measured less

calculated tide- and wind-induced currents.)

The current peaks discussed in §2.6, Other Currents' are,

I

almost without exception, associated with peaks in the

predicted tide-induced current and suggest that these have

been underestimated. This casts some doubt on the harmonic

I analysis. Again, presentation of the residuals would have

been helpful.

The currents associated with the long period shore-normal

waves (Fig. 14) deserve some discussion in view of the

contention elsewhere that no mechanism for offshore

transport exists in water deeper than 22m.

--

• The presence and significance of shore-normal currents has

I largely been ignored throughout §2. From the records

captured at Providential Head they appear to be of a similar

magnitude to the shore-parallel currents. It would be

I reasonable to expect that they would be of greater

significance from the point of view of beach stability.

Current Analysis for Providential Head.

A good deal of effort has gone into extending the Providential

Head data by correlation with data from the Bondi Ocean

I Reference Station (ORS) . The period of available data at the

ORS is not given except as "(approximately one year) ." No

I attempt has been made to assess the extent to which the one

year is representative of longer term behaviour.

The lowest current meter at the ORS is 15m above the bed

whereas the Providential Head one was only 1.5m above the bed.

I Given the complex current structure particularly for

shore-normal currents one would expect any relationship to be

I tenuous, at best. Contrast the current speed, direction,

frequency roses for the Bondi ORS (Fig. 55) and for

I Providential (Fig. AlO). The current regimes are obviously

quite different.

It is asserted that low frequency shore-normal currents are

exclusively associated with internal wave activity. This is

clearly not the case. Other than general discussion there is no

analysis of these currents (low pass, shore-normal)

That the relationship between the Bondi ORS and Providential

Head current meters is poor is reflected in the results of the correlation;

as little as 25% of the variation in the dependent variable

is explained by the independent variable.

the low coefficient obtained for Vblp when a much closer

relationship between the two sites could be expected. The

result probably reflects the current meters' location in the water column.

the high coefficient for Vw when that component should

already by included in Vblp. (The attempt to dismiss this

observation is not valid; the difference in depth between

the current meters is only lOm with the Bondi one being well

above the "steady' current boundary layers. It is not

unreasonable to expect both current meters to experience

similar wind effects for shore parallel winds.)

the 36 hour lag used for wind-induced currents is

unrealistic for transient events.

that the currents at Providential Head should be so strongly

and negatively correlated with the high pass Bondi currents

is surprising indeed. No physical basis for this conclusion

is offered.

what is the physical basis for the non-zero intercept in eqn (4)?

Reference §3.3.4 and §3.3.8: the fact that the nett currents

are close to the current meter's limit of accuracy is

irrelevant. They are the vector sum of much larger currents.

Note also that the progressive vector plot at Fig. All shows an

I offshore excursion of 80km over a period of 55 days giving an

average velocity of 0.017ms not the 0.013ms stated in

§3.3.8.

The so-called realistic model of the shore-normal wind driven

currents profile is in fact based on a uniform depth, closed

channel with a constant wind blowing directly down it; hardly

an accurate representation of the Sydney shelf. One could

expect the shore-normal wind driven current profile to be

dependent upon wind strength, duration and on shelf and

coastline morphology.

The analysis of shore-normal currents on p50 is puzzling. Why

I was the data set extended by a Monte Carlo technique when only

statistics were presented at the end? Such a technique cannot

I improve the original data. In fact in this case it appears to

have corrupted it in as much as the resultant shore-normal

I current in Tables 3-14 is .003ms 1 onshore, whereas for the original data it was .017ms 1 offshore.

3.4 Effect of Extraction

A generally superficial treatment based largely on idealised geometry.

Why wasn't the full 2D model run for the pre- and

post-extraction cases and perhaps for one or two critical stages during extraction?

The analysis is based entirely on depth averaged flows with no

discussion on the effect on current profiles.

4 Current Analysis for Cape Banks

1 4.1 The conclusion that the current structures at Bondi and

Providential Head are similar is not supported by the data.

I Whilst the current speed, direction frequency roses for the PWD

current meters are similar (Figs. 47, 49) those at Providential

I Head (Fig. AlO) and Bondi ORS (Fig. 55) are distinctly

different both from each other and the PWD data.

I 4.2 The drogue tracking revealed significant tidal flow at

I Point 4 (in 45m water) with a bias to the offshore. The

significance of this finding in terms of sediment transport is

I not discussed nor has it affected the "Developed Currents" in Table 20.

I 4.3 Again the contention that the PWD near-bed records at Bondi

are structurally similar to those recorded at an elevation of

I 15m at the ORS is not supported by the data.

IAnd again, the authors have avoided stating the period of record at the ORS ("about one year"). This "structural similarity" is used to justify the use of six months data for

I all further analysis of currents and sediment transport in the Cape Banks area. Even if such similarity existed it does not

I guarantee that the data is representative of the long term current regime. Certainly it does not include a major

I wind/storm event nor can it include sufficient EAC and CTW events to be considered representative of their long term

I influence.

The transformations from the Malabar Site to the study sites rely on the assumption that the measured near-bed currents are

proportional to the depth averaged currents produced by the 2D

I model. There is no discussion of this assumption nor of any variation in the transformation parameters with current speed. (It would appear that calculation of the parameters has been

1 done for a uniform coast-parallel current of 1ms 1) . Further, it is not stated on what the spatial variations in shore-normal

I currents were based. Certainly transformations based on the 2D model results for shore parallel currents would be quite

i

inappropriate.

It is not clear whether or not the current speeds (for point

1

10) presented in the table on p92 are maximums or whether the

average currents quoted relate to averages over a tidal cycle

I or over a year.

Summary - Current Regime

The current fields investigations at both sites suffer from

three fundamental deficiencies;

They have consistently ignored or dismissed shore-normal

currents and three dimensional effects which are potentially

very important in terms of beach stability.

They are based on very short periods of records (six months

in the case of Cape Banks and 'about" 12 months in the case of

Providential Head, the latter being based on a six week record

extended by a poor and inappropriate correlation)

The spatial variation was calculated on the basis of a 2D

(depth averaged) model of coast parallel currents (apparently

even for shore-normal currents in the case of Cape Banks area)

APPENDIX VIII

Nearshore Sediment: Transport

1. Introduction

In this Section the authors provide a definition of the beach

system and a qualitative model of beach fluctuation in response

to wave activity. This model, whilst describing sub-aerial

beach behaviour in a general sense, does not appear to provide

a balanced description of present thinking regarding the

processes of sediment transport on the shoreface. The model

describes an exchange of sediment between the beach and the

nearshore whereby shoaling waves transport sand onshore and

surfzone currents operating during storms transport sand

offshore. " .......the deposition of sediment offshore during

storms, therefore, does not occur to any great distance

seawards of the surfzone or beyond the offshore limit of rip currents". p2

The above model does not allow for offshore transport seawards

of the surfzone by mechanisms other than rip currents. There is

a large body of research that suggests offshore transport may

occur across the shoref ace by processes other than rip currents. These include:

I. offshore transport as a result of suspended transport over

rippled beds (eg. Johnson and Eagleson 1966; Tunstall and

Inman 1975; Nielsen 1988)

I seawards directed bottom currents, including downwelling

I currents, tidal flows, and shelf waves. (eg. Wright 1987,

Swift et al, 1985)

I • gravity assisted diffusion type processes resulting from

sediment suspension (eg. Wright, 1987)

I In addition some studies suggest that mass transport in wave

induced turbulent bottom boundaries need not be exclusively

I shoreward and nay act in either direction (Trowbridge and

I These mechanisrs do not limit offshore transport to the surf zone or just seawards of it and suggest that onshore/offshore sediment exchange may occur across a wider

zone than that assumed by the authors.

2.2 Survey Data

In this section data are presented from several studies which have involved :r-eeated bathymetric surveys. Profile closure

depths inferred from these surveys thus refer to bed elevation

of less than ±0.3m. When integrated over horizontal distances

of many hundreds or thousands of metres this may amount to very

large volumes A sand. The closure depths obtained from these

surveys are not likely therefore to represent an accurate

measure of the Limit of onshore/offshore sediment exchange.

Note that not alL profiles in Figures 2.2 and 2.3 close out in

12m of water as iadicated in paragraph 2, page 6.

Sedimentological llata

In this Section the authors present a summary of nearshore

sedimentologica2 data from the NSW coast and Pakiri Bay on the

north east coast of the North Island of New Zealand. Sediment

boundaries described in this Section include the inner

nearshore/outer nearshore boundary and the outer nearshore/inner s;he.lf sand boundary. Inner nearshore sands are

generally similar to beach sand, outer nearshore sands are

finer, while the inner shelf sands are generally coarser.

I 2.4.6 Sediments c1scribed for Newcastle Bight show a seaward

fining trend to 2mn-17m water depth and a coarsening trend

seawards of this epth. The authors interpret this boundary as

representing the bundary between the inner nearshore sands and

the outer nearshore sands. Outer nearshore sands are finer than

I the inner nearshc'e sands thus the interpretation that this

boundary corresponds to the inner nearshore/outer nearshore

I

boundary is not consistent with the characteristics of these I sediment types. A coarsening boundary of this type is more of the nearshore/inner shelf sand boundary.

Itypical

2.4.8 In this Section the authors indicate that the nearshore

Isands extend seawards of Malabar Beach and Maroubra Beach to

some 20 to 27m water depth while this may be correct at Malabar

I correspond

Beach, the boundary described off Maroubra Beach appears to inner with the nearshore/outer nearshore sediment

boundary. As seen in the Bate Bay sea bed information map (PWD,

I 1989), sediments seawards of 27m water depth and extending offshore to 37m water depth are finer and are similar to the

I outer nearshore sands found off other beaches in the Sydney region. Seawards of 37m water depth sediments are coarser and generally typical of the inner shelf sands. The inner nearshore boundary in this region is therefore in 22 to 27m water depth

and the outer nearshore/inner shelf sand boundary in 37m water depth.

I Note: The reference PWD (1989d) on p16 does not occur in the reference list.

I 2.4.9 The authors state that in the Providential Head proposed

extraction area a marked variation in grain size occurs at 22m

I water depth. While not specifically indicating the nature of

this boundary in the text it is presented in Table 2.1 (Marley

I Beach) as the boundary between the outer - nearshore sands and

the inner shelf sands. This boundary like the boundary in 27m

water depth off Maroubra Breach involves a fining in the median

I grain size. Its classification as the outer nearshore/inner

shelf boundary is subject to some question. A more justifiable

I conclusion would probably be that it represents the boundary

between the inner nearshore sands and the outer nearshore

psands.

In any case the significance of a 0.07mm change in grain size

is questionable, especially considering the limited number of

samples presented and that they were not obtained from a single

shore normal transect.

2.4.13 In this Section the authors present a summary of the

preceding sedimentological data which they say shows a

remarkable consistency between the Gold Coast and the

Shoalhaven Bight on the NSW coast and the north east coast of

New Zealand. Given the modifications mentioned above it may be

argued that sediment boundaries along the NSW coast actually

show considerable variability with the inner nearshore/outer

nearshore boundary varying from lOm to 27m and the outer

nearshore/inner shelf boundary varying from 12m to 37m.

The variability or inconsistency highlights some of the

deficiencies of the simple classification of the inner shelf

and shoreface sediments as inner nearshore/outer nearshore and

inner shelf sands. The generalisation of this model appears to

be inaccurate in regions where innershelf "sand bodies" are

found. Here, "sand bodies" refers to the large convex sand

accumulations found at various locations along the NSW

innershelf (eg. Roy, 1985) . In these areas sediment boundaries

appear to differ from the more generally accepted model. Finer

sands in the region normally characterised by the "inner shelf

sands" results in an apparent extension of the "nearshore

sands" into deeper water suggesting the possibility of offshore

sand transport. The relationship between the finer sand body

sediments and those of the nearshore sediments in shallower

water has not been discussed by the authors.

The reason for the existence of these sediment boundaries

I (particularly the nearshore/inner shelf sand boundary) has also

not been discussed by the authors. This may be particularly

I important in that some of the sites which have similar wave

exposure show different sediment boundary depths - suggesting

the importance of factors other than the hydrodynamics. See Roy

I & Stephens (1980), Hanslow (1989)

2.5 Geomonjhological Data

I In this Section the authors present the argument that the

diversity of profile shapes below 15-20m indicates that the

I seabed below these depths may be largely relict. This diversity

of profile shapes may be in part at least attributable to grain

I size variability, underlying bedrock or substraight controls,

I and the variation in shelf sediment volume found along the New

South Wales coast. Diversity in profile shape below this depth

I may not therefore be an indicator of whether this part of the

shelf is 'relict' or not. Examination of nearshore profiles

I

along the NSW coast by Poyitt (1982) showed that even that part

of the nearshore profile which is assumed to be in equilibrium

varies widely from beach to beach.

I In this Section the authors state that there is a consistent

I geomorphological discontinuity in 24m of water along profiles

in the study region. Profiles presented in Figure 2.20 show

that this discontinuity is not consistent between different

I transects and is extremely subtle in any case.

2.6 Hydrodynamic Data

2.6.1 An additional reference is Reimnitz et al 1976

suggesting rip current transport to 30m water depth off the

Pacific coast of Mexico.

3. Analytical And Laboratory Studies

3.2.1 Swart

Swart's 1974 work was based on uniform monochromatic waves. Its

application using significant wave heights is probably

conservative. Most comparative studies of sediment transport

under regular and irregular waves indicate that the relevant

wave statistic for the estimation of sediment concentrations and transport rates is more like Hrms or even H. (See for example, Rasmussen & Fredsoe (1981), DHI (1989) and Evans

(1991)) . If the ENS wave height had been used in Swart's

expression the depth limit would have become 17.6m.

Use of such a long wave period (14s) is however non-

I conservative as the calculated depth limit moves inshore for

increasing wave period (presumably reflecting reduced

I steepness) . The depth limit is more sensitive to wave height

than period. The most appropriate evaluation of Swart's

I

I I

expression would probably yield a result between 17.6m and 26. 5m.

1 3.2.2 Hallerrneier

IIn calculating ds in equation 3.4 the authors use a wave height

Hs = 6.3m which represents the height exceeded 12 hours per year and Tp = 9.7s. The period adopted from Lawson et al (1987)

I is in fact the mean period for all data collected at five sites along the NSW coast. The period corresponding to the

I significant wave heights for the same data was 8.03s. Lawson et al also analysed data from the same sites for which the

I significant wave height exceeded 2.5m. The mean period of the

highest one third of waves (ie. that corresponding to Hs) for

I Hallarmeier's

this data set would be the more appropriate one to use in fou1a. Its value, 9.45s is, quite fortuitously,

little different from the adopted value of 9.7s.

In the last paragraph in this Section the authors state that:

"In respect of determining depth limits for marine borrow or

disposal, ignoring the effects of wave refraction, Hallermeir

(1981) advises that borrow [that is extraction] or disposal

might be conducted seaward of the ds limit without adverse

effect on the nearshore sediment cycle" p45.

Hallermeier (1981) actually advises that:

"In a two-dimensional region , marine borrow or disposal of

material should be conducted well seaward of the water depth

ds, so that the activity does not interfere with the seasonal

cycle of nearshore processes and the shoal zone function as a

source or sink of littoral sands. .....Mathematical and

laboratory models (Motyka and Willis, 1974; Horikawa et al,

1977) have shown that shoreline effects can be negligible with

a relatively shallow dredged hole (less than 10% of ambient

water depth) in moderately deep water : d > 0.1, for present gT

purposes. This result is quantitatively consistent with

negligible shoreline effects with a relatively shallow hole

located near or seaward of d0. Also, with the present

I I at

viewpoint,

three- a

it seems that

dimensional borrow or disposal might be conducted

region seaward of ds without adverse effect on the nearshore sediment cycle, if bottom elevations

Iand thus shore exposure are not significantly changed". Hallermeier (1981) p270.

Hallermeier (1981) therefore advises that extraction may be

conducted within the shoal zone without adverse impact on the

nearshore sediment cycle if the bottom elevations are not

significantly changed. Further, extraction at or near the

seaward limit of the shoal zone may be conducted with

negligible shoreline effects if the hole is relatively shallow.

3.3 Onshore Sand Transport

The application of van Rijn's work to this area may be

inappropriate for a number of reasons;

Although it is not perfectly clear, van Rijn's work is based

only on shoreparallel transport. In this case wave induced and

"steady" currents are essentially orthogonal.

Van Rijn's formulation is only proven for fairly strong

currents, U > 0.4U. However the values of the mass transport velocity which were used for U are usually less than U. Measured shorenormal transport rates under such weak currents

are often in the opposite direction to u. (See, for example,

Nielsen, 1988 & 1992 & Forster, 1980)

I The use of the Lagrangran mass transport velocity (eqn 3.7)

over-estimates sediment transport rates. The evaluation of

1 dz ought to be based on the Eulerian drift velocity which

is smaller by a factor 3/5. On the other hand much recent data

I do not support Longuet-Higgins's expression for the mass

transport velocity. You et al (1991) found that drift

I

velocities were approximately proportional to H, not H2 as in Longuet-Higgin' s model.

I Alternative calculations based on Nielsen's (1988) "Grap and I Dump Model" for rippled beds and Ribberink and Al Salem's

(1990) expression for sheet flow conditions do however, confirm

1 the author's finding that potential shorenormal transport rates

are likely to drop by 1.5 orders of magnitude from 15m depth to

I 30m depth.

Given the uncertainty surrounding the estimation of mass

I transport velocity it would have been appropriate and

instructive to estimate sediment transport flux at the

I Providential Head meter site for the deployment period using

recorded data.

Onshore transport rates are calculated for beach size sand in

depths between 40m and lOm. These rates should also have been

I calculated for sand sizes actually located at those depths.

I In addition, analysis of transport rates for grain sizes within

the distribution of sizes found at each depth may be helpful in

I determining the potential for transport at each depth.

I The applicability of a horizontal bed model such as that used

by the authors to the sloping shoreface should also be

discussed in more detail. In this respect application of other

1 models including bed slope may be appropriate.

4. Discussion And Conclusions

I The authors provide a synthesis of the foregoing data and come

up with three water depths which are consistent through the

analysis. The inner depth (12±4m) represents the subaqueous

I limit of the active beach face on an annual basis and

corresponds to the inner nearshore/outer nearshore sand

I boundary. The mid depth (22n±4m) represents the absolute limit

of offshore sand transport under cyclonic or extreme storm

I events and corresponds to the outer nearshore/inner shelf sand

boundary. The deepest water depth (30rn±5m) represents the

I practical limit of significant reworking and transport of beach

sized sand onshore calculated on a flat bed under wave action

alone.

I

I

I- As indicated in the previous sections there are some problems

with the analysis and interpretation used to arrive at these

depths. In particular the inner nearshore/outer nearshore

I sediment boundary and the outer nearshore/inner shelf sand

boundaries show some inconsistencies that are not included or

I explained.

I The outer

indicator

nearshore/inner

of the absolute

shelf sand boundary is used as an limit of offshore sand transport

resulting from an extreme storm. This boundary is used to

I represent the toe of the beachface. The authors indicate that it is found in 22m water depth within the study area. As

I indicated in the discussion of §2.4 the nature of this boundary

is more consistent with the inner nearshore/outer nearshore

sediment boundary. If this is the case the sediments located

I immediately seawards of this depth are still very much linked

to the beach.

In any case the potential for transport seawards of the outer

nearshore/inner shelf sand boundary has not been addressed. In

this respect some bottom current data around this sediment

boundary would be particularly helpful. The higher roughness of

the inner shelf sands may increase the potential for suspended

transport of the finer nearshore sands which, given appropriate

currents, may be transported offshore. This may occur even

though the actual boundary shows little change.

Bottom current data in approximately 35m water depth within the

study area shows significant offshore bottom currents

suggesting the potential for offshore transport. This current

data demonstrated the large number of possible processes

operative on the inner shelf which may effect sediment

transport and highlights the possible deficiencies of the

assumptions of nearshore sediment transport made by the

authors.

In paragraph 1 on page 60 the authors argue that extraction

landwards of the practical limit of onshore sand transport.

(Hallermeiers seaward limit) may result in a disequilibrium in

the onshore sand transporting forces and the nearshore slope,

with the possible effect of undermining the toe of the

I beachface. The calculation of Halleeiers seaward limit gave

35m water depth, however in the following paragraph the authors

I argue that the practical limit of onshore sand transport

actually occurs in 25m to 28m water depth. This value is based

I on an inflection point in the shore normal profiles within the

study area. Inflection points on these profiles are not

consistent and are extremely subtle. The value of 25-28m water

I depth is not consistent with §2.4 which places it in 24 metres of water.

On the basis of the foregoing analysis the authors conclude that extraction beyond the 30m isobath would have no effect on I the subaqueous beach profile fluctuations and beach sand transporting processes and would not cause beach drawn down.

I This conclusion may not be valid given the foregoing arguments.

I Li I I I

APPENDIX IX - SHELF SEDIMENT TRANSPORT

3. Regional Sediment Transport

3.2.3 Bedform Structures

In this Section the authors describe sandwaves off the area

south of Providential Head. These sandwaves are for the most

part asymmetrical indicating bedload transport to the south.

The authors suggest that these features may have been

responsible for the reworking of surficial sediments to a depth

of at least 6m over the Holocene. This reworking is used to

explain shelly layers within cores analysed by Roy (1985) and

Hudson (1985) . These cores were taken north of Providential

Head in an area where no sandwaves of the type described by the

authors are found.

Roy (1985) provided carbon dating results from these cores

which showed increasing age with depth below the seabed. Roy

suggests that these dates indicate sedimentation both during

the post glacial marine transgression and during the still

stand. Still stand deposits make up a surficial layer which

thickens seawards. The composition of this layer indicates it

has formed as a result of offshore transport over the still

stand. This is supported by the seaward thickening, lobate

nature of the deposit.

The authors argue that Roy's surficial unit is actually a zone

of reworking. While the possibility exists, it is not supported

by the general consistency of the carbon dates obtained by Roy,

which would be expected to show a more random pattern if the

whole unit had been reworked. The absence of sandwaves of the

scale described by the authors in the Bondi/Malabar region also

points away from the reworking argument.

With this in mind a more detailed discussion of Roy's (1985)

and Hudson's (1985) results would be relevant to this section.

Roy's model suggests that offshore transport has been

particularly important in the formation of the inner shelf sand

bodies. Hudson (1985) also provides evidence of northward

transport at Bondi which is not discussed by the authors.

The authors provide no indication of the rates of movement of

the observed bedforms and their significance for the total

shelf transport. Bedforms of this scale indicate quite strong

currents operating over extended periods of time. Data is

presented which suggest that these features are not presently

active. Does this indicate that the available current data does

not reflect potential future shelf transport?

3.2.4 Dykes

The authors argue that sand build up on the southern side of

transverse dykes is evidence of southward suspended transport.

This argument is dependent on minimal bedioad transport and

significant suspended transport up to 2m or more above the bed.

(This being the height of some of the dykes). In the proceeding

section the authors provide evidence of large bedforms

suggesting significant bedload transport. No data has been

presented to indicate sand transport 2m above the bed.

3.3 Hydrodynamic Data

3.3.1 Direct Field Current Measurements

The authors make no reference to their own current data which

shows significant onshore/offshore flows. This data supports

the geological arguments of Roy (1985) which suggest that

offshore transport may be important for the formation of the

inner shelf sand bodies. Roy (1985) notes that the thickness of

the upper unit of the sand bodies increases seawards suggesting

a decrease in the competence of these currents in approximately

60m water depth. This may explain the diminished importance of

these currents at the 60m and 80m current meter stations.

Interdependence of Waves and Currents

Here the authors stress the importance of considering

background currents in assessing the relationship between wind

I events and the resulting sediment transport. However they

I provide no rational mechanism to achieve this.

I From examination of the sand transport calculations in

subsequent sections it is apparent that the authors have

I assumed that current speed and direction are totally

independent of wave height, direction and speed. Whilst it is

true that there is insufficient data to effectively evaluate

I the probability of occurrence of all wave and current

combinations, a comprehensive assessment of the validity and

effect of this assumption is required.

It is noted that in calculating the sand transport rates for

1 the 1974 storm (4.5.3, Extreme Events), the authors have

assumed a zero background current.

Sand Transport

The comments made on the nearshore sediment transport

computations (Appendix VIII) are equally applicable to those

I presented here in §5 & §6. Specifically;

The applicability of van Rijn's model to the case of

relatively weak superimposed currents and to shore-normal

currents is doubtful.

The work used a current field description which relies on

limited data and data extended by poor and inappropriate

correlation. The calculation of sediment fluxes based

directly on the Providential Head current meter data would

have been instructive.

I Further,

There is a degree of contradiction between the adaption of

the model to local data wherein 1% exceedence values for

wave height and period are recommended and the author's

dismissal of earlier work suggesting storm dominance of

shelf sediment transport. Also the expression for ripple

heights taken from Nielsen (1981) was derived using

I

I

I I I I

significant wave heights. Use of this expression with other

than significant wave heights is quite inappropriate.

The computed along-shore and cross-shore sand transport

rates in the Cape Banks area (Figs. 5-3-5.6) show a

consistent trend of increasing sand transport potential with

depth. Examination of fall velocities presented in Table 5.2

suggests that the decrease in grain size was insufficient to

account for this trend.

The significance of sand waves has not been adequately

addressed. How often and how quickly do they move? Are they

tied in any way to nearshore processes.

The use of a horizontal bed model for transport calculations

means that the ability of sediment to pass through the

depression is in doubt. It is conceivable that the

depressions could fill at a rate equivalent to the gross

transport rate. Similarly the depressions could act as a

diode, intercepting offshore transport but preventing its

return onshore thus possibly leading to beach drawdown. A

comprehensive assessment of what slopes and presently stable

in various depths would have been useful.

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Deift Hydraulics Report No. H840 pt 3.

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FORSTER, D.N., 1980. Review of "Nearshore oceanography and

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SWIFT, D.J.P., NIEDORODA, A.W., VINCENT, C.E., & HOPKINS, T.S.,

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VAN RIJN, L.C., 1989. Handbook of sediment transport by

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11