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H 85
Prepared for:
Queensland Government
Gold Coast Queensland Australia
Southern Gold Coast littoral sand supply
Final Report
Volume I, Executive summary
October 1992
delft hydraulics
•
Gold Coast Queensland Australia
Southern Gold Coast littoral sand supply
J.A. Roelvink and R.J. Murray
delft hydraulics
Gold COast Queensland. Australia H 66 Ootober 1992
delft hyd,aulic!)
Contents
List of figures
1 Introduction............................... . . . . . . . . . .. 1 1.1 Background....................................... 1 1.2 Approach......................................... 2 1.3 Lay-out of the report. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2
2 Sediment Budget Study ................................... 4 2.1 Method.......................................... 4 2.2 Area of Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4 2.3 Coastal Units ...................................... 4 2.4 Dates........................................... 5 2.5 Determination of accretion/erosion rates . . . . . . . . . . . . . . . . . . . . .. 6 2.6 Nourishment....................................... 6 2.7 Sinks ........................................... 7 2.8 Visualisation of bottom changes in plan view. . . . . . . . . . . . . . . . . ... 7 2.9 Inferred Longshore Transport Rates ........................ 8 2.10 Conclusions ....................................... 9
3 Process analysis study ................................... 10 3.1 Introduction...................................... 10 3.2 Conclusions...................................... 10
4 Coastal Engineering Interpretation of Geological Studies ............ 12 4.1 Introduction...................................... 12 4.2 Large-Scale Shape .................................. 12 4.3 Sedimentological Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 13 4.4 Conclusions...................................... 14
5 Conclusions .......................................... 15
Figures
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list of figures
1 Location of beaches and sounding lines 2 Fingal to Currumbin coastal units 3 Example plot of survey profiles, beta 4.8 4 Evolution of cross-sectional area 5a-d Measured volume changes and inferred longshore transport rates 6a Tweed River review, Phase IIa study; difference plot 1972-1966 6b Tweed River review, Phase IIa study; difference plot 1983-1966 6c Tweed River review, Phase IIa study; difference plot 1989-1966 6d Tweed River review, Phase IIa study; difference plot 1983-1972 6e Tweed River review, Phase IIa study; difference plot 1989-1972 6f Tweed River review, Phase IIa study; difference plot 1989-1983 7 Alongcoast volume changes per metre and longshore transport
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1 Introduction
1.1 Background
Queensland's Southern Gold Coast Beaches (from Point Danger to Currumbin) have experienced severe and progressive erosion since the time of the seawards extension of the Tweed River training walls (from 1962 to 1964) across the updrift littoral sand transport system. This erosion behaviour was not typical for these beaches, and had not previously been evidenced.
Responses to this erosion by the Queensland Government and the Gold Coast City Council have included:
(a) the construction of long seawalls to protect threatened properties and short groynes to retain beaches at selected locations; and
(b) substantial nourishment projects to offset the erosion deficit by obtaining sand from outside the active littoral system.
The latest of these nourishments has been the 3.6 million cubic metre Stage 1 of the Southern Gold Coast Besch Nourishment Project, the wo.rks of which were implemented from November 1989 to May 1990 during the course of the present study, and which has substantially offset the severe erosion from Kirra to Tugun.
In 1970 DELFf HYDRAULICS completed a report for the Queensland Government titled "Gold Coast, Queensland, Australia - Coastal Erosion and Related Problems" (Report R257) which evaluated data available at that time and recommended solutions to coastal problems for the entire Gold Coast. This report was a basis for the Beach Protection Authority's 1973 Coastal Management Plan for the city (originally titled Scheme of Works) which was a comprehesive plan of counter-erosion actions, and which has been a basis for plarming beach works.
A number of further studies of coastal processes in the Gold Coast region have been undertaken subsequently, and extensive databases of coastal phenomena and behaviour have been collected by the Beach Protection Authority and Council.
Further works have been proposed for the area including additional beach nourishment to offset the residual erosion and provide a storm buffer, and an artificial sand by-passing plant at the Tweed River (with associated dredging) to provide a safer and more efficient entrance without interrupting the transport of littoral sand past the entrance.
The aim of the present study was to provide a reliable understanding of beach processes in the Fingal to Currumbin area in order to aid the planning of future works and the proposed revision of the Coastal Management Plan for the. City of the Gold Coast.
A report by DELFT HYDRAULICS titled "Gold Coast, Queensland, Australia - Tweed River Review Study - Report on Phase I" (Report H85) was completed in January 1988, and consisted of an examination of existing data and reports, and recommendations for further
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investigation. The present detailed investigation (under the working title "Tweed River Review Study - Phase IIa) was initiated in June 1989 and has involved further data collection, further analyses of data and numerical modelling of relevant phenomena.
The study was carried out jointly by DELFf HYDRAULICS and Queensland Government officers. This report was prepared by J .A. Roelvink of DELFf HYDRAULICS and RJ. Murray of the Queensland Government. Quality assurance was by Prof. Dr. E. W. Bijker.
1.2 Approach
The mechanism of sand by-passing around the Tweed River entrance and Point Danger is extremely complex. The present state of knowledge of morphological processes and the accuracy of the available environmental data on waves and currents is such that mathematical/physical models exercises can improve the insight into this mechanism, but cannot be used to predict or hindcast the past, present and future rates of bypassing within the required accuracy.
On the other hand, a vast amount of data is available on the changes in the bathymetry over the period of 1966 to present, and on the sedimentological and geological characteristics of the area.
The approach that has been chosen is therefore, to set up a sediment budget model of the area from Fingal Head to Currumbin. Such a sediment budget can be used to infer the distribution of the longshore transport along the coast, given:
(a) a known input or output longshore transport rate; (b) the measured bottom changes over a certain period; (c) losses to or gains from adjacent seaward or landward areas.
Because of the amount of available data, the major part of the sediment budget can be based upon solid bathymetric data. Where there are gaps in the data, additional process analyses with the help of mathematical/physical models and sedimentological/geological data and analyses are applied to fill in these gaps.
The physical and geological analyses additionally serve to improve the insight in the mechanisms needed to extrapolate results or to evaluate the effect of future changes.
1.3 Lay-out of the report
This report presents the results of the detailed investigations. It considers and uses the large database on coastal processes in the area and utilizes advanced analysis and modelling techniques. The report's conclusions supersede those of the Delft Report of 1970 (where they consider the same matters).
The report consists of two Volumes: Volume I is an "executive summary", which briefly outlines the results of the study: Volume II contains the full Text and Figures. Both volumes have the following general lay-out.
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The analysis of the database on coastal profile measurements, which leads to a sediment budget model, is presented in Chapter 2. In Chapter 3, results of analyses of the underlying processes based on numerical models and process measurements are discussed. In Chapter 4, a discussion of geological and sedimentological studies from a coastal engineering point of view is given. The conclusions are outlined in Chapter 5.
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2 Sediment Budget Study
2.1 Method
For a coast which can be seen as a set of coastal units that are linked in along-coast direction, the law of conservation of mass dictates for each coastal unit:
INFLOW + SOURCES - OUTFLOW = ACCRETION
Where for this purpose,
INFLOW Net long-shore transport inflow OUTFLO Net long-shore transport outflow SOURCES Sediment inputs from other than long-shore transport; sources can
be positive (ie. nourishment) or negative (ie. wind blown losses landward)
ACCRETION - Accumulation of sediment in the unit (erosion is simply negative accretion)
The outflow from one unit is the inflow into the next unit. Generally, the sediment budget is used to derive long-shore transport rates from known accretion rate and sources. Starting at a coastal unit, where either the inflow or the outflow rate is known, the equation can be solved consecutively for all upcoast and for downcoast units.
2.2 Area of Interest
As the main emphasis of this study is on the effect of the extension of the Tweed River training walls in 1962-1964, the area for the sediment budget must be extended to places on either side where the influence of this extension has not (yet) been felt. Therefore, the study area has been chosen from Fingal Head in the south to Currumbin Beach in the north. The location of beaches and Beach Protection Authority sounding lines for the Gold Coast area is shown in Figure 1.
2.3 Coastal Units
The study area is divided into a number of units, and the time span into a number of periods, so that the sediment budget model can be used to calculate the time history of longshore transport at a number of points along the coast.
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The coastal units are chosen so that:
(a) the number of units provides enough spatial resolution; (b) the profiles in each unit behave in a similar way; (c) Each unit is (where possible) described by a consistent set of similar data (such as
parallel survey lines); (d) Breaks and control points occur at unit boundaries.
Of course, the combination of units must completely cover the study area.
The seaward boundary of each unit is taken at the 15 m depth contour, except at the Tweed Bar, where it is taken slightly deeper. From sedimentological analyses and model computations, it follows that no natural transport of any significance occurs across this boundary. The landward boundary of each unit is also chosen at the line of no significant change, which in the southern areas is in the dunes, and in northern areas is usually a rock seawall.
The coast units thus chosen are shown in Figure 2. In Table 1 below, some relevant data on the units are given.
Coastal Unit Location
Untit (-) Untit (-) Untit (+) Range Mid-pt. Unit Lines
(m) (m) (m) (m)
LETITIA -4400 BTA2 -1000 3400 -2700 ETA 4,5,6,7,8,9,10
TWEBD BAR -1000 PWD STNI -350 650 -675 TOMLINSON & HRANISANLEJEVIC
PUBLISHED ANALYSES
DURANBAH -350 PWD 394N 0 350 -175 ETA 12, 13; OMEGA 7 SNAPPER ROCKS 0 ETA 13 520 520 260 OMEGA 7,9,11; RB 1,2,3,4,5
COOLANOATIA 520 BETA 1.0 1200 680 860 BETA 1.2,1.5,1.8,2.1,2.4,2.7
KIRRA BAST 1200 BETA 2.7 1740 540 1470 BETA 2.7,3.0,3.3,3.6,3.9
KIRRA CBNTRAL 1740 BBTA 4.05 2220 480 1980 BETA 4.2,4.5,4.8,5.1
KIRRA NORTH 2220 BBTA 6.45 2700 480 2460 BIITA 5.4,5.7,6.0,6.3
BILINGA SOUTH 2700 ETA 18 3480 780 3090 BTA 16,17,18 B1LlNGA NORTH 2480 BTA 21.5 4880 1400 4180 ETA 18,19,20,21
TUOUN SOUTH 4880 BTA 24.5 6080 1200 5480 ETA 22,23,24
TUGUN NORTH 6080 BTA26.0 7080 1000 6580 BTA 24,25,26
Table I Coastal units
2.4 Dates
In order to suppress 'noise' in the results by averaging over a number of years, and still obtain a reasonable idea of the changes of transport rates over time, a time interval of 5-10 years for the analysis is desirable. Combined with the available data, this has led to the choice of the following standard dates:
1966 1972 1983 1989
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By interpolation in time if required, or (in special cases) space, survey results for these dates were obtained.
2.5 Determination of accretion/erosion rates
For each survey line, the profiles for the four standard dates were plotted in one figure and visually checked for errors; if necessary, bad data points were erased or datum errors corrected. An example is given in Figure 3 for line Beta 4.8 in the Kirra Central unit.
Changes in the cross-sectional area between subsequent surveys were determined for each profile; the results for all lines in a coastal unit were then integrated to produce the volume changes between surveys. Plots of the results for all lines per coastal unit are given in the Volume II (the full report); an example (units Kirra Central and Kirra North) is given in Figure 4. Indicated are the gross changes in cross-sectional area, the effect of sources on these areas and the evolution 'net of sources', which gives an idea of what would have happened without the sources.
For some units, no fully detailed information was available, so use was made of integrated results from other analyses.
The measured volume changes averaged over the whole period and over the separate time intervals are presented in Figures 5a through 5d.
2.6 Nourishments
From 1974 onwards, a number of nourishments have been carried out to mitigate the effects of erosion in the study area. A summary is given in Table 2 below, the results are shown in Figures 5a through d.
Nourishment quantities (thousands of cubic metres) Coastal urnt
1974175 1985 1988 1989
COOLANOATIA 22 KIRRA EAST 191 KIRRA CENTRAL 382.5 203 K1RRA NORTH 382.5 315 311 BlLINOA SOUTH 430 BlLINOA NORTH 352 50 TUOUN TUGUNNORTH
K1RRA NORTHKIRRA K1RRA NEARSHORE BlLINGA DEMONSTRATION
TOTALS 765.0 315 1509 50
Table 2 Summary of nourishment works
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2.7 Sinks
Possible sinks in the sediment budget are:
(a) Losses of littoral sediment to the Tweed River (b) Transport across the seaward boundary (c) Direct sand extraction from the beaches or losses in landward direction due to wind.
The losses to the Tweed River are mainly due to infilling of dredged channels. Based on literature it was estimated that this has led to the losses from the littoral system listed in Table 3.
Total volume Volume/yr Period
(1000 m') (1000 m'/yr)
1966 - 1972 220 38 1972 - 1983 640 56 1983 - 1989 300 55
Table 3 Estimated losses to Tweed River
Offshore losses due to transport across the seaward boundary are discussed in Chapter 3 and 4. In Chapter 3, it will be shown that it is highly unlikely that significant amounts of sediments other than fines are carried beyond the Tweed Bar unit by the current. Extreme flood events may be an exception to this; because of their limited frequency of occurrence, this influence would still be relatively small. In Chapter 4, sedimentological analyses show conclusively that there are no significant offshore deposits of nearshore sediments aged less than 6000 years, in the area from Currumbin to south of Fingal. Therefore no significant offshore losses need to be considered.
Losses in landward direction can be neglected; no significant wind-blown transport has been reported. Extraction from the beaches for mining purposes has been more than compensated for by the mining contractors and by sand excavated from building sites, which has to be put on the beach. These quantities are small and can be neglected for this study.
2.8 Visualisation of bottom changes in plan view
For the area of Coolangatta to Tugun, the depth data were interpolated onto a very fine rectangular grid. This enabled a different presentation of the bottom changes, as well as a check on the analysis of the volume changes. The latter showed an inaccuracy of the total volumes over the coast units considered of less than 5 %. In Figures 6a-f, the results are presented in plan view colour plots.
It should be noted that the coastal features shown on Figures 6a-f are those existing at present, and not necessarily at the dates to which the plots refer.
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The 1972 minus 1966 figure shows the very severe erosion at Coolangatta which led to the building of the Kirra Point groyne. There is significant erosion along all of Kirra. Some beach areas at Bilinga show accretion. The 1983-1972 figure shows the sand trapped at Coolangatta by the Kirra Point groyne. Most of the Coolangatta unit seaward of a line between the groyne head and Greenmount has eroded. In Kirra East, the sand passing the tip of the Kirra Point groyne has resulted in some small increases in level adjacent to the point, but offshore of this the seabed in nearshore areas has eroded. Kirra Central experienced substantial erosion. Kirra North also eroded.
The cumulative effect of these 2 periods is shown in the 1983-1966 plot (Figure 6b). The upper beach at Coolangatta has accreted because of the groyne, but offshore of this there is a large area where 0.5 to 2.5 metres of sand have eroded from the seabed. Kirra East shows even deeper erosion of the seabed. Kirra Central has a large area where in excess of 4.5 metres of vertical erosion have occurred, and the previous beach has been replaced by a gutter in front of the rock seawall. Kirra North shows erosion of up to 4.5 metres. It is noted that the shades of colour on these figures show that the magnitude of the changes show a general decrease in an offshore direction, and at the limit of the shaded area(which corresponds to the minus 15 metres contour delimiting the coastal units) the changes are virtually zero. The 1989-1983 figure (Figure 6f) shows accretion at Greenmount corresponding to a lobe of sand. There is an increase in the level of nearshore areas in the Kirra Unit, and partially at Coolangatta, as a result of the 1988 bottom dumping of nourishment sand in the nearshore zone.
The overall difference plot 1989-1966 (Figure 6c) shows an improvement to the upper beach in the Coolangatta unit. This is partially due to the groynes, but the accumulation at the Greenmount end of the unit is due to the lobe in 1989. Nearshore areas of the unit are still eroded by up to 2.5 metres. Kirra East is generally eroded except for a small tongue of net accumulation at the dump site. Kirra Central is eroded in excess of 1.5 metres for 200 metres from the rock wall, but there is accumulation at the 1988 dump site. Kirra North is similar but there is another tongue of accumulation inshore of the dumping.
Bilinga South shows these two bands of accumulation clearly, but the upper beach is eroded. Bilinga North shows the northern extent of the bottom dumping and the inshore accumulation. The upper beach is eroded.
The comparison 1989-1972 (Figure 6e) clearly shows the erosion of the upper beach of Bilinga in this period (it had accreted in the period from 1966 to 1972).
2.9 Inferred Longshore Transport Rates
As was explained in Section 2.1, the results we have now obtained can be used to infer longshore transport rates at the different locations, averaged over different time periods.
In Figure Sa, the inferred longshore transport rates, averaged over the whole period are shown; a long-term average input of 500,000 m'lyr at Fingal has been assumed, in accordance with the consensus in literature. The resulting transport rate at Tugun is 488,000 m'/yr. The long-term average transport difference between Fingal and Tugun is therefore a mere 12,000 m'/yr.
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The inferred longshore transport rates for the three periods are shown in Figures 5b-d. In Figure 7, the top graphs show the along-coast distribution of the gross and net volume changes for the three time periods, and the bottom graph the inferred longshore transport rates for these periods. This clearly shows how the effect of the Tweed Walls has progressed northwards in time. The effect of the interruption of the sand supply was initially felt by the nearest beaches such as Coolangatta which eroded, thus maintaining a moderate supply to Kirra. With worsening erosion, the ability to supply sand diminished, and Kirra eroded severely. The pattern repeated in a northwards direction with Kirra and Bilinga, then Bilinga and Tugun.
The bypassing rate out of Letitia had, over the last period, returned to the pre-1962 rate; bypassing out of Duranbah (over the NSW border) had returned to 57% of the pre-1962 rate over the last period, and was increasing as natural littoral sediment pathways reestablished.
2.10 Conclusions
The overall effect up to May 1989 of the 1962-1964 extension to the Tweed Walls has been the accretion of approximately 7.6 million cubic metres south of Point Danger in New South Wales and the erosion of 7.2 million north of Point Danger in Queensland (6.6 and 6.3 respectively in the period 1966 to 1989).
Natural bypassing of the entrance has shown a strong increase in the period 1983 to 1989 compared to the low values from 1966 to 1983, with the result being substantial accretion in the vicinity of Duranbah, Point Danger and Rainbow Bay and the re-establishing of sand transport pathways.
The erosion shadow has lengthened, deepened and moved northwards since the building of the walls. Beach nourishment, to May 1989, has replaced 2.6 million of the 7.2 million cubic metres of overall erosion to that time. Since then, Stage 1 of the Southern Gold Coast Beach Nourishment Project from November 1989 to May 1990 provided an additional 3.6 million cubic metres of nourishment, for a total nourishment quantity to date of 6.3 million cubic metres.
The virtually stable size of Letitia Spit and the approach of Tweed River infilling to previous equilibrium states are resulting in an increasing bypassing of the entrance with time.
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3 Process analysis study
3.1 Introduction
The process analysis study has involved a number of investigations with numerical models, and analyses of measurements of waves and currents. The purpose of these investigations was twofold:
• to improve the understanding of results obtained in the sediment budget study, in order to make better use of that model; and
• to investigate "missing links" in the sediment budget, such as a possible offshore loss at the Tweed Entrance or Point Danger area.
For a description of the investigations the reader is referred to the full report in Volume II. Here, only the main conclusions are presented.
3.2 Conclusions
The process analysis study, presented in Chapter 3 of the full report, has led to the following conclusions:
• The wave-induced longshore transport is the most important factor which determines the development of the coastline on a time-scale of years to decades.
• The shape of the coast arc between Tugun and Point Lookout is very similar to a computed equilibrium coastline for a realistic longshore transport rate which is constant along the coast. This indicates that there is no significant transport gradient along this part of the coast.
• Detailed longshore transport computations have been carried out for the locations Tugun and The Spit, using different transport formulations and estimates of the directional wave climate. These computations largely confirm the findings of the BPA 1981 report, and contradict the earlier conclusions in the Delft 1970 report with respect to the transport difference between Tugun and The Spit. The main reason for this is found in the limited data on the directional wave climate at the time of preparation of the Delft 1970 report.
• At present, the data on the directional wave climate are still not entirely conclusive. This, and the fundamental difficulty of accurately predicting sediment transport rates, lead to a relatively large error in the prediction of the absolute magnitude of the mean annual transport rates at both locations.
• Based on the computations only, the estimated transport rate at The Spit is 500,000 ± 250,000 m'/yr.
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• The difference in transport rate between Tugun and The Spit is very likely to be less tban 100,000 m3/yr, based on tbe longshore transport computations.
• Numerical simulations of tbe cross-shore behaviour of tbe profiles at Tugun and The Spit indicate no significant transport across tbe 15 m deptb contour line.
• Velocity measurements carried out at tbree points near tbe Tweed Entrance during very calm weatber indicate a general Soutbward transport trend under tbese circumstances. This is, however, not representative of general conditions. The importance of wave-induced orbital velocities for tbe transport rate is confirmed even under tbese conditions.
• Velocity measurements at 7.8 m water deptb near Snapper Rocks were carried out over a period of some montbs. These data were combined witb simultaneous wave measurements to provide an estimate of tbe variability of tbe sediment transport. From tbis it appears tbat a general, current-dominated Soutbward transport is episodically interrupted by large wave-dominated Nortbward transports. Nearer to tbe shore tbis effect can be expected to be even stronger, which confirms general observations tbat bypassing takes place in a slug-like fashion during storms, and is on average directed Nortbward.
• Principle computations were carried out to investigate possible mechanisms for a significant offshore loss due to tbe ebb jet over tbe Tweed Bar. These computations indicate tbat it is ratber unlikely tbat such a loss is of any significance.
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4 Coastal Engineering Interpretation of Geological Studies
4.1 Introduction
Although in recent years considerable effort has been put in measuring and analysing the coastal processes that lead to the present-day developments at the Gold Coast, the state of our knowledge is by no means enough advanced to make straight-forward predictions based on a given situation.
Therefore, it is necessary to use data on processes on a geological time-scale in order to test our concepts and to be able to accept or reject our hypotheses.
In the following, we shall look first at the large-scale shape of the coast. Here, the question to be answered is whether the coastal arc between Point Danger and Point Lookout has been eroding naturally or not.
Second, sedimentological data will be used to identify the paths along which sediment transport takes place; in some cases it can be used to quantify sediment budget terms.
This summary is based on work by many researchers, a reference list of the relevant studies is given in Volume II.
4.2 Large-Scale Shape
The evolution of the large-scale shape of the coastal arc from Point Danger to Point Lookout can be explained by an increasing longshore transport over the whole coast from southern New South Wales to southern Queensland due to the increasing angle of the predominantly south-eastern swell with the coast. This led to a retreating of the coast except near rocky outcrops. As the coast in each section retreats, the shape is adapted in such a way that the transport in each section is almost constant. In the case of predominantly oblique wave incidence, this leads to a zeta-shaped coastline between the headlands. For a large coastal area such as at the Gold Coast, this process must have taken in the order of 30,000-150,000 years, and has probably taken place during the interglacials in the Pleistocene.
During the last 6-7,000 years, the sea level has been almost constant at the present height. In this period, sand from the relatively shallow inner continental shelf has been moved onshore and deposited on the coast in the form of dunes and barriers, and in the nearshore zone. Elsewhere on the SIl Australian coast it has been shown that this took place mostly in the first 3000 years of this period. As the coastal profile approached an equilibrium shape, the onshore transport decreased and the coastline stabilised.
The result is a wedge of Holocene deposits overlying the Pleistocene deposits down to a water depth of approximately 20-30 m. The nearshore sediments were constantly reworked in their enviromnent of high wave energy and transported alongshore. Where there were sand losses from the system, this resulted in gradual erosion; in the case of some northern New
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South Wales beaches, virtually all Holocene deposits were eroded away. This has not happened at the Gold Coast, the Pleistocene coastline can be identified at approximately 200-500 m from the present shoreline. This indicates that the sand losses are small. Since at the Gold Coast sand losses could only be caused by longshore transport gradients, this means that such transport gradients must be relatively small. The conclusion to be drawn is that over the last 6-7,000 years, the Gold Coast beaches have accreted on average, mostly during the first 3,000 years. After this the natural situation has been quite stable, with an almost constant longshore transport rate along the coast and only little on-shore transport from the inner shelf to the nearshore.
4.3 Sedimentological Data
From many studies, it appears that three main "sedimentary units" (that is, clearly distinguishable types of sediment) are found in the depth zone from 0-30 m.
• inner nearshore sand (INS)
• outer nearshore sand (ONS)
• inner shelf sand (ISS)
Of these, the ISS sediment unit at present hardly interacts with nearshore processes, therefore we shall concentrate on the nearshore sediment units. The INS unit is found in the upper part of the profile, generally from the shoreline to approximately 10m depth. It indicates an almost continuous sediment pathway along the New South Wales and southern Queensland beaches. This unit is moved by the high-energy littoral drift which is northward on average, driven by the dominant south-eastern swell. The outer nearshore sediment unit is found in a continuous strip offshore of the INS unit, in depths of approximately 10-25 m. This unit is moved by currents (East Australian Coast Current, and occasionally wind driven currents) while waves act as stirring agents, especially during storms. The net transport direction for this unit is less clear-cut than for the INS unit; certainly it does not playa dominant role in the sediment budget for the Gold Coast.
In some cases, notably at Cape Byron and Point Lookout, the headland protrudes in such a way that the east Australian coast current can impinge on the coast and interfere with the nearshore sediment transport. In such cases, a lobe of nearshore sand is found south of the headland. This lobe can be seen as a loss to the littoral system. In the case of Cape Byron, this offshore accumulation of inner and outer nearshore sand amounts to approximately 510,000,000 m3 over the past 6,000 years, or an average yearly loss of approximately 84,000 m3/yr. At Point Danger, which protrudes much less sharply than Cape Byron and Point Lookout, no such lobe was found; only a slightly anomalous lobe of outer nearshore sand northward of Point Danger, with a total volume of approximately 3,000,000 m3 (170 times less than the lobe at Cape Byron) was found. This constitutes a negligible loss of approximately 500 m3/yr.
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4.4 Conclusions
The main conclusions for the Gold Coast area, based on the sedimentological analyses, are therefore:
• There are no significant losses of nearshore sediments to offshore at Point Danger.
• There is no significant onshore transport into the nearshore zone.
• There is a clear pathway for longshore transport around Point Danger; this is mostly confined to depths less than 10 m.
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5 Conclusions
The extension of the Tweed Walls in 1962-1964 has had the effect of reducing natural bypassing past the Tweed River Entrance. Due to this, Letitia Spit has accumulated, as has the Tweed Bar.
Natural bypassing at the Tweed Entrance has also been reduced due to infilling of the lower reaches of the Tweed River in response to dredging. The main dredging operation here has been in 1974-1975 and has been for beach nourishment of the Kirra beach. Although there certainly was a short-term benefit to the Gold Coast beaches, the long-term net effect of this nourishment has been nil, due to the reduction of the natural bypassing.
Since Letitia Spit and the Tweed Bar have almost reached a new equilibrium, and because the Tweed River is also approaching the pre-1962 situation again, natural bypassing has greatly increased, and has almost reached the rate of prior to 1962.
Contrary to the hypothesis stated in the Delft 1970 report, the conclusion is now reached that no significant offshore losses occur either at the Tweed Entrance or offshore of Snapper Rocks. This conclusion is based on sedimentological evidence and is substantiated further by 'worst-case' simulations of the sediment outflow due to the ebb jet at the Tweed Entrance.
The hypothesis of an offshore loss in the Delft 1970 report was based on computations of the longshore transport rate at Tugun and Letitia Spit, which resulted in a large difference in transport capacity. Since this difference should have resulted in substantial gains in the area between those points, and since these gains were not observed in the measured promes, the 'missing' sand was thought to be lost to offshore.
As was already stated in a BPA 1981 report, the Delft 1970 computations of longshore transport rates were (of necessity) based on scarce and inaccurate wave data from ships logs. The computations in 1981, and those carried out now, are based on much more accurate data from wave recorders. Although there is still some uncertainty as to the absolute magnitude of the transport rate, the present computations consistently show that the difference in longshore transport, i.e. between Tugun and The Spit is not more than 20% of the total. This is further substantiated by considerations on the large-scale coast shape, which has been shown to agree very well with an equilibrium coast for a constant transport rate of approx. 500,000 m'/yr.
Based on the sedimentological analyses and on model results, it can be concluded that the sediment budget applied in this study well covers the full littoral zone, which extends to approx. 20 m water depth at the Tweed Entrance and to approx. 10-15 m in other areas.
The sediment budget can therefore be used to infer longshore transport rates at various locations and time periods, for a given input transport rate at Fingal of approx. 500,000 m'/yr, according to literature.
Volume I - 15
Gold Coast, Queensland, Australia H 86 October 1992
The following results are obtained from this sediment budget:
• In the period from the 1962-1964 extension to the Tweed Walls up to May 1989, an overall gross accretion of 7.6 million cubic metres has occurred in New South Wales, and an overall gross erosion of 7.2 million in Queensland. This is clearly due to the interruption of the longshore transport by the extended Tweed Walls.
• The effect on the coast morphology and on the longshore transport rates is clearly shown in Figure 7. The erosion shadow has progressed northward and has deepened.
• The bypassing rate at the Tweed Entrance prior to 1962 is very likely to have been close to that at Fingal, i.e. approx. 500,000 m3/yr. This sharply dropped to almost zero directly after the extension. Over the period '66-'89, the by-passing rate increased: 110,000 m3/yr from '66 to '72, 220,000 m3/yr from '72 to '83, and 290,000 m3/yr from '83 to '89.
• The predicted bypassing rates, based on extrapolations of the further growth of the Tweed Bar and infilling of the Tweed River are 425,000 m3/yr over the period '89-'99, and 465,000 m3/yr over the period 1999-2009, provided no works are undertaken.
• This means that an additional deficit of an estimated 1.1 million m3 is to be expected for the coming 20 years.
Mitigating measures undertaken by various Queensland authorities to combat the effects of the erosion have included the building of groynes and sea walls and nourishment of the littoral zone with sand from the Tweed River and from offshore sources. Their effect is summarized below:
• The groynes have been effective in locally stabilizing or even creating a beach. However, they invariably aggravate the situation for the beaches to the North. Their influence extends only partly across the active beach profile.
• Sea walls serve only to protect property from direct attack by storms. They certainly have no beneficial effects on beaches.
• The '74-'75 nourishment of Kirra Beach provided short-term relief, but no long-term benefit, since the dredging in the Tweed River had the effect of decreasing the natural bypassing around the Tweed Walls.
• The nourishments with sand from offshore sources will provide a long-term benefit for Queensland's beaches. The nearshore nourishments were placed within the active littoral zone and thus provide a benefit to the beach system.
delft hydraulics Volume I - 16
~ .o~.t -----------______ ----u ------ _____ --u ------
--------------_ .. --------- ..
E :::=------- --------_ .. -------
QUEENSLAND
___ ----tJ -------- ___ 41
------------jn". "0" ____ U -----------
-------_________ 00
------- -------"
LOCA TION OF BEACHES AND SOUNDING LINES
DELFT HYDRAULICS H 85 FIG_ 1
0 rrl r 'l -I
I -< 0 ::0 » C C 0 (/)
.I
co (J1
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SCALE 1 : 55 000 Approx
o 1 234 5 I ! !
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EXAMPLE PLOT OF SURVEY PROFILES, BETA 4.8
DELFT HYDRAULICS H 85 FIG. 3
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EVOLUTION OF CROSS-SECTIONAL AREA
DELFT HYDRAULICS H 85 FIG. 4
• -8
-30
+137
1966 - 1989
MEASURED VOLUME
CHANGES (1000 m3 /yr)
500
1966 - 1989
:INFERRED LONGSHORE TRANSPORT
RATES ( 1000m3 /yr)
MEASURED VOLUME CHANGES AND INFERRED
LONGSHORE TRANSPORT RATES
DELFT HYDRAULICS H 85 FIG. 50
MEASURED VOLUME CHANGES
(1000m' /yr)
+254
-76
./ ,.-..:. --~ -,--
I /' +8 /
1966 - 1972
INFERRED LONGSHORE TRANSPORT
RATES (1000rT)' /yr)
(across unit boundaries)
/ , /
/
/
~28 '.34
'-46 34
-27 \
f \ -12
+142
1972 - 1983
MEASURED VOLUME CHANGES AND INFERRED
LONGSHORE TRANSPORT RATES
DELFT HYDRAULICS
73
+42 ......:.- 78
+49 ";--114 \" '~ " +48 -c--35
+62 ..L..4
+6
1983 - 1989
I
~\
1983 - 1989
H 85 FIG.5b-d
vnl I MF r.I-',,,,r.,,:: PFJ'l MFTJ'l1= --
~
~'1
r-------- - ----" l\
~ 1---------
~~ -
-,--------- , ., G n~",,4. t ALONGCOAST DISTANCE (m) ~ 66-72 -+- 66-83 -+- 66-89
vnll'ME r.I-'H''iE PER INETOF SOUl
/\ /~\ .
-- ~ \" - ~ l\
N- ... / .----:::::::::= "
" /I .
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ALONGCOAST DISTANCE (m) ~ 66-72 -+- 66-83 -+- 66-89
'" _._---
~ "" ~ ~
.., ~ . ",I", ~ ~
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. , -, ., , Th" .. J,
, ALONG COAST DISTANCE (m) ~ 66-72 --+-- 72-83 _-.-- 83-89
FINGAL POINT DANGER
ALONGCOAST VOLUME CHANGES PER METRE AND LONGSHORE TRANSPORT
DELFT HYDRAULICS
-------
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.-
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TUGUN
H 85 FIG. 7
ORIGIN Of THE XI-YI AXES: 20108.09 E I 21348.28 N .!; (BASE POINT Of ETA 25.9) accretion _ RBOUE 1.5
0 -i >= ANGLE BETUEEN XI-AXIS AND E-DI~ECTIOH : 38deg52'29"«I.<k~i5e) t [AliI 3.58 - 1.5
::ii:
r D 1.S0 - 3.5
TJ 0 1.58 - 1.5 TJ fT1 fT1 fT1
vertical 0 a.so - 1.5 ;;0 0 o 9.88 - 3.5 fT1 change 0 -9.59 - 0.0 z ;;0
(m) 0 -1.59 - -9.5 0 < 0 fT1 fT1 300°1
~ 8 -1.50--1.5 fT1 ;;0 -3 .50 - -1.S , '"0 . 0 -uo - -3.5 TJ , ;;0 -i 0 fT1 erOSion b _ I mOM -1.5
I -j <
.-< ~ fT1 I \ \ \ \ \ 0 (() ::ii: 2400 I \ \ \ \ \ ;;0 ~ I \ \ \ \ \ >- N '"0
1 \ \ \ \ c I , >- 1 \ \ \ \ 0 (/) 1 \ \ \ \ fT1 (/)
(() 1800 1 \ \ \ \ OJ I=l 1 \ \ \ \ OJ 0
I \ \ \ (/) \ \ \ 1 \
-i \ \ \ c 1 \
\ \ \ 0 1200 --1_ .. \ \ \ -< I ~
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\
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BILlHGA HOHH B ILlNGA sou TH
--I LJ TJ C)
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~I I I 0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000
e. XI (nil
ORIGIN OF THE XI-VI AXES : 29198.99 E / 21348 .28 N
~ (BASE POINT OF ETA 25.9) accretion _ ABOU( 4.5
o -i ;:: ANGLE BETUEEN XI-AXIS ANO E-OIRECTION : 38degS2'29"(tlotkwise) i ~ 3.se - '.5 I,, ··) l.se - 3.5
'1 :E
r ~ 1.se - 1.5 '1 ITl
ITl ITl vertical 0 e.se - 1.5 ;:0 0 h 0 e.oe - e.s ITl c ange 0 -9 .50 - e.8 z ;:0 (m) 0 -1.5e - -e.s o < t 1 ~'l -1.58 - -1.5 0 ITl ITl 3000
1 ITl ;:0 G::J -3.58 - -/ .5 r IJ _ -i.SO - -3.5 '1 r ;:0 erosion _ 8ELOU -i.S -i o ITl
I -i < I I \ \ \ -< ~ ITl
0 ill :E 2400 I I \ \ \ ;:0 (Xl I I \ \ \ }> (,oJ IJ
I I \ \ \ c I r }> I I \ \ \ 0 (J) I I \ \ \ ITl ,
\ \ \ , (J) ill 1800 I I m r=1 I • I \ \ V m 0
\ I \ \ .J. F \
(J) I I \ \ ..r \ -i \ ..r \ \ C I I \ ~ \ \ 0 1200 I I \ \ -< I I \ . .\
I I \ I I ' \ I
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I I I I I I
TUGUN~ \ ~ KrRRA EAST \ _ .' :.,
(Xl (.Jl. • •
°1 BILINGA Nom BILINGR sourn
....J LJ '1 G)
~ I I I -600' 0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000
XI (0)
0 rrI r '1 -;
I -< 0 ;;0 » C r 0 Ul
I
co (.J1
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'1 ~ '1 rrI rrI rrI ;;0 0 rrI
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(() m ):::1 m 0
Ul -; C 0 -<
3000
2400
laoo
1200
600
a
.5 ;:::
r
a
ORIGIN OF THE XI-YI AXES: Z818S .89 E / ZI34S.ZS N (BASE POINT OF ETA ZS.8)
ANGLE BETUEEN XI-AXIS AND E-DIRECTION : 38degSZ ' Z9"(clockwise)
I \ I \ I \ I \ I \ I \
\ \ \ \ \ \ \
\ \ \ \ \ \ \ I \
I \ \ \ \ .\
TUGUN SOUTH
600
I I
-. L iii" (. :"'~t;:'
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1200
I \ I \ I I
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B IUNGA NOR TH
1800
I 1 I " ,,'"Y' i>ccoo~""'- . :.:.. I
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BIUNGA SOUTH
2400 3000 3600
\ \ ~
\ \ .\ \
4200
\ \ \ \
accretion
t vertical change
(m)
~ erosion
\ \ \
A80UE 6.5 5.50 - 6.5 <.50 - 5.5 3. 58 - 1.5 1.58 - 3.5 1.58 - 1.5 0.58- 1.5 8.88 - 8.5
-8.50 - 8.8 -1.58 - -8.5 -1.58 - -1. 5 -3.58 - -1.5 -•. 58 - -3.5 -5.58 - -<.5 -6.58 - -5 .5 8ElOU -6.5
\ . \ #' \ \
\ \ \ \. \ \
:"'A<' \ '. \
KlRRA EAST
4800 5400 6000
----eD>~ XI (m)
accretion _ AiOi
ORIGIN OF THE XI-YI AXES: 28188.89 E / 21318.28 N accretion _ ABOUE 6.5 e t - 5.58 - 6.5 _ (BASE POINT OF ETA 2S.8) _ '.511 - 5.5
o -; ;::: ANGLE BEllJEEN XI-AXIS AND E-OI~ECTION : 38degS2'29'"«[o<kwise) _ 3.59 - ' .5
'I :E i CJ 1.58 - 3.5 'I fT1 D !.S8 - 1.5 fT1 8 vertical 0 8.59 - 1.5 ~ change 0 8.08 - 8.5 Z ;:0 (m) 0 -8.58 - 8.8 () < 0 -1.58 - -8.5
o fT1 fT1 3000 I ~,;.I -1 .59 - -!.S P -u;:O EJ -3.50 - -1.5
'I ,;:0 ~ ~ -us - -3.5 -; 0 fT1 - -5.59 - -' .5 I -; < . - -6.5& - -5.5 -< ~ fT1 I \ \ \ \ erosIOn _ mo~ -6.5
o <.O:E 2400 I \ \ \ \ ~ ~-u I \ \ \ \ c I I \ \ \ \ , }> I \ \ \ \
~ <.0 ~ 1800 I \ \ \ . \ '-I ~ I \ \ c\ .' N I \ \ _...r \
(j) I \ \ .r' \ ~ r "- I \ ,-J~ \ o 1200 ~- , \
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II I I ~~' I ", '~' ;;;;.,~~ K[~~A EAST TUGUN SOUTH - J__ .:- . Wb" L.' .~~\ . . ....
(Xl
J U VI 'SILINGA NORTH BILINGA SOUTH
'I C)
• • • -O\J\J
0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000
~I I I ,.. XI (0)
0 I'l r ...,., -i
:r: -< 0 :::0 }> c r () (f)
:r:
co U1
...,., C)
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-u r 0 -i
~
\0 CO \0
\0 CO 0J
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:::0
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:::0 I'l < I'l ~
-u :r: }> (f) I'l
~ 0
(f) -i C 0 -<
3000
2400
1800
~
;::
i
o
o
ORIGIN OF THE XI-YI AXES: 20108 .09 E 121348.28 N CBASE POINT OF ETA 25 .0)
ANGLE BETWEEN XI-AXIS AND E-DIRECTION : 38deg52'29"Ctlotk.ise )
. BILlHGA NORTH
600 1200 1800 2400
1\\ I \ \ 1\\ I \ \ I \ \ I \ I \ I \ I \ I \ I \ I \ I \ \ \ \
\
\
I \
BILINGA SOUTH
3000 3600
\ \ \ \ \ \ \ \ \
4200
accretion
t vertical change
(m)
~ \ erosion \ \ \ . -\
\ \
K1RRA EAST
ABDUl: 6.5 5.59 - 6.5 1.50 - 5.5 3.58 - 1.5 1.58 - 3.5 1.58 - 1.5 0.58 - 1.5 8.88 - 8.5
-0 .58 - 8.0 -1.58 - -8. 5 -1 .58 - -1.5 -3.58 - -1 .5 -1.58 - -3.5 -5.58 - -1.5 -6 .58 - -5 .5 BElOY -6.5
4800 5400 6000
----eE>~ XI Co)
