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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B7 - Waterborne Sediment Transport

1 INTRODUCTION

Waterborne sediment transport is a natural process whereby beaches continually adjust to the ever changing nearshore wave and water level conditions. Sediment is transported onshore, offshore and alongshore by the action of waves and currents. In response to this sediment movement, the beach undergoes a series of erosion and accretion cycles of short-term (weeks), medium term (years) and long-term (decades) nature. Man-made coastal works can disrupt this process.

An understanding of waterborne sediment transport processes is essential to the better management of coastal areas. Buildings constructed in the zone of active sediment movement can be imperilled by storm erosion of the beach. In some cases, beach accretion is beneficial, but it can also block stormwater outlets, reduce tidal flushing in coastal lagoons, inhibit safe navigation and exacerbate freshwater flooding. Coastal structures that interfere with waterborne sediment transport can markedly and permanently alter the shape and extent of a beach. A poorly sited port that interferes with sediment transport may require constant dredging to keep it operable.

2 PARTICLE SIZES

Unconsolidated sediment consists of particles of different sizes. Sediment size is an important factor in determining the ability of water to transport sediment and the rate of that transport. The smaller the particle sizes the greater the rate of transport. Sand and silt sized particles and to a lesser extent clay sized particles are of importance to waterborne sediment transport on the coast. "Clay" consists of particles smaller than 0.002mm; "Silt" consists of particles with sizes between 0.002mm and 0.06mm; "Sand" consists of particles in the range 0.06 to 2.0mm.

Figure B7.1

Figure B7.1 Particle Size Distribution, Coastal Sediments, New South Wales

Figure B7.1 shows the distribution of particle sizes of three sediment samples collected at different locations along the NSW coast. Sample "A" is a typical sand collected offshore from Sydney beaches. Sample "B" was collected from a beach along the lower estuarine reaches of the Clarence River. Sample "C" is a dune sand from Cronulla. Sediment "A" is a poorly graded medium sand. Sediment "B" is a better graded medium sand containing 5% coarse sand, 17% fine sand and 3% silt. The occurrence of silt in Sample "B" is typical of freshwater flooding effects along estuaries. Because of its finer nature, more of Sediment "B" than Sediment "A" will generally be moved by waterborne transport.

3 MOVEMENT OF SEDIMENT BY WATER

Flowing water can transport sediment particles as either "suspended load" or "bed load".

Suspended load transport refers to the suspension of sediment particles in the water mass by the action of turbulence (i.e. velocity fluctuations) and the subsequent transport of this material by currents.

Bed load transport is the process whereby currents "roll" sediment particles along the seabed. This process is assisted by turbulence, which can temporarily bring some particles into suspension. Currents cause these particles to "saltate", or move along the bed in a series of "hops".

Smaller particles are brought more easily into suspension than larger particles. Thus, suspended load typically consists of silt and fine sand particles (plus any clay if present). The more pronounced the turbulence, e.g. in the breaker zone, the more likely larger particles will be brought into suspension to be transported as suspended load or as saltating bed load. A minimum or "threshold" velocity is necessary to initiate bed load transport. This is of the order of 0.1 to 0.3 m/s.

4 SEDIMENT TRANSPORT IN COASTAL WATERS

The surf zone is characterised by high levels of turbulence caused by breaking waves, broken waves moving shorewards as "bores" and reflected waves moving back to sea. Vast quantities of sea bed sediments are brought into motion by this confused interplay of waves and currents. Much of this sediment movement does not contribute to net transport. However, we can distinguish two processes within the surf zone that do result in net transport:

The first of these is associated with longshore currents and the second with onshore/offshore currents (see Appendix B5). Seaward of the surf zone, inner shelf currents can also move sediment along the coast.

Finally, the various rivers and estuaries transport large quantities of clay, silt and sometimes sand into coastal waters, especially during times of flood. It should be noted that certain clay particles flocculate and settle out in salt water. This contributes to sediment build up off coastal entrances. The transport of sand sized particles by rivers and estuaries, although significant in the geological past, is now of lesser importance.

5 LONGSHORE TRANSPORT

Surf Zone

"Longshore Transport", or "Longshore Drift", refers to the movement of waterborne sediments along the coast. This occurs predominately in the surf zone, that is landward of the offshore bar.

The magnitude and direction of longshore transport depends in a complex way on wave height and period, the angle of incidence of waves to the shoreline, the bathymetry of the nearshore zone and the size and availability of sediment.

Depending upon wave and current situations, longshore drift will be towards one end of the beach or the other. On some beaches, sediment transport in both directions is balanced, resulting in zero net drift. On the NSW coast, there is commonly a potential for net northerly drift because of the dominant south-easterly wave climate.

Transport Rates

Longshore drift rates can be estimated from empirical formulae or by field measurement. The complexity of the processes that determine longshore transport precludes accurate estimates. (See Sayao and Kamphuis, 1983, for an extensive review of longshore transport).

Empirical methods, which are necessarily approximate, include the "wave energy flux" approach (CERC, 1984) and the "steady flow approximation" (Bijker, 1971).

Field measurement provides the most accurate assessment of longshore drift rates. Common techniques include sediment tracing (using dyed sand, radioactive isotopes, etc.), measurement of sediment build-up against coastal barriers (e.g. groynes, headlands), and the analysis of beach changes over time from survey plans or aerial photography.

Table B7.1 shows estimates of Longshore Drift at 32 locations along the NSW coast (Chapman et. al., 1982). These results indicate an increasing net northerly longshore drift in the North Coast Sector. Longshore drift rates in the Mid-North, Central and South Coast sectors are markedly less.

The higher rates of longshore drift in the North Coast Sector are attributed to the greater number of long beaches, fewer headlands, greater sand bypassing around headlands, greater availability of sediment and the increasing angle of incidence between the shoreline and the predominant wave direction.

It should be noted that the rate of longshore drift is highly variable, both over time and by location. Typically, storm conditions may cause an average year's transport to occur in a single week.

Table B7.1
Estimates of Long Term Longshore Drift and Shoreline Movement, New South Wales, Beaches
Location Distance From Sydney (km) Net Drift
(1,000 cum/yr)
Landward Movement of Shoreline (m/yr) AnalysisPeriod
(yrs)
Fingal 680 (N) 500 (N) Realignment 33
Dreamtime 675 (N) 350 (N) 0.6 33
Bogangar 668 (N) 250 (N) 0.8 15
Hastings Point 665 (N) 220 (N) 0.2 37
Pottsville 660 (N) 200 (N) 0.2 30
New Brighton 640 (N) 170 (N) 1.4 90
Byron Bay 630 (N) 80 (N) 1.0 90
Tallow Beach 625 (N) 65 (N) 0.2 26
Lennox Head 615 (N) ? (N) 0.9 26
Campbells Beach 448 (N) ? (N) 0.7 98
Macauleys Beach 445 (N) ? (N) 0.6 11
Boambee 440 (N) 75 (N) -4.5 38
Sawtell 435 (N) 75 ? 0.3 38
Old Bar 240 (N) ? 0.3 39
Diamond Beach 230 (N) ? (N) 0.2 42
Forster/Tuncurry 225 (N) 35 (N) Accretion 22
Boomerang Beach 215 (N) 30 (N) 0.3 25
Blueys Beach 210 (N) 30 (N) 0 17
Newcastle Bight 120 (N) 0 1.0 21
Soldiers Beach 65 (N) 5 (N) 0.1 36
Wamberal Beach 45 (N) 12 (N) 0.4 19
Avoca Beach 40 (N) 9 (N) 0.4 19
Palm Beach 25 (N) 11 0.5 37
Collaroy Beach 15 (N) 0 0.2 37
Bate Bay 30 (S) 0 0.5 90
Warilla 85 (S) 25 (N) 0.9 34
Shoalhaven Hds 125 (S) ? 1.0 33
Culburra 130 (S) ? 0.3 29
Narrawallee 180 (S) ? 0.2 35
Mollymook 185 (S) ? 0.2 35
Bermagui 310 (S) ? 0.5 20
Tathra 350 (S) ? 0.2 35

Note: N:North S:South ?:Not Assessed

6 ONSHORE/OFFSHORE TRANSPORT

Storm and Swell Wave Profiles

The steep waves that occur during storms erode sand from the beach berm and dune areas and transport it offshore to build a "storm bar". Rip cells are an important mechanism in offshore transport during storm conditions. A pronounced "dune scarp" in the foredune area commonly marks the landward extent of storm erosion. The resulting beach profile is termed the "Storm Profile". This is depicted in Figure B7.2. Multiple storm bars may be formed off exposed beaches or when wave energies are very high.

The effect of the storm bar is to widen the surf zone and flatten the slope of the surf and swash zones. This causes waves to break further offshore, which imposes a self regulating limit on beach erosion. Nevertheless, extensive erosion can and does occur before self regulation prevails.

At other times, ocean swell of longer period and lower height tends to rebuild the beach with sand from the offshore bar (see Figure B7.2). The "mass transport" current of these waves is the dominant process which transports sand shorewards (see Appendix B6). The rebuilding process commences immediately after a storm, but several years may be required to transport most of the sand back to the beach after a major storm event.

Figure B7.2

Figure B7.2 Beach Profiles: Accreted and Storm Conditions

Swept Prism

The area in which sediment is mobilised by onshore/offshore processes may extend seawards for several kilometres and landwards into the dunefield. This envelope of "active" sand is termed the "Swept Prism" (Chapman and Smith, 1980). Any buildings or structures erected within the onshore area of the swept prism, i.e. the active beach zone, will be exposed at some time to erosion. Structures erected within the offshore area of the swept prism may interfere with the onshore/offshore movement of sediment.

Equilibrium Beach Profile

For a given wave height, wave period and sediment size, there exists an equilibrium beach profile for which onshore and offshore transport are in balance. For medium and coarse sands, the equilibrium beach slope is about 4o and 7o respectively. Because of the ever changing wave conditions and the distribution of sediment sizes, the equilibrium beach profile is rarely attained. The ever changing nature of the beach profile is, nevertheless, the natural process of seeking this equilibrium state.

Transport Rates

Onshore/offshore sediment movement can be assessed theoretically or by field measurement. The equilibrium beach profile associated with a particular sediment and a given set of wave conditions can be derived from empirical formulae (Swart 1976; Vellinga, 1983). For any other beach profile and set of wave/sediment conditions, erosion, accretion and final beach profile can be estimated. Field measurements of beach and nearshore profiles made before and after storms indicate the extent and direction of onshore/offshore transport.

7 INNER SHELF TRANSPORT

Shoaling begins to significantly effect swell waves in water depths of about 60m, which marks the seaward limit of the Nearshore Zone or Inner Shelf Zone. The net seabed velocities of wave induced currents in these deeper waters are generally low. However, during storm wave conditions, wave induced oscillatory bottom currents can bring sediments into suspension, which facilitates transport by currents. The active transport of sand by this mechanism has been observed in water depths of over 60m (Gordon and Hoffman, 1984).

At headlands, large scale inner shelf currents can interact with wave induced longshore drift in the surf zone. This can lead to the interception of surf zone sediment, and its "loss" from the beach by offshore transport.

8 SEDIMENT BUDGET

A sediment budget is one means of assessing long-term shoreline recession, i.e. whether or not the coastline is gradually but progressively moving landwards. A coastal "compartment" or control volume is defined, the boundaries of which consist of physical or morphologic features across which the rate of sediment transport can be meaningfully assessed (see Figure B7.3). The net transport rate into the compartment from all sediment transport processes is estimated. If positive, the shoreline is accreting. If the net transport rate is negative, then the coastline is receding. Table B7.2 lists various sediment sources and sinks that can respectively supply and remove sediment from the control volume.

Table B7.2
Sediment Source and Sink Processes in a Typical Sediment Budget Study

Sources

Sinks

Longshore Transport Longshore Transport
Beach Erosion Beach Accretion
Cliff Erosion Deposition in Rivers/Estuaries
Onshore Transport Offshore Losses
River Supply Wind Blown Sand Losses
Beach Nourishment Wave Washover off Beach Sand and Mineral Mining
Figure B7.3

Figure B7.3 Typical Sediment Budget (after NSW Govt.,1987)

In undertaking sediment budget studies it is often difficult to distinguish between short-term fluctuations (e.g. storm effects) and long-term trends. A detailed investigation of both present day coastal processes and historical trends in shoreline change is necessary. Present day short-term fluctuations in the sediment budget can be identified from the relocation of sediment within the compartment. This in turn enables any long-term sediment gains or losses to be identified, thereby providing an indication of trends in shoreline stability. The undertaking of a sediment budget study and the interpretation of results requires considerable care.

9 IMPACT OF DEVELOPMENTS

All coastal, river and shelf developments need to be assessed to determine their impact on the sediment budget. This impact cannot be adequately assessed without a good understanding of the mechanisms and magnitude of sediment transport. This in turn requires data concerning offshore bathymetry, waves, tides and currents.

If an area is subject to longshore transport, great care needs to be taken with developments built in the surf zone. Structures such as a groynes will interfere with the longshore drift and are likely to cause accretion on the upstream side and erosion on the downstream side (see Appendix C2).

If significant onshore/offshore sediment transport exists, care should be taken with the siting and design of coastal structures. For example, the effects of seawalls and similar structures on waves may generate more turbulence and stronger reflected waves than would naturally occur. This may result in sediment being transported further offshore than would normally be the case. Recovery of the beach in front of the seawall will then take longer.

10 REFERENCES

Bijker, E.W., (1971). "Longshore Transport Computation". J. Waterways, Harbours and Coastal Engineering Division, ASCE, Vol. 97, pp. 687-701, 1971.

Chapman, D.M. and Smith, A.W., (1980). "The Dynamic Swept Prism". Proceedings of the 17th International Conference on Coastal Engineering, Sydney, March, 1980.

C.E.R.C., (1984). "Shore Protection Manual". Fourth Edition. Coastal Engineering Research Centre. Waterways Experiment Station, US Army Corps of Engineers, US Govt. Printing Office, Washington, D.C., 20404. Two Volumes.

Chapman, D.M., Geary, M., Roy, P.S. and Thom, B.G., (1982). "Coastal Evolution and Coastal erosion in New South Wales". A report prepared for the Coastal Council of New South Wales. Published by the Coastal Council of New South Wales, 1982.

Gordon, A.D., (1987). "Beach Fluctuations and Shoreline Change - NSW". Proceedings of the 8th Australian Conference on Coastal and Ocean Engineering, I.E.Aust., Australia, November, 1987.

Gordon, A.D. and Hoffman, J.G., (1984). "Sediment Transport on the South-East Australian Continental Shelf." Proceedings of the 19th International Conference on Coastal and Ocean Engineering, Houston, Texas, 1984.

NSW Government, (1987). "Beach Dunes Their Use and Management", Brochure prepared by the NSW Government Department of Public Works and Soil Conservation Service, 1987.

Sayao, O.F.S.J. and Kamphuis, J.W., (1983). "Littoral Sand Transport Review of the State of the Art". Department of Civil Engineering, Queen's University, Kingston, Ontario. C.E. Research Report No. 78, January 1983.

Swart, D.H., (1976). "Predictive Equations Regarding Coastal Transports". Proceedings of the 15th International Conference on Coastal Engineering, Hawaii, ASCE 1976.

Thom, B.G. and Hall, W.F. "Behaviour of Beach Profiles during Erosion and Accretion Dominated Periods". Earth Surface Processes and Landforms. (In press).

Vellinga, P., (1983). "Predictive Computational Model for Beach and Dune Erosion during Storm Surges." Delft Hydraulics Laboratory, Publication No. 294, February 1983.