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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix C: Coastline Hazards

Appendix C5 - Sand Drift Hazard

1 INTRODUCTION

Sand drift is caused by windborne sediment transport and is a seemingly inexorable coastal phenomenon: all sandy beaches experience sand drift to a greater or lesser extent. Moreover, it is a seemingly slow moving and gradual process, but short episodes of strong wind can move surprisingly large volumes of sand (transport is proportional to the cube of the wind's velocity).

Sand drift is a serious problem along the NSW coastline. A recent study has shown that 5,600 ha, or 11% of the total area of coastal dunes, is completely bare of vegetation and is undergoing active drift (Chapman, et al., 1987). A further 4% of the total dune area is in a state of incipient destabilisation leading to unimpeded drifting.

Sand drift creates a variety of hazards. At best drifting sand is a nuisance; at worst it represents a permanent loss of sand from the beach system and may completely overwhelm coastal developments. Detrimental effects include the abrasion of motor vehicles, buildings, vegetation and park and garden fittings; the burial of roadways, rail lines, agricultural land and coastal ecosystems; the blockage of street gutters and stormwater drains; and structural damage to buildings caused by forces imposed by the sand.

2 THE IMPORTANCE OF DUNE VEGETATION

In its natural state, dune vegetation provides an aerodynamic "cover" which deflects salt laden wind over the dunes. This minimises wind attack in dune areas and acts to trap any wind blown sand (see Appendices B8 and Appendices B9).

Breakdown of the vegetation canopy can expose sensitive foredune species to salt burn (especially rainforest plants). This can lead to irreversible degradation of protective vegetation, which in turn leads to dune "blowouts". Wind velocities are faster through the throat of the blowout, which causes more sand to drift. Thus, the blowout grows in a seemingly inexorable fashion and a moving "slipface" is formed at the rear of the dune (see Figure C5.1).

Dune Blowout Slipface

Figure C5.1 Dune Blowout and Slipface

The revegetation of exposed dune and beach areas is hindered by sand drift. Difficulties can also arise if exposed areas are first, remote from a source of nutrient supply (normally the hinddune and strandline areas), second, remote from the source of recolonising seedlings, and finally, exposed to a microclimate considerably harsher than that of hinddune areas.

3 FACTORS CAUSING SAND DRIFT

Sand drift in a coastal location is usually initiated by the degeneration or destruction of vegetation protecting the vital foredune. Common causes are foot and vehicle tracks devoid of vegetation running down the face of the dune. Studies have shown that as few as 50 vehicular passes per year are sufficient to prevent the revegetation of tracks.

Sand blown inland is permanently lost to the beach system and is not available to assist in the natural rebuilding of the dune system following wave damage. This leaves the dune system in a weakened condition to combat the next series of storm waves, which in turn results in progressively more serious beach erosion and may ultimately lead to shoreline recession.

4 SAND DRIFT PROBLEMS ALONG THE NSW COASTLINE

There are many examples along the New South Wales coastline of damage caused to adjacent land and property by sand drift. These include:

Dwellings threatened by sand drift, Anna Bay, NSW

Figure C5.2 Dwellings Threatened by Sand Drift, Anna Bay, New South Wales

Figure C5.3 shows the change over the period 1967 to 1987 in the area of active sand drift along the New South Wales coast. The quite striking decrease in the drift area north of latitude 31o is largely attributable to invasion of these areas by bitou bush.

Changes in area of active sand drift NSW coastline, 1967 to 1987

Figure C5.3 Changes in Area of Active Sand Drift Along the New South Wales Coastline, 1967 to 1987

Bitou is a vigorous coloniser. In addition to colonising bare areas, it also displaces native dune vegetation and destroys the habitat of native animals.

In the long term, bitou provides a less resilient protection against wind and wave erosion than native vegetation. The removal of significant areas of bitou by storm attack or through biological or herbicidal control could trigger widespread sand drift in areas where it presently dominates dune vegetation.

5 THE CONTROL OF SAND DRIFT

With regard to the control of sand drift hazards, it is a case of prevention being much better than cure. The dune system of a beach plays a vital role in coastal processes. Dune vegetation, especially native vegetation, can be quite fragile. Stabilisation of the source area to cut off the supply of sand is the only way to control a moving dune slip face. Any other means of control (short of complete removal of the sand itself) is doomed to failure.

The prevention and control of sand drift requires a dune management plan. This should incorporate a dune survey to delineate areas at risk plus appropriate management practices to sustain healthy dunes and revegetate blowouts. All of these issues are addressed in the Coastal Dune Management Manual of the Soil Conservation Service (SCS, 1990). Techniques of dune stabilisation and dune maintenance are thoroughly described in this manual. Officers of the Service are available to assist local councils formulate and implement dune management plans.

6 REFERENCES

Chapman, D.M., Wickham, H.G., Bergs, M., and Strike, T.M., (1987). "Coastal Dunes, New South Wales: Status and Management Directions". Proc. 8th Australasian Conference on Coastal and Ocean Engineering, Canberra. I.E.Aust. pp. 90-97

SCS, (1990). "Coastal Dune Management - A Manual of Coastal Dune Management and Rehabilitation Techniques", eds. P.A. Conacher, D.W.B. Joy, R.J. Stanley and P.T. Treffry, Soil Conservation Service of New South Wales, 1990.