To understand Tweed Sand Bypassing and why the project was necessary, it is important to have an understanding of coastal processes. The coastal zone can be viewed as an intricate system made up of different components that are all interconnected. The three main components of the coastal system are:
- the underlying geology such as coastal headlands and near shore islands;
- the nature and abundance of the coastal sediment; and
- the extent to which these controls are acted on by the waves and wind.
These components all interact to form what are known as coastal processes. These coastal processes operate over a range of temporal and spatial scales and are responsible for moving sand from one place to another, changing the shape of the beaches.
Longshore sand transport is the process that moves sand parallel along a beach or coastline. On long open beaches such as at the Tweed and Gold Coast, this process moves sand predominantly from the south to north, pushed by the prevailing south easterly waves.The crashing waves push the sand onto the beach; sand is then dragged offshore in the swash zone, before being pushed back onto the beach again. When the wave direction is from the north east, sand is moved in the opposite direction, from north to south, but this does not happen very frequently.
In one year anywhere between 200,000 and 1,000,000 cubic metres of sand moves northwards along Letitia Spit. The average volume is approximately 500,000 cubic metres, which is enough sand to fill 200 Olympic sized swimming pools. This river of sand starts in large sand reserves just north of the Clarence River in mid-northern NSW and flows parallel to the coast before slipping over the continental shelf and into very deep water, just north of Fraser Island in Qld.
Cross shore sand transport is the process that moves sand perpendicular to the coastline. Cross shore sand transport is the dominant process in smaller pocket beaches such as those in Sydney, where headlands impede the movement of longshore sand transport. During calm conditions when the waves are not very big, this process moves sand from offshore reserves, onto the upper beach above the water line. Conversely, during storm events, large waves move sand from the beach, causing erosion scarps to form and deposit the sand offshore in deeper water.
Coastal processes move sand from one place to another, and also over a range of time scales. The following provide examples of how coastal processes move sand over a range of different time scales.
- When a wave breaks, sand moves through the water, which happens almost instantaneously.
- During a storm event, large waves and high tides can erode sand from the beach and deposit it further offshore in deeper water. This process happens over a few hours.
- During calmer wave conditions, waves move sand from offshore sand deposits onshore, gradually making the beach wider. This process happens over several weeks.
- Beaches can change seasonally, with the beach increasing in width in summer when there are smaller waves pushing sand onshore and then decreasing in width in winter when large waves move sand offshore. This process can take several months.
- Climatic effects such as the Southern Oscillation Index which is responsible for El Niño and La Niña, can affect the amount of storms and cyclone events which are responsible for moving large amounts of sand in a short period of time. These climatic cycles can be many years long.