Narrabeen-Collaroy Beach Survey Program

Explore Beach and Climate Indices

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Subaerial Volume Index (SVI)
The Subaerial Volume Index (SVI) is a simple measure of the overall (i.e., for the five cross-shore profile lines) accreted/eroded state of the beach relative to the long-term (1976-2006) average. A positive SVI indicates that the subaerial beach (the section of the beach system situated above mean sea level) has a greater amount of sand volume relative to the long-term average, whereas a negative SVI means that the subaerial beach has less sand volume and hence in a more eroded state.
Beach Orientation Index (BOI)
The Beach Orientation Index (BOI) is a gauge of the average orientation of the beach, as measured by the mean sea level contour, relative to the long-term (1976-2006) average. A positive BOI means that the beach has a more clockwise orientation relative to the long-term average, whereas as a negative BOI highlights a more anti-clockwise orientation. The BOI is an important indicator of beach rotation, where sand within the embayment alternates between the two extremities and can expose one end of the embayment to erosion.
Southern Oscillation Index (SOI)
The Southern Oscillation Index (SOI) records the monthly difference in surface air pressure between Tahiti and Darwin as a measure of El Niño and La Niña events in the Pacific Ocean. Consistent SOI values less than -7 generally indicate El Niño events, while consistent SOI values greater than +7 generally indicate La Niña events.