Geophysical investigation of storm debris at Collaroy-Narrabeen Beach, Australia

This paper presents an innovative coastal management geophysical survey to detect buried storm debris at Collaroy-Narrabeen Beach, conducted following the most destructive erosion event experienced on Australia's southeast coast in the last 40 years. Collaroy Beach is characterised by legacy foreshore development within the active coastal zone, with the southern beach segment backed by informal coastal protection works comprising rock, fill and other structures. In June 2016, a powerful East Coast Low erosion event resulted in significant loss of beachfront land and damage to foreshore infrastructure. When beach access conditions were safe, Northern Beaches Council were quick to undertake large scale clean up works, but by that stage, natural recovery of the beach had already begun and some debris was concealed by sand. An environmental geophysical study was initiated to investigate whether any objects/materials that posed a potential safety hazard remained buried within the beach. The survey utilised a vehicle mounted 3D ground penetrating radar (GPR) system, magnetic gradiometer and electromagnetic conductivity meter. The survey had to navigate several site constraints including beach access difficulties (wave uprush, erosion gullies) and sub-optimal (saline/electrically conductive) environmental conditions. The 3D GPR clearly imaged the size, geometry and depth of many objects in high resolution including small scale features. Ferrous (magnetic) objects were readily identified by the magnetic gradiometer, which can also map areas such as the upper swash zone where electrical based methods including GPR and inductive conductivity are not effective. The survey indicated that large scale remedial debris removal works were not urgently required. The erosion debris mapping will help guide future clean up works in parallel with the construction of a future seawall. This paper also explores advantages and limitations of the applied methods, with reference to coastal zone applications.