Using remote-sensing techniques for the monitoring of existing seawalls

2013 
Maintenance and upgrading of existing seawalls for protection against storm erosion and adaptation to climate change are often limited by the availability of 'as-constructed' design drawings. The main failure mode of seawalls is undermining, particularly for rigid seawall structures, in which erosion by wave, tidal and current action causes the beach level to drop below the footing of the seawall. The standard methodology for determining buried toe levels of structures usually relies on the excavation of trial test pits, which require heavy machinery, can cause damage to the structure and be problematic due to access limitations. This paper presents a review of a combination of non-invasive, remote sensing techniques to quantify and assess the relevant characteristics of existing seawalls that may be buried, partially buried or inaccessible. The results of a buried seawall assessment in Bilgola Beach (NSW) are presented as a case study. The assessment was undertaken to review the suitability of remote sensing techniques based on Ground Penetrating Radar (GPR). The GPR surveys were accompanied by Real Time Kinematic Global Positioning System (RTK-GPS) measurements of the ground level and a series of jet probe test boreholes to confirm the presence and levels of hard substrate. The crest level, toe level and slope of the buried seawall were readily measured by the GPR and validated by the test borehole results. The advantages, disadvantages and limitations of using GPR for mapping existing buried seawalls are discussed.
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