GEOPHYSICAL EVDIENCE OF STORM IMPACT AND RECOVERY: A GROUND PENETRATING RADAR INVESTIGATION, BAY-BAY SPIT, BICOL, PHILIPPINES

The Philippines lie in the path of tropical cyclones, and many of its coastlines are prone to inundation due to storm events. On 30^thNovember 2006, typhoon Durian made landfall in Bicol province, on the east central Philippine coast. Google imagery from May 2007 revealed that Durian had breached the spit that runs southeast from the mouth of the Quinale River at Bay-Bay village towards Tabaco City. The imagery also showed that, although the breach site showed signs of partial recovery, there was still geomorphological evidence of the inundation event associated with typhoon Durian. A field visit was conducted in 2012 to map the geomorphological features. However questions remained unanswered. Had similar events occurred in the past? If so, was there any evidence remaining on the ground, and could this evidence be dated?

In June 2013 we conducted Ground Penetrating Radar (GPR) surveys on the Bay-Bay spit to investigate if there was any subsurface evidence of previous storm events. The surveys comprised five, 1.5 km, longshore profiles and 12 cross-shore profiles, with lengths ranging from approx. 50 m at the breach site, to approx. 200 m at the northern end of the study site. The GPR system used for this study was a Sensors and Software Noggin with 100 MHz antennae mounted on a smart cart. Antennae separation was fixed at 0.5 m and acquisition was controlled by odometer wheel. Hyperbolae matching was used to determine near surface velocities in order to estimate depth. Topographic and positional data were collected using a South Survey S82-T GNSS system. Processing and plotting utilised both EKKO_View and ReflexW software and applied dewow filtering, automatic gain control, horizontal and vertical averaging, to the data. Depth of penetration varied from 2 m to 8 m. The cross-shore GPR profiles reveal at least two erosional events prior to 2006 typhoon Durian, with approximately 10 m of recovery and progradation between each erosion surface. The GPR profiles that captured the erosional features were revisited in September 2013 for trial pitting, stratigraphic description, and sediment sampling. Sediment cores were taken horizontally from the trench walls and vertically from the trench bases to date sediments using Optically Stimulated Luminescence (OSL) method, which eventually could constrain the timing of the erosional surfaces.