Hurricane Response of Nearshore Borrow Pits from Airborne Bathymetric Lidar

Abstract

Airborne bathymetric lidar surveys taken in Florida before and after the severe 2004 and 2005 hurricane seasons show infilling of seventeen dredged nearshore borrow pits. During these seasons, groups of pits captured volumes that were the equivalent of up to four years of net longshore transport, even though only one of the seventeen pits studied was inside the presumed depth of closure. Unsurprisingly, dimensionless infilling increased strongly with the ratio of wave height to pit depth. For open coast pits with large alongshore lengths, cross-shore infilling appeared to dominate over longshore infilling but both processes may be of comparable importance in shorter pits. Infilling of three borrow pits adjacent to ebb shoals was found to be considerably larger than on open coasts. Bathymetric changes in borrow pits occurred at greater depths than on nearby undisturbed profiles. Crude estimates of the long term infilling rates from tropical cyclones indicate that annual infilling volumes may be equivalent to more than one quarter of the expected net longshore transport at some locations. However, the episodic nature of hurricanes means that infilling events will be highly irregular.