1 On the effects of wave induced drift and dispersion in the Deepwater Horizon oil spill 1 Eugenio Pugliese Carratelli, 1,2 2 Corresponding Author, epc@unisa.it 3 Fabio Dentale 1,2 4 Ferdinando Reale 1 5 (1) Maritime Engineering Division University of Salerno, Department of Civil Engineering, via Ponte don Melillo – 84084 – Fisciano 6 (SA), Italy 7 (2) CUGRI (University Consortium for Research on Major Hazards), P.zza Vittorio Emanuele – 84080 – Penta di Fisciano (SA), Italy 8 Abstract 9 The objective of this work is to provide an indication of the effects of wave-induced movement of 10 oil on the sea surface in connection with the Deepwater Horizon oil spill. By making use of 11 modeled wave fields, satellite altimeter and buoy data, mean trajectories and wave-induced oil 12 spreading are computed for some of the storm events which took place during the accident. The 13 effects of mean Stokes’ drift are confirmed to be an important element in most situations, causing 14 spill movements of 30 km and more in about 5 days. The diffusion due to random wave movement 15 is also shown to be relevant at least for smaller spills; for large accidents, its effects are less 16 important, but it still has an influence on some aspects of the oil spreading. 17 1. Introduction 18 The influence of waves on the movement of floating pollutants is, at least in principle, a well known 19 phenomenon. The effects of wave radiation stresses in inducing surges and currents, and therefore 20 advection, has already been demonstrated on the West Florida Shelf [Huang et al., 2010]. On the 21 upper sea surface, however, Stokes drift plays a very important role even if it is hard to distinguish, 22 conceptually and practically, between Stokes drift and wind effects; adding a percentage of the 23 wind velocity to the ocean surface velocity as described by Huntley et al. [2011] is indeed a 24 traditional method. 25 Recent oil spill tracking procedures, such as MOTHY [Daniel et al., 2003] and GNOME [Beegle- 26 Krause, 2001], include Stokes average wave drift in the calculation of forward movement of 27 floating pollutants, and an interesting example is provided by Daniel et al. [2004] who graphically 28 show the importance of Stokes drift in determining oil spill trajectories during the Prestige accident. 29 Less well known are the effects of horizontal dispersion due to the random nature of wave fields 30 and in particular to the angular spreading of the wave spectrum. As early as 1982, Herterich and 31 Hasselmann [1982] provided some formulas for the calculation of Wave Induced Surface 32 Dispersion (WISD) in fully formed seas. Later work by Buick et al. [2001] and by Pugliese 33 Carratelli and collaborators [Piro et al., 1992; Bovolin et al., 1997; Giarrusso et al., 2001] yielded 34 further numerical evidence. The importance of WISD in relation to other dispersive effects such as 35 underlying sea turbulence or current variability, was discussed by Herterich and Hasselmann 36 [1982], following Okubo’s classical work [Okubo, 1971], which suggested that the spatial scale is 37 of paramount importance and that other effects become dominant above the scale of a few 38 kilometres. 39 The recent progress in the measurement and computation of ocean physical parameters provides 40 opportunities for testing and re-assessing these conclusions; the Deepwater Horizon oil spill [Liu et 41 al., 2011a] is an occasion to improve oil slick modelling and forecasting tools. 42