A comparison of two different types of shoreline boundary conditions M. Brocchini a, * , I.A. Svendsen b , R.S. Prasad b , G. Bellotti c a DIAm, University of Genoa, Via Montallegro 1, 16145 Genova, Italy b CACR, University of Delaware, Newark, DE 19716, USA c DSIC, Universit  a degli Studi Roma Tre, 00146 Roma, Italy Received 2 January 2002; received in revised form 9 June 2002 Abstract Twodifferenttypesofshorelineboundaryconditionswhichcanbeusedineitherwave-resolvingorwave-averaging models of waves and currents propagation in the nearshore are compared here. The two techniques are essentially different:inthefirstcasethevelocityoftheshorelineisobtainedbythemomentumequationandtheshorelineposition istrackedbychangingthegridposition,whileintheothercasethevelocityoftheshorelineisobtainedbyamodified Riemann solver and the shoreline is defined as an interface between dry and wet fixed grid points. A number of test cases are described to compare the performance of the two techniques. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear shallow water equations; Moving shoreline; Moving grid; Riemann solver 1. Introduction Oneofthemostimportantproblemsfacedwhenmodelingnearshoreflowsistoestablishanappropriate representation at the shoreward boundary of the domain. Difficulties arise because the position of the boundary changes and the movement of the boundary is part of the solution itself. On a plane beach with a slope that is small enough waves break well before they reach the shoreline. However,onmanysandybeachestheslopeattheshorelineismuchsteeperthantheaveragebeachslope. Eventhoughwavesmaybreakjustbeforetheyreachtheshoreline,thetipoftheresultingswashmotionhas manysimilaritieswiththenonbreakingrun-upofwavesonslopeswhicharesosteepthatthewavesdonot break at all. For the infragravity components of the wave motion this is the case on almost all natural beaches. Theregionoftheswashandrun-upisalsowhereanoticeablepartofthesedimenttransportoccurs(e.g., [26,34]) and where long period infragravity waves generated in the nearshore by nonlinear interactions Comput. Methods Appl. Mech. Engrg. 191 (2002) 4475–4496 www.elsevier.com/locate/cma * Corresponding author. Fax: +39-010-343-2546. E-mail address: brocchin@diam.unige.it (M. Brocchini). 0045-7825/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0045-7825(02)00392-4