Post-nourishment beach scarp morphodynamics

Large and persistent beach scarps can be safety hazards to beach users and result in serious social and economic implications. In this study the morphological evolution of beach scarps of large dimensions is examined on a nourished microtidal Caribbean Mexican beach. Beach profiles were measured three-to-four monthly along the beach after the nourishment in December 2009 and over 1.5 years. A beach scarp was defined as a feature with a slope larger than the critical angle of repose of 32 degrees and a minimum height of 0.25 m. The top and bottom positions of the scarps were calculated from the minimum and maximum values of the second derivative of the measured beach profiles (slope gradient). The cross-shore morphological evolution of the scarps was related to wave runup (R-2) and tides, and also to both with the contribution of the longshore energy flux (P-t). During calm conditions characterised by longshore uniform mean and maximum R-2 of 0.73 and 0.83 m, and P-i=180KN/s, the scarps remained present along the beach. Energetic conditions with mean and maximum R-2 of 0.83 and 1.2 m and P-i=400KN/s, increased the longshore rythmicity of the beach and induced significant cross-shore erosion (over 20 m) and the disappearance of similar to 50% of the scarps. The added contribution of the longshore energy flux, wave runup and tidal elevation explain 40% of the morphological evolution of beach scarps over the study period.