Journal of Coastal Research SI 56 1666 - 1670 ICS2009 (Proceedings) Portugal ISSN 0749-0258 Wave Climate Clustering to Define Threshold Values with Respect to the Expected Morphological Response N. Valchev† and E. Trifonova‡ †Institute of Oceanology, BAS PO box 152 Varna 9000 Bulgaria valchev@io-bas.bg ‡ Institute of Oceanology, BAS PO box 152 Varna 9000 Bulgaria trifonova@io-bas.bg ABSTRACT VALCHEV, N. and TRIFONOVA, E., 2009. Wave Climate Clustering to Define Threshold Values with Respect to the Expected Morphological Response. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 1666 – 1670. Lisbon, Portugal, ISSN 0749-0258. The development of a flexible operational system providing warning of a potential morphological storm impact requires a preliminary assessment if a given wave regime may produce considerable changes in the coastal system. This study aims to present an approach to the increasing of the efficiency of such a system. It is achieved by setting up combined thresholds of storm impact the exceeding of which would bring a morphological response of a certain scale. The wave climate at the study site is estimated on the basis of hindcast data. It serves as a boundary condition for a cross-shore profile model calculating the wave transformation in shallow water. The model also takes into account the sediment motion regime and the linear length of the zone in which a sheet flow regime could be expected. A methodology for clustering of the wave climate is proposed, the sediment motion corresponding to each wave cluster is analyzed and the boundary thresholds are established. The latter differentiate the wave events that are capable of producing a noticeable morphological response from the rest of the cases and correspondingly provides a way to run the morphological module of the forecast system only in cases in which a discernible reaction is expected. ADITIONAL INDEX WORDS: Black Sea, numerical modeling, operational approach INTRODUCTION The coastal zone is exposed to storms of different intensity. Although the IPCC (2007) concludes that there is no pronounced multiplication of storm activity observed, an increase of the amplitude of extreme climatic events is expected to occur. An augmentation of damage as well as devastation of coastal infrastructure and even casualties is anticipated. The changes of the natural environment are also an issue that tackles the question of the morphological impact of storms with different probability of occurrence and the resulting risks. A flexible system warning of the possible morphological risk entails a preliminary assessment if storm waves of given parameters are capable of causing significant changes in the coastal system. Since the forcing of the morphological model requires considerable computing resources, the appropriate functioning of an operational system, forecasting the morphological storm impact, implies the imposing of certain restrictions. One of them is the area that is supposed to be covered by the prognostic models and for which the possible risk is to be estimated. Therefore, the efficiency of the operational approach drastically increases if the section over which the model would be applied is representative for the entire area and the results obtained could be considered reliable for any of its components. The system efficiency could be also improved by the setting up of combinations of thresholds of wave parameters the exceeding of which would cause a morphological response of a certain scale. The practical implementation of this concept will provide the grounds for the morphological module of the warning system to be run only in cases for which discernible impact is expected. Given that morphological changes are defined by sediment transport, the factors controlling this process could be taken as a point of reference determining the conditions at which bottom deformations are likely to occur. Traditionally, in literature, three regimes of sediment transport are distinguished (NIELSEN, 1992; LEONT’YEV, 2001). An initiation of sediment motion can be defined in terms of the critical value of Shields parameter. SHIELDS (1936) applied dimensional analysis to determine some dimensionless parameters and established the incipient motion diagram. The Shields parameter is the ratio of mobilizing and stabilizing forces acting upon the sediment particle in the bottom boundary layer. The intensification of the wave impact forms a pattern of ripples on the sea bottom, the geometry of which depends on the wave parameters. The further wave growth causes the surface bottom layer to be dragged into motion, the ripples are erased and the so called sheet flow takes place. From the coastal morphology point of view it is necessary to know the wave regimes within which discernible bottom and shoreline deformations could be expected. The critical value of the Shields parameter presents a clear view about the conditions for incipient sediment motions whereas certain considerable deforma- tions could be expected exactly in the sheet flow motion phase. In this study the threshold value, the exceeding of which switches the sediment motion regime to sheet flow, is assumed as the criterion for morphological impact. Hence, a set of threshold values needs to be established accounting for the variety of wave configura- tions. The present available data of wave measurements, however, constitute an unrepresentative excerpt. Moreover, in the majority of cases they do not refer to storms of lower probability of occur- rence but to those, which induce rather moderate morphological response. Such assessment for the western Black Sea is possible Journal of Coastal Research, Special Issue 56, 2009