The role of the fundamental seiche in the Adriatic coastal floods

The paper documents characteristics of the fundamental Adriatic seiche (period of 21.5 h) by analysing hourly sea level data at Split and Rovinj in 1957-2002 interval. The seiche is extracted by digital filter from residual (detided) sea level series. Basic statistics, both on seasonal and interannual scale, as well as flood (threshold-exceeding) analysis has been performed in order to visualise the importance of the seiche. Although mean seiche variance is as small as 2-3%, seiche energy can be much larger than tidal energy during extreme events. The largest seiche amplitudes occur in February and March, being almost two times smaller in summer season. A decrease of seiche flood amplitudes in time has been documented, coinciding with a decrease in the storm surge height. This is presumably due to weakening of atmospheric activity during investigated period. However, a reversal in that trend may result in disastrous and exceptionally high floods in the future, due to coupling with the global sea level rise and its regional (Adriatic) manifestation.