Morpho-sedimentary characteristics and Holocene evolution of the emergent part of the Ombrone River delta (southern Tuscany)

The Holocene evolution of the Ombrone River delta has been investigated using morphological and sedimentological evidence. The delta had a lagoon-like prehistoric evolution, while seawards the growth occurred mainly in historical times. The data obtained, together with the available historical and archeological documentation, enable a quantitative evaluation of the volume of the emerged part of the delta to be made. Moreover, for the apical area, it has been possible to calculate the annual average increase in the volume of deposition during Etruscan and Roman times, as well as during the Middle Ages and since the Renaissance up to now. It has been possible to estimate the periods during which wind exerted a more important role in shaping the delta morphology. The volume of sediments deposited during the last 6000 years has been estimated at 1766·106 m3. A combination of climatic conditions, littoral dynamics and human action appears to have driven the dynamics of the delta growth, which, however, was not continuous, but active especially during the last 3000 years. During this period, about the 80% of the whole delta volume seems to have deposited. High rates of sedimentation occurred between the 16th and the 19th centuries (a cold and moist period coinciding with the Little Ice Age), when the delta took the typical shape of microtidal wave-dominated deltas.