Human impact in the Holocene fluvial and coastal evolution of the Marche region, Central Italy

Abstract

Today, the coasts of the Marche region are represented by almost continuous rectilinea sand beaches except in the north, at the border with the Romagna region, and in the central part, around the Conero ridge, where there are active rock cliffs. The sandy coasts are protected almost everywhere by artificial barriers built up as a protection against the general tendency to retreat. Along some tracts, the coastal erosion started at the beginning of this century but became generalized after 1940–1950. This was mainly due, first, to the reduction in sediment supply, following the improvement in agricultural techniques after the Italian Unification, and, later, to the widespread extraction of gravels from the thalwegs related to the rapid increase in urban population in the 1950s. However, in the same period, along the rivers, there was a widespread creation of artificial levees to prevent lateral erosion or flooding and to extend areas suitable for agriculture. Following these practices, the lower-middle tracts of the rivers underwent a strong vertical downcutting which increased as a result of the creation of checkdams downvalley of many bridges which contributed to the store of the sediments in the upper part of the valleys. At present, most of the rivers have an irregular course and are deeply entrenched in the bedrock or inside their own sediments. Up to the beginning of the century, most of the lower tracts of the valleys were characterized by strong aggradation in a braidplain system. This aggradation started as early as the Middle Ages (1100 AD) but increased after the Renaissance as a consequence of a generalized deforestation of the Periadriatic Basin and the following severe soil erosion. In the mean time, the coast underwent a very fast progradation, in some places more than 500 m. Small deltas were created at the mouths of the rivers, and longshore bars and sandy beaches began to occupy the base of the active cliffs which previously extended between one river and another. However, in the mountain parts of the river basins, a meander course existed, and in many cases still exists, expecially where the human interventions were of limited extent. Before 1000–1100 AD, all the rivers had a meander pattern and, at the junction with the sea, entered lagoons and swamps in most cases protected by littoral barrier beaches. These conditions were established earlier than the third century BC and have been attributed to the first systematic land clearance and the following soil erosion during the Bronze and Iron Ages. At that time, the regional coastline became almost rectilinear but with many active cliffs between one river and another. Before being affected by a major human impact (earlier than 4000 years BP) the ‘natural’ coastal environment was represented by active rock cliffs which alternated with pocket beaches corresponding to a river mouth. In some rivers, embayments and beaches were located up to 4–5 km inland from the present-day position. After the Flandrian transgression the evidence presented reveals major changes that can be attributed to human impact.