Quantifying recent channel Incision in lowland Erzen River, Albania

While river channel change is a natural process for an alluvial river, increasing human activities such as sediment mining, construction of reservoirs and land use alterations can accelerate this process. Growing urbanization and socio-economic development in South-Eastern Europe over the last 30 years has shown a major impact on the river channel adjustments. New urbanization in Albania has rapidly developed after the collapse of dictatorial regime in 1990, with most rural population moving to the big cites, mostly the nearby capital Tirana and Durres. As consequence a boom in construction industry has occurred in this part of Albania, including new buildings and infrastructures. Rivers have been the primary source of building material. Here we analyse the channel adjustments that occurred on the Erzen River that passes nearby the two major urban centers of Tirana and Durres. The Erzen River has its origin at the Gropa mountain at 1200m asl, its length is 109 km, and it flows approximately westwards towards its mouth in the Adriatic Sea near the Lalzi Bay. The catchment area is 760 km² and the mean annual flow is 18.1 m³/s. We analyse the incision and channel narrowing at lower part of the river by using remote sensing, historical image analyses, DEM and survey in the field. Major hydromorphological pressures potentially affecting the flow and sediment supply regime have been also analysed. Specifically, sediment mining has been reconstructed by identifying the mining sites in contact with the active river corridor between 1990-2015 along 30 km river length from aerial rivers, and from technical reports providing estimation of sand and gravel removed from the river.

Our findings indicate rapid changes of channel morphology, with 20% up to 75% channel narrowing affecting the transitional and meandering reaches between 1968-2015 and high riverbed incision at the reach scale up to 5-6 m, which is also revealed by visual signs like increasing bed rock and bridge foundations exposure. Two cut-offs have been created at the meandering part of the river. Most of the main bridges in Albanian rivers have shown exposed foundation with 3-4 m on the last 15-20 years, where some of them are replaced by other bridges due to unstable structure condition. Compared with previous studies, narrowing and incision rates are among the highest observed in Europe after the 1950s. While on the upstream segment of the river two dams have been built, sediment mining appears as the main driving factors of the observed channel narrowing and incision. Twenty-two mining sites have been detected between 1995 - 2015 and 457,380 m³/year of sediments have been reported as withdrawn from the riverbed. The rapid channel incision has contributed to the increasing salty water intrusion and subsequent freshwater shortage in lowland part of the river. The observed narrowing and incision have likely played a key role also in the reduction of river sediment supply to the sea, which probably explains most of the very rapid coastal erosion that has been observed in the same period in the Lalzi Bay.