Regressió del delta del Llobregat. Efecte de les obres d’enginyeria al riu d’ençà del segle XIX

The recent human pressure upon the Llobregat River, an alluvial bed Mediterranean river, has changed its riverine and deltaic landscapes. The decrease in the coarse sediment yield into the sea has caused a shift in the delta evolution at the end of the 19th century which has resulted in a coastline retreat of 800 m (a set of data unknown so far). The paper concentrates on the fluvial component of the river-delta-sea system, trying to connect it to the delta evolution, and the possible causes of the delta retreat are analyzed. A method to compute the actual bed load transport with real information of the past river morphology and hydrological regime is developed. The encroachment (and channelization) by road and railway infrastructures built in the low valley in the last 70 years has reduced the sediment supply in the last 30 km of the river and caused important changes on fluvial morphology, but this is not the main cause of the coarse sediment yield decrease. Moreover, a number of weirs built throughout the 19th century and changes in flood frequency may be responsible for a baseline delta retreat starting at the beginning of the 20th century. The sediment accumulation behind the three dams built in the second half of the 20th century is less important for yearly sediment yield at the lower course than the flow regulation. Finally, the deviation of the final river reach and the resulting new river mouth, built in 2004, has become a sediment trap which poses a further thread to the future of the deltaic beaches since it has caused a decrease in the sediment yield into the sea of one order of magnitude.