Morphodynamics of the Bevano Stream outlet: the role of bedload yield
Résumé
The present work focusses on bedload yield estimations for the Bevano River, a small stream situated in the Emilia-Romagna Region of northern Italy. The Bevano’s final stretch is inside a natural park, one of the last unspoilt zones of the Emilia-Romagna coast. This stream resulted to have very complex outlet dynamics. The mouth is moving rapidly and constantly towards the north, eroding the dunes and the pine forest behind them. The bedload transport was correlated to the long and short term migration of the river mouth. The migration was also related to rainfall data, marine forcing agents and width variations of the beach close to the river outlet.
Because of a lack in direct measurements, the bedload yield was assessed using the ENGELUND & HANSEN formula [1972] (modified in MONTEFUSCO [1984]). This formula was considered the most suitable because it predicts the volume of the sandy fraction discharged at the river mouth.
Rainfall data were collected at the Bertinoro Station (180 m above m.s.l.) and at the Idrovora Fosso Ghiaia Station (2 m above m.s.l.), obtaining an overview of the rainfall for the entire Bevano catchment.
The long term evolution of the outlet was studied using four aerial photographs captured between 1943 and 1998. Short term monitoring of the mouth was studied between April 2003 and June 2003, using a Total Station. The influence of tidal currents and waves on the outlet morphodynamics were studied using sand tracers.
Both long term and short term analysis revealed that the river is moving rapidly northwards.
This trend fastened during the last years (from 1998 and 2003).
The bedload yield calculated empirically (Q355) resulted to be not significant during fair weather conditions. The river transports sediments only during periods of heavy rainfalls (Q10). This material is trapped on the submerged delta and moved onshore by marine currents, increasing the sediment budget of the study area. Most of the sand available for the Lido di Dante beach comes from lateral dune erosion, as it resulted from direct topographic surveys. Sand tracers revealed that the sand eroded by the dunes is transported outside the river mouth by ebb-tidal currents and then northwards by longshore currents, increasing the quantity of sediments available for the beach.
Local authorities intend to open a new river mouth, southward from the actual one, in order to avoid inland floods during periods of abundant rainfalls, and marine water ingression inside the pine forest behind the dunes during storms. As a result of this intervention the
beach northward from the mouth may start to retreat because of the lack of sediment supply coming from dune erosion.