The backwater effect as a tool to assess formative long-term flood regimes

The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. As a consequence of this disturbance in the flow, the shrinkage in the bedrock activates a backwater effect. Systematic water retention during extreme events has formative consequences. The process involves the creation of a helical flow for the structural misalignment of the channel at the point of narrowness. The backwater effect transmits the secondary currents backwards, resulting in the creation of a sinuous pattern upstream from the shrinkage. A tributary, the Gurri River, flows into the main river just before the constriction, and this too has been affected by the process of water storage and channel pattern change. A two-dimensional numerical flow model (Iber) has modelled several hypothetical cases of flooding. This modelling aims to test the reach of the hydraulic influence upstream from the constriction, both in the main river and its tributary, due to the backwater effect. Moreover, it sought to find the best balance of discharge between the two streams. The upstream reach of the backwater effect was considered as its endpoint. During the flooding, the system reached hydraulic equilibrium between the constriction and the two endpoints when both were at the same water level, and the flow regime was subcritical everywhere. The hydraulic conditions that drove the water flow to the equilibrium are thought to be the ones that promoted formative processes in a sinuous pattern on a long-term basis. The water discharge values obtained from this procedure are, in general terms, 50% above those considered to be a peak flood with a recurrence time of 500 years (Q(500)), and they accomplish the conditions of extreme events. Thus classified, the calculated discharges can be helpful for comparison with those measured in historical and systematic records, because a water discharge like the one calculated has never been measured at the Ter River.