Velocity-based approach for establishing a varying time of concentration: Α study in three Mediterranean countries

The time of concentration, tc, has a crucial role in hydrological design, as an essential input of rainfall-runoff modelling. In common practices it is considered as a characteristic property of the watershed, even though theoretical proof and empirical evidence imply that it is a function of flow, and thus varies within the same basin. Here, we implement a velocity-based approach, partially integrated in a GIS environment and show that the relation between tc and runoff intensity for a basin is approximated almost perfectly by a power-law function. The coefficient of this relation depends on the length and mean slope of the main stream and the exponent shows a small variability within the tested basins. Next, we propose a regional formula for the estimation of tc that is a function of runoff intensity, as well as, key geomorphological characteristics of the basin, calibrated and validated in a number of Mediterranean river basins in Greece, Italy and Cyprus. Lastly, we propose its adaptation in flood modelling, in particular in the SCS-CN method, using a parametrised Synthetic Unit Hydrograph (SUH) whose shape is dynamically adjusted according to the runoff produced during the flood event. The proposed methodology is tested in a number of observed flood events with very satisfying results in the majority of the cases.