The evolution of an ephemeral river during the rising and receding phases of medium and low magnitude discharge events

Abstract:

In addition to great flash floods, medium and low magnitude discharge events can also cause great morphological changes in ephemeral river channels. Despite the advances in measurement techniques, such as laser scanning, and simulation approaches, the channel evolution during the different phases of discharge events is still not well known in gravelly ephemeral rivers, such as Rambla de la Viuda (Spain).

The aim is to detect the temporal evolution of the ephemeral river channel during consecutive medium (March 2013) and low (May 2013) magnitude discharge events. The study is based on both accurate topographical measurements (laser scanning) and morphodynamic simulations (Delft 2D). Before the final analysis, the model’s performance was tested with different parameterizations. When compared to the observed channel changes, the transport equation had crucial role in simulation results. Engelund-Hansen equation succeeded the best. It was important to use spatially varying grain sizes.

Erosion and deposition (m³) were the greatest during the first hours of the rising phase of the discharge events. After this, erosion and deposition amounts, which were detected hourly, started declining. Thus, this occurred before the peak discharge, and erosion slowed down more than deposition. After the discharge peak, changes in deposition and erosion amounts were slightly more gradual than changes in discharge. The deposition during the receding phase was due to the advancing bar lobe frontier. River bed changes followed temporally the changes in discharges during the receding phase. This was different to the rising phase, when temporal differences occurred between changes in discharges and changes in deposition and erosion. This study shows that both rising and receding phases of discharge events are important for bar movement and channel evolution of the gravelly ephemeral river.