Morphodynamics of an ephemeral gravel-bed stream combining Mobile Laser Scanner, hydraulic simulations and geomorphological indicators.

In this study we document the role of moderate and small floods as effective agents for sediment transport and bedform change on Mediterranean ephemeral streams, with a study case in Rambla de la Viuda (Eastern Spain). Rambla de la Viuda is a gravel bed stream impacted by intense mining. The last significant variation in the river's bedform morphology took place in March and May 2013, during two rainfall events (daily accumulated rain of 70 and 40 mm) that produced flow discharge peaks of 45 and 25 m(3)s(-1) respectively. As a consequence of the flows, two new lateral gravel bars with a total volume of 2,323 m(3) were deposited on the area under study. The characterization of the flows was made by using Mobile Laser Scanner, RTK-GPS and Photographs surveys in addition to gravel size measurements prior and after each discharge. The integration of these techniques was used to trace the major fluvial geomorphic processes and thus implement a two-dimensional model that explains them. Change detection and grain size distribution pointed out the high availability of material (especially with sizes of 32-45 mm), the absence of a well-developed bed armoured layer, and provide information about hydraulic conditions that limited sediment transport during these events.