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In ephemeral rivers, channel morphology represents a snapshot at the end of a succession of geomorphic changes caused by floods. In most cases, the channel shape and bedform migration during different phases of a flood hydrograph cannot be identified from field evidence. This paper analyses the timing of riverbed erosion and deposition of a gravel bed ephemeral river channel (Rambla de la Viuda, Spain) during consecutive and moderate- (March 2013) and low-magnitude (May 2013) discharge events, by applying a morphodynamic model (Delft3D) calibrated with pre- and post-event surveys by RTK-GPS points and mobile laser scanning. The study reach is mainly depositional and all bedload sediment supplied from adjacent upstream areas is trapped in the study segment forming gravel lobes. Therefore, estimates of total bedload sediment mass balance can be obtained from pre- and post-field survey for each flood event. The spatially varying grain size data and transport equations were the most important factors for model calibration, in addition to flow discharge. The channel acted as a braided channel during the lower flows of the two discharge events, but when bars were submerged in the high discharges of May 2013, the high fluid forces followed a meandering river planform. The model results showed that erosion and deposition were in total greater during the long-lasting receding phase than during the rising phase of the flood hydrographs. In the case of the moderate-magnitude discharge event, deposition and erosion peaks were predicted to occur at the beginning of the hydrograph, whereas deposition dominated throughout the event. Conversely, the low-magnitude discharge event only experienced the peak of channel changes after the discharge peak. Thus, both type of discharge events highlight the importance of receding phase for this type of gravel bed ephemeral river channel. [ABSTRACT FROM AUTHOR]

Ephemeral rivers in the Mediterranean region have been exposed to significant human disturbance over the last century. Recently, there has been a growing interest in restoring their morpho-sedimentary condition, backed by the European Water Framework Directive. Previous research has highlighted the severe geomorphic impacts of sediment extraction, such as reduced channel width and riverbed incision, but the recovery of these channels during annual flows remains poorly understood. This study presents a methodological framework to investigate geomorphologic recovery (signs of landform change) and constraints in the Rambla de Cervera (eastern Spain) from 2018 to 2021. Aerial imagery, LiDAR topography, and field surveys were used for stream segmentation, fluvial landform classification, temporal landform change, and comparison with flow characteristics from a 2D hydraulic model. The study identified three groups of segments with different channel types and recovery mechanisms. Upstream (G1), unconfined valley settings showed minor changes, indicating morphological stability. Middle segments in confined valleys (G2) showed greater alluvial channel recovery and vegetation encroachment in response to flood/no-flood periods. Downstream segments (G3), incised on cemented gravel, were highly responsive to floods but lacked landform stability due to high energy conditions. The findings suggest that channel recovery during annual floods is slow and occurs mainly downstream in degraded areas (G3). Increased sediment flux from upstream and additional sediment storage in new forms are required to recover these channels. This analysis underscores the challenge of interpreting short-term morpho-dynamic changes for long-term river recovery, but provides insight into the morpho-sedimentary conditions required to accelerate recovery of ephemeral streams. [ABSTRACT FROM AUTHOR]