Using GeoWEPP model, high-resolution 3D models and ground-based survey to detect sedimentation changes and morphological adjustments in an ephemeral stream

The ephemeral streams, which drain steep and metamorphic catchments, experience rapid and torrential runoff with high sediment loads. These processes cause important morphological changes in the channels. This work proposes a methodological approach to verify the change patterns in the magnitude and frequency of the hydrological events that geomorphologically model this type of channels. A gravel-bed ephemeral stream, the Rambla de la Azohía, located in the coastal area of the Betic Mountains (southeastern Spain), has been chosen as a study case for the method validation. This approach focuses first on relationships between peak discharges and sediment budgets measured at checkpoints for specific events from 2018 to 2020 and then runoff data and sediment yields obtained using the GeoWEPP model for the same cases after calibration/validation. Water depths and concentrations of suspended sediment recorded during the events of 2018 and 2019 were used for model calibration and validation, respectively. For the calibration stage, a sensitivity analysis was carried out in order to detect the parameters that most influence the model output and are, therefore, suitable for calibration. Finally, the results obtained in the calibration and validation periods were evaluated using the Nash-Sutcliffe efficiency (NS) and percent bias (PBIAS). Values of NS and PBIAS equal to 0.86 and  7.81%, respectively, were found in the calibration period, while these indices were  0.81 and  -4.1% in the validation period. All these values confirm the model’s capacity to simulate peak flow and erosion in the experimental conditions. Topographical variations and sediment budgets, verified combining high-resolution digital terrain models (HRDTMs) with ortophotographs and point clouds dated in 2018, 2019 and 2020, and ground-based surveys, were analyzed in relation to changes in discharge in order to determine geomorphic flow thresholds. According to these thresholds, three classes of morphological adjustments were defined: 1. global changes caused by  discharges over the bankfull depth; 2. large alterations at the bankfull stage driven by a noticeable vertical bed accretion and lateral erosion; 3. moderate adjustments during sub-bankfull flows that are able to modify alluvial bars; and 4. minor events, in which the accretion of these bars ceases and shallow scouring and washing actions prevail. These geomorphic thresholds were then applied to the complete series of discharges simulated using GeoWEPP at the event scale during the period 1997-2019. The results revealed a significant increase in the number of events that are capable to produce bed aggradation and bank erosion. This research was funded by FEDER / Spanish Ministry of Science, Innovation and Universities - State Research Agency (AEI) / Projects CGL2017-84625- C2-1-R and CGL2017-84625-C2-2-R; State Program for Research, Development and Innovation Focused on the Challenges of Society.