Geomorphological impacts of an extreme flood in SE Spain

•A major flash flood on 28 September 2012 on the Nogalte in SE Spain attained 2500 m3 s− 1peak flow, with one hour rise.•Surveys before and after the event in monitored reaches provide rare data of morphological changes in ephemeral channels.•Major changes were net aggradation and massive sedimentation on large flat bars in the braided channel.•Calculated hydraulic values of peak flow were very high compared with published values and thresholds for change in floods.•Deposition and change amounts were related strongly to peak discharge and stream power and moderately to force variables.

Long-term field studies in semiarid ephemeral streams are rare. These geomorphic data are essential for understanding the nature of the processes in order to develop modelling for risk assessments and management. An extreme flood event on 28 September 2012 affected the Murcia region of SE Spain, including long-tem monitoring sites on two fluvial systems in the Guadalentín basin, the Nogalte and Torrealvilla. Detailed morphological data were collected before and immediately after the event; and the amount of morphological change, erosion, and deposition have been related to peak flow conditions at the sites.On the Nogalte channel, peak flow reached 2500 m3 s− 1 at the downstream end of the catchment in less than 1 h. The event had a recurrence interval of > 50 years based on rainfall records and damage to old irrigation structures. The major effect in the braided, gravel channel of the Nogalte was net aggradation, with massive deposition in large flat bars. The measured changes in bankfull capacity were highly correlated with most hydraulic variables. Net changes in cut-and-fill in cross sections on the Nogalte were highly related to peak discharge and stream power but much less so to measures of hydraulic force (velocity, shear stress, unit stream power). Relationships of amount of erosion to hydraulic variables were much weaker than for amount of deposition, which was largely scaled to channel size and flow energy. Changes on the Torrealvilla were much less than on the Nogalte, and net erosion occurred at all sites. Sites on the Nogalte channel in schist exhibited higher deposition than those of the Torrealvilla sites on marl for the same hydraulic values.Overall, less morphological change took place in the extreme event on the Nogalte than predicted from some published hydraulic relations, probably reflecting the high sediment supply and the hydrological characteristics of the event. The results demonstrate the high degree of adjustment of these channels to the occasional, high magnitude, flash flood events and that such events need to be allowed for in management. The detailed quantitative evidence produced by these long-term monitoring sites provide valuable, rare data for modelling morphological response to flood events in ephemeral channels.