Morphological impacts of flow events of varying magnitude on ephemeral channels in a semiarid region

Flows in ephemeral channels in semiarid areas are only occasional, and channel changes are episodic; but the flash floods can be devastating and have major geomorphological impacts. Data on morphological impacts of flows in semiarid areas are needed to increase understanding of the dynamics and variability of geomorphic responses in such channels. For this purpose nine reaches of river channel in three catchments in SE Spain - the Nogalte, Torrealvilla, and Salada - have been sites for measurement of flows and their effects over the period 1997-2012. The sites encompass a range of channel size, channel morphology, substrate, vegetation, and position within the catchments. A major difference is between schist and marl bedrock areas. Peak flow stage has been recorded and topography surveyed at frequent intervals and after major flow events. Over the 16-year period, an average of 0.5 flow events per year has been recorded at the schist sites, and an average of one per year at the marl sites; but occurrence has been highly variable from year to year. Threshold daily rainfall for channel flow is mostly 15-20 mm, but higher rainfalls do not always produce flow. One to two major floods have occurred in each of the catchments in the period, including the extreme flood event of September 2012 in the Nogalte catchment. Measured morphological changes have occurred between 2 and 10 times at the monitored sites. The same size flow can have differing effects depending on the state of the system. Low flow can mobilise sediment without producing much morphological change. The long-term trajectory of the reaches and the sediment substrate has a major influence on response to events. Channel change is governed by threshold values of hydraulic conditions. The measurements provide an indication of the scale of maximum erosion and deposition that occurs within the channels and on the floodplains over a range of flow magnitudes and the flow impacts that need to be considered in management.