This article reviews previous investigations into gully and badland research and discusses processes and definitions in the context of existing research in southeast Spain. The theory of badland development is summarized and definitions are proposed which draw on previous work and continuing studies. The processes influencing gully and channel head morphology are then discussed including overland flow, hillslope processes, pipe initiation and enlargement, mass failures and the magnitude and frequency distribution of storm events. Finally, modelling of badland landscapes is discussed. The article highlights that much detailed research has been carried out on badlands, but long-term rates of gully development are not well understood. There are also gaps in our understanding of pipe network formation and collapse. In the short term theoretical modelling may provide the way forward and a direction for more holistic investigations.

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Abstract. This paper examines river bank retreat rates, individual erosion events, and the processes that drive them in the Upper Severn basin, mid-Wales, UK. Traditional erosion pin networks were used to deliver information on patterns of downstream change in erosion rates. In addition, the novel automatic Photo-Electronic Erosion Pin (PEEP) monitoring system was deployed to generate near-continuous data on the temporal distribution of bank erosion and accretion: this allowed focus on the magnitude and timing of individual erosional and depositional events in relation to specific flow episodes. Erosion dynamics data from throughout the Upper Severn basin are combined with detailed information on bank material properties and spatial change in channel hydraulics derived from direct field survey, to assess the relationships between flow properties and bank erosion rates. Results show that bank erosion rates generally increase downstream, but relate more strongly to discharge than to reach-mean shear stress, which peaks near the basin head. Downstream changes in erosion mechanisms and boundary materials, across the upland/lowland transition (especially the degree of development of composite bank material profiles), are especially significant. Examples of sequences of bank erosion events show how the PEEP system can (a) quantify the impact of individual, rather than aggregated, forcing events, (b) reveal the full complexity of bank response to given driving agents, including delayed erosion events, and (c) establish hypotheses of process-control in bank erosion systems. These findings have important implications for the way in which bank erosion problems are researched and managed. The complex responses demonstrated have special significance for the way in which bank processes and channel-margin sediment injections should be handled in river dynamics models.

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