Slope-channel coupling between pipes, gullies and tributary channels in the Mocatan catchment badlands, Southeast Spain

The dispersive nature of the highly sodic silts of the Triassic-rich unit of the Gochar formation plays a fundamental role in the erosion of the Mocatan catchment badlands in Almeria, where a rejuvenating pipe and incised channel network is rapidly evacuating slope materials. Referring to concepts of medium- and long-term landscape evolution, and incorporating contemporary thoughts on the role of connectivity in system evolution from the geomorphological literature, this paper attempts to develop a conceptual model of the way geologic, topographic, material property and ecological factors combine to explain the complex geomorphological evolution of the site. An electronic distance measurement (EDM) survey was undertaken using a Leica TC3100, to produce a detailed topographic map. This database was supplemented by geomorphological, geological and ecological data derived from ground survey and remote sensing, and further morphometric analysis undertaken. Preferred orientations of channel segments, and the topographic distribution of pipe- roof-collapse features and outfalls in relation to known stratigraphic controls, suggests that, once coupled to the slope-base channel, pipe networks develop in a systematic, sequential way. A wave of incision moving up the main channel reconnects channels with slopes, and the resulting increased hydraulic gradients on sideslopes encourage extensive deep pipe development for the first time in these materials. Once major pipe development is possible, three-dimensional pipe networks enlarge and then collapse to form an extensive, partially coupled steep-sided gully network. From this perspective, the coupling of the pipe to a rejuvenating channel is a significant intrinsic threshold event and the main reason that badlands have developed locally in these dispersive materials. It is concluded that erosion in this landscape will only be suppressed after a considerable period of slope-base stability, which could allow a gradual loss of sodium from the surface by leaching or organic exchanges. Both theoretical and management implications are explored. Copyright (C) 2007 John Wiley & Sons, Ltd.