Characteristics and controlling factors of bank gullies in two semi-arid mediterranean environments

Bank gullies are gullies that are formed due to a height drop caused by a t errace or a river bank, which develop by headward retreat in erodible hills lopes. This study aims (i) to investigate the morphology of actively erodin g bank gullies, i.e., geometrical characteristics resulting from past erosi on and active erosion processes shaping the gully, and, (ii), to find relat ionships with environmental site characteristics, such as topographical par ameters, material properties and climate. The ultimate goal is to identify the most important controlling factors of past and present bank gully erosi on. Fifty-five active bank gullies formed in different lithologies by vario us erosion processes have been selected in the Guadalentin basin and the su rroundings of Guadix (Southeast Spain). For each bank gully site, geometric al and topographical parameters of both the channel and the drainage basin were measured. Erosion features indicating activity at the gully head, such as tension cracks, plunge pools, undercutting, fluting, piping and rill or sheet erosion on sloping side walls were mapped, and samples were taken fr om distinct lithological layers that were considered to influence the type and intensity of erosion processes. A relationship could be shown between t he presence of piping and fluting and a number of material characteristics, including particle size distribution, dispersion behaviour and electrical conductivity. On the other hand, lithology appeared not to be a differentia ting factor on gully development in the long run, as expressed by the total eroded volume (V). This parameter was most strongly related to the drainag e basin area in which the entire bank gully had been formed (A(o)), explain ing 66% of the variance. The relationship is V = 1.75*A(o)(0.59). No signif icant difference was found between regression lines through sub-datasets of different soil textural classes. Finally, multiple regression was used to include both topographical parameters and material characteristics in an ex planatory and/or predictive equation for the total eroded bank gully volume . The results of the analyses using the entire dataset, including the sites in the Guadalentin as well as in the Guadix area, have been compared with the results for the separate study areas. Differences are not only related to topographical and lithological characteristics. but may also be the cons equence of a different climate in the two areas. (C) 2000 Elsevier Science B.V. All rights reserved.