Some factors controlling gully growth in fine-grained sediments: a model applied in southeast Spain

Gullies in the Guadalentin catchment, SE Spain, are formed in a variety of lithologies, ranging from schists to marls. There are clear differences in valley and channel morphology between these two types. Gully erosion in sch ists produces large amounts of elastic material that restricts sediment tra nsport, landing to a relatively smooth transition from gullies to hillslope s, and to broad alluvial valley bottoms with laterally migrating channels. Gullies in marls contain only limited gravels, so gully extension leads to sharply incised gully heads. A two-dimensional model has been developed to investigate and simulate the distribution of gully heads in the Guadalentin . The model runs on a smooth initial surface with an added fractal perturba tion. Without some perturbation of the initial surface or material properti es, symmetry prevents the formation of any channels. Fractal perturbation w as used here to provide initial irregularities at all scales. This paper ex plores the dependence of model response on the form of the sediment transpo rt law. Initially, sediment transport was considered to be transport-limite d and so depends on the relationship between wash and creep/splash. This wa s modified by (i) altering the exponents for gradient and area to conform w ith experimental results, (ii) adding a critical tractive power fur the ini tiation of rillwash, (iii) including a more explicit treatment of mass move ments, (iv) including terms to reduce the dependence of the model on the co mputational grid size, (v) providing a basis for explicitly integrating fro m individual storms to average long-term behaviour and, most critically (vi ) introducing the 'effective bedload fraction' (ebf) as a simplifying conce pt to allow for the selective transportation of eroded sediment. This modif ication allows the model to represent removal of fines in mall-rich catchme nts, without the need to introduce a fully explicit travel distance formula tion. The steep gully heads and vertical side walls developed in marl catch ments in the Guadalentin basin are qualitatively well-represented by the mo del. Preliminary tests showed that steep-walled gullies are generated where the ebf falls to below about 0.5. If a tractive power threshold is also in troduced, gentler headcuts are produced. Selective transportation is thus s een as the critical determinant of steep headcuts in gully systems. (C) 200 0 Elsevier Science B.V. All rights reserved.