THE HEALING OF DISTURBED HILLSLOPES BY GULLY GRAVURE

Results of accelerated erosion on certain constructed surfaces in sout heastern Arizona appear similar to those described by Bryan (1940) as gully gravure. Twenty cross-section excavations in eight rills incised into silt-rich lacustrine and fluvial deposits reveal partial filling of the rills by debris derived from overlying fluvial sand, gravel, a nd cobbles. Interstices of the coarse material gradually fill with fin e-grained erosion products, decreasing permeability of the fill and de flecting subsequent runoff to the margins of the fill. Rills and rill fillings thus increase in width with time, and complete veneering of t he surface by coarse debris ultimately may occur. Through incision, fi lling, lateral planation, and armoring, channels of the dissected surf ace heal and the new hillslope approaches an equilibrium condition. Na tural hillslopes in the area with similar geologic conditions have inc linations of 16-degrees-22-degrees, have generally unbroken veneers of coarse debris, and appear subject to the same erosional processes ide ntified at constructed hillslopes. Field measurements and the revised universal soil loss equation (RUSLE) were used to estimate soil loss o n natural and constructed hillslopes. Results suggest that erosion may be almost three orders of magnitude greater on steep, nonveneered, no nvegetated constructed surfaces than on natural hillslopes. Attributes of stable natural hillslopes provide criteria for the design of stabl e constructed hillslopes.