Surface wash at the semi-arid break in slope
Kirby, A.V.T.; Kirby, M.J.
Zeitschrift für Geomorphologie, Supplementbände 21: 151-176
The break in slope in semi-arid areas is considered to represent neither a break in process nor a break in form but a local concentration of widespread slope concavities. The rapidly changing slope gradient within a narrow zone can be produced by the gradual transition of dominant process from gravity forces to surface wash acting together on material of 2-50 mm diameter. Field measurements of surface wash on unchannelled slopes in Arizona gave a mean transport rate for coarse material (> 1 mm) of 4-5 cm3/cm. yr. This is comparable to rates obtained for fine material on unchannelled slopes over 500-1500 years from archaelogical mounds in a semi-arid area of southern Mexico. For the coarse material the distances travelled increased with increasing slope gradient, amount of unvegetated surface and storm rainfall, and decreased with grain size and relative surface roughness. On natural slopes these factors interact to make overall sediment transport almost independent of gradient. Surface wash in shifting braided channels was found to be 100 times more effective in transporting fine material than unchannelled wash. Measured slope profiles across the break in slope show an overall correlation between slope and grain size which is independent of the sharpness of the concavity. Modelling of the break in slope as an equilibrium form produced by gradually changing combinations of gravitational and hydraulic tractive stresses provides an acceptable envelope curve to the empirical data. The sharpening up of the concavity at the break in slope is thus considered to develop through this transitional combination of present surface processes superimposed on a broadly concave wash slope.