Microtopography of hillslopes and initiation of channels by horton overland flow

Abstract

On long hillsides one can examine systematic downslope changes in the ground surface as Horton overland flow gathers into depressions and eventually incises the surface to form channels. Microtopography plays an important role in this process. We have sampled the microtopography of two long hillslopes in a savanna region of southern Kenya, and defined the spectral characteristics of its roughness at various distances from the drainage divide. The microtopography is fractal with a dimension that decreases systematically downslope, and the overall roughness varies between and along hillslopes in response to: (i) the weathering characteristics of the underlying bedrock; (ii) the type and density of patchy vegetation, and (iii) the tendency for wash to incise the surface with increasing intensity as runoff discharge increases downslope. The downslope decrease in fractal dimension reflects the progressive development of low-frequency roughness, here referred to as "swaley" microtopography, which the wash develops even far upslope of the channel head. The statistical analysis of surface roughness motivates a discussion of the role of microtopography in the interaction between wash and diffusive sediment transport processes that ultimately determines the critical distance from the divide at which channels begin. t Le e CD 0.02 5

Modeling depth distributions of overland flows

“…Such interactions between surface roughness and flow hydraulics have been discussed for several years (e.g. Dunne et al, 1995); however, the advent of rapid non-intrusive surveying technology permits more detailed analysis of these interactions than previously possible. The downslope transitions in the depth probability distribution observed here may also reflect stages of rill development where the channel network is expanding.…”

“…Montgomery and Dietrich, 1994) and the role of diffusive sediment transport processes highlighted. Dunne et al (1995) emphasise the role of microtopography in concentrating runoff and influencing hydraulics. The sensitivity of sediment transport to flow hydraulics (i.e.…”

“…The sensitivity of sediment fluxes to small changes in the probability distribution of flow depths places great importance on modeling surface roughness and the resulting flow depths for accurate estimates of flow detachment (Abrahams et al, 1989;Dunne et al, 1995). Flow depths, like most variables in geomorphology or hydrology, are necessarily zero or positive and in practice often highly skewed.…”

A rain splash transport equation assimilating field and laboratory measurements

“…Dunne et al [1979] estimated landscape-averaged erosion rates for various periods of the Cenozoic era at 8 -30 Â 10 À3 mm a À1 , indicating that rain splash transport alone cannot lower entire landscapes, even in the presence of preanthropogenic vegetation covers and infiltration capacities. On the other hand, 10 m long hillslopes could be eroded at 7 -13 Â 10 À3 mm a À1 , and 1 m long planes of typical ''interrill'' microtopography [Dunne et al, 1995] could be eroded at $70-130 Â 10 À3 mm a À1 , if the splashed soil from their ends is carried away by runoff. This comparison with long-term erosion rates indicates that the morphogenetic role of rain splash acting alone is limited to parts of the landscape that lower more slowly than the landscape average, or to short hillslopes (<100 m) from which rain splash feeds soil to overland flow in the process of rain flow transport.…”

Threshold phenomena in erosion driven by subsurface flow

“…Here we study the seepage case. Whereas the case of Horton overland flow has been extensively studied [ Smith and Bretherton , 1972; Dietrich et al , 1992; Montgomery and Dietrich , 1994; Dunne et al , 1995; Izumi and Parker , 1995, 2000], seepage erosion has received less attention. Dunne [1980, 1990] suggests that erosive stresses due to seepage are more widespread in typical environments than commonly assumed.…”