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Title: CYCLIC AND NON-CYCLIC PATTERNS OF BED ROUGHNESS IN ERODING RILLS

Author
item HELMING, K. - ZALF
item Romkens, Mathias
item PRASAD, S. - UNIV. OF MISSISSIPPI

Submitted to: Soil Erosion for 21st Century Symposium
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2000
Publication Date: 1/3/2001
Citation: N/A

Interpretive Summary: Rilling of soil by runoff is an important mode of soil erosion. Rills are formed in places where runoff concentrates and flow velocities and rates have reached critical values. The areal frequency and depth of rills obtained varies for each hydrologic regime (rainfall intensity, amount, and duration). The beds of the rills often show a surface irregularity or roughness. This roughness is a reflection of the local variation in the erosion resistance of the soil and the energy expenditure by the flow regime (flow rate and velocity). Therefore, a characterization of the bed roughness of the rill may yield valuable information about the soil susceptibility to erode under different concentrated flow regimes. This study analyzes the bed roughness of eroding rills in relation to slope gradient and runoff rate using digital elevation maps. The results of this study show that bed roughness consisted of three roughness components: a low-frequency component, a high-frequency component, and a cyclic component. The latter component reflects the presence of a series of regularly spaced micro-headcuts. This information will be helpful in interpreting the balance between flow expending energy levels and the resistance of the soil to rill erosion.

Technical Abstract: Rills are a major source of sediment in eroding systems. This study investigated the roughness of eroding rills as affected by the slope gradient and runoff rate. In flume experiments, loess soil material was subjected to simulated rainstorms. Roughness of the major rills was determined using digital elevation maps. Results showed that rill bed roughness significantly increased with slope steepness but was not affected by the runoff rate. Generally, three patterns of bed roughness could be distinguished: (1) a low-frequency component indicated the convex/concave shape of the rill length profile, (2) a high-frequency component of undirected roughness with an amplitude proportional to slope steepness, (3) a cyclic component that seemed to reflect a series of regularly spaced micro-headcuts. The frequency of the micro-headcuts tended to decrease with slope steepness, the amplitude increased with slope steepness. The results indicate that rill bed roughness is a dynamic parameter that is affected by slope steepness.