Erosional Cyclic Steps Governed by Plunge Pool Erosion: A Parametric Study Based on Field, Laboratory, and Model Data

For upland ephemeral gullies, gully erosion is strongly related to the formation and migration of cyclic steps. It is necessary to provide insight into the process of cyclic step development to accurately predict the pace of landscape evolution and soil loss. Information on the geometry of cyclic steps in subaerial environments is limited, and, to our knowledge, no model of cyclic step development considers plunge pool erosion. In this study, we analyze the geometric features and controlling factors of erosional cyclic steps through meta-analysis of measured data including new measurements in the Loess Plateau, China. We focus on cyclic step dynamics of fluvial beds controlled by bed shear stress and local plunge pool erosion. We develop a new theory to incorporate plunge pool erosion through adapting existing cyclic step and plunge pool models. Our method agrees with measured data, showing that a larger flow rate leads to larger step length L-d and height H-d and increasing erodibility increases step aspect ratio L-d/H-d. The method is also able to predict how the step length, height, and aspect ratio change with the average channel slope. Our results indicate that plunge pool erosion is an important mechanism of cyclic step evolution. However, plunge pool development alone is not sufficient to explain the wide range of L-d/H-d in the measured data. The posed theory relates to equilibrium conditions and thus cannot consider temporal adjustments in step geometry.

Automated summary

In this study, a meta-analysis of measured data was conducted to analyze the relationship between gully erosion and cyclic steps in upland ephemeral gullies. A new theory of plunge pool erosion was proposed and validated for its consistency with measured data.