Simulated headward erosion of bank gullies in the Dry-hot Valley Region of southwest China

Abstract

Although the development and migration of gully headcuts can increase soil loss and accelerate landscape degradation considerably, little attention has been paid to the spatiotemporal variations of the morphological characteristics of bank gully heads in the Dry-hot Valley Region of southwest China. This study explored the in-situ variations in soil loss rates and morphological characteristics in active bank gully heads, testing the overland flow discharge with a range of 30 to 120Lmin−1. In response to this flow, actively migrating headcuts developed with retreat rates ranging from 2.6 to 7.9mmh−1. All experimental runs resulted in a gradual increase in soil loss volume, incision depth, and retreat distance over time due to set flow rates. For the gully beds and upstream areas of gully heads, the soil erosion rates were greatest at the beginning of each run and progressively decreased during the scouring test. Non-steady state soil erosion rates were observed in the headwall for the flow discharge levels examined for this study. This was due to an abrupt slope collapse after long-term scouring effects. As degradation progressed, similar trends emerged for temporal variations within the fractal dimensions of topographic surfaces, in both the gully heads and upstream areas. After the scouring was run for a period of 90min, asymptotic fractal dimensions of topographic surfaces were attained in the upstream areas and gully heads, suggesting that steady state morphological characteristics had been realised. It should be noted that headwall collapses are typically associated with a substantial increase in sediment yield where no other obvious change in morphological characteristics occurs in the headwall. Therefore, even though a significant difference in the soil erosion rates and fractal dimensions of topographic surface values could be found between the bank gully heads and upstream areas, the temporal variation in the morphological characteristics of bank gully heads was similar to those observed in upstream areas where ephemeral gullies developed.