Effects of sediment yield fluctuations on the niche-like ephemeral gully formation in granite red soil

The granite red soil area in southern China has been afflicted by severe and large-scale gully erosion, locally known as Benggang erosion. It is generally believed that Benggang develops from the earlier gully; however, this remains disputed. Therefore, we investigated the rills and ephemeral gullies in the Yanglong watershed of the Hubei Province to explore the initial developmental process of Benggang. The results indicate that a unique gully with a niche-like morphology was found, which was named niche-like ephemeral gully (NEG). The NEG morphology was as follows: a distinct rill upstream, steep bank, a narrow upper bank, and a wide and deep gully bed. The NEG was formed on the exposed sandy soil layer and a local slope which was steeper than 30 degrees. The field runoff scouring experiment simulating NEG formation was conducted on two slopes (20 degrees and 40 degrees) and with two runoff flows (3 L/min and 6 L/min) on three naturally exposed soil layers due to early surface soil erosion (B1 and B2 red clay, and sandy soil layers). The experimental results revealed that the total sediment yield of the sandy soil layer was at most 24.39 times higher than that of the B1 layer under the same treatment. Furthermore, compared to upper red clay layers, the deeper sandy soil layer formed more bank collapse due to faster soil moisture variations and higher soil pore water pressure, thus prompting more drastic sediment yield fluctuations and NEG formation. Therefore, the NEG was primarily formed in the sandy soil layer and was influenced by gravitational erosion represented by bank collapse. The amount of bank collapse and sediment yield differed in the three exposed soil layers, which could be essential for the development of rill into the NEG. Therefore, the NEG is a critical stage where the rill evolves into Benggang from the predominant hydraulic erosion into gravity erosion. These findings provide a new possibility for the origin of Benggang.

Automated summary

A unique gully morphology called niche-like ephemeral gully (NEG) was found in the granite red soil area in southern China, which is primarily formed in the sandy soil layer and influenced by gravitational erosion. The amount of bank collapse and sediment yield difference in different soil layers play a crucial role in the development of gully into the NEG.