The study on morphological evolution process of gully headcut erosion in granite red soil hilly area based on an in situ scouring experiment

Abstract

Gully headcut erosion (GHE) is a key process of gully erosion. However, there is a lack of effective monitoring of the spatiotemporal changes in gully head morphological characteristics that occur during the continuous GHE process. Through a continuous scouring experiment in the field, GHE morphological evolution characteristics were examined for different exposed soil layers (laterite layer and sandy layer), headcut heights (25, 50, 75, 100 and 125 cm) and flow discharge levels (60 and 120 L/min) in typical granite red soil hilly area. The results indicated that the width–depth ratio (WDR) of the erosion gullies decreased in the slope direction and that erosion gullies violently developed on the downslope when stable runoff was generated across the upstream surface. The GHE could be divided into three periods, namely, the gully formation period, active headcut period and stable erosion period, which were mainly reflected in the gully head (GH) and plunge pool erosion (PPE) below the gully bed. The gully headcut retreat (GHR) distance and headwall collapse (HC) range increased with increasing headcut height and flow discharge. PPE exhibited a disordered downward cutting pattern, and the degree of development fluctuated. The GHE degree was jointly affected by GHR, HC and PPE, and these three processes interacted. The gully headcut erosion amount (GHEA) of the laterite layer was smaller than that in the sandy layer, but the average headcut erosion contribution rate (HECR) was 58.51 %, which was higher than that of the sandy layer (48.53 %). The GHE was the main erosion path in the two soil layers, and the HECR was not affected by a single erosion process. The sandy layer was easily eroded, and the GH morphology changed markedly during scouring, while the gully erosion amount (GEA) ranged from 0.02 m3 to 0.21 m3. This was mainly affected by GH and PPE. In this study, niche-like caves may form in the sandy layer to intensify gully erosion. When the headcut height and flow discharge reached a certain degree, the GEA of the laterite layer with strong anti-erodibility will also reach a higher level, and the GEA varied between 0.01 and 0.17 m3. GHR and HC were the dominant gully erosion factors. GHE prevention should focus on the laterite layer, as an effective protective layer in granite red soil areas.