Effects of grass density on the runoff hydraulic characteristics and sediment yield in gully headcut erosion processes

Vegetation plays a crucial role in gully headcut erosion. However, little is known about how grass influences runoff hydraulics, soil erosion and headcut erosion processes. A series of rainfall and scouring experiments were conducted on four plots to elucidate the effect of the density of Agropyron cristatum (L.) Gaertn (planting spaces: 20 x 20 cm(2), GA1; 15 x 15 cm(2), GA2; 10 x 10 cm(2), GA3; bare land as control, BL) on runoff hydraulics and soil erosion during gully headcut erosion. The velocity, Reynolds number (Re), runoff shear force (tau) and stream power (omega) of the BL increased exponentially with time (P < 0.01). However, for the grassland, most parameters increased linearly with time (P < 0.01). The Froude number (Fr) of the BL and grassland significantly decreased linearly and exponentially, respectively, with time (P < 0.01). For tau, the grass tableland needed to develop for a period of time to restrain it. The runoff of the BL tableland and gully head was supercritical turbulent, while that of the grassland was more dispersed. The existence of a gully head increased the jet velocity by 2.97%-67.19%, which increased the dispersion degree of the runoff. The runoff energy showed a significant exponential increase with time (P < 0.05). Compared with that of the BL, the energy consumption of the runoff from the grassland increased by 4.10%-31.81%. During gully headcut erosion, the BL was primarily cut down to the tableland by runoff, while the grassland was cut down to the headwall by on-wall flow. As the density increased, the benefit of sediment reduction increased, reaching a maximum of 66.79%. The collapse of the BL primarily occurred in the upper part of the headwall during the early experimental stage. The frequency and space-time ranges of the collapse of the grassland were larger than those of the BL. The gravity erosion types of the BL were headwall collapse and gully bank collapse, while those of the grasslands were headwall collapse and root-soil composite collapse.