Abstract:  The Yangjia gulley debris in Zhejiang Province is a typical low frequency gully debris flow formed by water erosion. Based on the background of geological environment, the subarea characteristics, types and formation mechanism of debris flow are analyzed, and the susceptibility and hazard of debris flow are also evaluated. Then the rainstorm intensity index is established by statistical analysis method. In addition, through mechanical calculation and limit equilibrium analysis theory, the hydrodynamic erosion initiation model is established to determine the critical runoff depth under rain condition. Based on these studies, the progressive hierarchical comprehensive early warning system is constituted by combining rainfall, mud water level and flow index. Finally the remote automatic monitoring network system of debris flow is built up from October，2012 for monitoring control and early warning. The results show that the Yangjia gully is a small enclosed watershed with distinct zoning characteristics of catchment area, formation circulation area and accumulation area. The debris flow is a low frequency, small scale and diluted water stone flow triggered by extremely heavy rainstorm. The susceptibility of debris flow is moderate, and it will harm the village area at the downstream of the gully, and the degree of harm is relatively high. When the surface runoff depth of loose material sources in the gully formation area exceeds 1.0 m, these loose materials are in a critical unstable state and easy to be eroded. This critical runoff depth can be used as an effective criterion to determine whether the debris flow is started by erosion, and the comprehensive early warning index of debris flow can be determined by combining the R value of rainstorm intensity and flood peak flow Qd of 31.83 m3/s. Through remote automatic monitoring, the R value （2.4） is less than 2.8 during the monitoring period（20121026—20221010）, the gully area is in a safe rain situation, and the gully mud level does not reach the warning limit of surface runoff, so there is no hydrodynamic condition for the formation and startup of debris flow. Moreover, the gully flow is normal, and the maximum value （1.70 m3/s） is much less than 31.83 m3/s, and there is no adverse phenomenon such as debris flow or channel blockage. The gully area is relatively stable throughout the monitoring period, and the possibility of debris flow occurring again is small. It is also confirmed that the comprehensive monitoring network system based on the characteristic parameters of hydrodynamic erosion startup is an effective technical method for disaster prevention and control of low frequency gully debris flow.

Key words:  , low frequency, gully debris flow, hydraulic erosion, critical runoff depth, remote monitoring, comprehensive early warning