The groundwater responses to loess flowslides in the Heifangtai platform

Due to the large area of agricultural irrigation on the Chinese Loess Plateau, the groundwater table has been significantly raised, resulting in dozens of flowslides in the Heifangtai loess platform, Gansu Province. The groundwater responses to flowslides in Heifangtai were investigated by field investigation and numerical modelling. These landslides happened at backscarps of the early landslides, indicating a geomorphological evolution. On this basis, the platform conditions in future can be evaluated, therefore, to establish the numerical model for groundwater simulation. The simulation results indicate that the continuous occurrences of flowslides can influence the hydrological boundaries of the groundwater system. It then improves the drainability of the platform by enlarging the seepage areas of the platform and decreases the total groundwater recharge by decreasing the irrigation areas of the platform. For a given irrigation intensity, the perched groundwater table firstly increases to a peak value and then decreases to an approximately steady state, close to the level in 2004. At that time, the accumulative volume of flowslides was up to about 4,600 × 10⁴ m³. After taking into account that the perched groundwater table in 2004 was still high enough to trigger flowslides, the rest area of Heitai (the modelled platform) is predicted to be largely reduced to at least less than 7.8 km² and will decrease continuously. Landslide risk in Heifangtai would be very high for a long time if no effective measures are adopted for irrigation control.