Effects of frequency and interval of groundwater monitoring on the inversion coefficients of permeability of materials of a landslide

Coefficient of permeability of materials of a landslide is one of the key parameters that control its response to rainfall infiltration and reservoir water level fluctuation. It has been well recognized to be one of the most effective and reliable approaches that the coefficient of permeability of materials of a landslide is obtained with methods of inversion based on groundwater monitoring data when the data are available and abundant. However, the accuracy of the inversion results largely depends on the sampling frequency and interval of the monitoring data. Therefore, it is necessary to determine an optimal monitoring frequency and interval to get a reliable and accurate inversion coefficient of permeability of materials of a landslid. In this paper, coefficient of permeability of the Lijiapo landslide, one of the representative colluvium landslides in the Three Gorges Reservoir area, was obtained with the approach of inverse simulation of seepage field and fitting of the inversion and monitoring data. Effects of frequency and interval of groundwater monitoring on the inversion results were evaluated based on their stability and discreteness, and the optimal monitoring frequency and interval were proposed. The results show that (1) the error of the inversion groundwater level is getting much less when monitoring frequency and interval are higher than 15 day/per time and longer than 810 days respectively; (2) when the monitoring frequency is each day or 5 day/ per time and the interval is 270 days, or the monitoring frequency is 10 day/per time and the interval is 540 days, the inversion coefficient of permeability is the most stable with the least discreteness; (3) when the monitoring frequency is twice a month or once a month and the interval is 1 170 days, the results are not stable and with much greater discreteness. Practically, it is suggested that 5 day/per time and 270 days could be the optimal frequency and interval of groundwater monitoring for the inversion coefficient of permeability of a colluvium landslide in the reservoir area, considering the error of the inversion groundwater level, the stability and discreteness of coefficient of permeability as well as the costs.