Rainfalls and groundwater influences on landslides in Northeast Bulgaria

Summary The main reason of landslide activation is rising of groundwater, saturation by rain, water infiltration and snow melting. One part of landslides in Northeast Bulgaria starts after heavy rainfalls, as was evident during the last 20-year wet period. The interconnection between the landslides in Northeast Bulgaria, rainfalls and groundwater maxima is investigated by variations of discharge for spring near Kotel and groundwater levels in two dug wells near Balchik, precipitation from meteorological stations Sofia, Varna, Veliko Tarnovo and Kazanlak, and Palmer Drought Severity Index (PDSI). The periods of maxima of precipitation, PDSI and groundwater time series are compared with the registered occurrence of landslides. Some part of landslides occurs after groundwater maxima, other part – after intensive short-time rainfalls without significant change of groundwater levels. It is necessary to analyze time series of precipitation and PDSI in order to provide comprehensive landslide forecast. The groundwater levels follow almost exactly the PDSI variations, while the short time intensive rainfalls are presented as significant spikes in precipitation time series. The hazard risk of landslides on the territory of Bulgaria should be estimated by the maxima of PDSI, real meteorological data from stations located near the Black Sea coast and inside the territory.