Rainfall intensity–duration thresholds for the initiation of landslides in Zhejiang Province, China

• We studied rainfall data with 1569 landslides in Zhejiang between 1990 and 2013.
• We defined the regional and 62 subregional rainfall intensity–duration thresholds.
• Spatially distributed rainfall threshold maps over the entire region were prepared.
• We discussed the correlations of threshold with soil types and slope gradients.
• Variation of threshold is affected by slope-forming materials and terrain slopes.

Zhejiang Province, located in the southeast coastal region of China, is highly prone to rainfall-triggered landslides because of its geologic, geomorphologic, and climatic settings. The rainfall–landslide relationship is critically important for predicting rainfall-induced landslides. This study defines landslide-triggering rainfall intensity–duration thresholds for the entire Zhejiang region; and the 62 individual areas that comprise the region, based on 1569 shallow landslides which occurred from 1990 to 2013 and their corresponding detail rainfall records from 2457 rain gauges in the region. The results indicate that the rainfall thresholds vary spatially over the region. For rainfall durations from 1 to 24 h, the threshold tends to increase from the northwestern part of Zhejiang to its southeastern coastal region; and it is lower in the central and coastal hill–basin regions than that in the western and southern mountainous regions. Variability of the threshold in space is mainly affected by the slope-forming materials and terrain slope gradients. Different soil types have different thresholds, and the thresholds for weathered rock slides are generally higher than those for soil slides. For the soil–weathered rock on slopes, the slope gradient has no obvious influence on the thresholds when the slope angle is < 30°; the thresholds have an obviously increasing trend with the increase of slope angles in the range of slope angles from 30 to about 40°; and when slope angle is larger than about 40°, the thresholds rapidly decrease with gradient on the whole. These findings will facilitate the improvement of warning systems for regional rainfall-triggered landslides.