Assessment of landslide-triggering factors and occurrence using morphometric parameters in Geyraz Basin, Tokat, Northern Turkey

The Geyraz Stream is one of the tributaries of the Yeşilırmak River, located within the southern of Middle Black Sea section, and enters the main river at Tokat city centre. Geyraz is a basin that has experienced numerous landslide events. In this study, morphometric parameters of areas prone to landslide occurrence were investigated and computed in GIS. These parameters included the Dissection Index, hypsometric integral, drainage density, Stream Power Index, and Compound Topographic Index, first and second orders. In addition, rocks, slopes and frequencies of first- and second-order streams in subbasins were considered and their correlations were examined. The moderate (15–20%) and high slope (20–25%) values with primarily clayey rocks (schist and flysch) correspond to high Di and Hi values, a maximum number of first- and second-order streams, and high landslide occurrence, while very steep slopes lead to low Di values and a low density of landslides. Results show that former landslides occurred in areas with Di values of 0.1–0.15, moderately steep and steep slopes, a maximum number of first- and second-order streams, high stream frequency on clayey rocks (schist and flysch), CTI values of 60–100, and SPI values of 75–85. In other words, landslide events are triggered by higher drainage frequency on primarily clayey rocks, intense fluvial incision and medium–high steep slopes. The location of agricultural land in the landslide area, reflecting lack of vegetation cover, is another landslide-triggering factor. Thus, the study area was ranked in six different classes and risk was rated as very low, low, medium, medium to high, high and very high. Determination of areas prone to landslide occurrence and their triggering factors is important for regional planning with respect to basin management and hazard prevention.