Qualitative assessment of geohazard in the Rječina River Valley, Croatia

The Rjecina watercourse is 18.7 km long and its river mouth is in the center of Rijeka city. The central part of the Rjecina river is 1.8 km long, and 0.8 to 1.1 km wide and represents a narrow flysch valley between a karst plateau. Maximal annual discharge is 248 m 3 s -1 during a 100 yr return period. Cretaceous and Paleogene limestone rocks are situated on the top of the slope, while Paleogene flysch forms the lower slopes, including the bottom of the valley. The south-western slope of the valley is covered by predominantly coarse soils of limestone composition. The crown of instability is clearly marked by cliffs formed in limestone rocks, which are very disintegrated with recently opened fractures clearly visible. On the north-eastern valley side, slope deposits are mostly a mixture of clayey silt, formed by weathering of flysch bedrock, and fragments to blocks of limestone originating from the cliffs at the top of the slope. On both slopes, different types of instabilities, according to type of movement, type of material involved and state of activity, can be found. Only in the above-mentioned part of Rjecina valley, are instabilities on both slopes formed. A potential geohazard event could involve movement of slope deposits towards the channel of the Rjecina River. Since both slopes are at or near to the limiting state of equilibrium for stability, preparatory factors already exist. Heavy precipitation and/or earthquakes may be potential triggers of rockfalls and rockslides. Daily precipitation higher then 100 mm is frequent in this area. This could cause two secondary effects: damming of the Rjecina River leading to the formation of landslide-dammed lakes; as well as formation of a flooding wave due to destruction of the natural landslide dams, and consequent flooding of the lower central area of Rijeka city.