Qualitative assessment of geohazard in the Rječina Valley, Croatia

Rjecina watercourse is 18.7 km long and its river mouth is in the center of Rijeka city. The central part of Rjecina river is 1.8 km long, and 0.8 to 1.1 km wide and it represents a narrow flysch valley between a karst plateau. Maximal annual discharge is 248 m3s-1 during the 100 yr returning period. The Cretaceous and Paleogene limestone rocks are situated on the top of the slope, while the Paleogene flysch is situated lower on the slope, including the bottom of the valley. Southwestern slope of the valley is covered by predominantly coarse soils of limestone composition. Crown of instability is clearly marked by cliffs formed in limestone rocks which are very disintegrated and recently opened fractures are visible. On the northeastern slope, slope deposits are mostly a mixture of clayey silt that was formed by weathering of flysch bedrock and fragments to blocks of limestone originating from the cliffs on 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, instabilities on both slopes are formed. Basic geohazard event could be movement of slope deposits towards the channel of Rjecina River. Since both slopes are at the edge of a stabile equilibrium state, preparatory factors already exist. Heavy precipitation or earthquakes may be efficient triggers of rockfalls and rockslides. Daily precipitation higher then 100 mm is frequent in this area. This could cause two secondary effects: damming of Rjecina and formation of lakes ; as well as formation of flooding wave due to destruction of natural dam, and consequently flooding of lower central area of Rijeka city.