Debris-flows, mudflows and submerged tree drifting in the plains

Abstract

Traditionally, it is believed that debris-flows are formed only in mountainous areas, and their occurrence in flat areas is impossible. These ideas about the conditions for the development and spread of debris-flow processes are reflected in the Russian Technical Regulations that determine the composition of engineering surveys for construction. In these Regulations, debris-flow and potentially debris-flow areas are identified only in mountainous areas, while flat and hilly areas are shown as non-debris-flow. Debris-flow risks are not calculated for flat and hilly areas both in Russia and in other countries. The vulnerability of objects, structures and territories to the impact of debris-flows is not taken into account either in the development of urban planning documentation, or in the design of objects and structures, or in the development of recreational areas. When designing hydraulic engineering and other structures, the loads created by water flows with a density of up to 1000 kg/m3 are calculated, and not by debris-flows, mudflows and suspended streams with a density of 1100-2000 kg/m3. The results of the research show that the presence, even in local areas, of slopes exceeding (depending on the type of rock) 5°-10°, creates the possibility of incoherent debris-flows (suspended streams) on lowland small rivers. For example, the number of such small rivers in the Novgorod region is 20%. The presence of even local sections with slopes of more than 100‰ on flat rivers creates conditions for the formation of debris-flows and mudflows. Debris-flows and mudflows are formed on plains not only in ravines, but also in the beds of permanent and temporary small watercourses and in erosion furrows on the slopes of hills and river terraces. The density of debris-flows and mudflows of small volumes formed in erosion cuts on the slopes of hills and river terraces can reach 2000 kg/m3, and sediment flows passing through the beds of small rivers-1100-1200 kg/m3. Submerged tree drifting on small watercourses in its composition are not water streams, and the incoherent debris flows.