Andrej Kryžanowski

Are Torrent Check-Dams Potential Debris-Flow Sources?

Check dams as a typical torrent structure behave as retention structures typically storing torrent sediments in order to stabilise the torrential bed. If such structures are damaged and partially broke down, stored torrent sediments may cause sediment–related disasters in the downstream torrent’s reaches or on torrential fans. A field study on the status of 22 check dams in the Upper Sava River in NW Slovenia with regard to their structural stability was performed to answer a question whether they should be taken as a possible debris-flow source. For this purpose we used the existing inventory (cadastre) of water infrastructure in torrents and field survey. The resulted hazard assessment showed rather high debris-flow risk for downstream infrastructure in the lower parts of the investigated torrential watersheds. That is why we should treat some check dams as “large” dams.

Hydro power plants on the Middle Sava River section

Construction of a chain of hydro power plants is planned on the Sava River from Medvode to the Slovenian-Croatian border which will, apart from the chain of HPPs on the Drava River, represent the linchpin of renewable energy production within the Slovenian power system. The mentioned chain of HPPs will also be one of the countrys main renewable energy sources that can still be developed for power generation. Three hydro power plants, Moste, Mavccice and Medvode, are already operating on the Upper Sava River section. Construction of the chain is underway in the lower part of the stream where Vrhovo and Bosstanj HPPs are already in operation; HPP Blanca is under construction and the site planning procedures are taking place for Krsko, Brežice and Mokrice HPPs. The planned HPPs on the Middle Sava River section between Medvode and Zidani most will connect the HPPs on the Upper and Lower Sava River into a closed chain which will operate on the principle of run-of-river type power plants with daily storage. Completion of all stages will enable optimal development of available hydro potential. Apart from the energy effects, also other beneficial effects of hydro power plant construction in the region can be expected: flood protection; better water supply; waste water treatment; development of transport and energy networks as well as positive economic and social effects.

More Room for LandslidesOpen image in new window

Since ancient times, and more intensively from the mid-19th century, land in the mountain region is developing and the space belonging to land and water has been reduced. On the surface that previously belonged to the river have developed agriculture, transport routes and settlements. At the end of the 20th century, development spread over hazardous areas, many torrent flowed in highly confined channels, and ground water recharges drop down, instability of land surface and security for inhabitants decreased. This resulted in the changes reducing water resources of appropriate quality, reducing space for sediment deposit, increasing erosion, affecting natural habitats, causing major flood damage, decreasing groundwater stock, and deteriorating water quality. The water regime integrates all events across space from landslides, debris flow and is manifested in river regime in low lands. This problem is partially covered by many United Nations (UN) and UNESCO documents and reports. Proper actions are also suggested in the Ministerial Declaration from the 7th World Water Forum, where the first mentioned action is the significance of appropriate land management in relation to sustainable water management and planning. More room for landslides control means more space for potential landslide control out man made impacts that cause land slope instability, more space for torrents, more space for water and sediment storages, less impact on the slope stability and higher security for the peoples. We should change paradigm of space planning and development, especially in countries in development under intensive urbanization. The aim of this paper is to present particularly bad examples from Slovenia in order to support this proposal.

Concrete Torrent Check-Dams and Debris-Flow Magnitudes

Large sediment-retention dams, built in a cascade (a chain of check dams), can impose hazard (hyper-concentrated sediment flow, debris flow), if they fail during torrential flash floods or when destroyed by overtopping by a debris flow initiated on slopes or in natural torrential channels. Using estimates of specific annual sediment yields in torrential watersheds on one hand, one can use the area of the watershed and the storage volume of torrential check dams in order to estimate the maximum potential of sediment stored in the retention volume of the check dams—as a first approximation for the magnitude of the debris flows initiated by the dam failure. On the other hand, one can use debris-flow susceptibility maps to estimate likelihood for triggering of potential debris flows in torrential watersheds under investigation (we used debris-flow susceptibility map of Slovenia in the scale 1:250,000). A field study on the status of several tens of check dams in the Upper Sava River in NW Slovenia with regard to the question whether they should be taken as a possible debris-flow source was performed.