Naslov
Torrential catchment raindrop erosion in the Central Istria (Croatia)

Autori
Jurak, Vladimir

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Riassunti delle comunicazioni orali e dei poster / Carulli, G.B. & Longo Salvador, G. - Trst : Edizioni Università di Trieste, 2000, 296-298

Skup
80a Riunione estiva

Mjesto i datum
Trst, Italija, 06.09.2000. - 08.09.2000

Vrsta sudjelovanja
Poster

Vrsta recenzije
Nije recenziran

Ključne riječi
Erosion; Raindrop erosion; Sediment delivery ratio; Erodibility map; Flysch; Central Istria

Sažetak
Flysch area in the central Istria has all the necessary physiogenetic predispositions for the raindrop erosion, which at many places has the character of excessive erosion. In the mentioned area the erosion is distinguished by various characteristic forms of surface wash particularly on the denuded land so called badlands (Jur , Fabić, 2000). The intensity of the rock and soil surface wash has been measured during certain time period in the Abrami test area near Buzet. The measurement results are also referential for the estimation of a possible specific annual sediment delivery ratio in the catchment of particular torrential streams. One of them is Posert pertaining to the spring of the Raša river. The prevailing physical - geological process in the catchment of the Posert torential stream is water erosion occurring as rainfall erosion - rill, gully, torrential and fluvial erosion. The practical aspect of the erosion status in torrential stream is shown by the erosion loading of the storage reservoirs and flood storages reducing their life. They are jeopardized by being covered up with proluvial sediment while, at the same time, these very reservoirs and storages are one of the stream regulation measures. One of them is the planned storage reservoir Grobnik because of which the erosion status in the catchment of the Posert torrent has been established by the relevant engineering-geological survey. The erosion loading of the storage reservoir as an engineering problem was considered on the basis of quantitative indicators of surface wash from the representative erosion areas of the Abrami test area. Translation of the mentioned measurement data is possible because of identical geological conditions and a relatively small distance (25 km bee-line) which also ensures the analogy of climate characteristics. Monitoring of pluvial erosion at six test areas with different geological-pedological-vegetational properties was carried out during a period of six years and the results were averaged (1971 - 1976). A total of 948 samples were available. The enormously high index of erodibility of the area, which is the representative of the surface wash erosion sources, was noted. It represents the areas of extremely excessive erosion, which exceeds 8000 times the sediment delivery ratio from the areas representing the lands under vegetation and with thicker pedosphere. This is to be expected since the denuded parent rock is directly affected by two factors - pluvial and thermodynamical. The entire catchment of the Posert torrent is formed in impermeable flysch deposits so that the water movement occurs on surface and near surface and hence the fluvial genesis of the relief. The catchment area of the Grobnik storage reservoir covering F = 24,7 sq.km has been considered. In the lithologic column of the flysch complex a subdivision to siliciclastic turbidites and carbonate turbidites is distinguished. Each of the lithologic members is distinguished by its geomorphological features resulting from its erodibility. Accordingly, the homogenous marls even without clearly pronounced textural elements can be classified into the category of weak/soft rocks in the geotechnical spectrum (Johnston et Novello, 1993). Their characteristic is maximum erodibility i.e. they are extremely liable to surface erosion - rill and gully erosion. After Magdalenić (1972) the mentioned sediments occur in the upper part of the flysch sequence (interval e). The phenomenon of the sediment delivery ratio is considered within the general denudation process as a geohazard. It is manifested as an engineering problem - filling of the storage reservoirs at a higher rate than predicted. The value of the specific annual sediment delivery ratio caused by rainfall erosion of only 450 cu.m/sq.km may be regarded as partial since the effects of the other types of erosion should be also taken into consideration. This particularly refers to the gully erosion through the network of the lower-order drainage gullies. Probably neither the debris fall from the denuded areas is negligible. An objection might be also made that the factography on the respective phenomenon has been obtained at relatively small areas and the monitoring result is quantitatively shown for a much bigger area by a specific sediment delivery ratio (cu.m/sq.km/year). Therefore, Racz (1997) reasonably warns that in spite of the advantage of the measurements carried out in the field, one should also be aware of the mistakes that could be made when the local measurements data are automatically applied to the entire catchment area. This would particularly refer to the area 1 (denuded land) representing an extremely excessive erosion. This is, however, a conventional manner of representing the sediment delivery ratio in hydrotechnical and agropedological practice (Racz, 1997). The represented quantification of the one of the physical-geological denudation processes is not far from that one recommended in publication RAC/PAP (1998). The erosion of various intensities and mosaic distribution, finally resulting in denudation takes place on nearly 90 percent of the catchment area while 10 percent represents the deposition and sediment transportation areas. The raindrop and gully erosion on the denuded areas generate 80% deposits by surface wash. It is quite evident that the sediments are predominantly generated from the denuded areas. By controlling the sediment amount transported from these areas (pluvial erosion and debris fall) a high quantity of information on the erosion phenomenon in a catchment area is obtained. Since the bare lands in the Grobnik catchment area cover only 6% of the surface area it is logical to direct the remedial measures to the erosion sources.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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