INFLUENCE OF THE SEDIMENTATION PROCESS ON THE RA?A RIVER MOUTH ON THE OUTFLOW REGIME OF COASTAL SPRINGS

摘要

The paper discusses the influence of the sedimentation process at the Rasa rivermouth on the outflow regime of several coastal springs important for the water supplyof Istria, among which some are subject to salination. The reason of existence of anumber of high-yielding springs in this area may be explained by the fact that theRasa river valley, whose canyon part is cut deep into the upper Cretaceouslimestones of the Istrian karst plain during the Holocene transgression is the lowestdrainage base for surface and ground water in the broader part of Istria. The valleybottom is covered by deep impermeable alluvial Holocene sediments, and the abovementionedascending karst springs upper in the zone of contact between the karstbackground and the valley, or the sea. The basic limestone rock mass in the Rasariver estuary is found at depths over 80 m, within the reach of intensive developmentof karst phenomena. The sedimentation process in the river mouth has resulted inbuilding up of sediment layers at the beginning of the estuary where the springs aresituated, and these layers reach to the depth of some seventy or more meters.The paper also discusses the process of formation of the valley zone and the effectsof this process on the water regime of the springs involved in these changes.Depending on the location-distance of individual sources from the sea and theirposition in relation to the sediment, the influence of the sea on salination of sourcewater is noted. The paper discusses the influence of the position of the sediment onthe hydraulic mechanism of water outflow from the sources. As examples, threelowest lopated important sources in the Rasa river catchment area have beenselected - the spring Rakonek as the most downstream source in the Rasa valley inwhich the influence of the sea is not felt, the spring Fonte Gaja-Kokoti where theinfluence of the sea is felt only in extreme hydrologic conditions, and the spring Blaz,in which salination is the regular seasonal problem.As the natural sedimentation processes affect the outflow regime of the sources andprotect them from salination, they may, in the upstream part of the Rasa river, serveas a model for future measures for protection of downstream springs from salination.