Abstract
Abstract. The Ombla Spring represents a typical abundant coastal karst spring located in the vicinity of the town of Dubrovnik (Croatia). Its outlet is at an altitude of 2.5 m above sea level (m a.s.l.) and the water from it immediately flows into the Adriatic Sea. The minimum and maximum measured discharges are 3.96 m3 s−1 and 117 m3 s−1, respectively. The Trebišnjica River traverses through its catchment. The mean annual discharge, after the canalization of over 60 km of its watercourse with spray concrete (in the time span 1981–2011), is 24.05 m3 s−1. Before massive civil engineering work which took place during 1968–1980, the mean annual discharge was 28.35 m3 s−1. There is a project for construction of the hydro-electric power plant (HEPP) Ombla, which will exclusively use groundwater from the Ombla Spring karst aquifer. The underground dam will be constructed about 200 m behind the existing karst spring outflow in the karst massif, by injecting a grout curtain. The top of the grout curtain is planned to be at an altitude of 130 m a.s.l. This karst system is complex, sensitive, vulnerable and ecologically extremely valuable. The grout curtain, as well as the HEPP Ombla development, could lead to extremely dangerous technical and environmental consequences. In this paper some probable, negative consequences of the HEPP Ombla construction and development are explained. The HEPP Ombla could result in many large and hard-to-predict negative consequences which are specific for this particular HEPP, for example (1) severe spring discharge change; (2) unpredictable regional groundwater redistribution; (3) threatening of endemic fauna; (4) induced seismicity; (5) induced sinkholes; (6) occurrence of landslides; (7) conflict regarding internationally shared karst aquifers; (8) intensification of karst flash floods; (9) sea water intrusion in coastal karst aquifer; etc.
Highlights
The natural behaviour of the karst aquifer through the Ombla Spring will be drastically changed by operation of the hydro-electric power plant (HEPP) Ombla, but is not easy to predict in what manner
Due to increasing of the groundwater levels (GWLs) caused by grout curtain construction, some temporary karst springs in the local Ombla Spring catchment area could become permanent
In the case of the currently active Ombla Spring, the possibility of inactive and blocked fossil conduits, which are expected to open under the influence of the new and much higher hydrostatic pressure caused by the grout curtain construction, cannot be excluded
Summary
Especially the construction of dams and reservoirs realized by creating large grout curtains (Bonacci et al, 2009), as well as interbasin water transfers through long tunnels and pipelines, can introduce instantaneous and distinct changes in catchment areas and boundaries, and, in turn, on the hydrological, hydrogeological and ecological regimes (limitation of energy flow, negative impacts on food cycle, blocking of convections between habitats and species, pollution of groundwater and underground environment, etc.) (Bonacci, 2004). The HEPP Ombla example, apart from being an interesting case study, can be valuable for many other karst areas worldwide where engineering works and construction are taking place, especially those based on grout curtains. From the hydrological and hydrogeological point of view, the poljes function as part of a wider system They represent more-or-less interconnected subsystems within the process of surface and groundwater flow through the Ombla Spring karst catchment. Most of the sediment (more than 90 % of whole annual amount) is delivered during the intensive short rainfall, in a few hours (Denic-Jukicet al., 2012)
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