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Possible negative consequences of underground dam and reservoir construction in coastal karst area: example of HEPP Ombla near Dubrovnik (Croatia)

The Ombla Spring represents a typical karst spring with limited maximum and mini-mum outflow capacity. The exit of Ombla Spring 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 mini-mum measured discharge is 3.96 m3/s. The maximum discharges never exceeded a value of 117 m3/s, despite of fact that the spring catchment area is large and is assessed to cover an area of about 1000 km2. The precipitations on it are very abundant and intensive. The mean annual discharge after canalization of the more than 60 km long Trebišnjica River watercourse with spray concrete (from 1981 till 2011) is 24, 05 m3/s. Before this massive civil engineering works in 1968-1980 period mean annual discharge was 28.35 m3/s. There is a project for construction of the hydro electric power plant Ombla, which will exclusively use groundwater from a karst aquifer which feed Ombla Spring. The underground dam will be constructed 190 m behind the existing karst spring outflow in the karst massif. It will be performed by injection as grout curtain. Projected total curtain area is about 300, 000 m2. Total length of the curtain will be about 1470 m, while its maximum depth will be 410 m. Installed discharge of the HEPP Ombla is 70 m3/s. Present-day ecological aspect of grouting has attracted little attention. Grouting is an extremely expensive and hardly foreseen procedure. In many cases it is not successful from the engineering perspective. After the construction of the grout curtain, the hydrostatic pressure upgradient of the curtain is increased to levels never previously encountered. This increased pressure can result in “clay plugs” being expunged from some of the cavities adjacent to the penetration of the grout, thus open-ing up new cavities that are gradually enlarged as effective conduits. In the paper will be explained many possible, but very probable, negative consequences of the HEPP Om-bla construction and development as for example: induced seismicity, intrusion of sea water in coastal karst aquifer, overflow of groundwater from Ombla Spring catchment to neighbouring karst spring catchments, landslides provocation, dangerous changes of suspended sediment deposition, collapse of dolines, induced subsidence, pollution of vulnerable and valuable karst underground environment etc.

karst aquifer; grout curtain; Ombla Spring; ecology; karst hydrology

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