The possible negative consequences of underground dam and reservoir construction and operation in coastal karst areas: an example of the hydro-electric power plant (HEPP) Ombla near Dubrovnik (Croatia)

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.

The Zrmanja River Estuary (Adriatic Coast, Croatia) – the Need for Interdisciplinary Approach to Protection of Coastal Areas

“…Accordingly, sediments of both the Novigrad Sea and the Karin Sea contain rather high shares of clay fraction, with an average of 16%. Although the drainage area of the Zrmanja River is a typical karst landscape [55], the lower part of the Zrmanja River estuary is surrounded by Eocene deposits (Figure 3), including flysch [32]. The higher content of clay fraction in the middle estuary is, therefore, the result of an additional input of terrigenous material through Karišnica River and small streams [24].…”

“…The prevalence of easily soluble carbonate rocks in the drainage area [55] is generally responsible for very low sedimentation rates in karstic rivers [51,65]. Specifically, in typical karstic rivers of the Dinaric region, only approximately 20% of the river-borne material is suspended and the rest is dissolved [47].…”

“…Peculiarity of karstic rivers, in general, lies in their intricate relationship between the underground and surface parts of their flow, but their uniqueness at the same time makes them extremely vulnerable to all forms of pollution. Namely, natural processes in combination with anthropogenic influence in karst areas can cause strong, sudden, unpredictable and generally immeasurable and dangerous consequences for both the environment and the society as a whole [55].…”

Karst spring catchment: an example from Dinaric karst

“…Fluctuations of groundwater level (GWL) are very fast and can have amplitude higher than 100 m during short time (about few hours) (Bonacci 1987(Bonacci , 1995Bonacci and Roje-Bonacci 2000;Roje-Bonacci and Bonacci 2013). During these changes, quick alteration of velocity and direction of groundwater circulation appear in some parts of karst massif (Hartmann et al 2014;Ravbar 2013).…”

Methodology for geohazard assessment for hydropower projects

“…Not only do geohazards pose a threat to reservoirs and dams (Sepúlveda et al 2005;Zhang et al 2010a, b;Huang et al 2011;Zhang et al 2013b;Sharma et al 2007;Cui and Zhu 2011), reservoirs can also trigger or activate geohazards, such as reservoir-triggered earthquakes (RTE) (Lomnitz 1974;Lane 1974;Meade 1991;Gupta 2002;Gahalaut et al 2007;ICOLD 2011;Rajendran et al 2013) and landslides (Schuster 1979;Fujita 1977;Wang et al 2005;Qi et al 2006;Singh et al 2012;Xiao and He 2013;Li et al 2013;Xu et al 2012). Furthermore, geohazards can affect the dam foundation (Yan et al 2004) and sensitive hydrogeological systems (Roje- Bonacci and Bonacci 2013). Moreover, the impounding of a reservoir by damming a river can impact the downstream river morphology (Ta et al 2008) by altering flow regime and flood peaks and trapping large amount of sediments previously carried by the river.…”