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The Ombla is a short river in Croatia, northeast of Dubrovnik. Its course is approximately long, and it empties into the Rijeka Dubrovačka, ria formed by the Adriatic Sea near Komolac in Dubrovnik-Neretva County. Rijeka Dubrovačka is actually a ria, a flooded river valley formed through changes in sea surface elevation on a geologic time scale. The river rises as a karst spring fed by groundwater replenished by Trebišnjica, which is an losing stream flowing in Popovo Polje, in the immediate hinterland of the Ombla. The elevation difference between the river's source and its mouth is just over . The average discharge of the river is per second. The drainage basin of the Ombla encompasses and, besides the short surface course, includes only groundwater flow.
The Ombla is used as a source of drinking water for Dubrovnik's water supply network, and construction of a Hydroelectricity has been planned for the past two decades. , the plans entail construction of a subsurface reservoir and a 68 megawatt power plant. The plan sparked controversy amid doubts raised with respect to environmental protection and biodiversity management, technical and financial feasibility, and procedural problems related to the project. A particular concern expressed was that the underground reservoir might trigger earthquakes.
The river rises in an cave whose roof has an clearance above the surface of the water. The primary source is located at an elevation of Sea level, and the secondary sources are found at above sea level (a.s.l.). The spring is the largest karst spring in Croatia, and one of the largest ones in the Dinarides. The surface of the water in the cave is a.s.l. The watercourse flows for approximately before reaching a weir across which the Ombla discharges into the Adriatic Sea, leading to claims that the Ombla is the shortest river in the world.
The area exhibits karst morphology, with bedrock largely consisting of and comparatively small areas of dolomites and Quaternary sediments. Eocene flysch forms the southwest boundary of the catchment area, towards which the catchment basin drains and where the Ombla rises. The rocks were formed as a thick series of carbonate rock between the Norian and Late Cretaceous as the Adriatic Carbonate Platform, up to deep. In the Eocene and early Oligocene, the Adriatic Plate moved north and north-east, contributing to the Alpine orogeny via the tectonic uplift of the Dinarides. The basin's karst topography developed from the carbonate platform's exposure to weathering. Karstification largely began after the Dinarides' final uplift in the Oligocene and the Miocene, when the carbonates were exposed to atmospheric effects; this extended to the level of below the present sea level, exposed during the Last Glacial Maximum. Some karst formations were created during earlier sea level drops, most notably the Messinian salinity crisis. The geological structure of the area indicates recent tectonic activity in the catchment, with a fault running between Hum and the Ombla. A recent strong earthquake in the wider region was the 1979 Montenegro earthquake, measuring 7.0 on the Richter scale. The only strong historical earthquake in the immediate area was the 1667 Dubrovnik earthquake, which was followed by a tsunami.
The Hydrology of the basin and Popovo Polje is determined by the flow of the Trebišnjica—a losing stream disappearing underground in the polje. The groundwater is distributed to a number of springs. Some of them are headwaters of watercourses discharging into the Neretva River to the northwest of the polje, or appearing as (submarine springs) or as headwaters of the Ombla river. Flow rate of the Ombla River, measured at the Komolac water intake plant, ranges from per second, averaging per second. The average has dropped by about per second since completion of Trebišnjica Hydroelectric Power Plant and the concreting of the Trebišnjica's river bed. On the other hand, the minimum discharge was not affected by the river engineering works.
The catchment basin area straddles boundary of two —the Mediterranean climate zone in areas at up to a.s.l, and the continental climate zone in other parts of the basin. Average annual precipitation varies depending on the climate zones: in Dubrovnik, at the coast, and in Hum, in Popovo Polje.
The development project became controversial as environmental protection non-governmental organizations (NGOs) drew public attention to a possible threat that may arise to seven species of . Subsequently, the NGOs alleged that the project is illegal, environmentally unsafe, financially not feasible, and technically questionable. Project legality was disputed because it is based on a 1999 environmental impact assessment (EIA), while Croatian legislation requires that EIAs must not be more than two years old. Flooding of Vilina Cave is cited as an environmental concern, while the financial issues are based on previous by Hrvatska elektroprivreda—the state-owned company planning the development—in other projects. One objection to the technical aspects of the project is the possibility that the groundwater may trigger earthquakes. Opponents of the project have urged the Prime Minister of Croatia to cancel development of the plant.
After the elections of 2011, Mirela Holy, who had declared her opposition to the project prior to the elections, was appointed Minister of Environment and Nature Protection. In 2012 the ministry commissioned four reviews of the EIA. The reviews—one of them supporting the EIA and three disproving the conclusions of the original EIA—were submitted to the Government of Croatia one day after Holy resigned her post, reportedly over an unrelated matter, on 7 June 2012. Objections were raised by authors of the EIA that the reviews were not published and the names of the authors have been kept secret. Prime Minister Zoran Milanović said he wanted to collect opinions from the foremost Croatian, European, and worldwide experts before deciding on the matter, which has been under consideration since the early 1990s.
In May 2013, the European Bank for Reconstruction and Development cancelled its proposed loan to the project, pointing at environmental concerns. Victory for civil society as EBRD cancels loan for controversial Croatian dam, Bankwatch blog, May 28, 2013.
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