Underground concept for Croatia's Ombla project Mar 2011
Z. Sever, Elektroprojekt d.d., Zagreb, Croatia; D.Kolic, Neuron Zagreb d.o.o., Zagreb, Croatia
The Ombla hydropower and water supply project in Croatia is conceived as an entirely underground installation - and that includes the upper and lower reservoirs. The design intends to harness the water of the underground Ombla River that springs at the Adriatic coast near Dubrovnik. Engineers working on this unusual and innovative project explain its concept, the main features of the installation and the proposed methods of construction and operation. The project and its development will be discussed further at the Using Underground Space conference in Dubrovnik in two week's time (April 7-8).
Fig. 1 The Dubrovnik area map

Fig. 1 The Dubrovnik area map

The Dinaric Alps karst running along the Croatian coast of the Adriatic Sea abounds in streams that sink underground at higher altitudes and reemerge at sea level. The Ombla project is a pioneering attempt to build a power plant entirely underground. This includes construction of an underground dam to create an underground reservoir on the Ombla River spring in which will discharge until under the natural conditions.
The Ombla spring is the discharge point of a large drainage system conveying water from the mountains behind the city, through underground passages, and into the sea at Rijeka Dubrovacka, a bay some 5km north-east of Dubrovnik city (Fig 2). Mean annual discharge is Qmean = 23.9m3/sec.
The fact that the spring does not dry out indicated the possibility of a high capacity underground storage basin that could be harnessed for water supply as well as hydropower generation.
Further investigations confirmed that rainwater penetrates the soil over a catchment area of 600km2 (Fig 3) and creates an underground reservoir. This reservoir discharges through a tectonically damaged Dolomite barrier into a conduit system that releases into the sea at the Ombla spring. The most important element in the system is the main conduit, which, according to investigations, conveys about 90% of the spring's discharged.
Fig. 2 Ombla spring outlets into Rijeka Dubrovacka Bay

Fig. 2 Ombla spring outlets into Rijeka Dubrovacka Bay

The project concept is based on building a grout curtain downstream of the Dolomite barrier to create an underground dam with the rock mass as the dam body. By impounding water to elevation 130.00m a.s.1., sufficient volume and hydrostatic pressure can be created to generate hydro power as well as supply water to Dubrovnik without the need for pumping, as is currently the case. Conditions will also be created for future supply of water for export to Mediterranean countries in super tankers. This innovative aspect of this multipurpose system is that all the installation structures, including the reservoir, will be located under the ground.
The entire Ombla underground river system is fed by a in which the average annual precipitation in the 600km2 catchment area is about 1,400mm in the coastal belt and more than 2,400mm in the mountains. This yields a maximum and minimum 100-year return period discharge of QmaxlOO = 113.0m3/sec and QminlOO = 2.4 m3/sec.
The rock mass in the greater source area can be classified into the following categories according to its hydrogeological characteristics:
Fig 3. Underground water catchment area

Fig 3. Underground water catchment area

• Eocene flysch representing an impermeable hydrogeological barrier;
• Triassic Dolomite which is a poorly permeable rock with local deviations appearing in the form of karst conduits; and
• Jurassic limestone which is permeable rock dominated by typical karst porosity with conduits and caverns of large sizes that act as hydrogeological collectors and aquifers.
The impermeable Eocene flysch is a backbone of the Ombla design concept, since the grout curtain is to bond with the flysch to create the dam. Based on these hydrogeological characteristics, the Ombla catchment area can be subdivided into two typical sections, one to the north and the other to the south of the dolomite barrier. The northern part of the catchment area is built of limestone and covers about 90% of the total catchment area. The Dolomite barrier encloses the underground reservoir in the limestone from the south with water seeping through the tectonically damaged central portion of this barrier into the southern part of the catchment area. Downstream from the Dolomite barrier is an area of limestone rocks, which, on the southem side, leans against the narrow belt of Dolomite rocks overthrown on impermeable flysch. The encountered is characteristic for cave conduits through which the underground reservoir is emptied. The main conduit through the karst limestone releases about 90-95% of the total Ombla River water.
Technical Solution
The basic technical problem that had to be resolved by the design concept was construction of a 130m high dam without causing environmental damage. The Ombla spring area is a nature park and home to protected plants and animals as well as to numerous important historical landmarks. Construction of a dam, with the compensating lower basin had to be built underground from various access adits and tunnels (Figs 4 and 5).
Fig 4. Conceptual design of the Ombla scheme

Fig 4. Conceptual design of the Ombla scheme

The grout curtain to create the dam perimeter against the impermeable flysch barrier has an area of about 300,000m2 and a maximum height of about 410m. Construction will be carried out from three grouting galleries set up at different elevations and with a total length of about 3,500m.Once it was decided that the dam will be located entirely underground, it followed that all other equipment installations and structures would also be placed underground. The only exception is the power plant control building, which is on the surface. With the plant hidden completely underground, the natural harmony of the Ombla spring area is conserved with only the main spring visible on the spring pond site, which corresponds with the present natural condition.
The power plant intake is located in a cave some 550m upstream from the proposed access tunnel entrance and above the main Ombla spring conduit. A 250m long headrace tunnel connects the intake structure and vertical shaft with a smaller intake structure access tunnel running parallel. The vertical shaft, located in the reservoir, is the central collecting and distribution structure of the multipurpose installation.
Fig 5.  Longitudinal section of the installation

Fig 5. Longitudinal section of the installation

The headrace tunnel, bottom outlet, penstock and access tunnel all meet at the vertical shaft, which also interconnects with a branch of the Dubrovnik Waterworks tunnel. The vertical shaft is 136m high and varies in diameter from 10m in the bottom to 3m at the top. It is used also as an air relief shaft, and is therefore located at the junction of the headrace tunnel and penstock. Additionally, in order to interconnect all the fossil caves at higher elevations, and to enable water intake for the Dubrovnik water supply from the cave on elevation 55m a.s.l., the shaft is located so as to enable the crosscut connections between the fossil caves on all the stages.
When the Ombla flow rates are low or when the water level upstream from the underground dam actually falls to the sea level, the vertical shaft shall also be a facility where the water flow can be shut off. Flow can also be diverted here to carry out inspections and repairs.
Fig. 6 Grout curtain longitudinal secion

Fig. 6 Grout curtain longitudinal secion

The penstock conveys water from the vertical shaft to the four generating units in powerhouse. Downstream of the butterfly valve chamber about 68m from the vertical shaft, the penstock branches to feed the four turbines. The powerhouse and 110kV switchyard are placed in the same 70m long, 18m wide and 30m high cavern. They sit opposite each other and separated by the 126m long vehicle and personnel access tunnel to them both.
Two of the four generating units have vertical Francis turbines with a rated discharge of Qj=24m3/sec and 30 MVA synchronous generators, and two have a rated discharge of Qj=6m3/sec and 8 MVA synchronous generators. The total installed power capacity is 68MW and mean annual output is about 225GWh.
Fig 7. Schematic of the fully underground Ombla installation

Fig 7. Schematic of the fully underground Ombla installation

After passing through the turbines and energy dissipater, the water is conveyed through the tailrace tunnel to the powerhouse stilling basin located in a natural spring cave. The cave goes about 60m deep and is about 40m wide and 10m to 15m high. The water goes from the spring cave, through the outlet structure into a spring pond, and over the spillway into Rijeka Dubrovacka Bay.
The Ombla Multipurpose Hydropower System concept is an innovative approach to harnessing water for power generation. Such a concept can only be implemented under adequate hydrogeologically conditions found typically and primarily in areas of karst limestone.
These ideal conditions exist in the mountains behind Dubrovnik. Locating the entire facility underground reduces the construction costs, and most importantly, prevents its environmental impact on an area of environmental protection and natural beauty.
1. Ombla HPP : Conceptual Design , Elektroprojekt Consulting Engineers 1998, Zagreb, Croatia
2. Ombla HPP : Tender Documentation , Elektroprojekt Consulting Engineers 2000, Zagreb , Croatia
3. Ombla HPP : Prefeasibility Study for the HPP and Water Supply Project for the Ombla river Spring in Dubrovnik , Elektroprojekt Consulting Engineers 2002, Zagreb, Croatia

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