The Preliminary Design for a canal in Dabarsko Polje to convey water from the Dabar HPS and from the Polje analysed hydrological, hydraulic, technical/technological and spatial-planning aspects that are important for the design of this multipurpose structure. The project area is an integral part of the ‘Trebisnjica’ hydropower system, which is being developed in stages on the basis of the adopted integrated principles of implementation and the prescribed water acts. According to the design, some drainage is provided by the improved river channel of the Vrijeka and conveyed towards Ponikva ponor. This river is a key water management facility for draining the waters of the northwest and central part of Dabarsko Polje, including the Pribita and Susica intermittent streams. In accordance with the requirements of the Terms of Reference, complex hydrological and hydraulic analyses were carried out as a basis for analysing the existing state and possible project solutions.

The following was concluded by hydraulic analyses which were carried out between the Dabar HPS and the Bileca reservoir, hydrological simulations in the wider area of ​​the Trebisnjica basin, and subsequent development of the technical solutions:

      • With regard to the study of hydrological conditions, the improved 3D NET-HET hydrological model was used to analyse the flooding of Dabarsko Polje and Fatnicko Polje, and the impact of Dabar HPS on the neighbouring basins when in operation and when the existing water conveyancing tunnels are completely free and open. The following was ascertained on the basis of the simulation results:
      • The analysis of the flow duration curves of the Bregava concluded that the structures (tunnels and Dabar HPS) have no impact on the Bregava’s low flows of a duration exceeding 75%, i.e. flows less than 3-4m3/s. Flood flows were somewhat lower than in the natural state before the construction. The operation of Dabar HPS with completely open tunnels almost brings mean flows of the Bregava closer to their natural state, whereas the appropriate operation of the tunnels will produce even greater mean flows, with the tendency to bring mean and flood flows closer to the natural state before the construction of the tunnels;
      • The flood duration curve in Dabarsko Polje moves closer to the curve in the natural state. In other words, the operation of Dabar HPS can significantly impact the floods in Dabarsko Polje in periods of mean and flood flows. Therefore, it is necessary to carefully plan the operating regime of Dabar HPS in these hydrological conditions. It is recommended to further improve the hydrological model as a tool and basis for planning the work and developing Operational Guidelines for Dabar HPS and other HPSs downstream, in particular Bileca HPS;
      • The impact of Dabar HPS on floods in Fatnicko Polje is significantly less pronounced, although not negligible. The flood period is extended by about a month, while the levels of water during flood flows are 3 to 5 metres higher than when Dabar HPS is not operating;
      • It is expected that, when the Dabar HPS is functioning, the appropriate operation of the gates will result in longer flood duration in both poljes during the conditions of mean and flood flows in karst poljes. Depending on the operating procedure, the flooding can be significantly longer than in the natural state, especially in Dabarsko Polje;
            • The requirements of and compliance with the prescribed and agreed water management conditions were fully respected during the design and construction of the integrated multifunctional ‘Trebisnjica’ system, all in accordance with the Water Acts No. UP-I-06-337-134 / 86 of 23.06.1986 and UP-I-03-78-1/69 of 29.07.1969;
            • The limitation of the existing facilities and drainage tunnels of the hydroelectric power system in Dabarsko and Fatnicko Polje will significantly reduce the efficient operation of the Dabar HPS in the planned capacity, especially simultaneous transfer of water from Dabar HPS and from Dabarsko Polje to Bileca HPS. The limitation is caused by the insufficient existing capacity of the tunnel Dabarsko – Fatnicko Polje, which was determined on the basis of a mathematical model and hydraulic analyses (complete water conveyance system from Dabar HPS to the Bileca reservoir was analysed) and confirmed by hydraulic analyses of the tunnel. It was determined that the capacity of the tunnel is 30m3/s in the non-submerged state. Smaller interventions, such as cleaning sediments at three sites would increase the tunnel capacity to 35m3/s, which is significantly less than the installed discharge at Dabar HPS of Qin = 55m3/s;
            • When studying the hydraulic performance of the canal, we analysed possible scenarios for increasing the capacity of the tunnel. This should be a subject of a separate detailed design, after defining the optimum capacity. This capacity should simultaneously convey the water from the Dabar HPS and some of the water from Dabarsko Polje. The designer is of the opinion that this should not be below 75m3/s. Due to the above-mentioned limitation, the design of the canal is technically adaptable and flexible to the phased development of the system and the possible transfer of water through the canal depending on the downstream limitation in the tunnel. This solution will be enabled by the construction of dykes of adequate height along the canal. The height of these dykes is conditioned by the stated limitation of the tunnel capacity in Dabarsko - Fatnicko Polje. If there are no downstream limitations (backwater due to the limited tunnel capacity), according to the designed technical solution, the canal will be able to convey up to 90m3/s;
            • After analysing the data for the design, an optimum technical solution was decided for the canal, with the following characteristics:
      • Option II (shorter route) was proposed as the optimum solution. The length of the canal is L = 6,407.34m. This option requires acquisition of 237.887m2 of land.
      • The finished level of the canal is set at a uniform drop of I = 0.23 ‰. This drop during the operation of the Dabar HPS of 55m3/s achieves a normal depth of 3.20m in the canal;
      • Option 4 was selected as an optimum solution for the drainage canal: the bottom of the canal is lined with RC slabs fitted in RC frame structure, while the slopes are covered with crushed stone (face of the stone smoothed) fitted in RC frame structure and pointed by cement mortar. The proposed option for the canal lining has a high degree of flexibility / adaptability, depending on the geological structure of ground in which the canal foundations will be laid. Due to potential risk of uplift pressure where groundwaters are expected, stones will not be pointed by cement mortar.
        • The following structures were designed for the canal:
      • Upstream transition section Dabar HPS – drainage canal;
      • Structure for redistribution of water the Vrijeka – drainage canal;
      • Intake structure on canal 1 (chainage 0+813.78) – irrigation of Dabarsko Polje and recharging Ponikva Ponor;
      • Two bridges with a request from the Investor to plan a third bridge. The bridge locations are at the following chainages: Bridge 1 at 1+365.50 (with a request from the Investor to design the bridge at downstream location 1+719.53), Bridge 2 at 2+827.93 according to the route from Option II (additional bridge proposed by the Investor), Bridge 3 at 4+446.11 according to the route from Option II (according to the route from Option I it is at 4+636.97);
      • Intake structure on canal 2 (chainage 1+885.82) – irrigation of Dabarsko Polje;
      • Stabilisation of the Ljeljesnica estavelle and conveyance of its water into the drainage canal;
      • Intake structure on canal 3 (chainage 3+870.47) – irrigation of Dabarsko Polje;
      • Gate on drainage canal (chainage 6+305.44) – collecting a part of the water drained in Dabarsko Polje;
      • Confluence of Subsystem 3 with drainage canal at chainage 6+542.88;
      • Downstream transition section at chainage 6+705.50 - 6+747.41.
        • The following hydro-mechanical equipment is planned, which will provide appropriate management of a part of the system in different hydrological states:
              • A plate gate on the structure for redistribution of water;
              • Two plate gates at the confluence of Subsystem III (water drained in Dabarsko Polje) with the canal;
              • A segment gate at the canal at P61 profile (alternatively at P55 profile);
              • Intake structures on the canal: plate gates at 0+813.78 (irrigation of Dabarsko Polje and recharging Ponikva ponor) and at 1+885.82 and 3+870.47 (irrigation of Dabarsko Polje).
        • According to the calculations and the selected technical solutions, a balancing reservoir of 90,000m3 described in the previous project documentation is not necessary, because the canal can hold about 180,000m3. The Preliminary Design for the irrigation of Dabarsko Polje will consider a location to be reserved for the reservoir, but without elaboration in the Detailed Design (as it is concluded at the presentations of the Preliminary Design);
        • It is planned to stabilize the channel of the Vrijeka in the length of L = 2,606.50m, as well as to improve the regime of other perennial watercourses (Pribita and Susica) using an 845m long drainage canal which will convey water to Ponikva ponor. This will include all accompanying structures at the stabilised channel, in order to start the development of a drainage canal network towards Ponikva ponor.

Construction of the drainage and irrigation systems for 700ha in Dabarsko Polje 

After the extensive and detailed field and office investigations, an optimum drainage and irrigation system was designed for the northeastern part of Dabarsko Polje, in accordance with the requirements and specification from the Terms of Reference. The design uses a canal with a drainage area of 700ha.

The technical solutions discussed one option to drain water using a canal network and two options for irrigation (canal network and a system under pressure).

The surface area of the 700ha in Dabarsko Polje is divided into four spatial units / subsystems for drainage and irrigation.

The basic concept of drainage was based on the existing drainage basins and runoff zones in this part of Dabarsko Polje towards ponors. This approach to drainage did not disturb natural runoff processes but created conditions to transfer some of the water from Dabarsko Polje to the Trebisnjica basin, in accordance with the Water Acts. Hydraulic analyses and flooding regimes showed that in certain periods of the year Dabarsko Polje will be flooded. The technical solutions are designed to ensure the average flow rates at the Bregava and the average interbasin transfer from the Bregava basin to the Trebisnjica basin during these periods, as well as enabling development of an irrigation system.

In general, the following conclusions can be drawn from the analyses and elaboration of the technical solutions:

  • The introduction of additional water from the Dabar HPS into Dabarsko Polje through the Nevesinjsko Polje – Dabarsko Polje tunnel will return the water level and the duration of flooding to the state before the construction of the Dabar-Fatnica tunnel, which was constructed to drain water from Dabarsko Polje. The duration of flooding in Dabarsko Polje will increase when the tunnels are completely open. When the flows through the tunnels are manipulated, the duration will increase further, to exceed the natural state before the construction of both tunnels;
  • The channel of the Vrijeka is stabilised from the source to the Ponikva ponor. This ponor drains around 90% of water from Dabarsko Polje and recharges the Bregava basin. The main channel is stabilised by natural materials that will stop further erosion and destabilisation of river banks. The stabilisation works did not disturb the existing drainage regimes. The introduction of water from the designed Pribita – Susica – Vrijeka lateral canal will positively impact the recharge of Ponikva ponor with water that was previously spread across the polje and retained in the area south of these sources;
  • The channel stabilisation works included the construction of the structure for redistribution of water, which enables the transfer of water into the canal and further into the Trebisnjica basin when the flow rates at the Vrijeka are high;
  • The designed 650m connecting canal between the drainage canal and the stabilised channel of the Vrijeka will recharge Ponikva ponor with the water from Dabar HPS, which will be further defined by additional studies and investigations. On the basis of these, Water Acts for the operation of the integrated Trebisnjica hydropower system during low flows will be issued;
  • The Preliminary Design retained to a large extent the existing gravity drainage canals from natural drainage basins to natural recipients (ponors), and the development of the primary and secondary drainage network will fit into the existing runoff regime in this part of the polje;
  • Taking into account the general conclusions from the agro-pedological data, it is evident that the reduction of the flooding period and the use of additional agronomic measures will improve the quality of the soil and agricultural production;
  • The drainage of water from the part of Dabarsko Polje covered by the design solution will enable a smooth development of the drainage system for the complete northwest part of Dabarsko Polje.

The following can be concluded from the elaborated technical solution for irrigation:

  • The development of the irrigation system follows the phased development of the ‘Trebisnjica’ hydropower system as a key source, as the development of irrigation system depends on the construction of the Dabar HPS and the key drainage canal;
  • It is possible to store around 180,000m3 of water for irrigation in the canal, using the segment gate in front of the Dabar-Fatnica tunnel. This quantity is a seven-day irrigation water need. It is evident that, in the course of irrigation, it will be necessary to supplement the above store of water with water from Dabar HPS (supply tunnel, Nevesinje reservoir);
  • In accordance with the conclusions and analyses, it is not necessary to construct a retention basin. However, the meeting decided to reserve a space next to the canal for the construction of a basin of the capacity up to 50,000m3;
  • Two irrigation options are planned: canal network and a system under pressure. As an optimum technical solution of Phase I, an adaptable, flexible and economically justified technical solution is proposed, where irrigation will be carried out from the drainage canal network with previously constructed gates for retaining water in canals, i.e. mini line reservoirs will be formed;
  • In Phase II, after reviewing the situation in the field, it will be possible to continue with the development and modernization of the system, by constructing modern systems under pressure. Also, if there is mutual interest of the users and electric power industry (which is the leader in the development of this area), it is recommended to expand the irrigation area in future to north-eastern parts of Dabarsko Polje.