By development of Preliminary design of Dabar HPP and part of Dabarsko polje waters drainage by energy channel in Dabarsko polje, hydrological, hydraulic, technical and technological and spatial and urbanism aspects have been analyzed, the ones important for designing of this multipurpose object. Project area is part of the integrated “Trebisnjica” hydropower system  which is developed in phases based on adopted integrated principles of implementation and proscribed Water acts. Drainage of the other waters gravitating toward Ponikva sink hole was developed by Vrijeka watercourse regulation, which are key water management objects of gravitating waters drainage of the northwest and central part of Dabarsko polje, including Pribitu and Susica temporary courses. In accordance with ToR provisions, complex hydrological and hydraulic analyses have been implemented, serving as base for comprehension of existing status and possible design solutions.

Implemented hydraulic analyses at the section of Dabar HPP to Bileca accumulation, and hydrological simulation at wider area of Trebisnjica river basin, and after that development of technical solution it is concluded as follows:

  • In regard to consideration of hydrological conditions, by application of improved hydrological 3D NET-HET model, flooding of Dabarsko and Fatnicko polje were analyzed and impact to the neighboring basins, when Dabar HPP is operational and under conditions of entirely passable/open existing derivations/tunnels. Based on the simulation results, following can be concluded:
  • Analyzing the flow duration curve for the Bregava river, it can be concluded that objects (tunnels and Dabar HPP) do not have impacts to the low water of Bregava river, in duration over 75%, i.e. to the flows less than 3-4 m3/s, while flood waters are somewhat lower than in natural conditions. Dabar HPP operation, under open tunnel conditions, practically brings closer the average water on Bregava to natural conditions, while in case of tunnel manipulation it will achieve bigger effects to increasing the average water level with trend of returning the mean to flood waters toward natural status before the tunnel construction:
  • To bring “closer” flood duration line in Dabarsko polje to duration line in natural status. In other words, operation of Dabar HPP can significantly impact to floods in Dabarsko polje in periods of mean and flood waters, therefore under those conditions-hydrological conditions it is necessary to carefully plan the Dabar HPP regime of operation. It is recommended to further improve stated hydrological model, as the tool and basic basis for planning the operation and establishing Operation Manual for Dabar HPP and other HPP upstream, especially Bileca HPP;
  • Impacts of Dabar HPP to floods in Fatnicko polje is significantly less although not insignificant. Flood duration is prolonged for approximately a month, while water levels during flood waters are 3-5 m higher compared to status without Dabar HPP;
  • In case of selective closure and manipulation with valves, and that is going to be case during hydropower system operation when Dabar HPP is in operation under conditions of mean and flood waters in karst fields, it can be expected that flooding is going to be prolonged in both fields. Depending on management regulations, flooding can be significantly longer compared to natural conditions, especially in Dabarsko polje.
  • During design solution development, requirements from harmonized water conditions have been completely respected during designing and construction of part of integrated multipurpose system of “Trebisnjica”, all in accordance with issued water acts no. UP-I-06-337-134/86 of 23.06.1986, and UP-I-03-78-1/69 of 29.07.1969;
  • Limitation on existing objects and drainage “reaches” of hydropower system at the section of Dabarsko and Fatnicko polje, will significantly reduce efficient operation of foreseen Dabar HPP in designed capacity, and especially simultaneous transition of water of Dabar HPP and waters from Dabarsko polje to Bileca HPP. Limitation is caused by insignificant existing discharge capacity of Dabarsko-Fatničko polje tunnel, identified based on mathematical model and hydraulic analyses (complete derivation from Dabar HPP to Bileca accumulation has been considered) and confirmed by hydraulic analyses of the tunnel. Discharge capacity of the tunnel of 30 m3/s was identified in unsubmerged status. With smaller actions-dredging on three locations, discharge capacity of tunnel would get increased to 35 m3/s, what is significantly less then installed discharge of Dabar HPP which amounts to Qin=55 m3/s;
  • For considering hydraulic performances of energy channel, possible scenario for increasing discharge capacity of the tunnel have been analyzed. Increasing discharge capacity of the tunnel should be developed in separate Final design, after defining values of optimum discharge capacity. Defined discharge capacities should simultaneously discharge water of Dabar HPP and Dabarsko polje waters. Designer’s opinion is that value should not be below 75 m3/s. Due to stated limits, design and technical solution of energy channel was foreseen as technically adaptive, adjusted to gradual development of the system and possible transfer of waters through the channel depending on downstream limitations in tunnel, such solution should be provided by construction of accessory dikes of appropriate height along the energy channel. Height of those dykes is conditioned by presented limits of the discharge capacity of Dabarsko-Fatnicko polje tunnel. In case that there is no downstream limitations-backwaters due to limited tunnel capacity, according to designed technical solution, energy channel can discharge up to 90 m3/s;
  • After implemented analyses of basis for designing, optimal technical solution of energy channel was adopted, pointing out following:
  • Layout technical solution of energy channel – option II – shorter route, was proposed as optimal, channel length of L= 6 407.34 m requires expropriation belt of 237 887 m2;
  • Finish grade of the energy channel is installed at the single fall of I=0.23 ‰. This fall for “Dabar” HPP has achieved normal depth in channel of 3.20 m;
  • As optimum selection of development of the standard profile of designed discharge channel is suggested Option 4 – covering bottom with RC slabs between framework RC structure, while slopes are covered in crushed stone (with front side of the stone processed) in cement mortar in established framework RC construction. Suggested standard profile represents technical solution of covering channel with high level of flexibility, depending on changes of geological composition where channel is founded, in case of potential risk of uplift, the stone is not going to be fixed in cement mortar;
  • Following objects are designed on energy channel:
  • Upstream transition section “Dabar” HPP – discharge energy channel;
  • Object for redistribution of waters of Vrijeka – discharge channel;
  • Water abstraction object on channel no. 1 0+813,78 – irrigation of Dabarsko polje and Ponikva sink hole replenishment;
  • Structures of two bridges, with Investor’s request to plan third one as well on following chainages: Bridge no. 1 on 1+365,50 (with Investors request to design bridge downstream at St.1+719,53); Bridge no.2 on St.2+827,93 on the route of Option II (additional bridge suggested by Investor; Bridge no.3 on St.4+446,11 on Option II of the route (Option I of the route on St.4+636,97);
  • Water abstraction on channel no. 2, St.1+885,82 – irrigation of Dabarsko polje;
  • Regulation and introduction of Ljeljesnica estavelle into discharge channel;
  • Water intake structure on channel no. 3, 3+870,47 – irrigation of Dabarsko polje;
  • Gate structure on discharge channel, 6+305,44 – accepting part of waters from Dabarsko polje;
  • Confluence structure on Sub-system 3 into outlet channel, 6+542,88;
  • Downstream transition section structure, 6+705,50 - St.6+747,41
  • At the following structures, hydro-mechanical equipment was foreseen for providing quality management over part of the system on different hydrological statuses:
    • Structure for distribution of waters- gate:
    • Structure of confluence for gravitational waters of Babarsko polje Sub-system III into energy channel – two gates;
    • Tainter gate on energy channel on profile P61 (alternatively on profile P55) of the channel;
    • Water intake structures on channel –gates: 0+813.78, - irrigation of Dabarsko polje and replenishment of Ponikva sink hole, and on St.1+885.82 and St.3+870.47 – irrigation of Dabarsko polje.
  • Planning of compensational pool (tail water reservoir) from earlier design documentation of 90.000 m3, according to calculations and adopted technical solutions is not necessary, since the channel itself can contain c/a 180.000 m3. Within the Preliminary design of Dabarsko polje irrigation, location for reserving the space will be planned, without further developing in Detailed design, as concluded on Preliminary design presentation;
  • Foreseen regulation of Vrijeka watercourse, length of L=2 606.50 m and regulation of other temporary watercourses regime (Pribitu and Susica by drainage channel, L = 845 m) i.e. drainage toward Ponikva sink hole with accessory objects on regulated channel, all with the purpose of creating preconditions for development of drainage channel gravitational network toward Ponikva sinkhole.


Development of the drainage system and irrigation of part of Dabarsko polje on the surface of 700 ha

After implementation of voluminous and detailed terrain and cabinet investigation and designing works, in accordance with requirements and specifications of Terms of Reference, optimum drainage and irrigation systems were planned on northeast part of Dabarsko polje, with energetic channel, surface of 700 ha.

Technical solution considered one option of channel network drainage and two options of irrigation (channel network and system under pressure).

Surface of part of Dabarsko polje of 700 ha was organized in four spatial areas – subsystems of drainage and irrigation.

Basic concept of drainage was based on existing catchment areas and run off zones from this part of Dabarsko polje toward gravitational sink holes. By this type of drainage natural runoff processes in the field are not disturbed, but, according to issued water acts, conditions have been met to transfer part of Dabarsko polje waters toward Trebisnjica river basin. Having in mind that Dabarsko polje drainage, as presented by hydraulic analyses and regime of flooding, has sense just to the certain limits, when flooding occurs, technical solution are designed to meet conditions, insurance of average discharges on Bregava, average transition of water in Bregava river basin and preconditions for development of irrigation.

Generally, implemented analyses and development of technical solutions lead to following conclusions regarding drainage on Dabarsko polje:

          • By implementing additional quantities of waters “Dabar” HPP, water level and duration of floods in Dabarsko polje are returning into the status before construction of Dabar-Fatnica tunnel, i.e. number of days of flooding the field are prolonged under open tunnels condition, and under condition of manipulation, number of days is going to be increased compared to natural status; Vrijeka channel is regulated from the source to the Ponikve sink hole which is draining 90% of Dabarsko polje waters, and it is key drainage toward sinkhole being replenished by Bregava river basin. Regulation refers to stabilization of the main channel by natural material, preventing further erosion processes and Vrijeka banks destabilization. Regulation of the riverbed does not disturb regime of drainage, while introducing waters from designed boundary channel Pribitu-Susica-Vrijeka has positive effects to replenishment of the Ponikva sinkhole with waters which flooded field south of those sources;
          • At the regulated channel there was water distribution structure established and over this structure it is possible to transfer water into the energy channel and further into Trebisnjica river basin under condition of increased discharges on Vrijeka;
          • By designing junction channel in length of 650 m from drainage-energy channel to the regulated channel of Vrijeka river provides replenishment of Ponikve sinkhole by waters from “Dabar” HPP. It is going to be defined by additional Studies and investigations based on which water act is going to be issued for functioning of integrated Trebisnjica hydropower system under conditions of low waters;
          • Existing channels of gravitational drainage from natural catchment areas are in Preliminary design contained mostly toward natural recipients – sinkholes, but upgrading of primary and secondary drainage network fits into existing discharge regime of this part of the field;
          • Respecting general conclusions presented in agro-pedological basis it is obvious that reducing of the flooding period and additional agro-technical measures is improving soil quality and conditions for agricultural production activities,
          • Drainage of water from the Dabarsko polje part covered by design solution, enables undisturbed development of the drainage system on entire northwest part of Dabarsko polje.

In the part of considered technical solution for irrigation following can be concluded:


          • Development of the irrigation system is following gradual development of the hydropower system “Trebisnjica” as a key source, since just after construction of Dabar HPP and key drainage-energy channel it is possible to plan development of irrigation;
          • In the energy channel, foreseen Tainter gate located in front of Dabar-Fatnica tunnel, enables to contain c/a 180.000 m3of water for irrigation, and that is meeting 7 day water demand. The fact is that during irrigation, it has to be counted with replenishment from Dabar HPP (supply channel, Nevesinje accumulation);
          • In accordance with conclusions and implemented analyses it is not necessary to construct retention pool, but in accordance with conclusions from the meeting, the area along the energy channel is reserved for construction of the object of volume up to 000 m3;
          • Two optional irrigation systems have been planned: by channel network and system under pressure. As optimum technical solution for the I faze of irrigation development adaptive, adjustable and economically justified technical solution was suggested for irrigation using drainage channel network with constructed objects of gates for keeping the water into channels i.e. establishment of the mini linear accumulations;
          • In the second development phase after reviewing realistic status on site, the upgrading and modernization of system can proceed, by construction of modern systems under pressure. Also, in next phases it is recommended to extend the area of irrigation to the northwest parts of Dabarsko polje, if there is mutual interest of beneficiaries and electro utility as leader of development in this area.