Impacts of partial water re-routing from the Buna and Bregava catchments in the catchment of Trebišnjica river

Abstract

The Trebišnjica Hydrosystem is one of the most complex development projects in that part of Europe known as the Dinaric Karst area. In this area, water is the only important and valuable natural resource and base for regional economic development. However, the proper planning and construction of water resource systems in any karst region, including Eastern Herzegovina, contains an inherent risk component. Situated in the area of Eastern Herzegovina, the Trebišnjica Hydrosystem consists of seven dams, six artificial reservoirs, six tunnels, and four channels. The ultimate aim of the Hydrosystem is to provide multi-purpose use of water resources from an elevation of 900 m a.s.l. to the sea level. Upon full completion, the natural regime of surface and groundwater will be completely changed on both a local and regional scale. The important issue for the future is how to keep the balance between the necessity for regional development and the preservation of the unpredictable and complex ecological system of Eastern Herzegovina. Construction of the Trebišnjica Hydrosystem started in 1959. A large part of the Hydrosystem is already operational. After many years pending the construction of the final stage of the Trebišnjca Hydrosystem, the so called 'Upper Horizons' have just started. The key structures of the 'Upper Horizons' are: PP Dabar, PP Nevesinje and PP Bileća, including the reservoirs Zalomka and Nevesinje. The main purpose of this part of the Trebišnjica Hydrosystem is socio-economic development of this area of Eastern Herzegovina (Nevesinje, Dabar, Stolac and Dubrave areas). To achieve this goal, the optimal use of regional water resources is necessary i.e. the Hydrosystem requires the transfer of 30% of water from the catchments of the Buna, Bunica and Bregava springs (Neretva River) to the direction of the already operational part of the Hydrosystem (Trebišnjica River catchment). The main natural properties of these springs are: Buna: Qmi^SnrVs, Qmax=380 m3/s, Qav=24 m3/s; Bunica: Qmin=0.72 m3/s, Qmax=207 m3/s, Qav=21 m3/s; Bregava: Qmin=0.4 m3/s, Qmax=59 m3/s, Qav=17.5 m3/s. After the transferred waters are used at the power-plants Trebinje 1, Trebinje 2 and Čapljina, it then has to be returned to the Neretva River to a point upstream of Metković town. The quantity of transferred waters is ten time less than Neretva average annual flow upstream from the confluence Neretva / Buna River. From the very beginning appears the crucial question of the possible environmental impact as a consequence of these changes to the natural water regime in the area of the 'Upper Horizons' and the previously mentioned springs. Changes in the flow regime of springs as a consequence of water re-routing in other directions often creates conflicts between the owners of reservoirs and the users of springs, particularly if it is a transboundary problem. To avoid possible conflicts, and to provide the sustainable and peaceful management of transboundary waters the designers of the Hydrosystem undertook a series of long lasting and complex hydrogeological/hydrological investigation programmes with the close co-operation of a wide spectrum of scientists and engineers. As often as possible, analyses included the application of a few independent mathematical models adapted to the complicated nature of the karst area (1985, 2003/04 and 2009). All hydrogeological and hydrological analyses and applied mathematical models confirmed that the integrity of the Buna, Bunica and Bregava springs would not be endangered as a result of part of its water being re­routed in the direction of the Trebišnjica catchment. Results of all analyses confirmed that the consequences of water transfer would be negligible on the Buna Spring discharge and without any practical negative consequence. In the case of the Bunica Spring the maximal flow (high precipitation period of year) would be reduced to about 70%, however in the period of low flow (dry period of year) a negative influence of water transfer is not possible. A similar result is applicable in the case of the Bregava sinking river. During operation of PP Dabar, as part of the 'Upper Horizons', a considerable decrease of the Bregava flow is to be expected as a consequence during the high precipitation period. This means that extreme flows (floods) in the Stolac urban area would be eliminated. According to the design solutions, the geotechnical measures are foreseen to improve water-tightness of the Bregava river bed, with the consequence that the flow in the dry period in the area of Stolac town, would increase to 3 to 5 times of that in its natural condition. At the same time, this improvement of the minimal flow of the Bregava would have a positive influence on the critical minimal inflow into the Hutovo Blato Nature Park. In most instances, the impact of the 'Upper Horizons' is positive, offering irrigation, flood reduction, water supply improvement, power production, infrastructure improvement, increasing of minimal river flow, reduction of deforestation, recreation and many secondary benefits. As a consequence of any human activities and engineering construction in karst areas, some negative impacts are unavoidable. However, the major aim in the planning of the entire Trebišnjica Hydrosystem is to minimize the negative and to maximize the positive environmental impacts.

Vodoprivredni 'Hidrosistem Trebišnjica', planiran kao sistem u etapnom razvoju, ušao je u završnu etapu izgradnjom Gornjih horizonta. Na taj način sistem postaje jedan od najvećih integralnih razvojnih projekata u ovom delu Evrope, sa brojnim ekonomsko-razvojnim, socijalnim, vodoprivrednim, energetskim, ekološkim, urbanim i drugim ciljevima. Pošto se sistem realizuje u jednom od najizraženijih karstnih područja Dinarida, jedno od centralnih pitanja koja su postavljena u fazi njegovog planiranja je pitanje hidroloških i ekoloških uticaja tog sistema na reke Bunu, Bunicu i Bregavu, ekološki veoma značajne pritoke reke Neretve. To pitanje je odavno fokusirano i u javnosti, sa zahtevom da se detaljno ispita uticaj sistema na vodne režime pomenutih vodotoka, posebno u periodu malih voda. U članku su, pored rezultata višegodišnjih istraživanja i analiza, prikazani ključni rezultati sudije iz 2009. (Energoprojekt Beograd i Zavod za vodoprivredu, Bijeljina). Nakon analize padavina, hidrogeoloških fenomena pravaca kretanja podzemnih voda i analiza hidroloških serija dnevnih protoka, urađeni su matematički modeli (klasa ARMA modela, homomorfne simulacije), koji pokazuju da se mogu uspostaviti vrlo pouzdane matematičke zavisnosti između dnevnih protoka na kontrolnim profilima na pomenute tri reke (tražena veličina), dnevnih padavina u pojedinim hidrogeološkim zonama i protoka u više uzastopnih prethodnih dana na pojedinim hidrološkim profilima. Vrlo visoki koeficijenti korelacije (0,96÷0,99) modeliranjem dobijenih i izmerenih vrednosti dnevnih protoka na pomenute tri reke pokazuju da se vrlo pouzdano može apriorno predvideti kakvi se hidrološki uticaji mogu očekivati nakon realizacije Gornjih horizonata (akumulacije, kanalske i tunelske derivacije). Zaključuje se da se velike i srednje vode smanjuju, jer se deo protoka akumuliše i usmerava prema već izgrađenom delu Hidrosistema Trebišnjica. Projektom Gornjih horizonata planira se prevođenje oko 30% (20 m3/s) voda slivova Bune i Bregave u već izgrađeni deo Hidrosistema Trebišnjica. To je deset puta manje u odnosu na srednji godišnji prirodni proticaj Neretve u Mostara (197,4 m3/s). To je manje je i od najmanje izmerenih proticaja Neretve u Mostaru (32 m3/s) s tim što tada (sušni period) na Gornjim horizontima nema vode za prevođenje. U tom periodu dotok u Neretvu sa područja Gornjih horizonata je takođe deset puta manji od proticaja Neretve. Interesantan je i podatak da je srednji godišnji prirodni proticaj Neretve, neposredno nizvodno od ušća Bune, 250,7 m3/s (Pavlović M. & Pavlović M.) što je dvanaest puta više od količina planiranih za prevođenje. Prema 'Okvirnoj vodoprivrednoj osnovi BiH' (1994) Qsr kod Žitomislića je 233 m3/s što je takođe u granicama gore pomenutih odnosa. Nakon energetskog iskorišćenja prevedenih voda na postojećim elektranama (HE Trebinje 1, HE Trebinje 2 i PHE Čapljina) ove vode se vraćaju u Neretvu kroz ustavu Krupa, uzvodno od Metkovića. Akumulacije Gornjih horizonata (Zalomka i Nevesinje) omogućuju da se u slučajevima ekstremnih padavina ublaži poplavni talas na podračju Čapljine i Metkovića, a u sušnom periodu povećaju protoci Neretve u Metkoviću i nizvodno. Nakon izgradnje Gornjih horizonata oko 65% voda Bune, Bunice i Bregave ostaje u prirodnom režimu i na njih se ne može uticati bilo kakvim tehničkim merama. Najvažniji je zaključak da se ne menjaju režimi malih voda, koji su i najvažniji za procenu ekoloških uticaja. Takođe, zaključuje se da se male vode Bregave na potezu kroz Stolac mogu poboljšati odgovarajućim geotehničkim merama regulacije korita, sprečavanjem poniranja u karstifikovanom koritu upravo u toj zoni. Smanjenja velikih voda razmatranih pritoka Neretve se može tretirati kao pozitivan uticaj sistema Gornjih horizonata, jer se za toliko smanjuje povodanj u donjem toku Neretve.Opšti zaključak je da su hidrološki i ekološki uticaji planiranog sistem Gornji horizonti bitno i preovlađujuće pozitivni i da se sa određenim radovima režimi malih voda mogu i poboljšati na Bregavi u urbanom delu Stoca.