PRELIMINARY DESIGN OF WATER REGIME AND VRBAS RIVER REGULATION THROUGHOUT BANJA LUKA CITY URBAN AREA

 

By development of “Flood Hazard Maps and Flood Risk Maps for the Vrbas river basin in BiH”, flood causes and volume of flooding have been defined by calculated flood waters of Vrbas in the area of Banja Luka city.

According to this planning and strategic document, flood risk from Vrbas river on the area of Banja Luka city was estimated as high and extremely high for the biggest part of the project area. Considering the fact that Banja Luka, being capitol of Republika Srpska, has realistic conditions for further urban development and progress of economic activities in mapped flood area, it can be concluded that it is necessary to start planned and extremely requiring measures on flood risk mitigation, in order to create preconditions for city development but also to protect population and its assets located on the already urbanized area.

Respecting general observations and directions from the stated document, and based on the Terms of Reference, the Preliminary design of “Water regime and Vrbas river channel regulation throughout Banja Luka city for 19.5 km” was developed.

Preliminary Design presents integrated technical solution of Vrbas river bed regulation, systematic measures against Vrbas flood waters, but also reconstruction and rehabilitation measures of flood protection objects upgrading, and planning measures with the purpose of increasing the flood protection efficiency in the Banja Luka city urban area.

Technical solution for flood protection objects on Vrbas in Preliminary design reconsiders following:

 

• Protection level.For Vrbas river main channel and water regime regulation on the project area from the bridge in Trn and upstream to Tulekova dam, following protection level is proposed:
  • Protection of the most downstream section X+ (downstream from the Široka river confluence to the bridge in Trn) 100-year flood protection level (Q1/100), upgrading the parallel protection objects of 0.80 m.
    • Protection of section X (upstream from Široka river to the bridge in Trapisti), protection level of 100-year flood water (Q1/100) with upgrading the parallel protection objects of 1.0 m. Upgrading of left bank of 1.0 m compared to 100-year flood water has its justification in foreseen construction of economic and industrial area.
    • Protection of sections VIII and IX (downstream from Vrbanja river confluence to the bridge in Trapisti), protection level is 100-year flood water (Q1/100) with upgrading parallel protection objects of 1.0 m. Preserving this protection level, as same as for the section X, is technically justified for creating unique “protected area” with mostly economic capacities in the hinterland area of Vrbas left bank.
    • Upstream sections from the section I to the section VII spread via deeply cut river channel with very narrow costal lower area, therefore for this section of watercourse it is suggested to implement 100-year flood protection (Q1/100) and upgrading the parallel protection objects for 0.80 m.
  • Route of regulated channel. Layout of the Vrbas river regulation throughout Banja Luka urban area, from the bridge in Trn (border with Laktasi municipality) and upstream to P1 profile, c/a 19.5 km long section to the Tulekova dam, was designed by the route of existing riverbed. For entire section of regulation, the route of the watercourse was aligned regarding designing of required curves and intermediate directions.
  • Longitudinal section. Vrbas river regulation level for the project area has ambiguous fall, overcoming the height difference of H=29.08 m, on L=19 514.08 m, therefore average fall of design level of regulated channel is I=1.49‰.
  • Standard section. Design solution, depending on level of existing and foreseen urbanization, adopts two types of Vrbas river regulation, urban and natural type. Types of regulation are selected considering the expected hydraulic effects, limitation of the regulation works corridor due to intense urbanization of the costal area in the urban part of regulation. Beside stated effects of regulation, the effects of foreseen bottle necks removal had to be achieved. One of the targets of standard profile section is creating mechanism for stopping the permanent trend of inundation belt degradation, but also of the water land degradation by constructing the permanent objects.
  • Implementation priorities. Since this is about very complex and requiring regulation works on entire Preliminary design area, with required significant financial support. Design defines phases of regulation work implementation. Multi-criteria analyses confirms practice of implementing works from downstream to upstream phases. Section X+ was marked as priority (3.5 km downstream from Široka river confluence) with most of the works in the streambed. Immediately after phase X+ there is phase X to the bridge in Trapisti. Works on this phase do not require bigger property issues and land acquisition or works on banks and achieve extraordinary effects to the flood risk mitigation for entire project area. Those two stages will be priority in detailed design implementation which will be implemented on the section 12.5 km long. By regulation of Vrbas river bed to the Vrbanja confluence flood risk will be reduced to the acceptable level, while continuation of the bed regulation on upstream stages will arrange hydraulic flow image having the effect of additional reduction of the flood risk and at the same time creating conditions to join part of minor and major bed to sport and recreation and other flood resistant purposes. Protection level. For regulation of the streambed and water regime of Vrbas for the project area from the bridge in Trn and upstream to Tulek dam, following protection level is suggested.

 

Other planning and water management instructions. Beside above-mentioned final considerations and directions which are result of design solution interpretation, for the next planning period it is necessary to consistently apply other important notes which have no direct connection with implementation of technical solution, but due to its importance in flood protection, they have been mentioned in Preliminary design such as:

 

• Technical solution of Vrbas establish appropriate discharge profile of minor bed and measures for maintenance of required dimensions of major bed, with defined areas of urbanistic elements for recreation and sport. Regulation works should perform expropriation of land which is not within water land or state owned.
  • After regulation of Vrbas channel it is necessary to perform maintenance of the Vrbas river channel and dredging in order to maintain discharge profile capacity of the minor bed. Removal of the sediment from Vrbas river bed is one of very important activities which will have to be implemented by plan after Vrbas river bed is being regulated in this area according to principles and Sediment Management Strategy.
  • Requirements and principals of flood protection for project area are completely respected within Preliminary design. According to recommendations from the Preliminary design it is necessary to continue with project activities to the downstream sections of Vrbas watercourse which is in Laktaši municipality.
  • Apart of design and construction activities on river bed regulation and regulation of the costal area of Vrbas, it is necessary to consistently apply other protection measures on discharge capacity of Vrbas. Surface occupied by existing riverbed and additional corridors required for water regime regulation should be included to primary purpose – retention area for acceptance and transfer of Vrbas flood waters in high water periods, as a first step by stopping further construction works and implementing foreseen regulation works and then in next phases by removal of permanent elements which disturb transfer of Vrbas flood waters. Competent institutions and public institutions authorized for issuing agreements and licenses for construction have to respect principles of watercourse protection and to stop practice of issuing any document which stimulate construction works within minor and major riverbed.
  • After the Preliminary design is accepted by Banja Luka city administration, construction of the detailed design is going to start for section 12.5 km long, starting from the stage X+, X, IX etc. After adopting Preliminary Design, Banja Luka city should submit the request for issuing Water guidelines for development of the detailed design as stated in technical specifications of Preliminary design addressed to competent Ministry – PI Vode Srpske