PRELIMINARY DESIGN FOR THE IMPROVEMENT OF THE WATER REGIME AND RIVER CHANNEL OF THE VRBAS THROUGH THE URBAN AREA OF THE CITY OF BANJA LUKA
The document ‘Flood Hazard and Flood Risk Maps for the Vrbas River Basin in B&H’ defines the causes and extent of floods by computed flood flows of the Vrbas in the City of Banja Luka, as well as the necessary measures for rehabilitation, reconstruction and upgrade of flood control structures in the urban area.
According to this planning and strategic document, the flood risk from the Vrbas in the urban area of the City of Banja Luka is rated as high and extremely high in the largest part of the project area. Bearing in mind that Banja Luka, as the capital of the Republika Srpska, has potential for further urban expansion and increase of economic activity in the mapped flood area, it is concluded that it is necessary to implement targeted and very demanding flood risk mitigation measures. This will enable the planned expansion of the city and also protect the population and their property in the existing urban area.
Respecting the general observations and directions from this document, and on the basis of the Terms of Reference, we developed the Preliminary Design for the ‘Improvement of the Water Regime and River Channel of the Vrbas Through the Urban Area of the City of Banja Luka’. This Design plans the improvement of 19.5km of the Vrbas river channel.
The Preliminary Design provides an integrated technical solution for the improvement of the Vrbas River, systematic measures for protection from the flood flows of the Vrbas, reconstruction/rehabilitation measures, measures for upgrading the flood control structures, and planning measures aimed at increasing the effectiveness of protection from flood flows in the urban area of the City of Banja Luka.
The technical solution for the flood protection structures of the Vrbas in the Preliminary Design elaborated the following:
- The level of protection
The following level of protection is proposed for the improvement of the main river channel and the water regime of the Vrbas from the bridge in Trn upstream to the Tulek dam:
- The most upstream section X+ (downstream from the confluence with the Siroka Rijeka to the vicinity of the bridge in Trn) is protected against a one-hundred-year flood (Q1/100) with 0.80m freeboard of flood control structures;
- Section X (upstream from the confluence with the Siroka Rijeka to the bridge in Trapisti) is protected against a one-hundred-year flood (Q1/100) with 1.0m freeboard of flood control structures. The freeboard of the left bank of 1.0m above a one-hundred-year flood is necessary because of the planned construction of an industrial zone.
- Sections VIII and IX (downstream from the confluence with the Vrbanja to the bridge in Trapisti) are protected against a one-hundred-year flood (Q1/100) with 1.0m freeboard of flood control structures. It is proposed to have the same level of protection in these two sections, as well as in the downstream section X, in order to create a uniform protected area which mainly includes industrial businesses in the riparian region of the left Vrbas bank;
- The upstream sections I to VII are located in a deeply cut channel with a narrow riparian lowland, so it is proposed to protect this area against a one-hundred-year flood (Q1/100) with 0.80m freeboard of flood control structures.
- The route of the stabilised channel
The channel stabilisation works of the Vrbas through the urban area of the City of Banja Luka are designed to follow the existing route of the river channel (from the bridge in Trn (borderline with the municipality of Laktasi) upstream to P1 profile (the Tulek dam)). The approximate length of the works is 19.5km along the axis of the stabilised channel. The stabilisation works across this whole section include minor changes to the position of the channel in terms of newly designed radius of curvature of the channel.
- Longitudinal section
The finished level of the stabilised channel of the Vrbas in the project area has a significant gradient with height difference of H=29.08m across the length of L=19,514.08m. The average gradient of the finished level of the stabilised bed is I=1.49‰.
- Standard section
Depending on the level of the existing and planned urbanization, two types of stabilised channel were selected for the Vrbas: urban and natural. The stabilisation types were chosen in accordance with the expected hydraulic effects and the space constraints of the area for stabilisation works due to intensive urbanization. In addition to achieving hydraulic effects, the stabilisation works had to remove all bottlenecks. One of the goals when selecting a type of standard section was to stop the permanent trend of degradation of the floodplain and the river, using permanent structures.
- Priorities of construction
Given that these extremely complex and demanding stabilisation works require significant financial support, the project defined stabilisation work stages. Multi-criteria analysis confirmed the practice of carrying out works from downstream to upstream sections. The X + section was marked as a priority (3.5km section downstream from the confluence with the Siroka Rijeka), where most of the works are in the main channel. The X section immediately follows after, which includes the area to the bridge in Trapisti. The works in this section do not require significant compulsory acquisition of land or works on the river banks, while achieving a remarkable impact on flood risk mitigation in the entire project area. These two stages will be a priority for the implementation of the Detailed Design, which will cover a section of 12.5km. The improvement of the Vrbas channel to the confluence with the Vrbanja will reduce the flood risk to an acceptable level. The continuation of the channel improvement works in the upstream sections will regulate the hydraulic flow and additionally reduce flood risks. Also, parts of the river and riparian area that are not flood-sensitive, could be used for sport and leisure activities.
Other planning and water management activities
In addition to the above-mentioned considerations and initiatives directly resulting from the interpretation of design solutions, it is necessary to take into account other important factors in the forthcoming period that are not related to the direct implementation of the technical solution, but, due to the great importance for flood protection, are specified in the Preliminary Design:
- The technical solution for the Vrbas River provides a corresponding main channel capacity and measures to maintain the required size of the river channel and floodplain, with clearly defined zones for sporting and recreational areas. Channel stabilisation works will require the acquisition of land that is not state owned.
- After the channel improvement works, it is required to plan the maintenance of the river channel and remove sediments to maintain the designed main river channel capacity. Removal of sediment from the Vrbas river channel is an extremely important activity which will have to be planned in accordance with the principles of sustainable sediment management.
- The requirements and principles of flood control in the project area are completely respected in the Preliminary Design, both in terms of the level and reliability of protection. According to the recommendations from the Preliminary Design, it is necessary to continue the project activities to the downstream sections of the Vrbas River that are located in the Municipality of Laktasi.
- In addition to design and construction activities for the improvement of the channel riparian area of the Vrbas, it is necessary to continue to apply other measures to maintain the channel capacity. The existing river channel and the additional space required for the improvement of the water regime must be reserved for its primary purpose, which is flood detention and conveyance of flood flows. Primarily, this means that all further construction must be stopped and planned stabilisation and improvement works must be carried out. Subsequently, permanent facilities that obstruct the conveyance of flood flows must be removed. Responsible institutions and public authorities that issue approvals and building permits must comply with the watercourse protection principles and not pass any legislation that stimulates construction in the main river channel and floodplain in this area.
- After the City of Banja Luka accepts the Preliminary Design, the development of the Detailed Design will commence, which will include a 12.5km section, starting with X +, and following by X, IX, etc. After accepting the Preliminary Design, the City of Banja Luka should submit to the relevant Ministry / PI ‘Vode Srpske’ a request for issuing water guidelines for the development of the Detailed Design, as stated in the Technical Specification of the Preliminary Design.