In Bobowa, at 58 + 700 km of the Biała Tarnowska River, there was a concrete weir built in the 1950s (photo 1). Its task was to protect the abutments of the road bridge and maintain a sufficient water level for the needs of municipal drinking water.
Photo 1. Barrier in Bobowa before modernization
The barrage height was 1.8 m, the length from the crown to the end of the stilling basin 43.0 m. The weir had a trapezoidal overflow 1.2 m high and 20.0 m wide. The stilling basin was 18.0 m long and the width varied from 23, 0 to 25.0 m. It ended with a 0.6 m high concrete end sill (photo 2). The banks of the riverbed directly below and above the building were reinforced with riprap made of stone and gabions (rectangular mesh-stone baskets) poured with concrete. The weir was destroyed and the bottom of the riverbed below the stilling basin was washed away due to vertical erosion revealing the rock outcrops. As a result, two high barriers were created preventing the migration of fish and other aquatic organisms, the first one below the crown of the weir and the second below the stilling basin.
Photo 2. The barrier before modernization – a visible drop-off point ended with a concrete buttress
In order to unblock the obstacle, the weir was rebuilt as honeycomb fish ramp. At the same time, the level of necessary damming was maintained for the water intake. Flood water flow conditions at the nearby bridge have also been improved. During the works, it turned out that the river flushed a deep gully at the right bank, threatening to damage the bridge pillar (Photo 3). The gap was filled during the laying of the rapids.
Photo 3. Probing the riverbed before starting construction works
As a result of the actions carried out above the road bridge, cascade rapids over 70 meters long were created (photo 4). It consists of several alternating pools surrounded by boulder walls, between which there are gaps to allow water to overflow. The whole mimics the natural current of the river, and the resulting slope in the riverbed guarantees the speed of water flow between the stones that gives fish the opportunity to migrate. Visible in the diagram below the photo, a light blue line with an arrow indicates the direction of this flow in low periods.
Photo 4. Built cascade rapids – view from the drone (Photo p.M.Borowiecki)
To improve the flow conditions of large waters, the area under the central span of the bridge was lowered by about 3.5 meters. This allowed for increasing the throughput under this bridge (Photo 5). The bridgehead and municipal infrastructure were also secured, strengthening the steep slope on the right bank with boulders.
Photo 5. Increasing the capacity under the road bridge
Below the bridge, the height of the concrete strap was lowered and an overflow was cut out in its central part (Photo 6). All the activities described have unblocked the obstacle to fish migration.
Photo 6. Work on lowering the step height
To secure the stability of the fish ramp, it is additionally supported by boulders anchored in the bottom. Over 600 stones with a total weight of around 3300 tonnes were used during the construction works. The weight of a single stone ranged from 4 to 12 tons. The rebuilt weir was solemnly commissioned for use on July 3, 2018 (Photo 7).
Photo 7. Meeting on the occasion of putting into service a modernized stage in Bobowa
As you can see in photo 8 taken at the end of September 2018, construction was made of natural materials. Unlike concrete constructions they quite quickly blend in with the natural landscape of the area. Their important feature is resistance to damage during river floods resulting from, among others due to the weight, the ability to dissipate the energy of flowing water and the ‘flexibility’ of the building (possible damage to the degree can be compensated by small shifts of boulders)