Liquidation of two migration obstacles at Florynka village
By the end of 2019 we completed all the planned reconstruction of the dams. Some problems resulting from demolition debris private concrete barrage which was fed the microelectric power plant in Florynka.In the town of Florynka, the commune of Grybów, in the mid-twentieth century on the Biała River a debris dam and a concrete weir supporting it (photo 1) were built.
The body of the weir had a trapezoidal overflow and the length together with side wings about 37.0 m. Its height from the bottom was 1.25 m. Below was a stilling basin and a concrete buttress insuring it (Photo 2).
The dam has a rectangular overflow and a stilling basin ended with a buttress. Its height was even greater, because it was about 1.75 m. The total length of the dam body was close to 52.0 m (Photo 3).
Both structures were severely damaged during the 2010 floods (Photo 4). The construction of the debris dam was washed out and water from upstream penetrated under the right wing. The bottom plate of the basin has been almost completely destroyed. The weirs completely prevented migration of fish and other aquatic organisms.
It was decided not to completely liquidate the dam and weir. They were afraid of destabilizing the Biała riverbed and starting erosion on a significant section of the river. As part of the investment, they were planned to be converted into “honeycomb” stone rapids close to nature (photo 5).
A rectangular 30 cm x 30 cm indentation was made at the bottom of the weir body and in the strap ending the drop basin. Above, the cross-section of the indentation changed to trapezoidal. Below the strap, at a length of about 50.0 m, “honeycomb” rapids were arranged with a slope of about 50 ‰. In order to protect the bottom against erosion, the rapids were deeply immersed in the Biała channel. At the end of the strengthening, the stones were wedged to a depth of about 1.0 m. Then the whole was covered with a gravel riprap (photo 6).
In photo 7 you can see the ready rapids created at the place of the weir, the correct position and size of the gaps between the boulders gives the right direction to the flowing water and reduces its energy.
In the case of debris dam its crown was partially bound and lowered (photo 8). To maintain adequate flow in the Biała riverbed, the beginning of the rapids was designed around 30.0 meters in front of its body, and the ending skipping bottom insurance over 70.0 m below.
At the height of the crown, migration channels were made at different levels, to provide the best conditions for fish to overcome the obstacle regardless of seasonal changes in water flows (Photo 9).
Stones along the entire length of the river were wedged deep in the river bottom (photo 10). This is to protect against erosion. In addition, below, at a length of about 30 m, a strengthening of boulders was made, the purpose of which is to improve the stability of the structure. Finally, the whole was covered with a gravel rip-rap, increasing its coherence.
Construction work on both partitions were carried out with the use of 4 heavy excavators and lasted nearly 8 months. Over 3500 m³ of stones were used. Rapids were finally completed at the turn of November and December 2018 (photo 11).
Finally, in order to protect against lateral erosion of the right sliding edge, it was secured with a stone band for a length of about 80.0 m (Photo 12). This action was taken to strengthen the escarpment. After the band was made, the rip-rap was covered with soil.
Pictures illustrating the text come from the resources of STAAND Ltd. and PGW Wody Polskie The Polish State Water Farm “Polish Waters”.
Binczarówka – added value of the project
Regardless of the construction works in the Biała riverbed carried out in accordance with the project, at the end of 2018, as part of maintenance works carried out by PGW Wody Polskie RZGW in Kraków, additional support was provided for the existing rapids on one of its left-hand tributaries – the Binczarówka stream (photo 1).
Rapids were established in 2014 on the occasion of the construction of a new, single-span bridge along the provincial road Grybów – Krynica No. 981 (photo 2). The old bridge was supported by a concrete block, in which a large gap was created during the 2010 flood. At the same time, the escarpment on the left side of the stream was damaged. When building a new bridge, nothing was improved, only the old barrage restored, the embankment was renovated, supplemented with native soil and reinforced with a stone coating. Twenty five honeycomb riffles were built, which was additionally secured with two 0.8-meter-wide concrete straps. The crown of this “rapid” has a straight shape with two overflows. The cascade has chambers about one and a half meters wide and 30-60 cm deep. The chambers are surrounded by stones over 60 cm in diameter. Fish ramp is a barrier impossible to cross by any fish.
The Binczarówka Stream is a fairly important breeding place for several species of fish, including the alpine bullhead, stone loach, common minnow, chub and spirlin (Alburnoides bipunctatus). In a situation of low water levels, they would not be able to overcome the existing device. In addition, below the concrete buttresses, are the beginnings of bottom erosion, which in consequence would threaten the stability of the entire weir securing the road bridge. Therefore, it was decided to modernize it.
Works were carried out aimed at, among others increasing the efficiency of buildings from 2014. The gravel bar has been removed, significantly reducing the light of the bridge (photo 4). The excavated material was used to backfill the gaps in the banks and bottom of the Binczarówka riverbed below the existing weir.
The new construction consists of four transverse stone straps spaced every 5 meters with an additional three oblique inserts, whose task is to increase the number of pools (from three to six). Such treatment significantly reduces the slope between individual chambers (Fig. 1). This structure was designed for the migration of weakly swimming fish species. To stabilize the bottom in flood conditions, the bottom of the pools was filled with boulders with a diameter of about 80 cm.
The task of the new rapid (Fig. 2) is to protect the estuary stretch of the Binczarówka riverbed against further erosion, protect the road bridge and the weir protecting it from damage, and increase the water level in the chambers of the old rapids from 2014. This is to enable fish to spawn upstream, even during periods of low water.
The task of the new rapid (Fig. 2) is to protect the estuary stretch of the Binczarówka riverbed against further erosion, protect the road bridge and the weir protecting it from damage, and increase the water level in the chambers of the old rapids from 2014. This is to enable fish to spawn upstream, even during periods of low water.
The obtained effect can be considered as an added value, affecting the integrity of the Natura 2000 area (PLH120090 Biała Tarnowska), which appeared to some extent during the renaturalization of the Biała River valley, which is the main goal of the implemented project.
Modernization of the weir in Bobowa – case study
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.
The barrage height was mmm1.8 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.
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.
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.
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.
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.
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).
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).