- 18th April 2019
- Posted by: Joanne Woodhouse
- Category: Blog
Jordan’s Dam fish pass is now officially open. The pass was constructed by Yorkshire Water as part of their National Environment Programme responsibilities under AMP6 (one of seven passes). Three have now been completed with a further two currently on site.
We provided design and site supervision services, our Project Manager (Dave Mould), Engineer (Ed Hogan) and site supervisor (Dave MacFarlane) providing brief reflections on the challenges faced and overcome.
Jordan’s Dam weir is around 2.5m high, with the upstream river used for navigation, therefore weir removal was not feasible. The pass was designed for a range of fish species, including salmonids, coarse fish, eels and lamprey. A double flight Larinier (with combined eel/lamprey pass) was the preferred option, the pass discharged into an existing stilling well downstream of the weir. The bend in the pass was required to ensure that the bottom of the pass finished within the stilling basin rather than downstream of the stilling basin. The downstream wall of the stilling basin was notched, and a slope of rock armour placed in front of the notch to produce suitable conditions for fish to access the stilling basin. The pass was designed to maintain a residual flow over the weir so that any impact to the valuable weir pool habitat downstream of the weir was minimised.
There were many constraints that governed the design and construction of Jordan Dam fish pass as each fish pass design is always unique. At Blackburn Meadows we had to consider the construction of the existing weir, the adjacent sheet pile wall, existing services and the access difficulties in getting the materials and equipment to site.
Unlike many structures built elsewhere, structures built in river environments are subject to highly variable loads and even though the worst case loading may be during a 1 in 200 year flood it may be subject to that extreme of loading many times in its design life. It is imperative to as accurately as possible determine the velocity and depth of the river for such an event at all locations along the structure. After assessing the construction of the existing structures it was deemed safe to remove a section of the reinforced concrete weir and part of the mass concrete to provide an adequate footing to the fish pass within the mass fill concrete. This was required as the fish pass invert was below the level of the weir. As the pass extended beyond the leading edge of the weir, a new piled foundation was required to provide support and prevent any differential movement.
A challenge of all fish pass construction projects is the tolerances required for the construction. Many people who are regularly involved with the construction of gauging weirs may be familiar with this difficulty. Despite no gauging requirements, the tolerances were tighter than the standard building tolerances familiar to many contractors or manufacturers involved with the production of reinforced concrete. This was to maintain the designed flows and velocities to allow fish migration across a range of flows. For this reason it was important to clearly communicate this requirement and ECI was helpful in determining a construction method and design that would facilitate this requirement.
We carried out the role of Site Supervisor during the construction phase of the Jordan’s Dam fish pass. Much more than simply watching the structure be constructed, our role involved ensuring that the fish pass was constructed as per the designs produced by the engineering team, alongside monitoring compliance with Environmental, Health and Safety, and CDM regulation and legislation.
To do this we carried out regular site visits, surveying, photographing and monitoring the works. Our approach was to work with the contractor to ensure the smooth delivery of the scheme. However, careful consideration had to be given to any design changes the contractor requested. As what may have come across as a simple change on site, which allowed for a simpler construction method, has the potential to have a detrimental effect on the operation or maintenance of the structure, all of which required strong team working skills between the design and supervision teams, and clarity on the design philosophy of the fish pass.
Our final task was to take measurement of the completed structure to gain approval to open the fish pass from the Environment Agency. Through working closely with the contractor through all stages of the design and construction of the scheme, the final levels were within +/-2mm of the design levels, which demonstrates the benefits of carrying out site supervision on fish pass construction projects. This has also helped in developing a working relationship with Ward and Burke, who we continue to work with on the delivery of further fish pass schemes for Yorkshire Water.
As the multi-disciplinary JBA team handled all aspects of the ecological surveys, permitting and design, it enabled positive partnership working with the contractor to ensure the smooth running of the build phase.
Want to know more?
For further information on JBA’s fish pass design capabilities, please contact Dave Mould at the Saltaire office.