Broadgate Weir Bypass
Broadgate Weir was the most difficult barrier on the River Deerness for fish to pass. A number of options were available to ease fish passage at this site but the area lent itself to the construction of a bypass channel which now enables all species of fish to navigate around the weir.
The weir is located near Lionmouth Rural Centre (formerly Broadgate Farm), between Esh Winning and Ushaw Moor. It consists of a 0.4m cross channel vertical brick face with 0.6m slope and 4m concrete apron. The combination of shallow water and vertical face makes this barrier incredibly difficult for even the largest of our fish species to get past.
Weir removal was considered but this would have threatened upstream infrastructure so a bypass channel was designed by Spanish fish-way engineer Pao Fernandez. She used hydraulic modelling techniques to predict flow regimes along the full length of the channel, allowing the final design to be based upon the creation of optimal flows to allow all species swim along it. The gradient and placement of the boulders was therefore of the utmost importance during the construction phase. A key element of the River Deerness Restoration Project has been to enable passage for smaller fish species such as bullhead, minnow and stickleback as well as for trout, eel and grayling.
Haywood Contracting were hired for this project. The bypass was built on the Lionmouth Rural Centre side and required considerable bank stabilisation work at the upstream end. A large-scale log jam was built into the bank before the bypass channel was created.
The bypass is 33.5m long and rises 1.5m to get the fish from below to above the weir. It consists of 12 small boulder weirs and 11 pools. There is a sluice gate at the upstream entrance and the river bed is reinforced at the bypass outlet. Dewatering of the sluice area was required to enable the bypass channel to be excavated in the dry.
To protect the underlying soil, a geotextile membrane was pinned into the bypass channel using 0.75m pins. Large boulders were then placed on top of the membrane and locked into place using rip rap along the banks. The banks were graded to a 1:1.5 slope and overlaid with an erosion control membrane to avoid wash-out during high flows. They were then planted with willow and grass seed to speed up the re-vegetation process. The sluice gate allows the Trust and the landowner to ensure that adequate flows enter the channel during important migration periods and that no damage is caused during high flow events.
Towards the end of construction, water was allowed to flow down the channel for a short period as a trial, before closing the slucie again. When the channel dried up, a dozen small fish had already entered the bypass and had to be rescued! The channel is now flowing full time and we are sure that the monitoring being carried out by Durham University will verify that all species of fish can now bypass the weir.
Funders and supporters of this project: