Improvise, adapt, and overcome...
In a recent monthly staff update, Gareth Pedley shared his experience of rethinking and redesigning a relatively cheap (but hopefully incredibly cheerful) fish passage solution at a challenging structure.
The importance of fish passage on small watercourses is often overlooked, despite a wealth of electrofishing survey data and scientific literature to demonstrate their importance as salmonid spawning and juvenile nursery areas. Making obstructions passable on those watercourses for dispersing juvenile fish, in addition to large adult fish on their upstream spawning migrations, is therefore vital.
Maintaining free passage becomes ever more pertinent when considering how widespread are issues such as siltation, dredging, ditch maintenance and general upland drainage work (past and present) leading to destruction of vital salmonid spawning habitat in the upper reaches of many catchments. Loss of such habitat is also likely to place an increasing importance upon fish being able to access restored habitats, including those on recovering urban rivers, many of which are littered with man-made obstructions.
With this in mind, the WTT recently allocated some of the funds raised in our annual auction to assisting Chester Le Street Angling Club (CLSAC) and Wear Rivers Trust install replacement baffles at a problematic culvert in the head reaches of the Cong Burn, Co. Durham, ahead of the 2018 spawning season. The site is particularly difficult for fish to pass, due to the high flow velocities that discharge from the raised culvert and the gradient of the stepped apron below it. Unfortunately, the gold standard, full removal, was not possible at the site.
Pre-works: with broken downstream facing baffles
CLSAC previously undertook improvements on the two components. They installed upstream-facing, paired baffles within the lower gradient culvert, which are still working well. The narrow aperture between the baffles increases water depth at low flows while the paired, upstream design deflects higher, overtopping flow inwards from either side of the channel, thereby dissipating the energy from one side with the other. In other words, a similar principle to a technical, bottom baffle fish pass at higher flows, but much cheaper! Diverting flow energy towards the centre of the channel also creates slacker areas along the culvert walls where fish can pass. A bed depth notch between the baffles helps maintain sediment transport and allows a smooth swim-through for fish.
However, downstream-facing baffles on the stepped apron had not fared so well; several had been lost during spate flow or damaged and required attention. Rather than replacing the apron baffles like-for-like, the decision was made to rethink and install an alternating baffle array. This would use more of the available space, slow the flow more effectively, hold back the water to retain depth, and hence reduce the overall gradient.
Elevated flows following installation of the new baffles. Note the increased water depth through the easement, without the requirement for full width weirs or additional steps
Maintaining a bed-depth notch at the end of each baffle promotes swim-through conditions for the fish (i.e. jumping is minimised) and greatly reduces the chance of the space between the baffles in-filling with sediment. The intention was to avoid a traditional pool-traverse design, which requires full width weirs that create additional steps for fish and are more likely to fill with sediment. The edges of the existing steps were chamfered, and a small concrete fillet added at the base, thereby creating a ramp (i.e. water flows down and fish can swim up) which also prevents air entrainment at lower flows.
High flows: demonstrating how the steps become increasingly drowned out while maintaining the flat, easy swim-through conditions between the alternating notches
Budget and a pragmatic approach to solutions dictated compromise at what was a particularly difficult structure. However, fish of all sizes now have a far greater probability of ascending the burn under a wider range of flow conditions. It will be fascinating to see what is revealed during future electrofishing surveys of the burn.