The concrete bridge apron under the A71 on the Harwood water, Polbeth, was highlighted as a barrier to fish migration after looking into mitigating the larger barrier about 500m upstream – Limefield falls. A low-cost baffle (LCB) fish pass was deemed the best option partly as it was something we could design and install in-house. Below, you’ll find a summary of the works and some before, during and after pictures. We have since hosted two site visits to the structure which were attended by West Lothian and City of Edinburgh councils, other trusts, angling clubs and interested members of the public.
The concrete bridge apron under the A71 in Polbeth (NT 03461 64444) measures around 6 x 40 metres, extends around a corner and is slightly U-shaped. There is about 2m of head over the whole structure.
Despite the U-shape (a cursory nod to fish passage), prior to installation, the flow over the apron could be defined as shallow and laminar. While probably passable at high flows, it was a barrier at normal and lower flows.
1. Planning & Design
Fish pass design is site specific. In this case, due to the shallow gradient, an existing concrete base which would make installation of baffles simple and a desire from the trust to carry out the work in house, a low-cost-baffle pass was deemed the best option.
Design resources – EA fish pass manual, industry peers, on-site trial and error.
Some rough design parameters for Low-cost baffle fish passes
>400mm (paired baffles may require notches as narrow as 400mm but alternate and angled will generally be much wider).
Standard sleepers are 125mm x 225mm. Minimum water depth requirements vary for different species and can be found in the EA fish pass manual. However, the depth of concrete – dictating the depth of fixings possible – is an important factor.
This is best tweaked on site or even laid out beforehand if possible. The basic principle is to work at low flow, start downstream and place the next baffle at the point where the water is backed-up to half of the baffle height.
(Notch width, spacing and baffle height are interdependent and site dependant. i.e. narrower width = wider spacing and steeper gradient = narrower spacing)
Material options include wood, concrete or plastic. The latter two would have the longest shelf-life, however, natural material was deemed more suitable. So, wood was chosen.
Green oak would be the best option but is prohibitively expensive. Larch was chosen for its durability in the water environment.
Expansion bolts could have been used but are more dependent on the strength of the concrete than rods and resin. Therefore, 20mm stainless steel threaded rods secured with resin were chosen.
There are many products available. In this case, we used RAWL R-KEX 2 RESIN which is suitable for use in flooded holes.
SEPA – CAR licence (registration)
Structure owners – Local Authority
Access requirements – Local landowner
- Measuring tape and pencil
- 110V SDS drill
- Auger drill
- Angle grinder
On the Ground
Ensure the team have read and signed the Risk Assessment and Method Statement.
Start at the downstream end. Work in low flow and layout some of the design beforehand (using sleepers or sandbags).
Sandbags and sleepers can be used to deflect flow in order to create a workable area for the installation of each baffle.
Pre-cut and drilled sleepers were used to spot drill holes in the concrete apron. Nuts were tightened and rods cut to length once resin had set. (Specification for the size of holes in the concrete apron and sleepers and for the resin curing time were found on the resin product card).