South East Rivers Trust (& the Wandle Trust)

Fish Passage at the Hogsmill Sewage Treatment Works (Part 3)

Having covered how the upstream barriers were tackled with the creation of the 60 m pool pass, it was time to focus on the furthest most downstream weir (Barrier 4).

This weir is formed when the concrete slab beneath a bridge ends, and the bed returns to being natural.  Over the years, as water has poured over the edge, a 400 mm vertical weir has formed with plunging, impassable flows.

Barrier 4. The furthest most downstream weir

To remedy these problems, the plan was to build a rock ramp immediately downstream of the weir. The rock ramp would back up water levels by a total of 870 mm, drowning out this weir and increasing the depth over the upstream slab by 470 mm to meet the bottom end of the pool pass. That was the theory, now to make it happen…

My primary concern above all else was the trickle of water we had been left to work with. Due a combination of the incredibly dry past two years and the abstraction from the groundwater chalk aquifers by water companies, the river had been reduced to a mere 10 litres a second, despite being 8 km from the source. In other words, the whole river was flowing through a space no larger than the size of a letter plate on your front door! Backing up such a minimal flow through hundreds of tonnes of rock was, to say the least, not going to be an easy task.

The whole of the Hogsmill River flowing over Barrier 4!

SERT has become pretty well-versed at designing and building this type of rock ramp but this was our biggest to date. Over the three weeks that we were on site, the SERT delivery team (oh yes, we now have one) consisting of Nick, Myles and myself used a total 285 tonnes of material. This consisted of 85t of the large (and in some cases very large) Purbeck rocks to form the boulder bars, 50t of rip rap and gabion stone, 60t of rock fill, 80t of clay and 10t of gravel.

The first load of rock arrives

Constructing the boulder bars is essentially like doing a jigsaw but on steroids. Admittedly this is no Stonehenge. Our rocks were smaller, and we had the added benefit of diesel and hydraulic power, but we probably didn’t take as long to build our structure either.  Each rock needed to be brought individually down into the channel, and offered up to nestle closely to the last. It must be stable and, importantly, set at the correct height. When one stone didn’t fulfil all of these criteria, it needed to be set aside and the process attempted with another. The joys of working with an irregular, natural material!

Checking levels as the massive jigsaw puzzle is built
The boulder bars in position

Each line of boulders is arranged in upstream facing arches so that each rock rests on the next and in turn the bank. This creates an incredibly stable structure. A primary notch and various secondary notches are incorporated into each line to provide a suitable depth for fish to pass. The gradient and hydraulics of the ramp are designed to be on the conservative side of what our native fish can handle.

The upper lines taking shape with notches in each

The space between bars is lined with a geotextile and then in-filled with a variety of smaller rock to lock everything into position, armour against erosion and to block the voids which would otherwise allow the water to pass through and disappear beneath the structure. A suitable depth of water is left between the bars which, with the inclusion of large boulders, provide slack refuge areas for fish to catch a breath as they ascend the ramp.

The space between another line is geotextiled before the rock fill and armour is introduces as it has in the previous lines

As we in-filled the ramp with rock, we watched cautiously as the levels ever so gradually backed up to meet the pool pass. Against the odds, it levelled out to the depth that was required. After years of problem solving, development and over three continuous months on-site, passage at the site was completed. It was definitely time for a celebration drink.

Our timelapse footage of the build.
Music: Ross Bugden – Run
The completed ramp following a bit of rain
Streaming flows through the notches in each line

Thanks this week goes to Lovell Stone for providing the rock and being so accommodating to our needs. To Thames Water for providing us access and work space. And of course to the funders, the Environment Agency, the Marine Management Organisation and Thames Water.

This project is part of a wider scheme where The Rivers Trust and the Environment Agency, supported by other partners, have been working with the Marine Management Organisation and Defra to develop a coordinated and funded programme of projects for 2018/19 with the aim of freeing migration routes of barriers to fish.  This project is part of that programme funded by over £1.6 million of European Maritime and Fisheries Funds, which is matched by more than £1 million of Environment Agency/Defra funding and £300,000 of other funds.