South East Rivers Trust (& the Wandle Trust)

The Hogsmill Fish Passage Mission Continues

The quest for complete fish passage along the Hogsmill takes another step forward. This time we have been addressing the A240 road bridge footing. The footing is wide resulting in a shallow flow of water, further exacerbated by it coming to a high point midway under the bridge. To finish the problem off, it comes to an abrupt end in the form of a vertical weir. All of these factors result in an obstruction to fish passage.

The structure before work starts

So to the process… we surveyed the site, undertook the required calculations, came up with a design solution, applied for the relevant consents and permissions, spec’d the build, bought the materials, hired in the plant and brought in assistance from Aquamaintain with Roo and Jack.

The solution, a rock ramp similar to the one that we recently built at Kingston University but this time on a grander scale. The aim being to increase water depth over the bridge footing and then stepping the head down by producing a passable cascade with a slope of below 1:20. Six lines of boulder bars (bed check weirs) stabilise the structure with each boulder weighing between 400-900kg. These were positioned in upstream self supporting curves and configured to create a series of notches that provide varying passage options under different flow conditions.

The first delivery of boulders arrive and are unloaded
and some of the rock fill material
The second line being placed into position
Jack lining the void between boulder bars with geotextile before the rock fill goes in
Setting the levels as another line is positioned

In total, 150 tonne of rock was used. Once each of the boulder lines were installed, the void space between them was filled with graded rock. This fill locks the structure together and armours it against erosion.

There is now a water depth of approximately 250-300mm over the footing. Water depths over the ramp are in excess of 250mm  in the form of passable streaming flows. In addition to providing fish passage for all species, the rock ramp creates interesting habitat that will rapidly be populated by an array of invertebrates.

1 before
Before the works…
1 after
and after.
4 before
Again, looking over the bridge, before…
4 after
and after. This time without a weir.
Various sizes and depths of notches set into each line

Once again, we had the great help from the guys from Aquamaintain to get the job done. Roo at the controls of the excavator, grappling with the grapple and Jack continually feeding us with rock . We also had the continued support from Epsom and Ewell Borough Council, of note from Stewart Cocker and Christopher Stone. A BIG thanks to you all.