The Hogs Mill is an Urban Chalk Stream that rises from underground springs in the town of Epsom in Surrey. It flows along its 6-mile journey through the towns of Chessington and Surbiton before entering the River Thames at Kingston. Because of its urban route, a plethora of modifications has hindered fish passage on the Hogs Mill, from the Thames into its headwaters. South East Rivers Trust has set about the mammoth task of returning fish passage to its entire length by removing, modifying or bypassing all of its obstructions.
Scope of the Project
The Hogs Mill River flows beneath the A240 between Ewell and Tolworth. 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. South East Rivers Trust had designed a boulder pass fish easement to address the issue, employing Aquamaintain Ltd to deliver the construction phase.
The aim of this fish easement was to increase the water depth over the bridge footing and then reducing the head of water down by producing a passable cascade with a slope of below 1:20. Six lines of boulder bars (bed check weirs) were used to 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.
In total, 150 tonnes of rock was used. Once each of the boulder lines was installed, the void space between them was filled with graded rock. This fill locks the structure together and armours it against erosion. Upon completion, a water depth of approximately 250-300mm over the footing had been achieved. 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 an interesting habitat that will rapidly be populated by an array of invertebrates.