South East Rivers Trust (& the Wandle Trust)

Fish passage and habitat works in the headwaters of the Hogsmill River

I am now back in the office and it’s time to update on the past six weeks which has been a busy time on the Hogsmill delivering a couple of projects in-house.  I will go back to the 11th of August when we started work at the confluence of the Hogsmill River and Green Lanes Stream. This is right up in the headwaters in Ewell within a kilometre of where the water flows out of the chalk to form the river.

In 1960, during the period when  concrete was considered the must-have look for most rivers, the entire confluence was dredged excessively deep, widened and entombed in a concrete cast. The best habitat within the Hogsmill River has since been cut off to fish by a sloping 35 metre length of uniformly wide concrete channel in conjunction with a small weir. Not wanting to do things by half, the banks were also cloaked in one metre high concrete and stone walls. The habitat was consequently considerably lacking, although nature has attempted to take a foothold with some sedge hanging precariously to cracks and crevices in the failing walls or with watercress clinging to silt deposits.

Picture 108
The channel before work.

The aim of this project would address both these issues. Fish passage would be returned to the upper Hogsmill with the removal of the concrete bed whilst a 150 m cumulative length of river bank would be naturalised. With the scale of the work beyond that of our normal trusted volunteers, we brought in the great assistance of Aquamaintain Ltd. The work took three and a half weeks on site in which time 200 tonnes of concrete were removed from site in addition to a further 40 tonnes of soil.

followed by more concrete, 200 tonnes in total.
You have to break a few eggs to make an omelette!
The new channel being formed. Note the previously high banks on the left that have now been regraded as seen on the right.

Most of the large pieces of sandstone contained within the walls were recycled on site in various guises. Some were used to fill the gabion baskets forming the headwall for a surface water drain perched high on the bank. Other bits were used to make rock rolls that would stabilise the toe of exposed banks most susceptible to erosion. First though, the stone had to be broken up ‘convict style’ with a sledge hammer, a painstaking chore as Jack will vouch for. Finally, other pieces were placed in the completed channel to provide habitat diversity and create a variety of flow types.

Jack at work breaking the stone up to fill the rock rolls
The gabion headwall going in before the banks get regraded to match the front slope.

The banks have been naturalised with a combination of hazel faggots, site won brash, rock rolls and coir geotextile for temporary soil protection. A sinuous meandering channel form has now replaced the former straight and wide channel. Variations in depth, width and the inclusion of gravel, small boulders and multiple pieces of Large Woody Debris and root wads have all provided a diversity of habitat and complex flows within the channel. The low lying wetlands have been seeded with a mix of native species in addition to several hundred plants being introduced as a bit of kick start. Give it a year or two for the vegetation to establish and this stretch will be looking like the chalk stream that it should be.

On completion. A new meandering and naturalised channel.
The impounded stretch is returned to fast flowing water.
Clean imported gravels, deeper holes, large wooded debris, root wads and planted marginal area.
Looking upstream.
The river already naturalising within a couple of weeks as watercress takes up a new residence.

The South East Rivers Trust has worked up this project with the continued support of Epsom and Ewell Borough Council who kindly contributed both financially and with their time, in particular that of Stewart Cocker and Christopher Stone.

Picture 108
Finally, the before shot…
and after.

This work is a continuation of the Catchment Restoration Fund Project funded by Defra, addressing fish passage along the length of the river. In combination with our two weir works last summer, also located in the Hogsmill Local Nature Reserve, this means that there is now a 1.5 km unobstructed stretch of river.

Without delay, work finished at the Green Lane on Wednesday and kicked off at Elmbridge Meadows on Thursday. Read on for what we are doing there.

A big thanks goes to the project partners, Epsom and Ewell Borough Council, especially to Stewart Cocker and Christopher Stone. Thanks to the EA for part funding the habitat element of the work and for undertaking the utility search. Thanks to Surrey County Council for permission to alter their asset. Finally, a BIG thanks to Ben, Jack and Tim from Aquamaintain for all the help, hard graft and good company.