Tag Archives: Silt traps

Pollution-Busting on the Wandle

Over the last 2 years, the Wandle Trust has been intensifying efforts to tackle the considerable problem of pollution in the river. Often contamination can be tackled by our partners in the Environment Agency, tracking down pollution to the source. However, this does not work for all sources of contamination.

For example, contaminants such as particles from car exhaust, the loss of engine oil and other contaminants from the roads can all be washed into the river from no one “point” source. This is known as diffuse pollution.

To illustrate this, there are about 2.5 million cars in London, and 16% of them leak oil. It has been calculated that this would equate to 261,635 gallons of oils dripping onto roads every year! Much of this oil will work its way into London’s surface water drains and then the rivers.


Although changes to the law and car technology may help one day in the future, we need to start acting now. It has been the Wandle Trust’s mission to find out how the contaminated waters from the surface water drains can be cleaned up before entering our river. This is vitally important because water quality is a major determinant of what wildlife can live in the rivers, how beautiful the rivers are, and how much the community value their local water landscapes.

In the current phase of our Pollution Busting Project, four measures are being installed and trialled to determine their effectiveness in reducing the contamination coming into the River Wandle. These measures are the most promising selected from several which were investigated by the Trust and they are called:

  • Downstream Defenders
  • Mycofilters
  • Siltex
  • Smart Sponges

There will be more information about these appearing on our websites in the coming months. We look forward to telling you more about this exciting new phase of our work!

Silt trap project: Update 6

The third Downstream Defender is in the road! Despite hitting some groundwater, progress to date has been very good (touching plenty of wood). We are currently in the process of building the monitoring chambers up and downstream of the silt trap and connecting up the pipes before it will come live early next week. R.J. Dance/Emerald Construction have done a sterling job and a big thanks to them. We also have to thank the patience of all those that the works have affected, in particular the residents of Mill Pond Place who have very kindly put up with our disruption for some time now. We are now into the final leg!

The silt trap being lowered into place

The silt trap being lowered into place

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The silt trap bedded in and pipe work going in


Excavations to lay the new pipes and monitoring chambers

Excavations to lay the new pipes and monitoring chambers


Silt trap project: Update 5

The installation of the third and final silt trap is underway on Butter Hill.

Work started on Monday with R.J.Dance being the Principal Contractor.  Progress to date has been great: the large surface water drain has been tapped into, a large deep hole has been excavated, and we are awaiting delivery of the chamber on Friday (21st June) morning.

Next week will see the pipes being connected and the Downstream Defender becoming active. Exciting times!!

Silt trap project: Update 4

The silt trap in Mill Lane is now in and functioning!

All that can be seen of it is a nice smooth patch of new tarmac and a manhole cover that will be used to clean the chamber every 6 months or so.

Here is  a picture of Councillor Hamish Pollock checking out the site of the chamber with Wandle Trust Director Bella Davies last week.


The second chamber on Denmark Road is now well under way and in fact our contractors F.M. Conway will be finished work on Thursday!  In contrast to the first chamber everything has gone smoothly with no surprise pipes emerging to slow us down!

This site has an important extra addition in that we have included two monitoring chambers into the design.  The surface water pipe first feeds into one inspection chamber then into the silt trap before passing into the second monitoring chamber and then on towards the river.

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The silt trap in the ground with the surface water pipe shown in the foreground.   This area of open pipe is where the upstream monitoring chamber is located.

The chambers will allow us to monitor the effectiveness of the traps by taking samples before and after the silt trap.  This will be carried out using automated sampling apparatus to allow analysis of the water quality and any particulates present.


The access chambers  (Two monitoring chambers and one for the silt trap) being built up to road height ready for the manhole covers.

The third and final chamber project which will be located on Butter Hill will start on 17th June.

Silt trap project: Update 3

The mystery pipe has been identified as a redundant water pipe that was first laid in 1868 and has now been removed.  All systems are now go, and the silt trap is being fitted as we speak!

A 3.5 metre hole has been dug:


A concrete slab laid:


The collection chamber inserted:


The hydrodynamic separator inserted:


The road should all be back to normal by Friday, and then it’s on to the next site in Denmark Road on Tuesday!

Silt trap project: Update 2

After some detective work involving a number of utility companies the mystery pipe has now been identified as a redundant water main.

This was removed on Friday 17 May but means we are now behind schedule with the works. Currently the aim is that Mill Lane works will be complete by Friday 24 May and we will be ready to start on Denmark Road on Tuesday 28 May.

Silt trap project: Update

As you may have seen, work started on the silt trap project on Mill Lane, Carshalton on Monday 13 May.

While digging up the road the contractors have found a gas pipe that is in a different location than was shown in the ground radar survey.

This has implications for the location of the silt trap chamber and will result in some delay to the project while this is resolved. There will likely be knock on delays to the work on Denmark Road and Butter Hill as a result.  We are working hard with Conways, Southern Gas and the London Borough of Sutton to minimise any  potential delay or disruption.  We will keep you informed as work progresses.