Stop Press! Silt traps and road closures: correction to the Sutton Guardian’s recent Public Notices

The Public Notices published in the Sutton Guardian on 14 March 2013 included details of some road closures proposed for Butter Hill, Mill Lane and Denmark Road in Carshalton.

These road closures are proposed so that the Wandle Trust can install silt traps (hydrodynamic vortex chambers) into the surface water drains as part of a vital project in order to intercept sediment and other harmful pollution which is currently getting washed off the roads and into the river.

Unfortunately, the details given in the Sutton Guardian were inaccurate, stating that the closures would last for a duration of 12 weeks.

In reality, the worst case scenario is that each road will be closed for 2 weeks.

Radar surveys are currently being undertaken to establish if we have the room to install the silt traps between the utilities in the road. We aim to know the answer to this by the end of the week (Friday 5th April).

With this information we will be able to identify if and where the silt traps can be installed. Where possible, if the roads can remain open they will be. If closures are required due to the only space being in the centre of the carriageway, we will give all concerned plenty of advanced warning of the disruption that may be caused.

The Carshalton arm of the Wandle is currently struggling to establish a sustainable population of fish, particularly trout. We believe that a very significant reason for this is the amount of road run-off entering the river. It is reported that between 20-40% of sediment in an urban river derives from road run-off. This has many negative impacts. Trout require clean, uncompacted gravels to spawn in. With so much sediment entering the river, these gravels can get clogged up, meaning it is difficult for the trout to dig redds (nests) and even if they are successful, the sediment limits oxygen from reaching the eggs, causing them to suffocate.

There is also the impact of contaminants being washed from the road into the river, including a wide range of organic contaminants (so called persistent organic pollutants, POPs) which include polyaromatic hydrocarbons (PAHs), polychlorinated biphenols (PCBs), hydrocarbons, dioxins, pesticides, herbicides and many heavy metals. This is a list of things that have no place in a river! The full impact of pollutants are yet unknown on most aquatic life but unsurprisingly they are considered detrimental, not only to fish but on the whole food chain from invertebrates upward.

The advanced design of the silt traps we’re proposing will help to limit the amount of these contaminants entering the river, and once they’ve been installed, the only evidence of their presence being a couple of manholes.

Installing these silt traps will help to greatly improve the condition of the Carshalton arm of the Wandle, making it a much more pleasant place for the local community to spend time around.

The Wandle Trust has been working closely with Sutton Council to draw up the Streetworks Application and other plans, giving careful consideration to limit the disruption caused by works.  

We apologise for any inconvenience caused but hope that you will support us with these actions which will greatly help to improve the Wandle!

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