Roads surface water drainage on A105 Cycle Lane and elsewhere.

Peter Johns made this Freedom of Information request to Enfield Council

This request has been closed to new correspondence from the public body. Contact us if you think it ought be re-opened.

The authority would like to / has responded by post to this request.

Dear Enfield Council,

Surface water runoff within the roads network is, by its very nature, polluted by solids, oils, dissolved air pollution, etc.. CIRIA C522 table 1.6 gives a comprehensive list of all of these pollutants. What mechanism exists within your roads surface water drainage design to handle these pollutants?
What mechanism is intended to get rid of the surface water runoff? Is it eventually passed into the storm water sewerage network? Is it intended to soak into the ground? Or is one or more of the other mechanisms outlined in the SuDS Manual C522 being used? Or is a combination of some or all of the above? If so, please can I have full details?
If subsoil infiltration is being used, have infiltration tests been carried out? If so, can I have details of these tests and the results?
Also how are pollutants being handled? What mechanisms are being put in place to remove and prevent a build up of litter and other sediments? What mechanisms, if any, are being put into place to deal with all the other pollutants?

Yours faithfully,

Peter Johns

complaintsandinformation, Enfield Council

Thank you for your email.

We will respond as soon as possible.


Complaints and Access to Information Team,
Enfield Council.


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Dear Enfield Council,
I am still waiting for answers and information that I requested in my FOI.
I did receive an email outside of official WDTK channels but the email left all of my concerns raised in my FOI unanswered.

Yours faithfully,

Peter Johns

Peter Johns left an annotation ()

On 12th December 2017 I received a response to this FOI request from Stuart Miller made directly by email to me rather than a response through the WDTK channels.

It attempted to explain the working of rain gardens. It said that geology was variable and full infiltration was not appropriate everywhere but, where it was used, infiltration tests had been carried out but none of the requested results were supplied. It also stated that rain gardens were equipped with overflow gullies or under-drains.

My enquiry concerned the whole of the surface water drainage on the A105 as modified in Enfield by the construction of cycle lanes.

Rain gardens account for less than 5% of this drainage.

It also explained the shortcomings with trapped gullies and catchpits and said “often silts surpass these systems and get washed through the surface sewer system causing blockages.”.

It is noted that the cycle lane project brought about a significant increase in the number of such gullies.

The four main questions asked, viz. How is pollution dealt with? How are solids and litter dealt with? How is rainwater disposed of? And if by infiltration, have infiltration tests been carried out? Were not answered.

This led to me asking on 15th December, through WDTK channels, for an internal review.

On 12th January 2018 I received a response, again directly to me, from Complaints and Information at Enfield Council answering my questions and I would like to thank Nicholas Foster for his candid answers.

He also sent me the drawings for 12 rain gardens.

The following matters arising from these answers I wish to record.

Rainwater runoff is shown to be allowed to soak into the ground and yet, contrary to the earlier answer by SM and the recommendations in various SuDS manuals, no check on the suitability of subsoil to be infiltrated has been carried out and no infiltration tests have been carried out.

The design shows that rainfall pollutants are allowed to enter the ground untreated because gullies are connected to perforated pipes.

Perforated pipes are not laid to falls and solids that pass the gullies are allowed to build up and eventually block the pipework..

Design details for the cycle lanes are creating difficulties with the street cleaning and the good housekeeping recommended by SuDS and other manuals is, so far, not being carried out although small trials of street cleaning have been observed.

The twelve drawings supplied with this response showed no under-drains to rain gardens. Overflow gullies were not specified on many of the drawings and many of the rain gardens were shown as constructed without them. Where they were specified it is also noted that, for these overflow gullies to become operational, water had to pond in the rain garden to a depth of 100mm.

In an earlier FOI asking for details of design standards... mention was made of drainage being designed using SuDS principles.
Enfield Council, by attempting to retrofit SuDS to an existing road, are allowing water to percolate into the road substructure with the possible softening and weakening the road foundation. This is contrary to recommendations given in SuDS manuals.

Dear Enfield Council,

Because you have chosen to try and answer me directly, I have posted an annotation to this FOI request summarising your answers.

Yours faithfully,

Peter Johns

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