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Report - River Fleet CSO, London 2011 - 2014.


Staff member

River Fleet CSO, London 2012 - 2014


(Sorry it's pic heavy, look away now)

The River Fleet is one of London's big 3 'Lost Rivers'. It was canalised between 1670/4 and in later years culverted

It's headwaters are two streams on Hampstead Heath, each of which was dammed into a series of ponds, the Hampstead Ponds and the Highgate Ponds, in the 18th century

At the southern edge of Hampstead Heath these descend underground as sewers and join in Camden Town

Today, the Fleet is a combined sewer, and any tranquil waters that once were are certainly no longer

Long before it was buried, it was an open sewer and contributed to the great stink and the great plague of London until J Bizzle came along with his interceptors in 1859

Google lost rivers/drains of London and the first indexed hit is almost certainly the Fleet

It's hardly surprising, as it's old and epic as far as London's main drainage goes

More so if you actually invest some time properly exploring anything other than the stretch around Farringdon Road that is :p:

That said, despite how awkward the Fleet can be it's certainly a rewarding one, especially if you throw the Fleet's storm relief in as well for good measure

I'd like to acknowledge some of the very few masters who have put some legwork into the place over the years; JD, Stoop, Siologen & the Otter

I've tried to include the more significant bits of the system (including the Fleet Storm Relief),
having traversed the majority of the system now over the last few years and more recently with Adders and The_Vicar

To make sense, I'll start it from the upstream, although we spent much of the earlier trips piss balling about on the downstream sections, but hey

Kentish Town is where we got as far as, before it got ridiculous in terms of stooping and getting dunked in London's finest nuggets

We entered the upstream section in Camden via a number of access points

One comedy gold moment as I almost catapulted Mrs Brown along with dog piddles across the road on one occasion


On the last trip we headed North from Camden through Kentish Town, already covering the other bit's and now Siologen had us intrigued with the staircase of doom

The flow was pretty rapid on this stretch, it took all our effort remaining vertical :D


Along the way, this overflow; Here it connects with the High Level Storm Relief according to the 1934 LCC map


A single sided weir to a spillway & drop shaft, with the overflow pipe further below




Eventually, we could hear "the rumble, the roar of nearly the ultimate sewer staircase of death"

And then it came into sight


Soo'ah staircase of doom!


I needed a shower anyway :rolleyes:



Once up, this junction


The middle pipe is filled with stagnant crap, looks like it's still occasionally used though!


The right hand sewer, a fast flowing 4ft egg, I went 10m and turned back as my lungs weren't in agreement with it's steamy content


On the left, the Fleet Mainline continues North towards Hampstead Heath


The sizeable RBP now becomes a slightly smaller, slippery egg for a good distance..


A number of connections along the way, some of which were truncated, others belching some dubious vapours





We continued further upstream until we hit a wall, that's right a fucking wall!

Where did the Fleet just go ? ;)


Of course we did traverse some ridiculous diameter pipes in order to chase it further but eventually gave up with the shrinking shit fest






Staff member
Re: River Fleet CSO, London 2011 - 2014

It's a good job we'd already looked at the rest of the Fleet at this point, or I might have had a different opinion of the place if this was all it had to offer :rolleyes:

The next leg of the journey wasn't that eventful with regards to features for some distance

On the night in question we managed to slip in just as the semi finals before Brazil V Germany kicked off

Great for access, but a total suicide mission for traversing a sewer with hindsight, especially with the amount of washing machines now belting out

..and then came half time, Good fun though :eek:

We headed downstream, having limited choice in terms of when we could explore the upstream bits, and far from ideal to say the least!

So, with work and adverse weather getting in the way, we just had to steal what ever opportunity came along, and with drains isn't always the best option..

Anyway we carried on down this slippery brick egg, in anticipation of something decent turning up, even a useful lid would have sufficed


A short downstream section was draped in rags and wipes, it now started to look a bit more inviting


We continued downstream a good distance through some peak flows before we managed to locate a suitable exit should something go wrong


Further down we encountered this 'orrible st00pfest



It took some encouragement from Adders for me to bother with this, seriously I was up for heading out to the nearest off licence on this particular jaunt

We eventually washed up at the first interaction with the fleet storm relief

That's right, another CSO almost identical to the one depicted further up, except this was slightly larger

I yearned for a Kronie at this point, instead I reached for my Nigon for some more snaps and whilst narrowly avoiding the bloody thing landing in the fresh again

The lens cap decided to leap off the Samyang and into the soup much to the amusement of Adders

There is a clip on the GoPro somewhere gathering dust of this event, more a reminder of how actual clumsy I am these days!



It's also at this point 2 branches of the storm relief merge below


We trundled down the upper reaches of the fleet until we reached the the Mid Level Sewer No.2


This connection would have been constructed between 1906 – 1911,
the section of sewer itself was one of the later upstream stretches to be built (c.1850) diverting the river’s flow off its meandering course 80yds to the east of the present sewer.
- Jon Doe

The thing with interceptors, although you wouldn't want to end up down one, their bark is often worse than their bite, and with a little care easy enough to pass

This ones a little more awkward, as the invert favours the mouth of the interceptor, making the flow even more ridiculous at this point in the proceedings

The handrail which proves useful also runs short of the manhole, making the stride across more interesting

I only managed the one pic as it just wasn't worth the associated fuss for imminent death :eek:


As always after an interceptor, the pipe remains dry a short distance and free of any flow until the system gathers momentum again from other connecting sewers

Here a brick junction, the first real feature we came across after a couple of smaller side pipes dumped their contents


We pushed on further downstream towards the mid section of the system, eventually arriving underneath St. Pancras

(1850's Tunnel Section)


Brick quickly turns to concrete, much has changed here as the Fleet was diverted in 2004, as construction was undertaken for the Eurostar rail link

Ahead, a 7ft RCP leads to a much larger concrete box section


Either side of the sewer was a raised spillway,
beyond which an extended 5ft RCP overflow which connects to the original Victorian brick storm relief a short distance away


An interesting change in scenery for sure and good to finally see, although the 1800's brickwork was a welcome sight once again


Downstream, a bricked up spillway which once connected to the Fleet Storm Relief, since modified to accommodate the 2004 St. Pancras diversion works upstream


Further down, another trunk joins the mainline

(It can just be made out on the left)


..Downstream continuation

WARNING, do not stand in front of this side pipe for too long..

That's right, a minute earlier it let rip whilst Adders was playing catch up :D


Further down a later edition of side pipes join in the shape of shrinking RCP's




Staff member
Re: River Fleet CSO, London 2011 - 2014

The next significant feature we happened upon was one of the better examples of a CSO

The engineering invested into this incredible example of an overflow is something to behold!


4 large arches allow any spill-flows when at capacity to enter the brick overflow chamber below & beyond


A large drop shaft leads to the relief sewer below


Also notice the smaller diameter brick overflow (mid right of the shaft) on the above pic ^

It's a continuation from another overflow weir from the North East branch of the fleet as seen in the pics below



The overflow on the right inter-connects to the afore mentioned drop shaft (see above) ^^^

We'd heard tales of sudden thunderous sounds of water, whilst I was taking the above shots, we witnessed the same phenomenon

Although on a later trip to the very same section, this time below in the storm relief a couple of months later I think we solved the mystery..

As it was getting late we headed for the nearest exit, discussing plans to return soon and carry on with some other bits & pieces within the system

The last feature I managed to capture before we retired for the evening was a section around Kings Cross where the Fleet mainline passes underneath the rail lines

Here, you can see the iron section implemented to strengthen the sewer


Next up is the downstream section, we originally dropped ROY in here at the ML1 a good few years back now, whilst we sat and drank Kronies


It's quite photogenic and ridiculously feature packed considering the shortish distance it covers (approx 2km)

Also, it lends itself as probably the easiest traversable section of sewer the big smoke has to offer,
and explains why it tends to be a bit of a tourist trail as far as wannabe drain0rs go..

(Only joking, I'm beginning to sound like Speed now) :p:

Anyway we dropped back in at the point where the Mid Level Sewer No.1 intercepts the sewage away to Beckton, under the watchful eye of J Bizzle


As always the line is dry after an interception, however within this stretch of the Fleet, you don't have to walk too far before a spaffing pipe and flow resumes


We continued a good distance via the extended brick arch, before the sewer curved beyond Mount Pleasant and stepped under Farringdon Road


Beyond which a 7ft RBP, heading downstream beneath Farringdon Road


A sizeable stretch of RCP and other related gubbins has been added as it passes by Farringdon Station,
presumably strengthening due to the ongoing Cross Rail works and associated infrastructure




(The last stretch just about visible in the background on the below pic)


Approx 200m later, the various attempts at strengthening and seismic measuring equipment bolted along it's length due to the works above ground came to an end

The mainline is once again back to it's original brick construction, this time in the form of a large extended brick tunnel


Eventually we reached the "Split"


In all the trips I've had down here, I've never been happy with my pics of this, the last time I was with The_Vicar I went for an un-expected swim trying to light it up :eek:

At around 150m (ish) long the Farringdon Split does exactly what it says on the tin..


A connecting passageway and manhole shaft


Furthermore, venture up the other stretch of the split and you'll be afforded access to the Fleet Storm Relief once more


We dropped into the downstream stretch of the relief briefly (a.k.a Wrens Cache),
and having since covered the system, grabbed a few more pics before carrying on with the sewer


Here, a diversion down a set of stairs ahead of the oak dam boards; at capacity the excess flow heading down to the outfall chamber (see further below)




Staff member
Re: River Fleet CSO, London 2011 - 2014

Anyway, back in the mainline...

Surface drains and a number of smaller sewers connect along the way, one of them this unassuming 4ft egg


A convoluted route via the Ludgate Hill sewer and others, eventually lead to the Pauls Wharf Sewer

My account of that can be found here >


As far as I was concerned, the Fleet just kept giving

Sure we've all seen the pics before, but the size of this brick chamber at the downstream end of the split was nothing short of amazing

The engineering/workmanship that has gone into these systems never ceases to amaze me

Naturally I spent a good few minutes looking around before even contemplating pics, And the best was still yet to come!


From the far end of the chamber, the Fleet mainline now takes shape via an 8ft brick arch on it's penultimate trip towards the embankment


Further down it's final interaction with the Fleet Storm Relief, just behind this series of arches, some kind of collection tank


The flow had picked up considerably, a mahoosive brick section designed for serious storm flows takes over, beyond which, the Low Level No.2 Interceptor



Beyond the L.L.2 is the outfall/end chamber, a seriously tidal affair; get it wrong here and yer fucked!

You wouldn't want to be in here, when

a.) It rains

b.) Father Thames rolls in

Starting with the primary outfall chamber let me explain why..


A two tiered double decker affair, home to a whole host of turds, crabs and tidal fudge

Each level has 2 smaller passages used to convey storm flows to each of the 4 flaps as and when required

Lower Section



Looking down from the top deck

It's been said before, but the amount of water during capacity required from the Fleet Mainline to call them upper flaps scares the shit out of me!

Now tell me you'd wanna be in here when it rains and the tide rolls in :p:


A further set of ladders lead to some defunct penstock winders in a small room above


At the far side, a small archway that leads to a narrow walkway and series of passages below the streets...


Further down; LHS Fleet Storm Relief, RHS Low Level Interceptor No.1


We first checked out the L.L.S No.1, a set of ladders from a smaller penstock room lead to the sewer below


The flow was pretty sketch and certainly wasn't something we planned on walking in :eek:


Our low tide window was approaching so we braved the steps down towards the outfall chamber


As it's tidal we had to wait for it to drain out, this takes some time

It doesn't look much when it's filled to the brim, but when you stand on the top gantry watching it retreat,
you soon get an idea how unforgiving this system could be


Once the tide had receded, we headed on down into the cavernous outfall chamber which serves the Fleet Mainline & Fleet Storm Relief


It's thick with tidal fudge and remnants of previous spill flows from the mainline sewer, and crabs, lot's of them!


Everyone likes a decent set of flaps :D


The 4 larger valves (flaps) in the background serve the Fleet sewer when at capacity

The 2 smaller ones to the left serve the Fleet Storm Relief, a separate system built in 1875 to further alleviate the system

Beyond the low ceiling in the end chamber as seen further up ^^ is the final overflow to the Thames resides, in the form of 2 brick arch tunnels

I'd been previously warned by GE about NOT passing a certain point in here

Even before I even reached that point, the meter was singing away to the dangerously high methane levels that were now kicking up underfoot :eek:

I got as far as the 'drop off' point grabbed the one pic and quickly retreated back to the main chamber

In the distance you can just about make out the final valves, beyond which the River Thames under Blackfriars Bridge


(WARNING, DO NOT, go beyond the silver lined section beyond where the yellow brick ends)

Aware of the time, I went for a quick look at the smaller outfall chamber which serves the storm relief..

Beyond the 2 smaller arches in the main chamber are 2 further valves which control the rate of flow at capacity,
the amount of water required to shift them beggars belief


The others had now left and I was alone on this particular trip as the tide started to roll back in, time to GTFO!






Mr Muscle
28DL Full Member
Re: River Fleet CSO, London 2011 - 2014

Rather bloody awesome mate :thumb

Good to see such a comprehensive view on a place most people only see a relatively small section of. I have to admit, having been in 4 times, I still didn't see half of what you guys got round to. Well worth the effort & long hours in the soup :thumb


living in a cold world
Regular User
Re: River Fleet CSO, London 2011 - 2014

Bob on that mate, shows the place well! We had some interesting times down there for sure.

Bring on the rest of them. ;)


28DL Regular User
Regular User
Re: River Fleet CSO, London 2011 - 2014

Was an awesome read, thanks for spending the time to type it out, thoroughly enjoyed it. :)


big in japan
Regular User
Re: River Fleet CSO, London 2011 - 2014

Excellent read and good work!
And why can't you head down that final section in towards blackfriars bridge? Just cos it's gassy and minging or does it drop off at a big step or something?


Bajo Tierra
Regular User
Re: River Fleet CSO, London 2011 - 2014

Top dollar as usual mate, it looks a great day out :thumb

The Lone Ranger

Safety is paramount!
Staff member
Re: River Fleet CSO, London 2011 - 2014

Very nice that, good to see another epic drain report :thumb

Oxygen Thief

Staff member
Re: River Fleet CSO, London 2011 - 2014

Thanks for documenting this ...finally :rolleyes:

Two amazing write-ups of the fleet in a matter of days. It's somewhere I choose not to go, so these comprehensive reports are appreciated.


Seriously, it fills up to the top with ming? Gross.


living in a cold world
Regular User
Re: River Fleet CSO, London 2011 - 2014

Excellent read and good work!
And why can't you head down that final section in towards blackfriars bridge? Just cos it's gassy and minging or does it drop off at a big step or something?
Both, but it's the drop that's worse as your legs sink into Thames goo that does not want to let go, like quicksand. Being stuck there with the tide coming back in wouldn't end so well.


A Porky Prime Cut
Regular User
Re: River Fleet CSO, London 2011 - 2014

Not bad that, our kid. First rate write up and pictures as always. :thumb

Squirrell 911

Regular User
Re: River Fleet CSO, London 2011 - 2014

Nice one dude. I see London sewers don't bother to spiral the fresh before it drops down but rather it cascades down.

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