The Bristol Frome

(yeah it's still pronounced Froooom!)​

The River Frome is Bristols second river, rising only 20 miles away it outfalls into both Bristol’s Floating Harbour and the River Avon New Cut. Until the Northern Storm-Water Interceptor (Motherload) was completed in 1962 the Frome regularly caused flooding in many areas of Bristol. Its course and mouth with the Avon has changed a number of times over the centuries.​

The Floating Harbour has had me intrigued recently. Its construction was completed in 1809 to allow bigger boats to stay afloat when the tide went out, while The New Cut (a man-made channel) was dug to bypass the river around the new Floating Harbour, which is on the original course of the Avon. The tidal range of the River Severn is the second largest in the world (upto 13m!) and high tides can have a dramatic effect on the Avon and the New Cut around Bristol city centre up as far as Netham Weir. Even then, Netham Locks occasionally have to stop extremely high spring tides that come up over the weir. There are locks at either end of the Floating Harbour which allowed small boats to continue their journey upstream passed Keynsham or some of the largest ships at the time to enter the Avon downstream to the Severn and beyond but only when the tide was in. When the tide’s out, the Avon isn’t really that big and certainly wasn’t navigable by the ships wanting to use the Floating Harbour. It was also difficult to navigate increasingly larger ships through the sharp turns in the gorge, even at high tide, so as time went on Bristol Docks started to lose international trade to Liverpool and later Avonmouth and Portishead, none of which are affected by the tide.​

As the Frome comes into Bristol near Eastville it enters a large concrete channel, which takes it under the elevated M32 motorway. The word Frome used to mean “fair, fine, brisk,” and is very apt. It flows very briskly through this channel until it arrives at Eastville Sluices. Here’s where stuff starts to happen. Seen below are six channels, controlled by sluice gates which, when open, divert a large part of the Frome down Motherload (bottom left of first pic) while the rest carries on in another large channel, through a debris collecting grill (top left second pic) and into a large concrete box culvert, which runs completely devoid of interest (and therefore photo’s) for a km or so towards town. At the beginning of this culvert is the mouth of the partially brick culverted Coombe Brook, that I’ve thus far been unable to access and at the end is the concrete box culvert of Horfield Brook, which looks accessible, I’ve just not checked it out yet, though I suspect it shrinks down to 3ft as I’ve seen a similar sized, submerged, heavy metal flap nearby​

Looking back upstream towards the debris collector (grease not completely wiped off lens!)​

The culvert starts beneath the M32 and is accessible behind the little fence if you really want to​

The boring concrete box culvert emerges in Riverside Park where it flows in the open air for another km or so before entering the mile+ long stone built culvert at the appropriately named Wade Street. Here’s where stuff starts to get interesting again​

The map below shows where the Frome used to join the Avon before 1240, when it was diverted along what is now its current course, into St Augustines Reach, to create a bigger harbour. No mean feat 750 years ago! Even before then, the Frome was partially diverted to create a mill pond and moat around Bristol Castle which was first built from timber by the Normans and rebuilt from stone and vastly improved in the 12th century. It also shows 3 other tunnels off the Frome, each with a concrete weir, acting as storm reliefs into the Floating Harbour. They are Castle Green Tunnel, Fosseway Culvert and the original Frome Culvert beyond Stone Bridge. The castle was demolished in 1656 and the moat culverted in 1847. When the Floating Harbour was built in 1809 the Frome flowed straight into it at St Augustines Reach. It was essentially just an open sewer, though, and with the Floating Harbour not being tidal any more the mounting sewage was becoming a health concern. Mylne’s Culvert and Stone Gates were constructed in 1825 to solve the sewage problem. Starting at Stone Bridge it runs parallel to St Augustines Reach and takes the last quarter of a mile of what was a sewage laden Frome to a siphon under the harbour and then directly into the tidal New Cut.​

I’ve visited the Frome several times now over the past few weeks, seeing how it all works or used to, between looking at old maps and photo’s showing its course. Only 90m in is what appears to be a bricked up bridge on the right. Old maps show the river used to flow under this into a channel which runs separate and parallel to its current course (shown on the map in red). The current course of the Frome was dug as a mill pond​

Somewhere near the end of River Street, the culvert changes to a stone arch​

On the left was this curious, rubber, one-way valve thing. I considered access using the reverse birthing technique but, er, it was too tighter squeeze. I couldn’t even prize it open enough for a pic of the inside. I think it’s just a manhole with a 3’ RBP in and out of it, running parallel to the river.​

A short way further and on the right was these outfalls for a concrete pipe. After a short(ish) stoop up it, concrete turned to brick. I managed to get gps signal here and found I was under the busy junction at the end of the m32. Trying to remember the twists and turns that I’d taken it all seems to match the route of Temple Way as roughly shown on the map above​

After a few hundred meters of two tone brickwork I could smell fresh and a small window came into sight, through the side of a concrete manhole chamber. It was Bristol’s main interceptor which surrounds the city centre taking sewage to a pumping station close to Black Rocks Quarry in the gorge, beyond the suspension bridge. Here it’s pumped to the treatment plant in Avonmouth. The tunnel continues, with a small flow in it, into the distance but is inaccessible from this side. I’m yet to locate this chamber from above ground​

Back to the Frome and further down on the left was another flap with a CSO behind it. This one wasn’t submerged and I could just squeeze behind it. 10m in was this small manhole chamber. I suspect this is the 3’ RBP I saw earlier behind the rubber valve.​

Further down, on the right, was another blocked up arch. Behind this, at some point, there was probably an overflow weir or mill tail race from when the Frome used to run in its old course and the current course was the mill pond. (See old maps!)​

On the left was this short overflow inlet into the moat. I went for a look but it was too deep for me and my waders. Hopefully we’ll spend some time down there in the summer when we intend to make the full through trip in inflatables!​

Then came the first significant junctions. This is looking upstream at two of the concrete weirs. Originally there would have been a weir at the mill pond head here as the moat was fed directly from the adjacent Frome (or maybe the Avon)?) and not the mill pond as I’d first thought. Behind me is the concrete Castle Green Tunnel pipe and hidden from view by the rusty old penstock is the Castle Moat. The Frome flows off to the left. Its difficult to see but notice the water levels are roughly even approx. 4” below the top of the weir​

The Castle Moat off to the right, looking upstream along the Frome (originally castle mill pond) on the left​

Looking downstream along the Frome, the Castle Green Tunnel on the left. Time to GTFO​

The staircase out from the metal inspection gallery, overlooking the two weirs (hidden behind the rusty old penstock) and the door that leads out on to Broadweir near Harvey Nichols. This is the ONLY easy way out until Stone Bridge still over half a mile away. There’s no other laddered access/ egress further on at all and very few good ‘refuge areas’ should something go wrong.​

Here’s the door from outside, on the left. The Frome continues ahead beneath Broad Weir. In the distance you can just make out the road goes uphill. The river bears right here contouring the high ground of Castle Park. The Fosseway Culvert, behind a weir, takes a sharp left under Castle Park​

On one of my visits, while I was messing about taking the staircase pic, I could hear a water rushing sound that I’d not heard earlier. Both the weirs were overflowing! The river must have been rising all the time I’d been down there, at least 4” in the 15 minutes I was taking pics from the river. There’d been no rain that day (in Bristol at least) so I was puzzled as to why the levels rose so rapidly. Obviously Eastville Sluices can control the rivers' flow from upstream but I thought they only came into play during severe weather, when they divert the Frome down Motherload. Having visited more recently with Seffy and Whodareswins (from above this time) I suspect it has more to do with the tide as both times I’ve seen the weirs overflowing there’d been a spring tide that day. There’d been some showers that morning but I don’t think they’d have caused the levels to rise that much. In normal conditions here, it’s over a foot below the weir. Maybe it all just backs up as the outfall flap from Mylnes Culvert to the New Cut is several meters below high spring tide level, as is Motherload’s, in which case normal flows cause already high levels to spill over. Here’s a short vid of it overflowing​

Here’s what it’s like in normal conditions!​

I returned again a few days later but checked the sluices to Motherload first. They were all closed and the whole river was flowing down towards town. Again, there’d been no rain so I went back to the door, which was still off the catch, locked me bike up against the railings inside and went down for a look. I was half expecting it to be full but it wasn’t, it was the lowest I’d seen it. I went down as far as I could, not stopping for pics, as I’d get them on my return. I’d no idea if it was rising but it was low for now so I didn’t hang about. I didn’t get to Mylnes Culvert as eventually it got too deep but I could hear a loud rushing sound of water, the infall for the culvert, so I must’ve been close.​

Just down from the two weirs at Castle Moat is another overflow weir into the Fosseway Culvert. Apparently there’s still a dry ditch around here, that once served the castle as a sally port, whereby soldiers could “sally forth” and surprise any would-be attackers. That needs findin'​

Beyond the weir, you just make out the harbour lights in the distance. I didn’t venture down here. Again, maybe in the summer, floating on a blow up dolphin or something​

On a visit from above with Whodareswins. On another visit with Seffy as well we could only stand on the viewing platform, what with this one overflowing as well. One of the lights comes on for about 30 seconds every 10 minutes or so. Just enough time for the camera inside to take a pic! The Frome’s on the right, below the Floating Harbour on the left​

The river turns right here, the bed rises up, the width narrows and the flow increases substantially. At least the base is solid still so the going isn’t too difficult. This is one of my favourite sections, it’s all a bit bodged and nothing seems to line up! Judging by the visible score marks there used to be a mill wheel here too as the river passes under what used to be Castle Mill. I think this was the main feed for the moat (in which case water used to flow the other way), however I read somewhere that the Avon fed the moat so I'm not sure now!​

At the downstream end of the fast-flowing stretch beneath what was Castle Mill (centre) another large culvert entered from the right (left of pic!) as the river turned left​

Up this other large culvert was lots of stalactites and what was presumably a live CSO. This short stretch was lifting! It absolutely stank and got worse the more I disturbed the “river” bed. Basically, I wanna know what’s behind the wall in the pic below as the Frome used to flow down there (see maps) Did the old river bed just get filled in? How much of it was culverted? Did any of the modern shopping centres destroy the culverts when they were built? I dunno ....yet. The old bridge, though, I suspect is under Merchant Street​

Hold yer horses there’s more to come yet, its another bloody long one......​