We'd known of the existence of the Holloway storm relief for sometime, having first stepped foot in it around its mid
way point where the West - East running Holloway
crosses the upper section of the North - South running Deep Ochre.
The Holloway/Heavy Mettle was always going to be interesting,
even if it was relatively featureless, simply by virtue of it being contemporary with Bazalgette's intercepting
sewers. For most of its length it is a circular pipe of yellow brick in its upper two thirds and brown brick
in its lower section, approx 7ft in height throughout and 36ft below ground level at the
point where it intersects Deep Ochre.
Statistics done with let's move onto the exploration. Entering downstream of the
Deep Ochre junction the first thing that struck us was the massive
amount of silting, a good two foot deposited at this point had pushed the water level upto around mid
thigh height. Ordinarily we'd do our utmost to avoid disturbing such turd-tastic matter, in this case though it was pretty well
impossible, but gas monitor armed we weren't overly concerned. Heading downstream the tunnel
splits into two 5ft pipes running parallel to each other,
rejoining approx 300 metres further along as can be seen in the last pic. Stooping through this section with
the silting, water level and the reduced height
presents a major splash back hazard, nobody likes a face full of poo cocktail.
Following on downstream from the split gets more and more silted, at some points it's of a porridge like consistency
making progress incredibly slow
and other times it is solid underfoot reducing the tunnel height to a mild stoop but making progress way speedier.
For most of the downstream length we trudged through varying stages of these two states, which seemed to interchange
without warning and you'd find yourself stepping from solid ground to having your leg swallowed upto thigh height!
Although there were no significant features for quite a while, the tunnel itself was very cool if slightly annoying
having also to contend with massive amounts of ground water seeping in and dripping from every conceivable surface and angle.
The last couple of hundred metres of the downstream end are likely the most interesting. A later added red brick extension starts
in a stepped junction chamber and runs on to a further junction with a smaller storm relief that receives overflow from the
Northern High Level sewer. It's a weird little set up, the smaller relief being at a higher level than Heavy Mettle has an
overflow slide down into the main Heavy Mettle tunnel, whilst most of the overlfow received into the smaller relief is channelled away in a raging
torrent, as can be seen in this video footage. This point marked the end of our downstream journey as the
main tunnel is sealed all but for a tiny slot to allow some flow to continue on, that likely explains the silting! We've yet to explore the remaining upstream third
from the Deep Ochre Junction onwards, but shall return soon enough.
