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Wallbridge Lower Lock Update No. 6

By-wash & Fish pass progress at Wallbridge Lower Lock

Work at Wallbridge Lower is back on schedule with fish and eel pass apparatus now fitted at the upper/eastern end of the concrete channel. Eels will use the green material while fish will use the black zig-zags. Photo 1.

Final formwork installed

At the other end of the 75 metre channel the final formwork is being installed to cast the channel down to the water level of the pound below the lock. Photo 2.

Source of curiosity & comment

The construction of this concrete channel has been the source of curiosity and comment from visitors to the Cotswold Canals Trust Visitor Centre at Wallbridge. 

One such comment was couched in the form of the humorous question…

Just how many whales will be migrating along the canal?”

The humourist was referring to the size of the concrete chamber which featured in the last Wallbridge Lower Lock update. Photo 3.

The rationale explained

Sam Chapman, Technical Officer (Fisheries, Biodiversity & Geomorphology)
from the Tewkesbury based Environment Agency explained the rationale behind the construction of the concrete channel running alongside the lock. 

Dual purpose construction

This construction serves two purposes.

  1. By-wash to carry heavy flows of water from Slad Brook which is classed as Main River
  2. Fish-pass to facilitate the migration of fish and eels


Dictionary definition: The outlet from a dam or reservoir; also, a cut to divert the flow of water. In this case around Wallbridge Lower Lock.

Main River

In England the Department for Environment Food & Rural Affairs (DEFRA) decides which watercourses are the main rivers.

Main Rivers are usually larger streams and rivers, but some of them are smaller watercourses of local significance.


Main Rivers that affect the restoration

Slad Brook is classed as a Main River. The River Frome is also Main River from the Severn all the way up to Whitehall Bridge in the Golden Valley.

It follows therefore that any section of canal that also contains a river will also count as 'Main River' itself.

The smaller watercourses of local significance to which the title 'Main River' applies include Slad Brook, Painswick Stream, the Nailsworth Stream and even the much smaller Ruscombe Brook at Dudbridge.

Main rivers can include any structure that controls or regulates the flow of water in, into or out of the channel.

Slad Brook gathers water quickly

Slad Brook has a total catchment area of 1,500 hectares and a channel length of 31 km running along the bottom of the steep sided Slad Valley to the east of Stroud.

The brook joins the Thames and Severn Canal immediately above Wallbridge Lower Lock. Photo 4. This photo of Stroud Brewery Bridge shows the blue brick arch adjacent to the canal channel.

The photo was taken in the summer of 2013 when the flow was not fast. However, there are times when the stones just outside the arch are covered by a lot of fast running water. Heavy rainfall quickly creates a very fast flow of water through the arch. This is why the By-Wash element of the channel is important. It will permit the westward passage of excess water from Slad Brook and the Thames and Seven Canal above Wallbridge. That flow requires a lot of space hence the large chambers in the By-wash.

Fish Pass

This is often referred to as a 'fish ladder' or 'fish steps'. It is a structure on, or around, artificial and natural barriers (such as dams, locks and waterfalls) to facilitate the migration of fish from the sea to freshwater to spawn.

Species supported by the canal

In addition to Brown Trout the canal also carries Roach, Pike and Eels.

The Fish pass is a legal requirement covered by two pieces of legislation:

  1. 1975 Salmon & Freshwater Fisheries Act (SAFFA). This legislation protects the migration of the salmonids, that is to say salmon, trout, chars, freshwater whitefishes, and graylings
  2. The Eels (England & Wales) Regulations of 2009. These regulations afford new powers to the Environment Agency to implement measures for the recovery of European eel stocks

Sam Chapman says… “We have found Salmon Parr at the bottom end of the River Frome. (‘Parr’ is the term used to describe the fish’s next step in development from ‘fry’.) The salmonids migrate from the sea to spawn in fresh water from December to February while elvers migrate upstream between February and May which is the time of the high tides in the Severn Estuary.”

Sam also explained that the European eel has a mysterious life cycle. It spends its early years in our rivers before heading out to sea and across the Atlantic to the Sargasso Sea in the western Atlantic near the Bahamas. Here, it is assumed they spawn and lay eggs; however this has never been witnessed.

Sam sums up the justification for the fish pass… “Fish are good indicators of the general health of a watercourse, and installing this fish pass is not only a legal requirement, but also helps the restoration partnership to display its own environmental ethos.”
Commenting on our use of the term Larinier Fish Pass Sam Chapman added…

“We have heard that the inventor Michel Larinier of The Institute of Fluid Mechanics in Toulouse, France prefers the pass to be referred to as a Super Active Baffle Pass.”

Excuses M. Larinier. Désormais, 'Pass Baffle Super Active' il doit être .

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