Bevere natural bypass channel Fish Pass from unlocking the Severn

Thank-you to everyone who attended the 3rd talk in our Severn Series – all about Fish Passes with Richard Leigh, Project Engineer for Unlocking the Severn.

We received an overwhelming number of questions during the talk – over 50 in fact.  And more than we are able to respond to in the allotted time. So, we promised to follow up in more details.

Here are the questions and answers that weren’t able to be fully covered in the talk.

If you have further questions or suggestions for future talk topics, please do drop us an email: [email protected].

You can also scroll to the bottom of this post to view the recording of the talk if you missed it.

Q:  Can elvers and eels use our fish passes or do they have separate structures to use?

In addition to the main fish passes we are building; we are also installing separate eel pass (containing tiles) on the opposite bank to the fish passes. All the fish passes we are building will allow eels to swim through them, but eel tiles are specifically designed for eels, with projecting studs of two different sizes. The eel tiles will work much better for smaller eels to maintain their purchase against the current.  It is also important to have an eel option on the opposite bank as eels do not necessary cross a river to actively seek the outflow of the fish pass.  And as the Severn is a wide river, a proportion of eels travelling up the river on the other side would not find the fish pass, but they will be able to use the eel tiles on that side.


Q:  What was the most difficult constraint in the design process of the fish passes?

In terms of fish passage, the hardest part is getting the flow conditions inside the pass suitable for all species of fish, whilst also maintaining sufficient attraction flow at the downstream end. It is important to have a good attraction flow to allow the fish to locate the pass entrance.  Best practice recommends that the flow exiting the pass should be around 10% of the average daily flow (ADF) down the river. To achieve this balancing act between flow velocity and attraction in our deep vertical slot fish passes, two auxiliary flow channels down each side of the pass have been included in the design and flows modelled using a scale model to make sure the injection of the water did not upset the flow dynamics at the downstream end of the pass.

In terms of construction, whereas canal piles (the steels used to secure the banks) are normally 1.8-2.4m long light section (L8) trench sheets. This project used 11m long heavy section (VL606) piles.  This involved a lot greater logistical challenges and much larger plant machinery (such as cranes) to bring them onto site and install them.


Q: Did you consider removing the weir rather than installing the fish passes?

In an ideal world, the best thing for migratory fish and the whole health of the river would be to remove a weir.  Weirs provide a barrier to fish.  In some cases, this is effectively an unpassable barrier, and in other cases, they delay migrating fish, and even those fish that can pass over the obstacle, do so at a cost of extra energy expended and the fish can also sustain physical injuries in their attempts to swim or jump over a weir crest. Weirs also disturb the natural sediment transport of rivers and thus affect habitat both up and downstream.  At the two sites on the Teme we were able to install a rock ramp and do a partial weir removal at Powick. However, the River Severn is used by boats and its navigation status is protected by an act of parliament. Installing the fish passes was something that could be done and would have an immediate positive impact, providing fish passage past these barriers to historic spawning grounds.


Q:  If water levels are constantly rising and there have been an increase in flooding year on year, how will this impact on the speed and success of the fish passes and will you need to make changes as the years progress if water increases and you are unable to maintain a specific speed of water that is compatible for the fish to be able to swim upstream?

Rising water levels, do not necessarily pose a problem for fish on the river.  The design of the passes is such that the flow conditions will we maintained even in elevated flows to allow successful fish passage.  The auxiliary flow channels in our deep vertical slot fish passes also allow us to alter the attraction flows if necessary.

In a flood event, once the water level overtops the weir, fish can simply swim straight over the top.  However, the speed and temperature of the flow everywhere in the river during flood events would be a concern if they happened during the shad run as they would make the conditions very unfavourable for the shad migration and successful spawning.  A greater risk for shad, related to climate change, is the risk of more frequent summer droughts which are more likely to affect the shad run in May and June.  Our fish passes have been designed to allow fish to pass even in very low flow conditions.


Q: In the challenges of the build and complications you have experienced (such as flooding), did this significantly impact on the allocated budget?

We had calculated prices for each fish pass construction and on top of that we had a risk pot as contingency for any change/variations – including some allowance for flooding. Our fish passes at Bevere and Lincomb have come in on budget.

We are 2.5% over the originally submitted budget (including contingency) at Diglis. This seems like quite an achievement considering the exceptional conditions that we’ve faced.

We incurred significant flood related costs and extra costs due to COVID (particularly the showdown last spring).  But, we have been able to offset these additional costs via changes in design, working with our stakeholders and identifying potential efficiencies throughout the duration of the project. Working collaboratively has enabled us to bring about efficiencies that enabled us to stay line with the budgets.

Our fish pass in construction at Holt remains the biggest budget challenge the river conditions we experience this spring/summer will be crucial as all the plant equipment and materials to this site can only be bought in by river.


Q: How will the success of the pass be measured?  What would success look like for the project?

There are a number of options for monitoring the return of the shad.  These include acoustic monitoring which allows us to track a sample of fish and see where these individuals travel on the river.  We will also use monitor another sample of twaite shad through smaller pit-tags which are detected by a  energised loops  as the fish pass through the pools of the Diglis fish pass. This allows us to understand how the fish approach and pass into and through the pass before continuing their journey further upstream.

If shad successfully pass the weirs, then our project will have achieved its aims.  This does not have to be all shad.  It is likely to be a smaller number of pioneers initially, and over time we hope to see a gradual recolonisation of high-quality spawning habitats beyond the barriers.  This will allow the River Severn shad population to substantially recover in numbers over a period of several generations of fish.

We will also record film footage from our underwater monitoring window in the Diglis fish pass and the use of the pass by other river fish will also be keenly observed.

eDNA – environmental DNA sampling – where river water samples are scanned for genetic evidence of certain species, is also another potential method to determine the expanding range of twaite shad and may be possible to obtain insights from this linked to related research and funding streams.

We will be measuring the fish pass efficiency for the first two years and then we will rely on other techniques to see how distribution and returning number of shad increase in future years.  This will include such as film from the window and eDNA analysis described above, plus other exciting opportunities that form the legacy monitoring of our project


Q: Who will do the longer-term monitoring of the success of the fish ladder i.e., increased numbers of fish higher upstream?  How long will this programme continue for?

The project is funded for a further two monitoring seasons.  After this period a legacy monitoring proposal will be formulated, this might involve collaborative projects with universities, new funding sources, citizen science monitoring through third sector organisations, like Rivers Trusts and Wildlife Trusts, The Environment Agency is likely to continue with monitoring in the viewing gallery, possibly in partnership with universities.  The field of eDNA is also rapidly developing which may prove to be the easiest technique to show changes in distribution and possibly abundance over time.  Certainly an appropriate level of monitoring will continue, as reporting to comply with the Habitats Directive needs to be completed every 6 years.


Q: Can small-scale hydroelectric schemes be incorporated into design (if non-impacting on ecology)?

Unfortunately, it would need to be a separate structure. Hydro and fish passes have different – generally competing – hydraulic requirements.


Q: The historic Bevere Island Bridge is in a very poor state of repair.  Will anything be done about this?

The Bevere Island Bridge is privately owned, with no public access, as such it does not fall within the remit of our project and would need to be the subject of separate funding bids by the landowner if that was something they wished to pursue.


Q: I have been paddling on Diglis and Bevere weirs and also the Teme at Powick for nearly 60 years. The Bevere fish pass is excellent but it would have been so easy to create a potential white-water site for Canoeists which would benefit all.

Unfortunately, white-water channels for canoeists has significantly different hydraulic requirements to a fish pass. Our passes have had to be very carefully designed to the specific flow requirements that make them suitable for twaite shad, and at a range of river levels.  For example, we would not have been able to make the gaps between boulders in Bevere bigger, without significantly negatively impacting the flow dynamics for the fish.


Q: What archaeology was found?

At the Bevere site, where we had extensive archaeological survey carried out, with a number of trenches dug in the field adjacent to the fish pass.  Archaeologists found evidence of medieval grain stores and some medieval pottery.


Q: Will these designs be shared with other rivers/waterways that have had a reduction in specific fish species due to the similar issues?   Both in the UK and abroad?

Yes absolutely, we are doing a lot of knowledge sharing with projects across Europe and beyond. Sharing knowledge, insights and best practice internationally is one of our project’s aims as specified by one of our key funders – EU LIFE project. We have hosted lots of visits for interested parties from other rivers across Europe. We will also have an end of project technical conference for those in the field, including projects working on similar shad species in North America.  This is provisionally planned for the week beginning 23rd May 2022.


Q: Can Bevere weir be visited or is it private and all fenced off?

You can walk alongside the fish pass by following the public footpath which gives good views of the structure in operation, but please respect the private land either side of this right of way. Unfortunately given particularly wet conditions this winter it has suffered from mud and the fence was damaged in the last flood event.  We will be back on site in Spring for remedial works on those.


Q: Do you expect much predation in the Diglis pass from herons/egrets etc.?

We expect to see very little predation in the pass will be very low. Herons and egrets do not fish in that way so will not access the pass, cormorants might go through the pass, but it is too turbulent to provide good fishing for them. The benefit of the pass and free movement of fish preventing delay at the barrier will far outweigh any possible predation in the pass. The reason you see fish-eating birds such as herons at weirs is the fish have nowhere to go so are easy pickings.

Fish predators like pike and zander will find the pass flows challenging to ambush prey and otters will go through the pass but again will find it hard to trap fish in corners etc.

The fish passes are designed to maintain a deeper body of water (in summer low flow conditions) to help protect the fish from predation.


Q: Are there plans to do something about the weir in Shrewsbury too?

Shrewsbury weir does not fall in within the scope for this project.  The fish pass at the weir is owned by the Environment Agency and is of a deep vertical slot design similar to Diglis but was designed principally for salmon.  Once shad reach the pass its effectiveness will be assessed and at this point any improvements will be considered.


Q: Are you aware of any disadvantages to the either the river or anglers of building fish passes?

The ideal situation for overall river habitat health and river ecology would be to remove weirs. However, the River Severn weirs were installed to protect the depth of the river for boat traffic, and since the navigation of the River Severn is protected in statute, if we had only considered the option of weir removal, this project would probably never have got off the starting blocks.

The fish passes will ensure that we provide fish passage past barriers.  But unfortunately, the pass will not negate the interruption to the natural riverbed processes this are normally involved in healthy, natural river habitat.

Anglers who have favoured fishing where fish are collecting in the pools beneath weirs (because the fish that are blocked or delayed from progressing further upstream), may find that they fishing becomes a little less productive in the short term as fish readily pass upstream of the weir on their migration runs.  However improved connectivity on the river will benefit all river species – including coarse fish.  Thus, in the long-run, anglers will benefit from healthier fish populations on the river.


Q: Could sturgeon use the fish pass one day?

Breeding populations of sturgeon have been absent from the river Severn for at least a century and probably longer.  Restoration projects are ongoing in Europe and sturgeon are more and more likely to revisit the river Severn in the coming years.  The current understanding is the deep vertical slot fish passes of the size constructed by Unlocking The Severn and the pass at Bevere would be suitable for sturgeon. It will be a very exciting day when the first sturgeon is spotted at one of the passes.


Q: Are anti-poaching measures built into these structures?

Poaching inside the passes is likely to be very challenging and not very effective.  For public safety the deep vertical slot fish passes will have high fences which will also prevent easy access for any potential poachers..


Q: What are the width and length of each cell in the vertical slot pass at Diglis?

6m long and 5 m wide – but we are planning another talk specifically about the Diglis Fish pass so do join us for more detailed insights in that talk!  Keep an eye or our website or social media, but we hope to hold this talk around the end of April.


Q: When will the remaining fish passes be opened and when and how can you visit the viewing window?

Diglis fish pass will be completed very soon (by the end of April 2021), with Lincomb fish pass also due for completion this spring. We will return to site at Holt in April and expect to finish the pass there in the Autumn this year (2021).

Visits to the viewing window at the Diglis Fish Pass will be piloted in a COVID secure manner this May and June and we hope to open public booking later in the summer. Please keep an eye on our website, follow our social media and subscribe to our newsletter (at the bottom of any page of this website) to be kept up to date!

You can watch the replay of our talk on the Unlocking the Severn YouTube channel :



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