Our Position
Our Position Statement
WEIR WATER ABSTRACTION AT ABERDULAIS
The Tennant Canal has historically been fed from a weir located at the confluence of the Rivers Neath and Dulais at Aberdulais. Until recently this was achieved using a weir to raise upstream levels and provide sufficient hydraulic head to allow flow to gravitate to the Tennant canal through two manually adjustable penstocks.
The weir was in place and functioning, coincident with the canal construction, and thus has been in place for almost 200 years. Flow from the Neath/Dulais passed through a dual penstock arrangement and flow was conveyed into the Tennant Canal via a short open channel (see Fig 1). A degree of flow control was afforded by manual adjustment of the penstocks. The channel contains a removable weir at the downstream end to maintain a minimum water level – presumably to allow flow to be measured.
It is assumed that the penstocks were manually modulated to regulate the flow from the Neath/Dulais and that they were completely closed at times of high river level to avoid contributing to flooding risk.
Fig 1 – Tennant Canal Feeder Channel
The weir failed due to damage in 2015.
Prior to failure, the weir on the Neath/Dulais was in poor condition but functional. Most of the weir is still in place but a section on the Eastern side of the river Neath has been washed away. This has unfortunately compromised the ability of the remaining weir to elevate river levels and allow flow to be readily supplied to the Tennant Canal as has been the case historically.
Subsequent to the weir being damaged the arrangement has fallen into disuse. The channel and forebay leading up to the penstocks are blocked with stones and sediment preventing any flow entering the feeder channel regardless of river levels (Figure 2 and Figure 3).
The implications of the damage to the weir, silting of the channel and blockage of the penstock forebay are that the Tennant Canal no longer has a source of water from the Neath/Dulais confluence.
This is apparent during summer months when levels in the Tennant Canal drops alarmingly, threatening cherished environmental habitat all the way along the canal, including the Internationally important and protected RAMSAR site & SSSI (Crymlyn Bog) and the very rare species that exist in this ecosystem. The Eastern section of the canal is particularly impacted by low water levels, far lower than designed Canal operating levels.
The Tennant Canal Association(TCA) is committed to the urgent restoration of a sustainable water supply to the Tennant Canal from the Neath/Dulais. TCA’s view is that this should be achieved by fully restoring the weir. However, TCA are also open to considering other options that achive the same outcome.
Fig 2 – View of remaining weir and inlet channel showing sediment build up
Fig 3 – – Penstocks at inlet to channel feeding the Tennant Canal – clearly non-functional due to stone/sediment accumulation.
A fish pass was constructed on the Eastern side of the weir in 2002 (see Figure 4). The construction of this fish pass involved working on and alongside the weir and integrating the fish pass into the weir arrangement. Concerns regarding the impact of the fish pass on the impact on the weir and aqueduct were raised by Peter Ricketts of the Neath and Tennant Canal Preservation Society at the time of construction.
There is no indication from construction drawings, or visual inspection on-site, of proper provision for adequate tying in of the structure of the fish pass to the weir. Evidence from witnesses also suggests that very large, tracked excavators were observed to be tracking on the top of the weir during the construction of the fish pass.
Site observations indicate that the weir failed at the interface between the original weir and the reconstructed section (believed to be the remaining length shown in Figure 4). This suggests that there was inadequate tying in of the original weir. There is no reference to tie bars on the drawings, and no evidence of such an arrangement on the remains. This suggests that a proper structural tying in detail was not utilised.
TCA’s view is that the construction of the fish pass has contributed to the failure of the structural integrity of the weir.
Fig 4 – View of remaining weir alongside the fish pass
The Tennant Canal has historically been fed from a weir located at the confluence of the Rivers Neath and Dulais. Full restoration of the weir and clearance of the penstock forebay and channel would reinstate the water feed arrangement to its previous state.
The section of weir on the Eastern side of the river was damaged in 2015. This prevents a full weir effect forming across the whole river. The weir is effectively breached (see Figure 5). As such the water levels at the entrance to the penstocks are not as high as required to ensure consistent flow through the penstocks into the canal.
The damage to the weir also currently negates the intended purpose of constructing the fish pass.
TCA’s primary position is that the weir should be fully reinstated to a condition that will provide a reliable water feed arrangement for years to come.
The concrete channel leading to the Tennant Canal will also require cleaning out to prevent large quantities of silt and mud entering the Tennant canal upon re-commissioning of the arrangement. Some repair work to the channel sidewalls is also required where masonry has failed. It is envisaged that some maintenance of the penstocks would be needed as they appear to have been inoperable for several years.
Figure 5 – View along the damaged weir from the fish pass on the Eastern bank showing breach of the weir
Whilst the restoration of the weir will bring a clear benefit to the Tennant canal and preservation of the biodiversity therein, there have been concerns expressed previously by Natural Resources Wales (NRW) that a rebuilt weir may contribute to increased flooding in the area.
TCA’s position on this is as follows:
- The intention is to reinstate the weir to provide the same function as prior to the flood damage. The weir had been in place for almost 200 years and was not intended to be a temporary structure, being part of the permanent canal infrastructure. It is only reasonable that it should be returned to its former state without the need to validate flood consequences.
- The weir is set at a relatively low level in relation to the flood embankment. Due to the variance in height of the weir relative to river bed and variable nature of the weir along its length, it is difficult to provide a ‘drowned out’ weir calculation to evidence the impact of the weir. However, it is clear by observation that the hydraulic loss that would be generated by the full weir is negligible at times when the river level approaches the upper embankment.
TCA’s opinion is that the weir provided no or negligible restriction to river flow under imminent flood conditions. This is partially evidenced by a view across the weir (in its current state) in a reasonably high river level (well before risk of flooding to property occurs). Whilst this photograph (Figure 6) was taken prior to the weir damage, it does illustrate that there is no evidence of any significant hydraulic impact on upstream river levels across the remaining length of the weir.
As river levels rise further the effect will decrease even further as the weir gets increasingly drowned out.
Figure 6 – View across weir from Eastern back illustrating negligible hydraulic impact of current arrangement in partial flood conditions.
A basic level survey has been undertaken on the existing weir and channels (See Figure 8). This indicates that the existing arrangement currently has the ability to provide water to the Tennant Canal in most, and possibly all conditions.
During relatively low flow conditions the level of the River Dulais appears to be at sufficient top water level to enable flow to enter the channel that feeds the penstock forebay.
This is due to a weir effect at the exit of the River Dulais due to build-up of shale, stone and sediment, raising the water level at the exit from the Dulais above that of the River Neath. This build-up is likely transitory in nature, so it is not clear whether this is a reliable longer-term solution without considering the sheet pile reinforcement described in Section 3 a.
* Levels were taken during a survey in February 2022. All levels relate to the weir crest level of 6.72m AOD indicated on record drawings of the fish pass.
Unfortunately, the channel feeding the penstocks is currently completely blocked with stone and sediment, as is the penstock forebay. These should be cleaned out at the earliest opportunity as has been customary to allow flow to reach the penstocks. The inlet channel leading to the Tennant also requires cleaning out.
Levels indicate that the Dulais top water level is typically at least 500mm above the level of the bed of the channel feeding the Tennant Canal. The concrete channel feeding the Tennant also contains a small removable control weir immediately before flow enters the canal (see Figure 8). This provides an additional hydraulic loss of around 300mm. However, the remaining 200mm (approx.) head would be still enough to provide flow into the Tennant Canal and the weir could potentially be lowered, or even removed to increase passage of flow.
Figure 7 – Survey showing key levels*
Figure 8 – View of exit of channel feeding the Tennant Canal and removable control weir.
When the major road works on the A465 were carried out, a culvert was installed in the River Dulais. This culvert runs from the embankment underneath the A465 road bridge. It is understood that this culvert was used during the road construction as a means of supplying water to the Tennant Canal.
After a 90 degree turn, and via control valves, the culvert delivers flow into the feeder channel of the Tennant Canal.
A steel stop gate is currently used to isolate the inlet to the culvert. Flow can be controlled using the valves located in a chamber near the outlet to the Tennant Canal feeder channel.
The pipe potentially delivers behind the penstocks into the Canal feeder channel. Flow into the canal would be regulated by the culvert control valves.
Some minor work to clear the bed of the Dulais in front of the culvert would also be required to optimise the flow.
If the culvert were re-opened it would allow water into the Tennant Canal with little effort and cost – an ideal temporary solution.
The inlet is located near the end of the Dulais, hence impact on water levels throughout the Dulais is minimal.
TCA wish to understand whether NRW would allow use of this as a temporary solution until the weir can be repaired.
Figure 9 – Engineered Culvert & stop gate
Figure 10 – Culvert exit in Canal feeder channel.
TCA recognise that reinstatement of the supply to the Tennant Canal involves a licenced water abstraction being in place. Previously such a licence was maintained by the Tennant Canal Company and was financially viable due to industrial customers buying the water. This business no longer exists, and funds are not available to secure that water supply on the same basis.
If the supply to the Tennant Canal is reinstated it is important to recognise that the water transferred into the canal is transitory in nature and returned back into the River Neath and estuary at various locations such as the Bridge Street overflow. The water would have no other use, other than to serve to preserve the Tennant Canal and ecosystem before returning to the river/estuary. The water is simply ‘borrowed’ in that respect.
Consequently, TCA’s expectation is that the license provisions do not seek cost for the actual water entering the canal, only the costs associated with administering the license.
Natural Resources Wales (NRW) advises that a ‘Transfer License’ would meet this need.
There are several distinct options for a solution to reinstate flow to the Tennant Canal of ascending complexity, cost and reliability. None of the three solutions impact water levels upstream in the River Dulais to any notable degree as they are all based on undertaking work at the confluence with the River Neath.
Immediate Solution
The first ‘Immediate Solution’ option is that the long-standing, documented routine maintenance be reinstated by Leeder Property Consultants. This involves clearing out the shale & sediment in the feeder channel leading to the sluice gates, and in the feeder channel behind the gates.
Maintenance of the penstocks would be required, together with a review of the manual operating procedures to ensure adequate, reliable flow control, and closure of the penstocks at times of high river levels.
Interim Solution
As an ‘interim’ solution, TCA proposes:
The re-use of temporary supply vis the existing engineered culvert in the Dulais.
The long-standing maintenance and operating procedures described for the ‘immediate solution’ would be required to continue.
Complete Solution
Thirdly TCA proposes as the ‘Complete’ solution, reinstatement of the weir to its historically correct condition (pre 2015).
The long-standing maintenance and operating procedures described for the ‘immediate solution’ would be required to continue.
Save The Tennant Canal | Made for The Community*
*By local tech Steve Thomas who gave his time and skills for free to help save the canal.