An Industrial Legend
Sanctus were commissioned by the Wildlife Trust for Birmingham & the Black Country (WTB&BC) to remove Ackers Weir in Tyseley, South-East Birmingham. This unique dam removal incorporated the bioremediation of 500m3 of impacted silts, allowing not just the reconnection of 14.5km of the River Cole, that reinstated fish passage, once again, connecting human and natural transport corridors but also resulted in the creation of new hibernacula whilst providing tangible biodiversity and chemical and oxygen level improvements.
Historically, the River Cole has been impacted by industry, straightening, and oversteepening with Ackers Weir forming complex 3-tiered, 2.6m high structure, with a high hydraulic jump, first recorded on site around 1852. The weir formed a small dam upstream, known as the Mill Leat which was used as a water supply for a former metal works, located to the north-east of the weir. The river would wash and cool metal products as they were formed, producing industrial effluent within the watercourse. Additional contaminated run-off was subsequently created from other nearby historical industrial operations including the Birmingham Small Arms factory, a fireworks factory, railway and welding works.
Sediment Contamination
The contaminated sediments that were produced as a by-product of Birmingham’s industrial legacy, settled on the river bed and gradually accumulated behind the weir, creating a uniform habitat, unable to support wildlife diversity. Previous investigation works spanning a 2km stretch of the River Cole had produced some 6,324 environmental monitoring data points identifying elevated Lead and Hydrocarbons within sediments samples in and around the Weir. Over time these sediments leached contaminants into the river, effecting water quality downstream of Ackers Wier.
Leachate analysis and water sampling found leachate levels contained concentrations of contaminants in exceedance of the Environmental Quality Standards (EQS) for freshwater bodies and water samples recovered from both upstream and downstream of the weir found the highest concentrations of contaminants being identified at the weir and downstream of the weir.
Weir removal was essential for suitable habitat creation. However, contamination presented a significant risk – weir removal would re-mobilise and cause downstream migration of the impacted sediment.
Why Remove Ackers Weir?
Sanctus are committed to removing barriers to fish migration, improving sediment connectivity and restoring natural river environments, these goals align with the Water Framework Directive and 2019 Cole Valley Catchment Landscape Vision Report, aiming to reconnect the river environment, create habitats, and improve fish passage along the River Cole and the surrounding catchment. It also supports the vision for the Tyseley Environmental Enterprise District (TEED) to become Birmingham’s Green Innovation Quarter, as set by Birmingham City Council, Tyseley Energy Park, and the University of Birmingham.
Under the Water Framework Directive (WFD) Cycle 2 classification from 2019, the river ‘Failed’ the assessment based on its chemical composition indicating that the River Cole required remedial works to improve its water quality. A hydraulic head difference of circa 2.4m in impounded contaminated sediments from the River Cole was considered a major barrier to the movement of fish up and downstream. This was demonstrated by the rivers ecological classification under the WFD as ‘Poor’. Furthermore, surveys carried out previously identified a significant difference in biodiversity between the river channel upstream and downstream of the weir demonstrating that the weir presents a defining barrier to the migration of species.
By retaining the weir in place, the cost of maintaining the structure and managing the risk associated with its misuse was considered by Birmingham City Council to be unsustainable and a significant social and economic risk for the local community. The removal works therefore aligned with the areas wider Environmental Enterprise Improvements scheme and the Tyseley Local Development plans.
Characterising the River
To reprofile the river upstream of the weir, Sanctus commissioned a detailed hydraulic model extending 1.5km upstream of the river including a physical survey of the river channel to determine how the flow would change a naturalise post weir removal. This also identified the extent of sediment removal required to achieve a natural profile and prevent the mobilisation of large volumes of sediment in a short period, should the river be allowed to reach sediment equilibrium on its own, determining that 440m3 of sediment would need to be removed.
Regulatory Liaison
Prior to the start of site works Sanctus collected water quality data to monitor the quality before, during and after the works. Water quality monitoring data was also used to monitor the mobilisation of sediments throughout the in channel works, which was a major risk due to the contaminants that had accumulated behind the weir structure. Silt curtains and bubble curtains were deployed in the channel as with continual monitoring and clearance of debris that may hinder the effectiveness of these sediment control measures.
Fisheries Mitigation
A variety of permitting processes were deployed including; FR2 fish rescue, electrofishing, translocation and stock exclusion outside of spawning months was of paramount importance to improvement objectives for the scheme and to protect species. This facilitated many translocations of fish and control of invasive American Signal Crayfish. All fish were translocated downstream of the works.
Biosecurity
A biosecurity area was set up for use on entrance and exit of the site. Sanctus employed the same biosecurity measures in regards to the invasive non-native North American Signal Crayfish (Pacifastacus leniusculus), vegetation clearance for access and protected species. Sanctus actively practice robust biosecurity practices in all waterway projects to reduce the risk of spreading the disease carried by the Signal crayfish that harms the native species. In accordance with Schedule 9 of the Wildlife and Countryside Act 1981 (as amended), no Signal crayfish were re-distributed or released into the wild. The site was resident to the three major invasive plants: Himilayan balsam (Impatiens glandulifera), Japanese knotweed (Reynoutria japonica), and Giant hogweed (Heracleum mantegazzianum). Sanctus were not instructed to treat or remove any of the invasive plants, yet some Himilayan balsam clearance was required to gain access to the channel and biosecurity measures were implemented.
Weir and Contaminated Sediment Removal
Sanctus used a series of bespoke permits to re-naturalise the riverbed and remove redundant structures. Dewatering of the working area was conducted by gradually notching the weir on one side of the structure and diverting the river flow to a singular deeper channel thus lowing the water level very gradually behind the weir to reduce disturbance to the contaminated silts, before it was safe to excavate.
The material was then removed from the river to a soil treatment area and subjected to bioremediation from Sanctus’ bespoke Mobile Treatment License to become physically and chemically suitable for re-use on site. The demolition of the weir included the removal of the weir wall, apron, slab and foundations. All concrete debris was removed from site to a suitably licensed recycling facility.
Bank Stabilisation and Riffle Installation
Sanctus implemented green engineering techniques to stabilise the riverbank by securing organic biodegradable coir erosion control blankets to the new slope cuttings which incorporated a specifically selected native seed mix. Suitably sized, locally sourced rock armour was placed into the reprofiled channel to form riffles, creating small pools and regulating the flow and merging into the existing riverbed downstream.
Rock Armour was fixed along the reprofiled banks working upstream as the riffles were completed. This erosion control will be helped with bank stabilisation from carefully selected native wildflowers, shrubs and trees. Imported river gravels were then spread over the adjacent riverbed.
Following treatment of the sediments, the suitable material remained and was used to cover site won logs to create hibernacula; forming new wildlife refugia.
Monitoring and Aftercare
A 2022 (before inchannel works) and 2023 (post inchannel works) MoRPh survey was conducted with the data confirming re-naturalisation and improvement in visible natural processes and the effectiveness of the project on the River Cole.
The number of surface flow types increased in five of six locations upstream of the former weir location, and one location downstream of the former weir location. This greater diversity of flow types now allows for greater oxygenation of the water which enables the inhabitancy of a greater diversity of aquatic species which have greater dissolved oxygen requirements. Dissolved oxygen concentration was measured before and after the installation of riffles which created surface agitation and subsequently increased the amount of dissolved oxygen for significant areas of the reach.
The weir removal and addition of numerous different flow types of different velocities allows for the natural process of erosion and deposition. This is a dynamic process that is integral to the hydromorphology and ecological function of the river. Areas of high flow velocity will allow for erosion, and areas of low flow velocity will allow deposition and provide refugia for fish to rest and shelter. This is a natural and desirable process within rivers that often increases the diversity of geomorphic units within a river system.
Sanctus used automated continual monitoring upstream and downstream of the works. This ensured that the works were not impacting the water quality, providing real time data for project partners, often showing a near immediate benefit in oxygenation levels. Data was shared between all parties involved to allow re-search and knowledge sharing.
Sanctus also provided 4 months of aftercare and bioremediation to materials taken from the river channel after the weir removal to improve contaminated material to form new habitats. Further increasing the net gain potential of former derelict land.
Ongoing monitoring and rigorous management plans have been implemented for INNS (Invasive Non-Native Species) targeting the Himalayan balsam (Impatiens glandiferula), populations of Giant hogweed (Heracleum mantegazzianum) and Japanese knotweed (Reynoutria japonica).
Conclusion
Sanctus achieved a fully approved and commercially viable proposal for the removal of Ackers Weir, to solve a generation legacy problem which continues to pollute the natural aquatic environment every day. This was achieved through the extensive review of historic information and completion of supplementary information gathered to fully characterise the site resulting in a robust conceptual site model.
Sanctus excavated 500m3 of silt to extend the life of the river by preventing succession, reducing the flood risk through increasing the water carrying capacity and promoting biodiversity by improving the levels of dissolved oxygen in the water. Restoring soil health in the riparian zone, slowing the flow and installation of more habitat variety is known to help mitigate the effects of climate change. The approach of improvement of sediments made this process scalable, economically viable, and sustainable, with minimum disruption to the local environment whilst providing maximum benefit.
Employing natural solutions to improve riverbank stability with biodegradable coir blankets and rock armour prevented adding unnecessary pollutants into the environment and increased vegetative diversity which was previously lacking. The re-use of soil to create hibernacula addressed the habitat needs of local wildlife and removed the carbon intensive process of transporting large volumes of soil. In addition, Sanctus sourced labour and materials locally, when possible, which contributed to lowering the carbon footprint of the project.
Sanctus recruited through local suppliers and the locally sourced materials to provide opportunities within the area for businesses benefit as well as contribute to a sustainability project.
