How will the river change?

Our current preferred design option will involve installation of a rock ramp directly downstream of Bucks Pool where it narrows. At the upstream end of the rock ramp there will be a partially submerged weir which has been hydraulically designed to ensure existing flows down the Mill Stream and the River Thame are maintained, while allowing passage to all fish species.  The new weir will be sensitively integrated into the riverbed / banks and faced with natural materials.  

Directly downstream of the new weir and for approximately 20m there will be a gradually sloping rock ramp, comprising stones, boulders and naturalised bank protection, creating an attractive cascade with flow characteristics to enable fish passage.  

The sloping sides into Bucks pool with be landscaped to have a shallower gradient and there will planting in keeping with the existing environment around the pool and the rock ramp. The barrier will have the effect of increasing the water depth in Bucks Pool by up to 75 cm.

How will risks of flooding change and what about the risk of the Mill Stream drying out?

The fish passage solution has been hydraulically designed to maintain the existing low to high flows and not affect the current levels of flood risk in Dorchester upon Thames. Extensive hydraulic analysis has been conducted to evaluate the design. Topographical survey has been completed of the entire section of the channel, hydrometric data has been gathered and a 2D hydrodynamic model has been built to simulate flooding both pre and post rock ramp. The approach has been verified with the EA. The design has been developed to ensure no impact to existing flooding mechanisms.    

Additionally, the replacement of the currently failing, and highly vulnerable weir structure with a new structurally sound and purposely designed fish pass will remove the risk of an uncontrolled and unpredictable failure of the existing weir with unknown consequences for flood risk.

For low flows, hydraulic analysis has been completed to ensure that the low flow notches in the new weir are designed to maintain the existing flow splits and ensure the Mill Stream receives the same low flows as it does currently. In addition, the design will include flexibility so that the flow down the Mill Stream can be adjusted if required into the future.  

Will the bridge and existing deteriorated weir structure remain?

The current preferred design option will leave the existing weir fully submerged and move the control of flows downstream of the Bucks pool to the new weir. So the presence of the existing weir and bridge will not impact the efficacy of the design and the associated hydraulics.  

The proposed design ensures that the high velocities currently present at the existing weir during non-flood flows will be reduced significantly. Reduced velocities in turn lower the risk of obstruction by fallen trees (and the associated increase in flood risk from the Mill Stream), plus reduces risk associated to maintenance/ obstruction removal.  Nevertheless, we are conscious that aesthetically this may be of concern to the local community and therefore any changes to the existing weir and bridge will be subject to stakeholder input and availability of funding.  

Who will be responsible for ongoing maintenance?

Following an initial verification period, in which RTCT will gather data to confirm functionality. Maintenance responsibilities will be with the riparian landowner. RTCT will develop a maintenance plan along with the riparian owners as part of the project delivery. 

How will this improve the River when water quality is an ongoing concern?

RTCT acknowledges that poor water quality is a major concern across the entire Thame catchment.  We continue to engage with Thames Water, river users and landowners and have ongoing projects conceived to improve the overall water quality in the Thame. Facilitating fish passage is one of the most effective means of improving fish populations and their resilience.  RTCT will continue to pursue the longer-term objective of improved water quality, whilst also pursuing river habitat restoration such as fish passage.  These objectives are not mutually exclusive.

What about people who use the area for recreation and swimming?

Consideration for recreation and access has been part of the design process. In response to concerns raised in community consultation, the banks of Bucks Pool will be landscaped with a shallower gradient and clear signage will be installed. Ongoing collaboration with the Hurst Water Meadow Trust and engagement with local communities will inform implementation of these considerations.

What about angling?

The hydraulics in Bucks Pool will change and as such the characteristics of the Bucks pool will change from an angling perspective.  Bucks Pool will remain fishable and the project will have an overall positive impact on fish communities in the River Thame.  Access for angling is not anticipated to change.

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