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Re-Routing of the River Thet

Erosion of the river bank at Ford Meadow during Summer 2018 has resulted in the main flow of the River Thet being diverted from its historic course under the Coffee Mill. The main flow now follows what was a smaller drainage channel. This passes under the middle bridge of Nuns' Bridges, before joining the Little Ouse just below the Thetford Pulp Mill Sluice. The old course between Ford Meadow and the Tricorn bridge by the Riverside development now has a lower water level, and is almost stagnant with very little flow. On the map below, the old course is shown with a grey dashed line, while the new course is shown with a deep blue solid line.

The flow along the new course and the lack of flow along the old course has the potential to cause several issues:

The Environment Agency is aware of the change in the course of the flow, and provided the following summary to Thetford River Group in September 2018:

The low water levels upstream of Coffee Mill, and the slow flows downstream are the result of a breach in the left bank upstream of Nuns’ Bridges. Most of the flow from the Thet is now being diverted down a smaller channel and into the Little Ouse at Pulp Mill leaving a 'backwater' between the breach and coffee mill.

The exact cause of the breach is unknown, however rivers are not static, naturally erosion will occur. There is also a lot of erosion of the light soils in the banks at this location caused by dogs accessing the river — this may have contributed to the eventual bank failure and reconnection of secondary channel.

There are a few solutions to this problem, one of which is to seal the breach in the bank and revert back to the previous management situation. However, there is a notable benefit of this recent diversion — it allows free movement of fish between the Thet and Little Ouse — something previously impossible.

A preferable option, which takes into account the biodiversity of the river, would be to build back the levels in the Thet, but to allow some water down this diversion channel to allow fish to move through Thetford. Ultimately though, this land belongs to Thetford Town Council, so any solution will have to be agreed by them.

A further update from the Environment Agency was provided on 15th October 2018:

The low water levels upstream of Coffee Mill, and the slow flows downstream are the result of a breach in the left bank upstream of Nuns’ Bridges. Most of the flow from the Thet is now being diverted down a smaller channel and into the Little Ouse at Pulp Mill.

The Environment Agency is currently working with Thetford Town Council to find a solution that will restore the levels and allow fish to move freely between the Little Ouse and Thet rivers. The Environment Agency has some funding available this financial year to address the problem and is currently working with consultants to determine the feasibility of several options. Depending on the final option and cost, we hope to implement it before the end of March 2019.

The breach in the bank of the River Thet at Ford Meadow, taken in November 2018. The original river course flows from the top of the image to the front left. The new flow through the breach goes under the footbridge to the bottom right.

The view from the breach along the now flooded drainage ditch, which is carrying the flow from the River Thet, taken in November 2018.

The view from Ford Meadow of flooding caused by the breach in the bank of the River Thet, taken in November 2018.

The daily average river level in the Thet in 2018, measured by an Environment Agency gauge just upstream of the Coffee Mill Sluice. The breach in the bank at Ford Meadow appears to have occurred around the middle of June, and led to a significant drop in the water level. Explore the data further at River Levels, or use the Environment Agency API.

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