SRA Annual Report 2021-22: Dredging and silt monitoring
Dredging
Maintenance dredging in January 2022 covered 3.75km (2.33miles) of the River Parrett between Saltmoor Pumping Station and Andersea. It was organised for Somerset Rivers Authority (SRA) by the Parrett Internal Drainage Board (IDB), working as a partner in the SRA. The Parrett IDB liaised closely with the Environment Agency and Natural England, and again deployed water injection dredging specialists Van Oord.
Between 2017 and 2021, the Parrett was dredged by Van Oord’s vessel Borr. In 2022, Baldr was used instead (pictured with tugboat Havik pushing from behind).
Baldr is a newer and smaller craft than Borr, built specially by Van Oord to serve places like the River Parrett. Baldr does not have its own propulsion system. Instead, it is driven and manoeuvred by the tug boat Havik. Three advantages of this arrangement are that Baldr can access places Borr cannot; it can operate at lower water levels; and at the start and finish of jobs, it can be lifted in and out of the water by a smaller – and therefore cheaper – crane. (Text continues below photo).
In the water, Baldr operates in a similar way to Borr. It has a dredging bar with nozzles through which a high volume of water is pumped out, so it forces sediments off the river bed and they disperse through natural processes, downstream as the tide goes out (text continues below graphic).
The Parrett IDB watched Baldr working on the Parrett in summer 2021, clearing silt from around Combwich Wharf for EDF (text continues below image of Baldr and Havik at Combwich).
As part of a continuous push for operational improvements, they decided to see how Baldr would fare doing maintenance dredging for the SRA.
The exercise was successful. More than 20,000m3 of consolidated silt deposits were removed from the Parrett. However, while Baldr is lighter and nimbler than Borr, it is also less powerful, so which vessel will return for maintenance dredging this coming winter has not yet been decided. A lot depends on water levels in the river.
The purpose of water injection dredging is to help maintain the benefits of all the dredging that has been done along the Parrett since the floods of 2013-14. Parrett maintenance dredging reduces flood risks for properties, and helps to reduce the risks of agricultural damages, which tend to be worst from spring and summer floods (as seen in 2012).
Silt monitoring
The Parrett IDB has continued silt monitoring along 12.15 kilometres (7.55 miles) of the Parrett and 3.35 kilometres (2.08 miles) of the Tone to help shape the SRA’s dredging programme.
Since 2016, surveys have been carried out twice a year, at the end of summer when silt deposition tends to have reached its annual peak, and at the end of winter when silt levels are low because of natural processes of scouring.
Through building up a detailed picture of seasonal and year-on-year trends in siltation, the SRA and its partners’ long-term ambition is to get a better understanding than anybody has ever had before of how the tidal River Parrett-River Tone system really works.