This special edition of Somerset Rivers Authority’s newsletter The Stream tells you about two forthcoming dredges of 3km of the River Parrett, one maintenance dredge and one new dredge. It also refers to the wider future of dredging as a flood protection measure in Somerset.
Extra flood protection work along 3km of the River Parrett begins at the end of November, with two contracts for dredging being funded by Somerset Rivers Authority.
One contract – awarded by the Parrett Internal Drainage Board – is for 2.2km of maintenance dredging upstream of Northmoor pumping station, the other – due to be awarded imminently by the Environment Agency – is for 750m of new dredging downstream of Northmoor pumping station.
The work follows the 8km dredge of the Parrett and Tone in 2014 that was identified as producing the biggest reduction in flood risk on the Somerset Levels & Moors.
The River Parrett after dredging in 2014, looking upstream from Westonzoyland Pumping Station.
The two new contracts are designed to produce even greater benefits, particularly when combined with a wide range of other improvements, such as the Environment Agency’s £2.5million investment in temporary pumps and pumping facilities.
Recent hydraulic modelling has shown that the 8km dredge – combined with extra pumping power – would make a big difference if there were to be a repeat of flooding on the scale of winter 2013/14. Effects would include:
- significantly cut the risk of flooding to 129 of the 142 properties reported to the Environment Agency as having flooded in Northmoor and Saltmoor
- reduce closure of the A361 to about 3 weeks instead of the 9-10 weeks experienced
- clear the Moors of water more quickly.
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River Parrett Maintenance Dredge 2015-2016
A contract for the maintenance dredging of a 2.2km stretch of the River Parrett upstream of Northmoor pumping station has been awarded by the Parrett Internal Drainage Board, acting on behalf of the Somerset Rivers Authority, to WM Longreach.
Parrett IDB staff have specified this maintenance and will supervise its progress, with WM Longreach working for them.
The main technical aim is to ensure that the design profiles achieved by the 8km pioneer dredge of 2014 are retained.
The River Parrett looking upstream from Lake Wall with Westonzoyland Pumping Station Museum in the background and a fairly substantial berm of silt on the right of the river. Photograph taken 25 September, 2015, by project engineer Rob Kidson of the Parrett Internal Drainage Board.
The River Parrett viewed from approximately the same Lake Wall location as the photograph above, but on the opposite bank looking downstream. Picture taken on 1 August, 2015, by Rob Kidson.
The scheme will not go beyond the limits of the pioneer dredging work carried out in 2014 and it largely follows the dredging methods used then. One important aim is to place most material removed on the back of the river banks, reducing the need to haul material along the road and thereby avoiding disruption.
The site compound is at the Environment Agency’s Westonzoyland Pumping Station, accessed from Westonzoyland via Lakewall and Hooper’s Lane. The contractors started mobilising and bringing in long-reach excavators on Wednesday, November 25. Excavators will have reaches of up to 22 metres.
The River Parrett maintenance dredge site compound is by the Environment Agency’s Westonzoyland Pumping Station and near the Westonzoyland Pumping Station Museum.
The compound will also be the lifting point for floating plant. Around four weeks of work will be done from floating platforms near Westonzoyland Pumping Station, as access from the river banks is severely restricted there.
A small satellite compound will be established on EA land near Meadow View where plant for right hand bank works downstream to Ham Lawn Farm will be mobilised. This area will be accessed from Westonzoyland via Lakewall and Hooper’s Lane or from Burrowbridge via Riverside.
Safety and information signs will be prominently placed to show access points and communicate site hazards to workers, visitors and members of the public.
Works will start with two dredging gangs focussed on the right hand bank (facing downstream), working from the river bank that runs parallel to Hooper’s Lane. As this work is being done on private land there should be minimal impact on local traffic.
River Parrett New Dredge 2016
A contract for 750m of new dredging downstream of Northmoor pumping station is expected to be awarded by the end of November to Galliford Try, Black & Veatch (GBV) and Land & Water by the Environment Agency, acting on behalf of the Somerset Rivers Authority.
Work is due to begin in February 2016. Following the maintenance dredge, contractors will be based at Westonzoyland pumping station.
The river downstream of Northmoor pumping station is top of a list of 10 priority sites for further new dredging.
Work could cut peak flood levels by between 50-80mm in Northmoor and reduce the duration of a flood like that of 2014 by 3 – 5 days, in combination with the 8km dredge of 2014 and additional pumping. As with the 8km dredge and this year’s maintenance dredge of the Parrett, the aim is to achieve a cross-sectional area of around 70m2 in the most efficient way possible, by dredging one or both banks. Protected species will be safeguarded and the impact on important wildlife sites nearby carefully managed.
GBV and Land & Water are delivering the ‘design and build’ of this dredge. It’s a more complex and costly job than the 8km, which was also carried out by GBV and Land & Water.
Land & Water dredging the River Parrett with floating equipment just upstream of Westonzoyland Pumping Station in 2014.
150m will be dredged by a bankside excavator with Huntworth Lane being closed to allow material to be removed safely. Other access problems mean that work will be done almost entirely from a pontoon-mounted excavator floating on the river. This is expensive to set up and much slower to operate than bankside excavators.
Silt downstream of Northmoor is saltier. This means it has to be spread more thinly on agricultural land to avoid reducing crop yields for the first couple of years – and this in turn means that more land has to be used.
The future of dredging
The Board of the SRA voted in October to approve the scheme because it will bring benefits – albeit at a high cost – but members also agreed to investigate more cost-effective dredging techniques and sites before deciding on a dredging programme going beyond the financial year of April 2015 – March 2016.
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Click to download a PDF of this special Dredging Briefing issue of Somerset Rivers Authority's newsletter The Stream.