Pages 28 to 33. Future Tidal Barrier at Boston. The following information was obtained at an Environment Agency exhibition in the town, 11 November 2014.
The barrier will be located across the River Witham, a little downstream of the access lock into the Black Sluice Navigation, thereby managing the tide in the Witham and making access to the Black Sluice available for a longer period each day. This will be more important when the Fens Waterways Link, via the Black Sluice, is completed to the Nene near Peterborough, via the rivers Welland and Glen - making a navigable circular route of the fens waterways.
Although the new barrier will be an aid to inland navigation its primary function is to prevent flooding in Boston, the town suffering greatly during the east coast tidal surge in December 2013 when over 800 properties were extensively damaged.
The barrier will rise up to block the water flow, and sink down to allow the natural flow of the tides, plus passage of boats through it.
A bird's-eye view of the planned barrier, raised up across the River Witham. Downstream is on the right.
From inland the Witham is already controlled by the Grand Sluice and its lock, on the western edge of Boston, holding the levels of freshwater. The stretch between the Grand Sluice and the planned barrier passes through the centre of the town and is currently fully tidal, therefore resulting in extensive, some say "unsightly", mud-banks, during much of the day. Therefore, many factors have to be considered when deciding on the levels at which the water will be held between the two control points, and at what times - boats going to/from the North Sea via The Wash and the lower Witham, those to/from the Black Sluice lock, fish habitats, high river flows from inland, the positive effect on the appearance of Boston's riverside areas, and the crucial flood-prevention in Boston and surrounding areas.
No definite decisions have yet been made about water-level management but it's likely that the new barrier will be closed during daylight hours. The morning high tide will flow up to the Grand Sluice, as it does now, but its waters will be captured by the barrier when the levels have fallen to the desired level. On the rising evening tide, when the levels are equal on both sides, the barrier would be opened. The high and low tide would then occur naturally through the town overnight. However, the barrier would not operate during high river flows, so allowing maximum discharge of freshwater out to the North Sea. No decision has yet been made on the levels which will be held in the freshwater Witham above the Grand Sluice. It may also be that annually, from 1 November to 1 April, the barrier will only operate when a tidal surge is predicted.
Project Timescales.
Nov 2014 - late 2015. Preliminary designs and the preparation of a Transport and Works Act Order (TWOA).
Late 2015 - spring 2017. TWOA application considered by the Secretary of State.
Summer 2017 - late 2019. Multi-functional Boston Barrier and associated works completed.
The "associated works" will be raising the lowest sections of the Witham's banks upstream of the new Boston Barrier.
An Environment Agency video, using a 3D-printed model and a graphics mock-up, shows the planned enviroment of the barrier and a boat passing through. tinyurl.com/omytyk5
The barrier closed, with raised river banks.
We wait to see - but the prevention of tidal-flooding aspect of the scheme will ensure something is done.
It is a pity that no one appears to have done due diligence. A simple check of the tide tables tells us that only twice in a 29 day Luna cycle will we have a morning and evening tide. If the EAs proposals are followed, as stated we will have a morning retaining on the first day with an evening opening. On the 14th / 15th days we will have a morning opening with an evening retaining (o dear). Just check the tide tables and work it out for yourself. It is impossible to safely open the barrier if the retained water does not have equal level with tide. Get in really wrong and the Grand Sluice will look very sorry for itself with the tide heading for Lincoln. The best place for the barrier is at the entrance to the Haven at the Wash.
ReplyDeleteDavid, I've passed your interesting reply on to EA. I think I overheard a conversation at the barrier exhibition during which it was stated that the geography at the mouth of the River Witham, at the Wash, is unsuitable for the barrier. But I may be mistaken and await EA's reply. I don't have any first-hand knowledge of that area, it being difficult to access.
DeleteHi Mr Matthews,
ReplyDeleteYou are correct that every two weeks we will not rise the barrier for two low tides so that the impoundment can resynchronise with daylight hours.
A multi-functional barrier downstream of the port has many disadvantages which were identified in the Boston Combined Strategy. A lock at the barrier was initially considered but was rejected because the impact on commercial river users was identified as too great.
Some of the other reasons include:
1. To maintain navigation we would need to create a new bypass channel in the Haven large enough to let shipping vessels safely pass during construction. This would be very expensive and not viable for technical and environmental reasons.
2. The cost and difficulty in getting mains electricity supply, construction plant and machinery to the mouth of the Haven.
3. Higher cost of building a larger and more complex barrier to prevent collision damage and allow commercial vessels to navigate through.
4. A multi-functional barrier holding water to the mouth of the Wash would affect land drainage and gravity discharge of rivers – increasing the risk of flooding.
5. Environmental damage to The Wash.
If we were only building a tidal flood defence barrier, many of these reasons would still apply - which is why the proposed location (just downstream of Black Sluice Lock) is the best possible.
Best wishes,
Dominic Burton, Environment Agency