Bridgwater Tidal Barrier Final Public Consultation

Ahead of the forthcoming last public consultation on the Bridgwater Tidal Barrier (BTB) the rate and tax payers might consider asking what becomes of Bridgwater after the BTB reaches the end of its design life?

The answer is simple.  

if there  is no barrier capable of keeping the rising sea out there will be no house insurance, no mortgages and Bridgwater the town will not be a viable place to live. 

Our present proposed EA flood defence scheme is based on retreating as far back from the sea as we can, it deliberately  puts our backs to the wall  On one side of the barrier we will ultimately have the inter tidal habitat (the sea)  and on the other our housing and industry.

No space is being provided behind the proposed barrier to create another bigger one. As a consequence there will be nowhere to go with a replacement that does not have a huge and unnecessary price tag. There is an irony that land at Chilton Trinity returned to the sea under the present scheme  and the planned managed realignment in 2055  will be inadvertently reclaimed  under a downstream replacement barrier albeit land now poisoned with salt.

With sea level rise continuing for hundreds of years past the design life of the BTB one would expect that as responsible people we would give some consideration to what happens next. Unfortunately, like so many things in need of fixing and investment we seem content to kick the problem into future refusing even to consider how we might make some provision to help those that follow us.

Many years an Arup engineer called Poul Beckmann wrote  the opening sentence of a  document with the line “Human nature is that we put off until tomorrow what we should do today”. Never has that been more true today than in Somerset. Somerset’s failure to develop a realistic, affordable and sustainable long-term strategy is unforgivable.  It really is time for some of the promises made by our political leadership on this issue to be delivered and people stop making excuses why things cannot be done. Simply working our way through the Somerset 20 years plan and saying the future is not our business is irresponsible and unacceptable.

what was the question?

The SRA was asked “What is the SRA and its partners working assumption for the replacement of the existing scheme and barrier?

The written answer received from the SRA  answer is in blue

From day one, the tidal barrier will be designed in such a way that it will still provide a 1 in 1,000-year standard of protection in 2125. The design includes an allowance for climate change up to 2125.

  • Climate change does not stop in 2125!!!! 
  • 1:1,000 sounds good but its just the margin on the starting point and degrades over time. The Dutch use 1;10,000 and maintain it as the level of protection.

The downstream defences will be designed in such a way that they will still provide a 1 in 200-year standard of protection in 2055, with allowance for further adaption for climate change over time.

  • 2055 is important because this is the date from which managed realignment is implemented under Policy 7d39  of the Shoreline Management Plan.
EA explaining the scheme in 2017

It is to be expected that any piece of infrastructure would need updating after 100 years. We cannot decide now how future generations may wish to live or what their priorities for infrastructure will need to be. This is why, for example, there are plans for the Thames Barrier to be re-built in future decades but decisions on exactly where and what will be required are deliberately being left for later.

  • Building the barrier in the town as planned does not  give those in the future choices but leaves them no choice to but to go downstream and  use land we have previously given back to the sea and to abandon the £100M investment the present scheme will have cost.
  • The response attempts to conflate the need to replace the  1970’s Thames Barrier as a reason not to make provision today for a replacement  in Somerset. if you know you have to replace something why would you not plan for the replacement? Its an excuse. 
  • We should be learning the lessons of the past rather than making a  virtue of repeating the failures that the SRA reply implies.

The Thames Barrier Myth

The continual portrayal Thames Barrier shown on TV as an iconic single barrier solution is misleading; the barrier is actually part of the Thames Estuary flood defence system comprising not one but eight individual tidal barriers that all shut together. Five of the eight barriers are closer to Bridgwater’s  situation than the “Thames Barrier”. They are however where they should be at the mouth of the rivers they defend not 5 miles upstream as with Bridgwater’s Tidal Barrier.

Thames Estuary Flood Defence

Using the Thames Barrier as an excuse for not planning a replacement in Somerset is particularly disingenuous  for a number of reasons.

  1. Nearly 60 years ago  when the Thames Barrier was designed climate change was not understood as it is now. There was little provision for sea level rise just the post Ice Age tilt of the south of England to consider.
  2. The  Thames Barrier is upstream of what were active docks  that had navigation rights in the Thames Estuary.
  3. In Somerset we have a topography in the form of the Polden Hills, Pawlett Hill and Stockland Bristol that we can link and use to our advantage.  London does not have such a luxury.
  4. All the Thames Barrier replacement options  move the replacement barrier  downstream perhaps as far as Tilbury. (July 2016 update)
  5. The  most frequent use of the Thames Barrier is now the management of fluvial flood water rather than tidal surge. Something Bridgwater is not designed to do.
  6. Sedgemoor District Council is able to reserve space for a bridge over the Parrett south of Dunball should its not be doing the same for a replacement barrier?

The collective response is No ; there is no plan.

The SRA, EA and SDC were conformed as having contributed to the response

Bridgwater Tidal Barrier; the Questions the public need to Ask

Once  the question “how will the proposed Bridgwater Tidal Barrier be replaced?” the present  scheme simply looks out of date and poor value for money.

The Bridgwater Tidal Barrier is repeating the same mistake as was made with the Thames Barrier system setting it as far upstream as possible. Because of the selected location  a replacement would like the Thames Barrier have to go downstream towards Combwich. Confused? We should be.

When you go to public consultation some questions worth asking;

  1. How will the proposed Bridgwater Tidal Barrier be replaced?”
  2. What becomes of Bridgwater after sea level rise exceeds the design basis of the Bridgwater Tidal Barrier?
  3. Why is there no long-term plan or vision for what comes next?
  4. Why is the ratepayer required to provide money to raise river banks money protecting land at Chilton Trinity when the planned sea water flooding will make it of no economic value?
  5. Why is not improving fluvial flow in the river system  part of the scheme?
  6. Why did the cost of the scheme almost double after the location was decided on?

The River Parrett – how it works

Rivers that discharge into the Severn Estuary are very different from most UK rivers. With a tidal range in the estuary of around 11m  MLWN -3. 0 and MLWS -5.7 AOD and the river bed level at around 0.00 AOD.  These levels means that at low tide in the estuary the water level is always below the bed of the River Parrett and the river simply empties into the estuary. In the case of the Parrett its a process that goes as far upstream as Oath and the changing tide sees the river refilling from the sea.

What is seen looking over the Bridgwater’s bridges is just one half of the normal tidal cycle you would see at the coast; at low (ebb) tide its purely fresh water going out and that can be seen on the West Quay tidal gauge where the graph flattens. Equally the point at where incoming seawater (saltwater)  takes over from the  freshwater of the Parrett and becomes  a relatively small component of the rivers water can also  estimated.

At low tides the river empties so completely that during periods of low fluvial flow it can actually dry out so much so that upstream of Combwich there is an old iron age path where people simply walked across the river at low tide.

High tides can be nearly 2m above the surrounding land and at Bristol Road nearly as high as the head of the doors and over the next hundred years that will  continue to grow .

Ecologically it is largely a  dead river. Nature likes, however bad, a consistent environment and the Parrett offers the greatest of changes twice a day from empty to full and freshwater to saltwater. The large amounts of Seven Estuary silt that arrives with the tides exclude sunlight and there is nothing to generate oxygen. Fish(salmon) and Elvers just transit the river on their way to the fresh water of the River Parrett and its tributaries.

It is a true challenge for The EU Water Framework Directive as  there is little negative actions that can be done to it. The lack of water does not deter unrealistic claims that fisheries exist in the river.

The images below show River Parrett tides ( courtesy of  GaugeMap)

West Quay One days Tides
West Quay One weeks Tides
HPC Tide Gauge

The Hinkley Point tide gauge above shows the full cycle of the tide in compoarison to the truncated cycle at West Quay which is caused by the river bed being higher than the sea bed in the Seven Estuary.

West Quay- One Months Tides

Looking along the river it could be shown

River Parrett at Low Tide

2014 Flood

In 2014 the Somerset Levels flooded, normally excess water is stored on the levels and then pumped back into the river  when the weather improves and fluvial flows have reduced. The floods occurred because the flood storage capacity  of the land was exceeded.

The response to the flood and still the current plan is pumps some more fixed ones and temporary platforms for hired pumps such as the permanent platform at Dunball made ready for the next time the levels flood and improvements made to how fast the flood water can be drained off the Levels.

One important question is whether its  possible to increase the  amount of water that flows  down the river without pumps? 

The answer is yes;  the Thames Barrier is used in that way when there is flooding and the Thames and its tributaries are at capacity. The barrier gates are closed at low tide creating a shallow reservoir normally filled by the incoming tide and floodwater coming down the river fills it up. Previous references to the Thames Barrier and the River Parrett were in relation to the type  of gate rather than how the system operates.

How might it work in the River Parrett?

There are two sources of fluvial floes into the Parrett. The Parrett and King SedgemoorDrain (KSD) /Sowy River.  Water unable to get down the River Parrett is diverted around Bridgwater via the KSD/Sowy and re-enters the Parrett at Dunball.

Dunball Clyce (River Parrett side)

Both  sources can discharge when the tide is out  but become tide locked when the sea re-enters the river.  Creating a large temporary empty length of river downstream of Dunball would allow both sources to continue flowing and when full and matched with the tide allowed to go to the sea.

River Parrett – Increasing the Flow

To do this a gate is needed downstream of Dunball and the best location would be Combwich or somewhere in between.  The Bridgwater Tidal Barrier is a gate that could  have been used to do this.

EA  Downstream  Scheme 

The   current plan is to build the Bridgwater Tidal  Barrier upstream of Dunball Clyce and with sea and tide levels continuing to rise  allowing the sea to reclaim the farmland  at Chilton Trinity and Pawlett Hams.  The existing  system returning to an inter tidal saltmarsh/mudflat covering the existing farmland at Pawlett Hams and most of the land to the north of the village of Chilton Trinity. The drawing below shows how the regime will change over the years to come.

River Parrett at Chilton Trinity & Pawlett Hams showing planned next 100 years

The information can be found in

The Parrett Estuary Flood Flood Risk Management Strategy

Full copies of the EA policies can be found at

Steart Peninsula (Stolford to Combwich) 7d34 to 7d37

Parrett Estuary (Combwich to River Brue) 7d38 to 7d42

further information with flood maps can be found in the article on

Chilton Trinity and Pawlett Hams

 

 

 

 

A Tidal Surge Barrier, A Bridge & Dunball Wharf

One of the problems with building the Bridgwater Tidal Barrier downstream of Dunball Wharf is the marine traffic to Dunball Wharf. Sedgemoor District Councils current draft local plan states “The Council will support the continued operation and potential development of Bridgwater Port including Dunball Wharf and Combwich Wharf (Policy D16 applies). It will also support the re-establishment of active commercial wharf’s at suitable locations elsewhere on the River Parrett. In all cases the construction or operation of new wharf’s should not adversely impact upon the Severn Estuary internationally designated site.”

The reality is that  the Dunball Wharf’s with their tidal drying berths, dependency on  high tides and difficult navigation is never going to be a commercial success but the 200m wide river at Combwich might. Construction of  the tidal surge barrier could enable the relocation of Dunball Wharf to a better location.

Much hangs on what is considered to be adverse.

Concepts of what the river structures might look like have not been forthcoming since 2009. Apart from the tower  and turning vertical gate Bridgwater’s gate structure will be considerably shorter than the Hull Barrier structure we have  been repeatedly shown.

What would a barrier with a road bridge  options look like?

Another wharf at Combwich, EDF have  one there already, would not require an opening bridge or clearance much more than Drove Bridge on the NDR. A tidal surge barrier with a 40m fixed navigation span and a road deck might look like

http://https://www.youtube.com/watch?v=sjX20a1a7sg

Keeping Dunball Wharf  would require a one larger gate and an opening bridge. Dunball currently sees 40 or so vessel movements a year so its operation would not affect the bypass traffic.

The engineering is relatively simple and with a purpose built wharf everything could be shipped in during construction.

 

 

The Great Bridgwater Gate Debate

Bridgwater’s has had a long running debate over the type of gate for the proposed tidal barrier. Bridgwater Town Council and the Inland Waterways were led to believe that only a rising sector gate, similar to the Thames Barrier, could deliver the penned solution which they want in order to maintain the water level in the river and that would be delivered. As an an attendee to the September consultation the message of a rising sector was firmly reinforced to the public when a video of a rising sector gate was continuously shown at the now selected site and various supporting images included on the static displays.

The rising sector gate aka Thames Barrier was always a red herring and never going to be built. Only a barrier downstream of Dunball where the Hanson dredgers transit to Dunball might  have supported an argument for it.

Regardless of the merits of Penning the River Parrett the point made by the EA that penning could be incorporated at a later date is well made. To illustrate how that might be achieved can be seen below

Typical proposed Vertical Gate
Modified Barrier with Penning weir added, gates omitted for clarity.

Once the planned barrier has been built it would not be difficult to do  this if it was wanted.

What is more concerning is that after 2  years of presentations  people deeply involved in reviewing  the scheme  still do not understand  it. That they feel they were led to believe  a rising sector gate would be provided is completely understandable. It is also something that despite the issue  being closed in  November 2014 the EA have chose to keep open until now.   References to The Thames Barrier, Boston and Ipswich  were at best inappropriate. The Thames Barrier actually consists of a number of  other vertical gate  such as  Barking Barrier, Dartford Barrier as well as the more famous one.

Does  the gate type actually matter? No.

It is a  lesson to those presenting options to the public and our representatives.