General Discussion

ICE recently hosted a lecture (08/05/14) entitled 'Failures of Seawalls - what do we learn from Dawlish?'

I will try to listen to it online and summarize.  In the meantime you may be able to register yourself to hear it.  The lecture is available to view online.

Attendees had the opportunity to put questions to the panel.  The questions (copy printed below) were not covered during the lecture due to time limits.

1. Paul Canning Principal Consultant, Atkins ( to all panellists): Do the speakers have any view or thoughts on the design requirements, and whether they are realistically workable, for the seawall in the longer term, bearing in the mind the likely predicted sea level rise of at least 0.7m by c2110.

Finch replies: The Shoreline Management Plan and Exe Strategy have proposed a ‘hold the line’ strategy for the frontage for both the short and long-term. We understand Network rail are in general agreement with this. We have not attempted to review this strategy and have identified schemes that could be designed to provide this function. These can be designed to allow for sea level rise and further loss of beach material from the foreshore. Whether these schemes are realistically workable, comes down to a cost benefit assessment. In this assessment it is important that both direct and indirect benefits are considered. For example, very significant numbers have been mention in the press relating to how the Economy of the SW has been affected by the recent events. These benefits are not necessarily identified in a cost benefit analysis that may only look at effects of flood and erosion risk on local infrastructure.

2. Maria di Leo ( to all panellists) : Would you please expand on calculation of downfall pressures?

Allsop replies: There are very few researchers ever to study this problem. Probably the best papers are those given below by: Bruce and co-workers (2001, 2002, 2005) and Wolters et al (2005).

3. Katja Leyendecker ( to all panellists): How has the new Flood and Water Management Act helped to get the right people round the table, to connect agencies and bodies more effectively?

Owens replies: The FWMA 2010 represents a significant step forward in the right direction as the original Pitt Review recommended. Whilst these relationships are continually evolving I have little doubt that there is a very positive move forward but with significant challenges to be faced.

4. Sunday Orimadegun (to all panellists): Please advise on possible solutions to the Somerset levels and how the models for Dawlish seawalls could be applicable in this instance? To what extent have you been advising relevant government agencies on the best possible solutions to the flooding as experienced last winter?

Allsop replies: The processes and responses at the wave-attacked site at Dawlish have no bearing or relevance to the fluvial / tidally dominated flooding in the Somerset Levels.

5. Simon Dart ( to all panellists): Have 'underwater' breakwaters been considered? Such a surf reef like the one built in Bournemouth.

Allsop replies: We do not know what options have been considered by Network Rail and their advisers, although it has been rumoured that a Transport minister suggested 'a breakwater'. Any breakwater, submerged or emergent, must be able to provide protection commensurate with its cost over the full range of water levels of concern. In a macro-tidal situation, that generally requires that the crest of the breakwater is very close to or above an upper tide + surge water level. Such a breakwater, or breakwaters, will therefore be difficult and expensive to construct, requiring marine plant, and suitable weather and tide windows. Segmented or sets of nearshore breakwaters can adversely affect the beaches behind leading to increased erosion unless the beaches are nourished. Some examples without beach replenishment in Italy, Spain and Japan have caused significant erosion.

6. Elizabeth Johnson ( to all panellists): During the extreme weather events were there any unexpected observations?

Owens replies: I believe that the Environment Agency and the Plymouth University Marine Observatory were very surprised by some of the wave heights being measured around the Cornish coast. The analysis of these data could potentially have significant implications for wave prediction and consequently on predictive modelling, structural design and how we approach coastal management.

7. Viviana Russo (question for David Finch specifically): Did they quantify already the increase in the construction costs with the new seawall he suggested in comparison to the standard types?

Finch replies: No, the high level option assessment was focused on providing a review of ‘technically viable’ options. The options will be associated with different costs, but these will need to be considered against the different benefits provided by the various schemes.

8. Andy Jackson, Atkins (to all panellists): You mentioned a few times that the wall profile at Sea Lawn was different (lower than the rest) - was this a contributing factor to the failure, and why?

Allsop replies: We suspect so, but our present calculations do not show conclusive differences just because of the small toe berm. Any effect appears to be masked by the effect of beach level variations.

 

What on earth is a small toe berm?

Think i had that once but the doctor gave me a cream for it..

 

@Cassandra. google "small toe berm" and there are plenty of texts to read and images to see.

So here is my best effort, as promised, at capturing as much of the lecture as possible without too much of the techi bits.  I have put in quotes directly transcribed parts of the lectures, the rest is paraphrased as best as possible.  Skip to the five paraphrased solutions (which I think was the bit that AMM was presented with?) if it is all too much to take in!!

 

Presenters:

William Allsop – Technical Director, HR Wallingford

David Owens – Assistant Head of Environment, Cornwall CC - How local gov. manage

David Finch – Coastal Specialist, HR Wallingford + Dawlish Warren expert.

 

William Allsop

"...and we were asked to go meet Ann Marie Morris in Newton Abbot, who I suspect was a little concerned at all the talk of running a railway route inland and abandoning Dawlish.  So David (Owens) substituted for me which is a very good thing because David has a somewhat more holistic view of developing."

 

Prediction methods and gaps:

Main Failure Modes:

 

"We have inherited infrastructure from our predecessors, we’ve inherited problems from our predecessors, frankly a lot of our seawalls are too far seaward and in an era of climate change we might wish from a hydrodynamic point of view to retreat inwards.

That doesn’t work with a society in general, it doesn’t work with transport / infrastructure owners, it doesn’t work with those who are on the ... so we need to explore how we can hold the line and in some instances still advance the line.

Many of you, who like I work around the world, know there are still a lot of nations that are developing and advancing out into sea.  It’s all possible, does it make sense?"

 

Wave Effects on seawalls include:

Horizontal impact of waves

Scour of fill and beach material

Internal Pressures - very little known about this

Overtopping

Downfall pressures

The latter is one obvious source of concern in predicting it before it happens.

 

"Scour – one of the big problems with seawalls is where is the toe?

It certainly isn’t at the point where the sand is now or last week.

One of the really important points is ... you will never see, you will never survey the sand at its lowest.

There is a mechanism by which the sand will recover...and you can see that what it was doing (the sand level) is going down as the wave comes up but it always recovers afterwards.  So there is a beach rebuilding mechanism that will always fill in your scour trench.  So it’s no good saying the scour hasn’t got down to the toe of the wall.  You don’t know that.  That it didn’t do it during the storm...how far the sand goes down during a storm."

 

"Absolutely classic failure ...scour is the most prevalent cause of failure of seawall.

This is Shingle Beaches and this is a very convenient graph, anyone who hasn’t used it, it tells you where you stay in an attrition zone and outside of it where you erode.

It’s worth looking at the condition of your seawall and running that through, just to see whether you’ve got... there is a nasty flick through failure and once it goes, once the bed drops by a certain amount then nothing apart from strong longshore transport or you putting shingle back is going to fix it.

There is a mechanism to analyse what you will do and give you advice on when to take action and that’s included in ‘Toe Structures Guide.’...available before 2009...and I think there’s a practical version that’s out now."

 

David Owens

"I was actually the duty director in Cornwall during our incident. Obviously the north coast /east coast surge started on the 5th December, the issue in Cornwall actually started on the 21st ... actually went through to the 5th March.  The Gov’t think it finished on the 19th February but it did go on longer....actually now gone to 12 storms in their own definition.  We’ve got issues with the Met. Office as well.... Just to give you the context, Cornwall, we had the heaviest rainfall for 240 years.  That all seemed to fall in December.  It was the stormyist weather for 52years though again the Met. Office might change that.  The Environment Agency have said in the past few days that of all the damage nationally to coastal defences 30% actually happened in Cornwall.  You didn’t see much of it on the TV, because the railway line was broken at Dawlish and maybe it couldn’t get through (laughter).  We actually had 364 property flooding incidents...."

 

Continues with details of Cornish experiences.

 

"...So some brief conclusions..."

• Cornwall is a dynamic coastal environment shaped by the weather and the sea
• Good response and multi-agency working
• Significant cost implications for the Authority
• Settlements remain significantly exposed to flood risk (28,000 properties).
• Is this a blip or a new trend (IPCC Report)?
•  

William Allsop – Technical Director, HR Wallingford

"Let’s remind ourselves of bits of history. First Isambard Kingdom Brunel took a bit of a punt.

He thought that the bastians, sometimes things that stick out normally from the coast are called breakwaters.  They are not, but some people have sort of evolved that as local terminology, but let’s call them bastians or groynes.

So they would gather sand, hmm bit of a belief in magic there, so the wall would never be touched by the sea.

Hmm, yes, ok that is certainly is an overoptimistic view of the amount of beach material available and it certainly doesn’t take any account of....

And in 1845 frankly their ability to predict waves, wave forces, wave breaking was really rather sketchy at best."

 

"However, they could build walls, and they could build railway lines and they started trains running.

And then almost immediately a section collapsed, undermining, so then the classic procedure that all British coastal contractors know and love, and some designers too, called ‘chasing the toe’ in which you build a toe and then it erodes and then you build another one seaward of it in a smaller tidal window and then that undermines, and then you have to chase further down.

The contractor has to buy kit with longer arms and spend more time working in the wet."

 

"1867 – and then this is the tricky one - there was a particular guy who owned Sea Lawn House.  And somehow he managed to petition not to have the high level promenade in front of his house, because persons passing by would look into his house, so somehow he managed to get the promenade dropped down lower to the bottom of the sea wall, just where it may trip the waves up so they break onto the wall.

Sea Lawn has always been a difficult point and you can see that almost immediately the seawall and a hundred yards of line was removed."

 

"There was a collapse further back up at Rockstone, 1872."  

 

1930 - "This is the one that got glossed over, the seawall at Sea Lawn was undermined again and a breach existed in the seawall and the tracks collapsed.

They did at that stage pre-Beeching have other lines, so the line wasn’t cut per see because they could go around the back inland."

 

"I am quoting from New Civil Engineer and one always hesitates when quoting journalists but this is what they wrote.  They said that advice had been given to the Transport Select Committee that over 120 years the sea wall has never collapsed. Not sure what happened in 1930!"

 

"So Sea Lawn always been a bit of a weak point, we know what happened, well do we?

Let’s have a look at the wall. One of the tricky things here is that you have got two stages of berm in front.  This is the lower promenade, but down under here is another part of the structure and that 1 on 15 bedslope may come and go."

 

"I need to congratulate a train designer, because some time ago Bombardier trains built the most wonderful mobile wave overtopping detector.  I am sure that’s what they did build, well I did know they built it.  I am not sure they intended to !

What they did is, they designed a train so that when you immerse it in salt water it trips all the electrics out.

And the train operator then ran it on a line where waves break violently over the seawall, sometimes perhaps more so where the toe or the upper beaches, and therefore increases the amount of violent overtopping onto the train.

And in 2003, 2003 they had quite a lot of these trains stopping."

 

"I think they bought three old diesel locos that they kept parked in some sidings near Exeter and used to motor down and toe the trains out, once they had acted as a mobile detector.

And there is in the records, you could tease out, when it was overtopping strongly at this point from the records of train fails."

 

"At that time some valiant Network Rail man, I think, was given the task of dipping down to the beach.

Now they logged it on an arbitrary scale which gave us huge difficulties.

But eventually we worked out that all he had done, is he had leant over the edge of sea wall, dropped his tape with something heavy on the bottom and read off the reading on the top of the wall, if you know where the wall is you can level it back into a datum that those mere engineers amongst us can use.

The crucial thing is just how much of that beach is po-going up and down.

And what we think is happening is that there is a fillet of upper beach material that zooms up and down the coast....So we do actually have some beach information."

 

Continues as a very technical presentation.

 

David Finch - Possible Long Term options

 

"Looking forward into future schemes..."

Management plan for the frontage:

• Shoreline Management Plan SMP 2, Durlston Head to Rame Head (2011)
• Exe Estuary Flood and Coastal Erosion Risk management strategy (2013)

- Hold the line strategy, for both short and long-term horizon

- Level of defences to be sustained and improved in the short term

- And then sustained in medium to long-term

 

Five Options in summary for Sea Lawn (paraphrased):

1. Sea wall with new concrete curved and higher profile to reflect waves, tight against the existing.  Some reduction in pedestrian access. Small footprint, little loss of beach.
2. New walkway further out to beach creating a two tier wall, each with reflective curves.  Better pedestrian amenity width of walkway but wall is further out to sea.
3. Rock armour revetment sloped against existing wall, has a larger footprint, but leaves existing wall untouched.
4. Increase beach levels to push sea much further out + use of groynes to protect.
5. Continuous offshore breakwater, 6m height, some 50m out to sea.

 

Key Comments:

• Several technical solutions can be designed to sustain and improve
• Need to think long term
• Include adaptability to address uncertainties
• Ensure appropriate stakeholder involvement
• Continue to monitor & make data available for the future

Questions (paraphrased):

Comments:

Immediate solutions by 1st Oct...

The numbers (water volumes landing on the train) are quite frightening, I am glad I wasn’t in a train under it all.

 

Qn.1:

David Owen said you would need a continuous breakwater...

is it there a possibility of an intermittent breakwater?

Ans. Where segmented breakwaters are deployed there are very few places where they work.  A lot of scour can occur.

 

Comment:

Stability of cliff behind - used to cut the unstable cliff behind in the 1930s and put it on the beach.

There was your supply of material.

 

Qn.2:

New flood and water management act, has it helped get the right people around the table?

Ans. Yes it has improved the situation.

 

Comments by Network Rail:

Have kicked off a study – Exeter-Teignmouth to examine sustainability

Railway vs amenities.

Wall as we have it, it is still standing today, whether it stands tomorrow is another matter.  (Saturday is it).

Climate change – sea levels will rise but also beach levels may lower

 

Comment:

Newquay Fistral Beach we lost 6metres.

We brought in JCB to replace 2metres of it.

Penzance to Marizion is an absolute....

 

Comment:

In addition to the failure list presented, 60 years ago we also lost the downline for a chain.

 

Qn.3:

Who do you think should be looking after the storm tourists?

 

Ans. At Portreath

We had 450 people watching.

Police got a lot of abuse.

It’s a real issue now.

The power of social media is great but...

When there is a storm they know where it is going to be the best location.

Parked cars stop emergency services getting through.

People had watched the storm and left their cars and not only will it stop a train but a BMW alarm.

 

Qn.4:

Is a considerable number of groynes being considered as a solution?

 

Qn.5:

Section that was dropped down at Sea Lawn.

Were the waves creating higher pressures?

 

Ans. Hard to prove by formulae but if you push a berm out at the toe you may get adverse conditions. Complicated, it depends on water levels

 

Qn.6 (Italy online) complex – very pleasing to get questions from that far.

 

Qn.7 (India online) – any opportunities for solutions that include energy generation?

Ans. Water column caissons - Not likely economically, but this sort of thing is under consideration generally.

 

Qn.8:

Have heard a lot about volumes of water landing on the line.  How was run off allowed for?

 

Ans. There is a metre depth of permeable ballast to soak this up and 450mm diam. back to front drainage and some ‘scuppers’ to discharge at 20m intervals.

 

Chair summing up:

Impulsive loading is an issue.

1 in 52 year event – a big one

Difficulties of not being allowed to get funding for betterment

Improvements by 1st Oct aimed for.

Is this a blip or a trend?

The problem of ‘chasing the toe’

My Father – They are not going to shut the railway - I am going to buy my house at the top.

1930’s  - Urban district council opposed the railway behind the town because it was going to ‘disturb the town’.

Good luck on how we put it all back by the first of October !

Interesting presentation on what the future solutions may be.

The solution for this issue is:

Simplicity,

Agility

And more speed.

 

@Clive - I have a copy of a letter dated 1st april sent to our MP from Network Rail. In the letter NR outline the works and investigations they will be doing with regard to improving the resilience of the railway between Exeter St.Davids and Newton Abbot.  

NR say the study will take about 18 months to complete.

18 months - long time.

Do I take it you were, or are, a civil engineer? From what I assume then is your knowledge of engineering can you say if you think 18 months a reasonable period? Could it be shorter?  

I can quote to you more details from NR's letter if that would be of help. Let me know if it would.  

@Lynne - i would be very interested to see a copy thanks - presumably it will have some overlap with the lecture.  1st october was mentioned in the lecture in the context of immediate improvements (presumably 'sea lawn path raising' in particular).

Would guess that the 18months is for long term major solution decision making, although I seem to recall that there was a pledge when the 'wall was down' that options would be presented this autumn?  i.e. NR already had ideas half formed anyway.  Or maybe was it a timescale of options on table this autumn, but decisions autumn 2015 (i.e. 'conveniently' post-election  ). 

Reasonable to wait 18months? - Quite possibly, if it is to decide on a major undertaking that involves an awful lot of money and stakeholder consultation.

Okay this is the relevant bit from the letter.

"With respect to the future improvement in resilience of the railway between Exeter St Davids and Newton Abbot my Route Assessment Team is currently commisioning a major Engineering Feasibility Study. This study has been initiated by the Route to assess a series of options to increase the Geo-Environmental Resilience between Exeter St Davids and Newton Abbot. A range of potential engineering solutions are available including raising the wall height, contruction of toe revetments and the installation of an offshore breakwater. The geomorphology of the coastline along which the railway line between Exeter and Newton Abbot runs results in the Project being divided into four discrete but linked study elements namely:

Exeter  St Davids - Dawlish Warren. Objective - resilience to manage estuarine and river flooding.

Dawlish Warren - Kennaway Tunnel. Objective - resilience interventions to manage coastal erosion (primary) and cliff stability (secondary).

Kennaway Tunnel - Teignmouth. Objective - resilience interventions to manage sea cliff stability and coastal erosion.

Teignmouth - Newton Abbot. Objective - resilience interventions to manage estuarine and river flooding.

 

Each of the above elements will generate differing, but linked infrastructure resilience options that will require assessment, review and cost estimation against the benefits arising. At present Network Rail has no formal cost estimates of the Works required.

It is estimated that the Study will require around 18 months to complete. Following this, Network Rail will identify the preferred engineering solution within each Study Element and then work to secure funding to deliver the Works required. It is probable that funding to deliver the whole range of Resilience Works required will be sought over several Regulatory Control Periods and that phasing of the Works will be concentrated on the areas of highest risk."  

   

The letter then goes on to say this with regard to alternative/additional inland routes;

"We have also launched a wider study looking at the impact of the resilience issues of the route between Exeter and Plymouth. This study will provide an early indication of the viability of additional/alternative route alignments (including Tavistock) and this, combined with inputs from the study referenced above and analysis of the potential socio-economic impacts on the South West of rail service resilience, will help generate an options report due to be published late June 2014. We have set up a Study Management Group to control this work and source the necessary data, with representatives from Local Authorities, Local Enterprise Partnerships, Train Operating Companies and the Department of Transport all invited to participate. The remit for this piece of work is published on our website at:

www.networkrail.co.uk/Long-Term-Planning-Process"

 

Oh that's good, NR still 'striking while the iron is still hot' to get it's South West route options tabled.  It seems as though there are three strands of work in progress.

- NR Optioneering in the broadest sense (June 2014)

- Immediate make do and mend on the Seawall (1st Oct 2014)

- Detailed coastal/hydrology/cost study for 18months by HR Wallingford focussing solely on improving the long term (climate change etc) hard defences of the seawall and coming up with a solution.  This may be the one and only investment that actually moves forward from NRs June 2014 options, which previously 'reportedly' included major hard defences as an option to be listed, (not necessarily mutually exclusive though).  This is of course the study that provides Dawlish with the solution to save it (railway, cliffs, etc) from the waves and seems almost certain to happen in one form or another - depending on cost/benefit/funding etc.  Another way of looking at it, with an engineering hat on, is that the time is neigh to convert the seawall from a 'glorified Victorian garden wall' into a 21st century mass concrete structure fit for purpose. 

LTPP - Excellent cross-reference Lynne.  Reading between the lines of it, it seems to me that the real win that NR would like materialised from the point of view of their long term responsibilities is to be found in Scenario 2 para.(iii). i.e. ‘...improved access and availability for regular engineering works’.  Put another way, given the notorious level of difficulty maintaining the seawall (reportedly the most expensive stretch of line in the whole country to maintain, plus add on the ‘Dartmoor slopes’) I personally think that NR would dearly, dearly love to be freed up a bit from all the severe possession time constraints and lack of access issues that the current topology restrictions of the railway imposes on them.

 

Scenario 2 - ‘Alternative Route with Coastal Route Retained’.  This is clearly the (money forthcoming) win-win outcome with no losers, except some unfortunate landowners.   From a Dawlish point of view and treating it as a strategic game of chess, then you folk should absolutely be backing reopening the OKE-TAVY line. This in no way threatens the long term commitment (nor does it risk any ‘back tracking’ on commitments, pardon the pun) to maintaining the coastal route, as NA, Torbay and Totnes services would still be more than sufficient raison d’etre to justify its retention.  Also, as above NRail would have much improved engineering track access to maintain the coastal route (and south Dartmoor) in an orderly fashion.

 

As a highly speculative and long term view point, for Dawlish, perhaps the option to fear, is a fast DALine round the back of the town.  Why?  One day this could potentially weaken the resolve to maintain the coastal line.  Playing the numbers game of those being served by rail, Dawlish and Teignmouth would be left standing alone as the only rail losers.  You may however, be placated by a new station on the DAL.  No Mrs C, this isn’t evidence that I am plotting the demise of the coastal route, au contraire, I am offering a view on how to play the chess game of life to guarantee its long-term future .

And if a DAL were to be an option then that particular option might need to be thought about seriously now as there is an awful lot of house building that is due to take place in Dawlish and the route of the DAL and the location of that new house building could be ever so, ever so, close. Wouldn't want 'em building new houses only for those houses to have to be demolished in order for a DAL route would we?

or imagine somebody buying a house in a quiet part of the Country to find that a railway line arrives at the top of their garden

@Lynne & roberta - absolutely don't want uncertainty blighting new housing.

I think we can be fairly sure that a west of DAL will be 'an option' in the report.  Luckily we only have a few weeks to wait now until finding out how it's cost/benefit is ranked by NR as an option.  I will reserve further comment until then!