Modern Roads Flooding

[Barkstar]

Not long after the M60 was completed the then new section closed due to flooding. At the time many questions were asked as to how this had happened and we got the usual obsequious hand wringing and excuses about the unusual amount of rain blah blah blah. You could argue that back in 2000 the regularity of very heavy rainfall wasn't what it is now but even so it seemed something had failed in its design or execution when a brand new piece of motorway closes because the drains are inadequate. So lessons learned? Seems not.
This weekend the media was full of half a month rainfall in 24 hours. Which is a lot but to my untrained eye it was fairly normal rainfall, it just didn't letup. But here we are again. The 10 year old Alderley Edge bypass closed and the A555 closed for the second time since it opened this time last year - both under very considerable depths of water. Surely we can do better than this? It really smacks of spoiling the ship for a h'porth of tar. Especially as we now seem to get 'once a century' downpours at least every other year and the general opinion is it will only get more frequent.

[Jeni]

Put too much money into catering for these events and the climate change deniers will moan

[Berk]

Nevertheless... does drainage need to be upped by a power of 10?? And how expensive (not to mention disruptive) will it be to retro-fit it??

[DavidBrown]

What strikes me is that all these modern roads that are massively failing are all in a relatively small area of Greater Manchester. I can't say I've noticed similar problems on new roads in the rest of the country - certainly not on the scale or frequency of what we're seeing up there. I do wonder if there's any key features of the drainage that they're doing differently that simply isn't working?

[Alderpoint]

When they put the Toll Bar underpass in on south-east Coventry I commented that I hoped they'd put in plenty of drainage as it created a massive dip in the road and would be bound to flood. However, having for the last year passed through it most days in both directions, it never has. So they must have got the drainage right.

[WHBM]

Having been involved with such events over years, things like parsimony with maintenance, leading to not rodding the perfectly adequate drainage, or having a useless contractor who does not do what they are paid to do, are far more likely than the drainage being overwhelmed.

There's also the basic design, where adequate Balancing Ponds etc that were typically provided a generation ago, get omitted or downsized to save budget in construction.

The A40 Hanger Lane Underpass in West London used to regularly flood, after construction in the 1960s, because of poor design putting it down under the water table, then providing an electric pump which was located in a non-waterproofed location and itself used to be flooded and ruined by rising water just when it was needed. It took decades before it was sorted out properly.

[RichardA35]

Having been involved with such events over years, things like parsimony with maintenance, leading to not rodding the perfectly adequate drainage, or having a useless contractor who does not do what they are paid to do, are far more likely than the drainage being overwhelmed.

Most maintenance activities other than simple cyclical activities are planned and funded by the asset owner - for instance on the section of dual carriageway I am working on, the drainage appears to have not been surveyed or cleaned out since it was installed over 30 years ago.

There's also the basic design, where adequate Balancing Ponds etc that were typically provided a generation ago, get omitted or downsized to save budget in construction.

This doesn't happen on any new infrastructure that needs any sort of permit from the regulators such as LLFA or EA. IME the standards are not compromised and have taken account of increased rainfall and a climate change allowance.

The A40 Hanger Lane Underpass in West London used to regularly flood, after construction in the 1960s, because of poor design putting it down under the water table, then providing an electric pump which was located in a non-waterproofed location and itself used to be flooded and ruined by rising water just when it was needed. It took decades before it was sorted out properly.

Poor advice from their design and maintenance consultants who were likely in-house council employees in those good old days before they were privatised, out-sourced or otherwise motivated to perform properly.

[Conekicker]

Snip...

Poor advice from their design and maintenance consultants who were likely in-house council employees in those good old days before they were privatised, out-sourced or otherwise motivated to perform properly.

You think modern private sector consultants always know what they are doing? Really? No offence but what planet are you living on?

[WHBM]

Poor advice from their design and maintenance consultants who were likely in-house council employees in those good old days before they were privatised, out-sourced or otherwise motivated to perform properly.

I think it was a Trunk Road when built, so a Ministry of Transport design.

The railway bridge HERE https://www.google.com/maps/@51.523832, ... 384!8i8192 (Burnham, Bucks) regularly floods (note elevated footpath inserted), elderly locals said it has done it all their life, so presumably even Brunel didn't always get it right !

[Bryn666]

You don't need to be chartered to know if you build on a low point it will pond.

The classic example is virtually every 6mm kerb upstand at pedestrian crossings. Who actually has ever found one that does not collect a giant lake?

[KeithW]

Nevertheless... does drainage need to be upped by a power of 10?? And how expensive (not to mention disruptive) will it be to retro-fit it??

No for the simple reason that we do not get 10 times as much rain. It is a fallacy to suggest roads did not flood in the past as a Google Search for 'flooded roads 1970s' will demonstrate. What happens now is that news spreads farther and faster. The amount you spend on flood prevention is a matter of judgement and is influenced by several factors including money. Ultimately if the entire district is flooded the only way is to put the road on a flyover and even then you probably could not get on or off it. Just after the A1(M) from Alconbury to Peterborough opened it was closed by flooding but then so was the A15 and a good part of the surrounding area including Huntingdon racecourse. The Huntingdon Viaduct section of the A604 (now A14) was of course high and dry but that did not make the road usable as the road near the racecourse was under water. I knew there was going to be a problem that year as looking across to Potton from my bedroom I could see flooded fields and this was on high ground near Sandy Heath ! I did not see that happen again in 20 years of living there. I did get back to the NE that year by taking the B roads to the A1 then the A604 to Kettering and the A6 to the M1 at Leicester.

The reality is that sometimes roads have to built across a flood plain and if a 100 year flood occurs protecting the road is pretty low on the priority list. Now in the case of the A1(M) some work was done to improve drainage but the odds are that a similar weather pattern would also produce flooding. If you build on a flood plain you have to expect floods. I note the new A14(M) crosses the Great Ouse flood plain at a higher level than the old road as will the new A428. The problem now in that area is not the roads but the new houses being built on the flood plain here.
https://www.google.co.uk/maps/@52.21278 ... 6656?hl=en

Historical flood examples
http://datchethistory.org.uk/general-ar ... ents-time/
https://www.blackburnfirehistory.org.uk ... -blackburn
https://www.walesonline.co.uk/news/loca ... hs-6712149
https://www.rushdenheritage.co.uk/history/floods.html

When floods occur without extreme weather being involved the reason is often neglected maintenance. This was which made the 2014 floods on the Somerset levels so bad. The environment agency had badly mismanaged matters by discontinuing the dredging needed to keep the rivers flowing and had neglected the sluices with some actually being locked in place.

Poorly designed roads can often make things worse, when the A165 south of Scarborough was improved it was raised to be above the surrounding area so as to reduce closures due to flooding. It worked in that regard but unfortunately it acted as a dam causing damaging flooding in residential areas where it had previously not flooded. Culverts to carry away winter rain fixed that one.

[rhyds]

When floods occur without extreme weather being involved the reason is often neglected maintenance. This was which made the 2014 floods on the Somerset levels so bad. The environment agency had badly mismanaged matters by discontinuing the dredging needed to keep the rivers flowing and had neglected the sluices with some actually being locked in place.

Also, the EA had replaced some old Diesel powered pumps with grid electric powered ones, problem was the flood knocked out the power supply to the pumps...

[Chris5156]

Poor advice from their design and maintenance consultants who were likely in-house council employees in those good old days before they were privatised, out-sourced or otherwise motivated to perform properly.

I think it was a Trunk Road when built, so a Ministry of Transport design.

The Hanger Lane Underpass was one of the earliest trunk road schemes to be handed out for design work to a firm of consulting engineers. They were also entrusted with drawing up the contract documents, bills of quantities, etc. and got the whole business spectacularly wrong.

When the underpass was finished, the completion of the job was followed by several years of litigation which went all the way to the House of Lords, so badly written were the contract terms and so poorly defined were the scheme designs and construction methods.

The whole thing was an acute embarrassment to the MOT who learned a lot of lessons about what we would now call outsourcing, deskilling and oversight.

I didn’t know it had such elementary problems with its pumping system, but it’s no surprise to me that it did!

[lefthandedspanner]

When floods occur without extreme weather being involved the reason is often neglected maintenance. This was which made the 2014 floods on the Somerset levels so bad. The environment agency had badly mismanaged matters by discontinuing the dredging needed to keep the rivers flowing and had neglected the sluices with some actually being locked in place.

Similarly, a significant part the 2007 floods in Wakefield were caused by partially blocked drains which had not been kept clear, and where overwhelmed with the volume of rain, so they backed up onto the street - the area where the worst flooding occurred (Agbrigg) was nowhere near the river, or any other large body of water.

[rhyds]

Locally there was extensive flooding of the then new Glasdir estate in Rhuthun. The site had been built on a flood plan, and the planning dept had mandated various mitigation measures be designed in. Unfortunately no-one had given thought to clearing out the culverts under the road that ran near the site, so the whole site flooded badly. Oddly, the council depot nearby had very clear culverts and didn't suffer much at all...

[WHBM]

I think the last time the Hanger Lane underpass was flooded was in the late 1980s**. It flooded twice then. First time the pump was ruined (again, apparently), and the second time was before they had mobilised to get a proper replacement. The second time it was right up to the roof, and ruined all the M&E fittings as well (lighting and fire protection - I don't think it has ventilation). It was closed for a good while afterwards.

** : I was mistaken. It flooded again in August 2004.

http://news.bbc.co.uk/1/hi/england/london/3533104.stm

[KeithW]

Also, the EA had replaced some old Diesel powered pumps with grid electric powered ones, problem was the flood knocked out the power supply to the pumps...

I was in Weston Zoyland at the old pumping station earlier this month. The old steam pumps had been built well above any flood level on a raised platform so never flooded. The new electric pump was installed below the top of the embankment, guess what happened in the floods. The new electric pump is on the left.


The old pumping station stayed dry and the EA ended up installing emergency diesel pumps brought in from the Netherlands on the old embankment area.

[AndyB]

The classic was the M1 underpass at Broadway, which had two once-in-100-year events in five years - 16 August 2008 - see also nirs's site - and 1 August 2013

[Barkstar]

There are plenty of established roads that will inevitably have flooding issues and where opportunities to do remedial works are limited. There we are relying on regular maintenance to make the best of things. And after a decade of cuts in spending what hope have we of that. I wonder that not unlike the cash for repairs to pavements often going to payout people who injured themselves on un-repaired pavements the cost of sorting the clear up of major floods comes out of a budget that would otherwise be spent on trying to alleviating these trouble spots.
My concern is that with all the computer modelling and design we have had in these last 20 years, improvements in materials and the like new stretches of road are still substandard. We know weather patterns are changing and I can remember that the sort of deluges we get quite regularly now were things of wonder decades ago. Perhaps situations like the Boscastle flood are once a century events but last weekend was only unusual in that the rain went on and on. It will never be practical to stop all flooding on our major roads but when we build new ones lets make them fit for purpose.

[RichardA35]

The classic was the M1 underpass at Broadway, which had two once-in-100-year events in five years - 16 August 2008 - see also nirs's site - and 1 August 2013

It may be over pedantic for a hobby site and teaching people to suck eggs but to be clear it's a probability based forecasting approach. A "one in a hundred year" event doesn't relate to a historical event that has only occurred once in the last hundred years, but to the probability of the event occurring annually.
AIUI given the base data in the estimation handbooks has been revised to account for certain factors, the probability going forward is different to that looking back.