Flooding in the era of global warming

Gloucester c.1725. Source

In some ways we started building our towns and cities in the right places: at crossing points on major rivers; at the confluence of smaller rivers; and at the head of river estuaries. They were places excellent for communication, trade, reliable supplies of water, and surrounded by fertile, flat land. But they were also prone to flooding. However, being ingenious, we humans diverted rivers, built flood walls and generally managed the flow. For thousands of years we put up with the inevitable ‘once in a lifetime’ floods and just made the best of where we lived; for the benefits far outweighed the problems. Then in the last few years it seems something has changed, for we’re now confronted by new phenomena: the repeated and record-breaking extreme flood event.

The immense rainfall in the UK over Christmas 2015 — first in Cumbria and then in Lancashire and Yorkshire — has been an eye-opener and, hopefully, a game-changer. For the people living through it as I write, it’s upsetting and costly; and very worrying because there’s every chance it could occur again—as it already has at places like Cockermouth and York. It will also be worrying for people in Gloucester, Swindon, Surrey and Hull, for they know it was just lucky that Desmond or Eva didn’t dump on them, as storms did in the recent past. So it’s vital now to take stock and look carefully at what’s going on so that hopefully we can start to make the changes necessary to reduce the severity of future re-occurrence.

Hamm Court and Hamhaugh, Weybridge, Surrey : January 2014

First we must accept the fact that the climate has changed, not just in the UK but across the planet. The world’s scientists are unequivocal in reporting a 1°C rise in global temperatures since pre-industrial times. Of course this doesn’t mean that the temperature has risen everywhere, at every instant, by 1°C more than it would have been otherwise. It’s a global average, trending upwards. That means, from day to day, in some places it’s less, while in others it’s more. And, remembering that every additional 1°C that the atmosphere warms allows the take-up of 7% more moisture, it means that from now on extreme weather events will take place in the climate of a warmer world.

But there’s another contributing factor which makes for more extreme weather at this time. Every few years off the west coast of South America there’s an up-welling of warm ocean water that the locals call ‘El Niño’. And it’s of such a scale that while it’s happening it can increase the surface temperature of the entire planet.

Graph from NASA

El Niño last occurred in 1997 and is clearly seen at that date as an upwards spike in the global temperature charts. Today it’s occurring at this very moment, which is why, currently, so many temperature records are being broken.

Of course each time El Niño is over, the world drops back to the steadily rising temperature trend it was following before the up-welling of ocean heat occurred. So it’s a temporary phenomenon. But while El Niño is here it gives us a taste of what a warmer world could be like in a few years’ time.

El Niño 1997 and 2015 compared. Source

Given that the world is increasingly warmer, putting more energy in the system and thus making weather more extremeand allowing more moisture in the atmosphere to fall as rain — unprecedented flooding events are phenomena that a wet island like the UK should expect. Bearing that in mind let’s look next at the basic mechanics of river flooding.

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In river valleys — which are exactly the locations for the events we’ve seen in Cumbria, Lancashire, Yorkshire and many other Northern areas — flooding occurs wherever water arrives from up-stream faster than it can escape downstream. It’s the result of a constriction. Where there was once a wide, shallow floodplain spreading right across the valley, with the water making its way slowly downstream, there’s now a town or city. Looking at these centres of habitation today, invariably the river running through them has been contained within what amounts to a high-sided conduit crossed by numerous bridges. And there are shops, houses, municipal buildings, theatres and factories built right up to the watercourse—some even over them.

The Waterside Pub over the Irwell in Summerseat near Manchester: demolished by floods in 2015

As I mentioned at the start, these concrete and stone-lined channels, and the bridges crossing them, have been designed over the centuries to cope with water volumes which are the maximum within the living memory of those who commissioned and built them, perhaps plus a few per cent if the designer was prudent. Occasionally after heavy rain when the containing walls have proved to be inadequate, they might have been rebuilt or strengthened—often by just raising the height of the parapet. Almost invariably the needs of the surrounding buildings usurp those of the water flow.

A small section of Google’s satellite image of the River Aire as it passes through Leeds city centre.

In modern times the Environment Agency has been tasked with installing flood defences to protect those towns and cities most prone to flooding, at significant cost. Flood defences are usually installed upstream of habitation to stop water inundating suburbs and to reduce peak flow through the restriction formed by densely-built town centres. Defences consist typically of high earth banks to contain water on large areas of farmland that can be temporarily sacrificed to the flood; with strategically positioned sluice gates to control the flow of water to be released down stream.

Morpeth flood defence scheme. BBC. Water is held back behind the large bank to the right and left and allowed to leave at a controlled rate through the sluice gates in the centre

In town centres the constriction point is made able to take additional water flow by the erection of strong retaining walls to increase the height of the cross-sectional area of the channel.

Raised and strengthened town centre flood wall in Keswick, Cumbria. It was over topped on the night of 5th Dec 2015 Source

Of course, all this effort results in a vicious circle: the more floodwaters are restrained to prevent them from spreading across the entire floodplain then, by definition, the greater that depth of water will be. So planning flood defences is a delicate balance of constraints. And it all comes back to the fundamental starting point: “what is the level of flood we can afford to contain”. This has been usually gauged by the frequency the flood is expected to occur—once every 100 years; 200 years; 500 years; or whatever seems politically expedient. Underestimate the increase in flow due climate change, so that the defences cannot cope, and you might as well not have bothered. And unfortunately, perhaps because governments have been slow on the up-take or even in denial about climatic change, flood defences are currently proving woefully inadequate. The current floods have hit some places three times in less than a month. How can anyone live somewhere they’re likely to see repeatedly disappear under water?

Source BBC Website

I started this section with the statement that for flooding to occur, water must be arriving from up-stream faster than it can escape downstream. Recognising this very basic physical reality produces only two long term solutions. Either the constriction must be removed; or the rate of water arriving at the constriction must be reduced.

We’ll start by dealing with constrictions. In 1952, the village of Lynmouth on the North Devon coast was inundated when water poured off the hills during a freak storm. Thirty four people died, with a further four hundred and twenty made homeless. Bearing in mind my previous comments, pictures of before, during and after will demonstrate both the problem and the solution.

Lynmouth before the flood: Source
Lynmouth following the flood. Source
Lynmouth today. No longer any constriction for the flow of water. Source

Of course once the water has passed through Lynmouth it immediately joins the sea, so this fix has solved Lynmouth’s problem for good. However, it’s worrying to note that while deployed immediately at Lynmouth after the events of 1952, this solution wasn’t applied to any other villages in similar danger of flooding. Unsurprisingly almost identical events led to the destruction of Boscastle in 2004. So much for the lessons of history (and ‘freak’ storms).

I mention that Lynmouth is — as its name suggests — on the coast, because removing a constriction results in the flow of flood water increasing for any further constrictions that lie downstream of the first. Clearly that’s not a problem for a coastal town where water discharges straight into the sea. So improving the water-handling capability of river courses in towns and cities needs to start at the coast and be applied, in strict order, to every town as we travel up stream. Linear waterside parks through the middle of towns following the course of the river, complete with cycle ways, resting places, wild flower meadows and waterside seating should be considered. We can do such major engineering projects to improve the routing of cars, why not for flood water?

A basic linear park constructed to enlarge a river course to cope with flooding. Seoul. Source

Here is such a proposal I found for Lewisham. And another for Bath.

The consequential nature of flooding defences constructed all down the course of a river is a vital consideration for, in flood conditions, an improvement scheme for one town is almost certain to radically disrupt the expected flow in the next town downstream. The costs for a whole-river solution, as described, will be potentially enormous. The current total cost of the British Government’s flood defence spending has been just £1.7bn annually, with the current government’s proposed spending dropping to just £2.3bn over the next six years. And yet it’s estimated these floods alone will cost £6bn in damage just until December 27th—and they’re not even over yet. Changes things, does it not? Clearly engineering solutions to reduce such constrictions will take time to implement, but they should now become a priority whenever any new development is planned in a city or town that’s prone to increased flooding.

Thanks to climate change, costs of flooding are bound to rise in future, though it’s difficult to make reliable estimates, such are the uncertainties. Rather like the man falling off the cliff, he can’t predict the injuries he will sustain, but he knows for sure it will bloody-well hurt. Watercourse widening is sure to pay for itself one way or another in the long-term.

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There’s another cause of river flooding which I’ve not yet mentioned: the type which we saw in winter of 2013/14 on the Somerset levels. It’s different to that described so far, yet still follows the rule that water must be arriving from up-stream faster than it can escape downstream. The Somerset Levels are unusual in that they’re around the height of sea level, so they rely on the low tide to allow floodwater to escape. How can we remove the constriction if the high tide is preventing flow downstream? There is a solution which will require high expenditure but which, for once, has the advantage of payback. I propose the building of a tidal lagoon off the Somerset coast encompassing the mouths of the main rivers and drains for the Levels. This lagoon would be constructed like the proposed Swansea barrage, complete with hydro-electric turbines and sluice gates. At times of flooding in the Somerset Levels, sluices could be closed at low tide and then this lagoon could be used to hold back the high tide and create an immense ‘sink’ into which the floodwater could escape, thus maintaining river flow round the clock. Such a lagoon has already been suggested elsewhere in Somerset, but the difference would be to make this barrage provide a way to eliminate flooding.

Clearly with sea level rising at 3mm a year and increasing, coastal flooding is an inevitability in time, so this is only a temporary solution. Managed retreat might be the only realistic outcome in the long-term.

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Fields of winter wheat growing in sodden ground high in the catchment of the River Torridge, Devon. Pic: John Russell

As I keep repeating; flooding occurs in any place where water arrives from up-stream faster than it can escape downstream. I’ve dealt with the ‘escaping downstream’ side, so now I need to examine the ‘up stream’.

For several years now it’s become increasingly argued that the land use in the higher reaches of the water catchment area can dictate the speed at which storm water is shed and thus the rate of flow in the rivers below. The volume of run-off that is sometimes produced by bare moorland has been astonishingly-well described during the December 2015 floods in a You Tube video shot by someone in the Greater Manchester area calling themselves ‘MrDazp1’. Please do watch it: it’s jaw-dropping to see hills behaving like saturated sponges. Undoubtedly many bad practices for managing upland have been introduced over the centuries, and they are starting to be publicised, most notably by George Monbiot. Pulling no punches, his latest article outlines the side effects of grouse moor management.

The idea of slowing water down high up in the catchment area usually leads to a call for planting woodland. Trees, by their very nature, tend to produce an uneven and absorbent soil that slows run-off. In fact, according research quoted by George Monbiot, “full reforestation would reduce the peaks [in flow downstream] by about 50%”.

During research for this article I came across another example which adds to the evidence. Titled ‘Eddleston bends save the Cuddy’, this article explains how it’s believed a flow-restriction and tree planting scheme that was completed only two years ago has managed to reduce flooding over the last week. It’s still too early for conclusive results as data gathered has yet to be written up.

It seems some MPs might already be realising that tree planting can make a difference, for according to ‘Farming UK’, Anne-Marie Trevelyan, Conservative MP for Berwick-upon-Tweed, is suggesting existing plans for the planting of 11 million trees in England by 2020 should be increased almost 20-fold to 200 million trees.

Owning, myself, woodlands in the catchment of both the Torridge and the Tamar in West Devon and having seen the flow rates out those woodlands into the local streams in times of flood, compared to flow rates off neighbouring pasture land, I’m sure the 50% reduction mentioned by George Monbiot is likely to be no exaggeration. However, I’m also certain there are more ideas that can be relevant.

Water flowing out of woodland after prolonged rain in the upper reaches of the Torridge. Picture: John Russell

One of the features of the British upland landscape is the many reservoirs built for the purpose of holding water supplies for use in the crowded industrial conurbations down the valleys. Frequently they’ve also been constructed as compensation reservoirs to maintain river flows in times of low rainfall. These reservoirs, particularly in winter — thanks to our frequent and heavy rain storms — stand brim full of water.

Agden Reservoir, Sheffield. One of numerous Victorian dams built to provide drinking and compensation water. Source

So when rain falls in the uplands and flows into these already-full reservoirs, they can do little in the way of retaining additional water. Surely with the lead time that can today be provided by the Met Office, these — and perhaps even new reservoirs like them — could be partially emptied over several days in advance of unusually heavy rain, and then used as buffer storage to reduce flow? This is not a new concept and indeed in some countries dams are constructed specifically for the purpose of holding back flood water in the higher water catchment area. On a smaller scale, features are also now being constructed as a requirement of new infrastructure development schemes, where they are called retention basins. Perhaps there are opportunities to create more in areas where the density of habitation downstream makes it viable?

I’ve really only scratched the surface of the ways water can be held back. I’m no expert on hydrology, but in researching this article I came across a list entitled ‘Killer Facts’ by Alastair Driver of the Environment Agency. This compilation demonstrates the utility and value of taking an Ecosystem Approach to slowing water flow. As well as quantifying improvements from growing trees, the list of examples includes data on the benefits of blocking drainage ditches; restoring peat catchments and installing ‘woody debris structures’ in headstreams (bring back the beaver?). It makes for impressive reading and I’m sure all these ideas, appropriately combined, have their part to play in dealing with the problem of accelerated run-off.

* * *

Unfortunately there are as yet no firm estimates of the costs we might expect as a result of future flood risk in the UK. Significant cuts in greenhouse gas emissions, notwithstanding the Paris agreement, seem a long way in the future. If the worst scenarios suggested by the IPCC come to pass — considering what we’ve already experienced — the scale of future flooding in the UK could add up to something as yet unimaginable by the time the world passes 2°C.

Planning for this future is not something that can be put on the back burner: urgent action is needed now. However, the decisions to be taken need to driven by science, taking a whole-river approach to water management, cutting through all barriers of red tape between the owners of land, local authorities and government agencies. The solutions should take every short-term opportunity to hold back water in catchment areas, while at the same time making longer-term plans to ensure water can flow without restriction through the centres of our towns and cities. This need for adaptation is a problem with no magic bullet but instead with a number of relatively straightforward, some relatively cheap, some undoubtedly costly, solutions — all needed. What’s now required is the political will to identify them all and start the ball rolling. Otherwise, when another El Niño comes along, we’ll be surprised all over again — but next time it will be surely much worse.

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