Reflections on the first weeks

(Written 29 April)

It seems like months since we set out from Saul Junction, but it's only been two weeks. Since then we've cruised around 80 miles, gone through 54 locks, emptied the toilet tank twice and stopped at a few pubs along the way.

Waiting for other shipping in Sharpness docks - thought
we'd better give way to this one

The highlight for me was the leg from Sharpness to Bristol. We were never planning to do this - Gloucester lock has been out of action for several months, meaning we couldn't get out onto the Severn to head north on our journey. We had options: put our trip on hold, hire a crane and lorry to get us to Evesham, or head south into the Severn estuary to Bristol. We chose the estuary, and I'm very glad we did. It took a lot of organising to book pilots (all done by Janey, I only fitted some navigation lights), and we were both slightly nervous about it, but turned out to be a wonderful experience. Our pilots Tim and Steve were reassuringly expert, the weather and tides were in our favour, and after a 6 o'clock start we found ourselves happily cruising down the Severn on the ebb tide. Seeing the motorway bridges from below was a surreal experience, as was the feeling of being very much out to sea on a boat designed for 3 foot deep canals.

Under the second Severn crossing

And under the Clifton suspension bridge

Caen Hill flight was another high point (literally) - 21 locks in less than two miles, going up through a marvel of 19th century engineering. We aimed for an early start, and were the first boat through when the gates were unlocked by the lock keeper at 8:00. All the locks were set for us, we had Scott our willing friend to help, plus excellent Canal and Rivers Trust volunteers were on hand to guide us through, open a few sluices and generally chat about the canal. Hard work - but a fun day.

Waiting to start Caen Hill flight

Janey expertly steers Manzanilla into a lock

There have been some more testing times. Cruising up the Avon in the rain was not fun - it was that peculiarly wetting English rain than seeps into every crevice. We’ve also had some technical problem with the boat – we lost all domestic power at one point due to a faulty isolator switch, which I managed to bodge and then replace with a new one from the well stocked Hilperton Marina chandlery. But it looks like we'll need to buy a new central heating unit - this died for some unknown reason, then I hammered a further nail into its coffin by breaking off one of the power supply pins. More expense, but looking forward to having heating and hot water – it’s been a cold end to April.

Delightful cruising between Bath and Bradford on Avon

Overall it’s been a great start to our canal journey – not always easy, but always interesting.

Unprecedented flooding caused by climate change? Not really.

Liz Truss, Secretary of State for Environment, Food and Rural Affairs, linked extreme weather with climate change (albeit implicitly):

The hon. Lady is absolutely right about the extreme weather patterns that we are seeing. As we say, that is consistent with climate change trends. Climate change is factored into all the modelling work that the Environmental [sic] Agency does, but in the light of this extreme weather we must look at that modelling and ensure that it is fit for purpose for future decisions. We constantly review investment in flood defences. It is important that we remain fair to people across the country, and that the people of Cumbria understand why decisions have been made and get the proper protection they deserve.

(From Hansard, viewable here.) As an aside, I take exception to her statement that "climate change is factored into all the modelling work that the Environmental [sic] Agency does" (see my blog here), but in this post I mostly want to look into the link between extreme flooding and climate change.

A team from Cambridge, Aberystwyth and Glasgow universities have looked at geological evidence for historic floods (article here) and come to the conclusion that current flooding isn't unprecedented. I thought it'd be interesting to try to do the same with recorded river flows - so I've looked into how often we'd expect to see an extreme flood (i.e. a flow bigger than we've ever seen before at the gauge in question) across the UK gauging station network.

The number of times a record breaking flow is observed is actually an interesting statistic to work with - because it's independent of how the flow is distributed statistically. Fitting distributions to short series is fraught with problems (see here), so not having to worry about this is a definite advantage. The probability of seeing a record breaking value is simply 1/N, where N is the length of the record¹ - if we assume that there are no trends in the data. This probability decreases as we collect more records - as we'd expect, because in a long record we've seen it all before, and the probability of a record being broken is therefore small. If there is an upward trend in the data, we'd expect records to be broken more often than this - so this could be a useful test for increasing flows coming, for example, from climate change.

I've calculated how many record high flows we'd expect to see in UK flow gauging stations assuming no climate change, along with how many we actually have seen over the past - shown in the figure below. Unfortunately it takes a while for new flow data to get incorporated into publicly available data sets, so this data doesn't include the most recent 2015-6 floods.

The expected number of maxima (grey line) peaks in the 1970s. This is purely due to the increase in the number of gauges (blue line) between approximately 1960 and 1980 - there were a lot of new gauges around in the 70s which caused a lot of record flows to be recorded. The observed incidence of record breaking high flows (orange line) show a lot of variation - with big peaks in 1960, 1968, 1974, 1979 and 2000, corresponding to big flood years. Interestingly, the peak corresponding to 2007 isn't that big - perhaps reflecting the significant role of surface water flooding in that year.

There's a lot of variation in the signal, and we expect some years to show more records than others. Is it possible to pick out a signal from climate change? I don't think so - the observed number of records follows the expected number pretty well, but with a lot of scatter. I've simulated what we might expect to see if there were a trend in flows (plot below), increasing by 2% per decade (this is broadly in line with guidance on current rates of climate change effects for river flows). The difference is much smaller than the natural variations in maxima we see from year to year.

So looking at the record breaking flows we see every year, I don't think we can assign these to climate change - it's just natural variability that leads to some record breaking years.


¹ One of the series has got the be the maximum - and the probability that it occurs at the end, i.e. we've just broken a record, is therefore 1/N. You can prove this with fancier maths too.

Climate Change - it's happening now

The winter 2015-16 floods in the UK have sparked the usual debate - we're not spending enough on flood protection, building on floodplains has made things worse, it's all because we're paving over driveways etc. Many commentators (including my old colleague Reza Ahmadian) have cited climate change as a contributory factor - but how true is this?

Lots of work on climate change seeks to look well into the future - some work I recently did for the Committee on Climate Change (report here) focused on changes by the 2050s and 2080s (although we did look at the 2020s too). But the recent news that we're already approaching 1°C above the pre industrial average prompts a question - should we therefore also see a change in flood risk? The link between global temperatures, rainfall and river flows is well established (in the minds of climate scientists at least), so in this post I'll try to understand what this means practically for flood risk.

It's important first to understand a key concept in climate studies - the baseline. While we're almost at 1°C above the pre industrial average for global mean temperature, that's only around 0.7°C above the temperatures in the period 1961-1990, which is typically used as the baseline in climate studies (like the IPCC). The chart below shows global mean temperature relative to this 1961-1990 baseline, showing that the baseline almost corresponds to a relatively stable period for global temperatures between 1940 and 1980, but that there was significant period of warming before that. I've added a thick red line to show a "cartoon" of running average temperature, which is constant before 1920 and from 1940 to 1980.

This global mean temperature history has two potentially significant effects for how we understand flood risk. Firstly, our records of flows from gauging stations might not be stationary - there might be trends in there which we need to correct for before doing any statistical analysis. Secondly, there has been an increase in temperature since the 1961-1990 baseline which could have caused an increase in flood flows.

What does this mean practically? I've done some calculations for 5 gauging stations across the UK, using the latest advice on climate change impacts on river flows, and an assumption that increases in flows are roughly proportional to global temperature increase. I've expressed the results as the 1 in 100 Annual Exceedance Probability (AEP) flow for the baseline period 1961-1990, and the annual exceedance probability of that flow now in 2016.

The results are shown in the table below - for the Eden at Sheepmount in Carlisle, the baseline 1 in 100 flow of 1405m³/s now has an AEP of 1 in 62 - an increase in frequency of ~70%. The Blackwater in Northern Ireland shows a doubling of frequency for a big flood. The effect is less pronounced for the Tay, and the Severn and the Wye show no increases (because the climate change impact guidance indicates no sensitivity to climate in the short term up to 2025). 

So for some catchments, we're seeing significant increases in the likelihood of large floods compared to the baseline period 1961-1990. We need to think about these effects when we undertake flood risk analysis - rather than assuming our records of flow are representative of conditions now, and that climate change is something that only happens in the future. 

What are the chances? Trying to make sense of the Cumbria floods

Rain radar for 2015 Cumbria floods

Last week saw devastating floods in many parts of Cumbria and elsewhere in northern England - the latest in a series of big flood events to hit the region. So questions are naturally being asked about why this keeps happening. Probably the most important things to remember here are that hydrology is difficult, and that we should therefore expect weird things to happen.

What's so hard about hydrology? For flood protection, we're typically trying to build defences to a standard of "1 in 200 AEP". The AEP - Annual Exceedance Probability - is the probability that a flow or water level will be topped in any given year. So we're looking at rare events, and we simply don't have long enough records of hydrological events to estimate rare flows, rainfall, river levels etc accurately.

The longest records of river flows in the UK are at Kingston on the Thames and the Lee at Feildes Weir in Hertfordshire - both with ~130 years of record. The Eden at Carlisle has almost 50 years of record, meaning our estimates of extreme flows and probabilities are inherently uncertain - and when the next big flood comes along, our understanding of risk could change significantly.

Take flows on the Eden in Carlisle as an example. At the time, the 2005 event (with a peak flow of 1520 m3/s) was calculated (using the standard methods and data available at the time) as having an AEP of 1 in 150 - meaning that we'd expect a flow greater than this with a probability of 0.67% each year. This analysis used an approach called pooling - we combine data from other similar catchments to make up for our short record. But using pooling means we're mixing in data from other rivers which may not behave in the same way - what if Carlisle is unique? What if everywhere is unique?

I've estimated the AEP for a 1520 m3/s flow, using flow records up to, but not including, the 2005 event; including the 2005 event; and including an estimated flow for the 2015 event (I've used 1600 m3/s as the measured flow isn't available yet). Results are in the flood-frequency plots below - before the event, we would think a 2005 type event would happen once every 370 years. After the 2015 event, we think this will occur every 70 years - the probability for this event has increased by a factor of 5!

If we accept a figure of 1 in 70 as a reasonable estimate for the AEP of a 2005/2015 type event, then the probability of seeing two of these events in a 10 year period is approximately 1% - rare certainly, but not impossible. And given the number of gauges in the UK (around 1500) we might actually expect this kind of thing to happen at least somewhere every year or so.

There's also been a lot of talk about the new record rainfall (341mm over 24 hours) seen at Honister - a few km from Seathwaite where the previous record (314mm) was set in 2009. These are huge values - more like we'd expect in tropical countries like the Philippines. What's more, they're off the scale for the standard method for estimating extreme rainfall, the FEH DDF method, which only covers events no rarer than an AEP of 1 in 1000. It's difficult to unpick the significance of this - partly because rainfall data isn't openly available in the same way that river flow data is. But a quick back-of-an envelope calculation that if the AEP for 300mm is around 1 in 1000, then the probability of seeing 300mm twice in 10 years is around 1 in 20,000. So either this is a true freak event, or we need to reassess our extreme rainfall estimates.

So the science of hydrology says that any estimate of rare events is very uncertain - but as engineers we still need to be able to manage the flood risk. We need to acknowledge the uncertainty and build it into our decision making (we do this already, but perhaps not enough). And perhaps we also need to shift focus away from hard defences, whose performance depends so critically on estimating rare events, to other more robust measures (such as building resilient properties) that work no matter what the probability of flooding is.

The Last Post

The last post from the Philippines, just a few photos of some recent outings - then off to Indonesia for a few weeks. 

Rice Terraces of Banaue

I spent the easter weekend at Banaue, a small town up in the Cordillera of northern Luzon. The region is famous for its rice terraces, some of which were built around 4000 years ago. The farming methods being used haven't changed much since then either - most of the work is done by hand without the aid of tractors, artificial fertilizers or pesticides.

The town of Banaue, from the hotel terrace.

Banaue terraces, looking like a contour map of themselves.

The terraces in Batad are the most famous. The village, like many others in the area, is inaccesible by road, so everything is either grown locally or carried in by hand. This also means the farming methods being used haven't changed much since the terraces were built. But even in these remote places, electricity and radio masts are becoming common - stone age farming methods with mobile phones and Manchester United shirts.

Amphitheatre of terraces in Batad.

Mount Pinatubo

In 1991, Mount Pinatubo volcano erupted, spewing 10 billion tons of ash into the atmosphere, producing debris flows that devastated nearby villages, and lowering global temperatures by 0.5C. It also produced this rather nice crater lake, which is now a major tourist attraction.  

Lahar deposits eroded into fantastic shapes.

Trekking up through the debris deposits.

And into the jungle.

We did it in 18 minutes.

And arrival at the lake.

Return journey by 4x4.

Nothing to do with rice terraces or volcanoes, but here's a picture of my colleague's dog and her new puppies. Aw, bless.

Half way

I'm halfway through my placement, and my! hasn't the time flown. This is of course assuming I manage to get my visa extended and don't get deported at the end of next week. Here's a collection of random images to celebrate. 

View over Laguna de Bay, to the east of Manila. This enormous lake is the remnant of a volcano crater.

Pagsanjan Falls. The final scenes of Apocalypse Now were filmed here.

Boatmen row, haul and drag tourist boats up the rapids.

Meet of the United Folding Bikers club. Stridas are very popular, Britain is known as the home of the Brompton.

Landslides on the road to Infanta.

Infanta - a narrow strip of habitable land between the mountains and the Pacific.

A visit to some badly eroded river bank protection.

This is what it should look like - the has only just been built, and has yet to see a wet season. The volcano in the background is Arayat National Park.

House abandoned after being hit by a debris flow following heavy rains two years ago.

Evening ride through Intramuros, the old heart of Manila. The horse was called Dave.

Manila Bay by night. Sinister squat building on waterfront is US Embassy.

Fieldwork

Rice fields, mountains

Last week was my first trip to the field: 2 days in Nueva Ecija and Pampanga provinces with a team planning to take river flow measurements at locations where water level sensors have been installed. This work is actually going to be quite important - we'll be using the measurements to improve the hydrological models which estimate flood flows in response to typhoon events.

Levels on lamp post by road bridge

Local resident cools off in the shade

Most of the time was spent driving to and from the mall for lunch; waiting around for other people to turn up; and, inexplicably, a trip to a buffalo milk processing plant. We did actually set up 3 GPS stations for static survey measurements (probably about 2 hours work all told); one of these blew over after 5 minutes, so we gave up on it.

GPS survey set up - this one remained upright

This is reminiscent of some of my experiences in Indonesia (see here for for a typical example), and brought with it a possible revelation. Some people not only don't mind the endless waiting around, faffing and generally being a bit disorganised - they actually enjoy it.

Not much water in the rivers

So in the end I didn't get to see any flow measurements being made - and this is something that I might have been able to help with. As it is, measuring flow at this time of year is a bit pointless anyway - it hasn't rained in 6 weeks and the rivers are almost dry. Hoping for better when I go to inspect a flood defence next week.

Popular tourist resort - especially with watersports enthusiasts