Category Archives: science

Below Sea Level does not mean Below the Sea.

This post represents my own views and is not intended to represent the views of my employer, present or past.

I’ve been umm-ing and ah-ing for a couple of months now about whether to write this blog, but I think I have finally had enough. You see, in Hull, we are at risk of flooding from the sea, or more specifically, the Humber Estuary. This risk emerges when low pressure out in the North Sea, caused by the storms, which can be common in the winter, effectively suck up the sea causing it to raise a little. High winds whip up waves, and these add a little more height to the water. All of this has the potential to raise the level of the sea, for a few hours, by up to a couple of metres. On December 5th 2013, a storm surge (as these events are called) raised the water level in the Humber by 1.7 metres.

The added complexity to this are the tides. The difference in the water level between low and high tide at Hull, according to the Associated British Ports (ABP) is between 3.5 m for a neap tide, and 6.9 m for a spring tide – this staggers the level we have determined to be 0 m, or sea level. This means the risk of flooding is all a matter of timing. If, on December 5th 2013, the storm passed by a few hours earlier or later the surge would have aligned with the low tide, and the additional 1.7 m would have barely been noticed by anyone. However, it was timed with a high spring tide, resulting in record water levels in the Humber and caused flooding in Hull and around the Estuary.

Coastal flooding

Graphic showing how coastal, or tidal, flooding forms. This was the type of flooding which occurred around the Humber in 2013. Thanks to NERC for producing these great resources. 

When we design and build flood defences on the coast we don’t build them to just hold back tidal levels of the water, but also to defend against enhanced water levels produced by storm surges. Since 2013, the defences around Hull have been updated and a repeat of the event would result in little or no flooding in the city – I don’t know the exact level of the defence, but we can say that it is able to contain sea levels of at least 1.7 m higher than the highest natural tidal level.

A big issue facing Hull is sea level rise. Sea level has been rising since the end of last ice age, and is set to continue in the future. On top of this, the climate change caused by our industry is accelerating this. Our best estimates for the Humber area, assuming that as a species we continue increasing our influence on the climate, suggest the sea level will be around 1 m higher in 100 years than they are today – this will increase the risk of flooding and we need to ensure that the public understand this and that we continue to invest in improving the standards of our defences to keep pace.

On the first point, talking to residents of Hull about the risk of flooding from the Estuary provokes two responses. (1) There is a lack of appreciation of the risk from the Estuary, and when I start to talk about the 2013 flooding, people tend to share with me their experiences of the 2007 flooding (a surface flooding event). (2) People tend to feel that there is no point in doing anything as “Hull will be underwater in 100 years”. This latter point is what I want to discuss here, it’s a common perception and leads to a kind of apathy where people become disengaged with flood risk and actions to mitigate for it, but it is wrong.

It is a deeply held belief that goes beyond even the city – in 2015, Dr Hugh Ellis, the now Head of the Town and Country Planning Association (TCPA), made the claim that the city would be underwater in 100 years –

“We need to think about moving populations and we need to make new communities. We need to be thinking, does Hull have a future?” (Source – Daily Telegraph)

Ok, he was trying to make a valid point, one that sea level rise is going to increase the risk of flooding for coastal cities, but I don’t think bold, and inaccurate statements, like this are helpful, and they only result in residents of the areas becoming disengaged – why do anything about the problem if it is futile?

But where does this idea come from? Why are people convinced Hull will be underwater in 100 years? Why do people think it will become the “Venice of the North”? Well, look at the map below –

surging seas

Screenshot from Climate Central’s Surging Seas Risk Zone Map – this shows the Humber Region, UK, with a 1 m sea level applied.

This is map of ‘risk’ taken for the Humber area. For areas outside of the US, the Risk Map has been produced using a map of land heights obtained from space by the Shuttle Radar Topography Mission, which mapped the entire globe at resolutions between 30 m and 90 m. The areas shaded in blue are all those ‘below sea level’ – normally 0 m, but in the map above I’ve set it at 1 m to represent the predicted sea level in 100 years time. Hull isn’t labelled on that map, but it basically the large blue area between North Ferriby and Hedon – very clearly ‘under water’.

But the method is problematic, it’s too simple. An average measurement of land heights over a 30 m area is fantastic when considering it is for the whole planet, however for determining flood risk it’s a bit rubbish. It smooths the land surface, removing obstacles, like wall, roads and buildings, and crucially, flood defences. The method also ignores ‘hydraulic connectivity’*, basically meaning that for water to flood an area it has to have a source of water and a route for it to get there – flood defences work by removing this hydraulic connectivity and this is why today the Humber region, and much of Holland, is close to or below sea level, but not under the sea.

To understand the actually risk posed by sea level rise requires a more complex model, one which accounts for tides, contains more detailed data, and more importantly includes flood defences. Our model (paper here behind paywall) does this, and a version of it is incorporated into Humber in a Box – with both of these we observe no flooding around the Estuary for natural tides with a 1 m sea level rise. This is because the defences are built to hold back the much higher water levels caused by storm surges.

Climate Central have been careful to refer to this shading as ‘risk’, and not direct inundation by the sea, but the use of blue and not making this explicit anywhere opens this up to mis-interpretation where ‘below sea level’ means ‘below the sea’. This is clearly happening – see this article in the Conversation, which made the BBC Sports pages, which used the app to suggest Everton’s new stadium “could end up underwater” in the future, or this article shared by the awesome Geomorphology Rules  Facebook page, suggesting that coastal cities in the US will be “drowning in water”.

Sea level rise is going to increase the risk of flooding in coastal cities but they are not going to be under water. The risk does not emerge from the tidal water levels, which will most likely be contained by present defences, or those to be built in the future. However, the risk from storm surges will increase – the likelihood of events like December 5th 2013 is set it increase, both in strength and frequency, and with 1 m extra sea level in 100 years our defences will need to be updated to cope with the enhanced levels. This will take a lot of money, a lot of effort, a lot of political will, and this requires the buy in and support of the residents of these areas. Telling them, or suggesting, that they will be required to relocate will only achieve the opposite.

Sea level rise and the related flood risk is a complex issue and we can’t keep trying to find simple answers.

*For areas within the US, the method uses much higher resolution height data, and accounts for hydraulic connectivity by shading areas differently.

Find me at #EGU17

It’s almost time to go to Vienna again for the 2017 General Assembly of the European Geoscience Union, or #EGU17. It’s promising to be another awesome week of science, schnitzel, and the collection of cold bugs from around the globe. Incredibly, it will be my fifth EGU, and I have the pleasure of being joined by a couple of first-timers from my research group – I’m looking forward to showing them the ropes.

I have two oral abstracts and a poster at this year’s meeting –

EGU2017-15699 | Orals | GM3.3/SSS3.13/TS4.6

LEMSI – The Landscape Evolution Model Sensitivity Investigation
Christopher Skinner, Tom Coulthard, Wolfgang Schwanghart, and Marco Van De Wiel
Wed, 26 Apr, 16:30–16:45, Room N1

This talk will show the results from our global sensitivity analysis of the CAESAR-Lisflood model. This has been a large piece of modelling work, and seems to have been going forever. Our computers have been busy for well over a year, so it’s great to get the results out there.

EGU2017-12624 | Posters | GM3.3/SSS3.13/TS4.6 | | Highlight

Influence of Rainfall Product on Hydrological and Sediment Outputs when Calibrating the STREAP Rainfall Generator for the CAESAR-Lisflood Landscape Evolution Model
Christopher Skinner, Nadav Peleg, and Niall Quinn
Wed, 26 Apr, 17:30–19:00, Hall X2

This poster has been selected by the session conveners as being of public interest. We’ve used a rainfall generator to produce ensembles of high spatial and temporal resolution rainfall, and used this to drive the CAESAR-Lisflood model – the results are very interesting indeed!

EGU2017-764 | Orals | EOS5

SeriousGeoGames – Geoscience Virtual Reality Experiences for Festival Settings
Christopher Skinner
Thu, 27 Apr, 10:45–11:00, Room L4/5

My final talk is something a little different, and will be summarising the SeriousGeoGames project the best I can in 12 minutes! I will show a little of Humber in a Box and Flash Flood!, and sum up their successes. For a preview, check out the brand new Flash Flood! YouTube Free60 –

 Please do come find me and say “hi”, or “Oi, your research is rubbish”, and if you have something you think I should see, let me know.

See you in Vienna!

Storm Surge Jan 13th 2017- Stay Alert This Evening.

It’s 6.45 AM as I’m writing this, listening to Radio Humberside and watching the snow fall on my garden in Barton-upon-Humber. For the past hour I’ve been following the news, updates from Twitter, and the reporting from live tidal gauges, trying to get an idea of the materialising image of this storm surge.

A storm surge occurs when low atmospheric pressure during storms causes the sea level to rise. This is because the low pressure draws up the water level. This is a basic explanation, but a 1mb reduction in atmospheric pressure will result in a 1cm increase in sea level. On top of the sea level increase caused by this, storms are usually windy and whip up significant waves, adding further level to the water.

To cause flooding, a significant surge needs to coincide with a high tide, usually a high spring tide like we are experiencing at the moment. If the surge coincides with the low tide, it results in nothing more than an unusually high low tide level but is not a risk. This page from the MetOffice provides a great description of them.

This combination was what drove the warnings for flooding along the East Coast of England this morning. Fortunately, the flooding feared did not occur. It is important for me to be very clear now that the main risk, and the flooding warnings for today, refer to the tide this evening – keep listening to the Environment Agency, do what they say, and keep an eye on their warnings here

From the National Oceanographic Centre we can see forecasts for storm surges at tidal gauge sites. Below are the forecasts for today for the Immingham gauge – the peak surge is forecast to be over 2m. If this was to occur with the high spring tide due at Immingham this evening, over 7.4m, it could produce water levels in excess of 9.4m which would be higher even than 2013. Thankfully, the peak of the surge is forecast to occur prior to the high tide. This has happened numerous times in the past, where large surges have coincided with low tides.

surge-forecast

This does not mean there is no risk by any measure – the water levels this evening will be higher than this morning. This forecast could turn out to have predicted the timing wrong. Surges can also have the effect of drawing in the high tide, causing it to occur slightly early as demonstrated by Horsburgh and Wilson (2007) (Paywall). The winds forecast also did not occur this morning but the could arrive during the day, adding to the waves.

Storm surges are complex, with numerous facets combining to cause the risk. This makes them difficult to forecast and can evolve and change quickly. The Environment Agency have done a superb job is warning and preparing these past few days and should be commended for this work. It is also encouraging to see the greater appreciation for tidal flooding risk from the media and the public – awareness is vital component in reducing the dangers of flooding.

Stay alert and stay safe.

Hurricane in the Humber : Modelling the Unthinkable

We’ve all been stunned by the images of Hurricane Matthew tearing through the southern States of the east coast of the USA, and the footage of the resulting storm surge sweeping into these coastal areas. We should not forget Haiti and the carnage once again unleashed on this nation, and the ongoing struggles the people will have there for years to come. The power of nature can simultaneously be awe inspiring and horrendously destructive.

In the UK we are relatively blessed in our sheltered position from natural disasters – it is difficult to imagine just what it is like as a nation to suffer an event on this magnitude, just as we could scarcely imagine what the impact of an earthquake or a volcano might be. But what if the unthinkable did happen? What if Hurricane Matthew did hit the UK with the full force of a Category 4 or 5 storm? How would the storm surge look like?

My research involves using numerical (computer) models to understand how nature works, in particular the movements of water. In the past I have used these models to simulate the workings of the Humber Estuary, UK, and some of that work includes simulating “worst case scenarios”. Before the 2013 storm surge this was often thought to be equivalent of the 1953 event, but now the baseline is 2013. On December 5th 2013, a storm in the North Sea caused a storm surge of around 1.8 m to form, coinciding with a high-tide resulting in the storm tide1.

1To pose a threat a storm surge needs to coincide with a high-tide. This combination is called a storm tide. A surge which coincides with a low-tide probably will not pose a risk, and the peak water levels will usually be lower than that of a normal high-tide. This obviously depends on the size of the surge and local difference between low- and high-tides.

A category 4 or 5 hurricane hitting the Humber and the UK at that strength is way beyond our “worst case scenario”, and reveals little to us about the nature of the Humber and the state of our defences. However, simulating it does provide prospective of the scale of the event and helps us understand just how powerful and destructive they are. At St Augustine, Florida, the surge was estimated to be 2.75 m, adding this swell to the tidal sea level – looking at the surge from 2013 this is nearly 1 m greater.

The represent this in our Humber model I have done nothing more sophisticated than simply adding 1 m height to all the water level data we use to simulate the 2013 flooding. The video below shows the results – it looks pretty bad and it would be, but we need to consider some aspects of the model to fully understand what we are seeing. The model uses a smooth representation of the land surface, as in it has no buildings, walls, roads, hedges, tree etc which would stop or slow the flow of water, although it does have a representation of flood defences. This means once the water levels exceed the defences and spill over on to the land the water can just keep flowing, when in reality it would be stopped by obstacles – so the area flooded in the model is larger, yet probably shallower, than we would expect.

This is truly an unthinkable event and we would not expect a surge of 2.75 m to be seen in the Humber. However, global sea levels are rising and our best predictions suggest that the base sea level in the Humber will be around 1 m higher in 100 years time – from this point, the 1.8 m surge from the 2013 event would cause water levels of the same height as a 2.75 m surge in the present day. As our climate warms, providing more energy to the atmosphere, we can also expect our weather to become more stormy and events like 2013 will become more common. This paints a bleak picture and presents coastal areas like the Humber a major challenge for the rest of this century.

The good news is that those responsible for our flood defences are aware of this challenge and are developing their plans to help us face it. Our model is already out of date as several areas around the Humber have had their flood defences improved since 2013, and there are plans for more – this process will be continuously assessed and developed in the future to keep people and property safe. Models such as our will be used to test those plans and the contribute to designing new schemes. The challenge is great but we can meet it.

Hull University Science Festival with @SeriousGeoGames

It’s that time of year again, one of the most exciting times of the year for me. This weekend, on Friday 18th and Saturday 19th, is the Hull University Science Festival. It’s my fourth time presenting an exhibit here, and it’s come a long way. This year, I will be leading a team of helpers with an exclusively SeriousGeoGames exhibit – on behalf of the Department of GEES.

In 2013, when the event was called the Science Showcase and took part in Hull Town Hall, I was inexperienced (still to submit my thesis!), and was asked to present the Dynamic Humber Project I was employed on. Our exhibit was not very good, composing a stale poster and an informative, but not very interesting, presentation we had to project on to the ceiling! We were later joined on the ceiling by the plastic balls from Becky Williams’ awesome liquid nitrogen volcano demonstration!

My Mother-in-law, Beverley, trying Humber in a Box

In 2014, I stepped up my game a little and so did the event. The first true Hull University Science Festival, we were housed in a large marquee on Campus. I presented a hacked version of the Humber CAESAR-Lisflood model, with a slider to raise sea level – it was the precursor to Humber in a Box. The participants found it interesting when they used it, but many people bypassed us as we were sandwiched between a 3D Printer and HIVE – to be honest, I would have done the same.

My favourite ever interaction came during this event, which went something like this –

Me – You move the slider and you raise the sea level in the Humber.

Schoolgirl 1 – (With full sincerity) In real life?

Me – (Struggling to come up with a reply) No, just in the model.

Schoolgirl 2 – (Said in a way only a close friend could) Don’t worry about her, she’s really thick.

I’m not sure you can every really train or prepare yourself for conversations like this! She wasn’t “thick” as her friend cruelly suggested, and I am constantly blown away by how bright and engaged the school pupils who visit us are – it really is a joy explaining our science to them.

Last year, I had Humber in a Box at my disposal for the first time. The exhibit I was part of also featured the River in a Box mini-flume, and the this and the excitement of the VR headset ensured a busy couple of days!

Now in 2016, we will be presenting Humber in a Box, but also Flash Flood! for the first time. We have be demonstrating the application on a large TV, and you’ll get the chance to explore our virtual river valley. We will also have some of the field equipment used to make the application on display. I’m really proud of Flash Flood! and am very excited about demonstrating it in public for the first time.

Looking forward to seeing you there! Details here.