Tag Archives: rivers

Another Storm Surge? Keep your eye on the forecasts!

There are warnings emerging of a storm surge along the East Coast of England for this Friday (13th January 2017). At the time of writing these are a low risk warning, but the situation can change so keep your eyes open for updated warnings from the MetOffice and Environment Agency, such as the Flood Information Service.

If you’ve read this blog before you will know that I have performed plenty of computer model simulations of the 2013 storm surge in the Humber Estuary. Thankfully, it does not look like this Friday’s surge will result in flooding, and not on the scale of 2013, but I thought I’d share a simulation of the latest model anyway. This is a simulation of the 2013 storm surge –

There still, as ever, more work to do but it’s getting there.

Hurricane in the Humber : Modelling the Unthinkable

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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 5^th 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.

New Discussion Article in @EGU_ESurf

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Some of Prof Tom Coulthard‘s and my own research has just been published as a discussion paper in the European Geoscience Union’s Journal – Earth Surface Dynamics. It’s my first proper open-source paper, so this release is not yet peer reviewed but will be reviewed in the same way with anonymous reviewers. However, it is also open to anyone to make comments (but these are public, so no hiding). After review, and edits, hopefully it will be published fully later in the year.

Kisdon Force on River Swale

Here is where I try to write a ‘plain English’ summary of the work and the backstory. The work was conducted as part of the Natural Environment Research Council funded project, Flash Flooding from Intense Rainfall. The project hopes to improve our ability to forecast the intense, rapidly forming, but small and short-lived thunderstorms which can trigger flash flooding in the right conditions. We want to be able to predict their occurrence better and also understand the conditions required for flash flooding. We (Tom and I), in particular, look at the erosion and deposition which occur during the flash floods.

The computer model we use (CAESAR-Lisflood) was only able to use an input of rainfall which is averaged out over the whole area covered. These areas can be quite large, and as you probably know, if it’s raining in one part of the town you live, it might not be raining over another part. With the storms we are looking at they exist at a scale often much smaller than a whole river catchment, so that intensity is smoothed out by the model. This will likely reduce local river flows (in the model) and consequently reduce the amount of material (rocks, stones, mud etc) moved around (in the model).

Clearly, we needed to add the ability to represent rainfall in much greater detail, so I came up with a plan and arranged to meet with Tom to discuss how I was going to build this into to the computer code. I sat down with Tom and told him my plan, and in typical Tom fashion he tells me “I’ve already done this, I’ll send you the code”.

This single sentence saved me several months of coding and debugging and banging my head on my desk.

We used rainfall records taken from the MetOffice’s archive based on weather RADAR measurements. For the River Swale catchment (the catchment of choice for testing CAESAR-Lisflood), this data was available in grid squares of 5 km x 5 km, and recordings every 15 minutes. We wanted to test how the model reacts to the same rainfall data but applied in different resolutions, so we averaged out this data to various resolutions, both spatially (5 km, 10 km, 20 km and full catchment), and temporally (15 min through to 24 hours).

Incredibly, it made a big difference, with the best resolution (5 km every 15 minutes) moving over twice as much material as the worst (Full catchment every 24 hours) in some cases! We then looked at the longer term impacts by repeating our rainfall record (but jumbling up the locations at the end of each ten year cycle) for 1000 years (in the model). This showed that using the best resolution rainfall instead of the worst predicted more erosion in upland areas, and more deposition in lowland areas – this has implications for studies looking at the long term development of landscape that often use averaged rainfall records which miss out this detail.

This is because of the relationship between the discharge of a river (the amount of water flowing past a point in a specified time) and the amount of material moved is disproportionate. We called it ‘non-linear’, in that a small increase in the discharge results in a big increase in material moved – by representing the rainfall in greater detail, the model focusses it over a smaller area for a shorter amount of time, increasing the discharge in that section of the river.

The research also highlights the need to consider how our rainfall is likely to change with climate change. Often, only the overall change in volume of rainfall is considered but if this is in the form of frontal rain which covers large areas over long periods, the rain is low intensity and will unlikely cause flash flooding or move much material. If we are to expect an increase in the intense thunderstorms then we can expect our rivers to become more active in the future – the implications of which are as yet unknown.

The paper is free to read, so does not require a subscription, and can be viewed here.

Flash Flood! from @seriousgeogames

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As I have a brief hiatus whilst I wait for ArcMap to select a few million Lidar points, I thought I would share a post from the SeriousGeoGames blog. It’s all about the new application I’m developing with BetaJester Ltd.

“Flash Flood! Our new project with @BetaJesterLtd #MadewithUnity

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We are pleased to announce that we have started working with developers from BetaJester on our latest project, Flash Flood!

Flash Flood! is being produced as part of the Flash Flooding from Intense Rainfall (FFIR) research programme, funded by the Natural Environment Research Council (NERC), and is designed to highlight the destructive power of flash floods. This work has taken particular significance in light of the recent flooding in the UK over December.”

Read the full post, here.

Cumbria Flooding 2015 – @geophemera Press Release

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It is with great shock that we are witnessing the third period of intense flooding in the North-West of England in the past decade. The rains brought by Storm Desmond have been record breaking, and simply too great for most flood alleviation schemes to fully hold back.

The flooding has also brought vast quantities of sediment and debris with it, and has destroyed bridges, roads and other important infrastructure. The changes floods cause to rivers, valleys and the flood plain are often overlooked in reporting, but can have very long lasting influence.

In response to this, myself and Lynda Yorke wrote a press release for the British Society for Geomorphology –

“Flooding and Geomorphology – Dr Chris Skinner (University of Hull) and Dr Lynda Yorke (University of Bangor) on behalf of the British Society for Geomorphology

The past weekend has seen record breaking levels of rainfall fall upon the North-West of England. Storm Desmond, as named by the MetOffice’s ‘Name our Storms’ pilot project (http://www.metoffice.gov.uk/uk-storm-centre), has brought with it scenes of devastation as flood defences overtop and water spilled into people’s houses….”

You can read the full release here.

For more detail on the flooding and why the defences could not hold back all of the water, these BBC articles contains some superb analysis –

How do you stop flooding?

Storm Desmond: Defences against indefencicble floods