Storm Surge 2013 : One Year On - Part One : Modelling the Surge

by @cloudskinner

This is the first post of a four week mini-series looking back at the storm surge of 5 December 2013. The surge caused extensive flooding along the East Coast of the UK but our focus has been on the area immediately around the Humber, and you can read our reaction shortly after the storm surge in this older post. Over the next few weeks we will be discussing the research that has been ongoing since the event, how it affected and continues to affect local residents and businesses, the community resilience that has been built and finally we consider the damage done to Spurn Point and its potential future.

This week the focus will be on a paper recently published by myself, colleagues at the University of Hull, the Association of British Ports (ABP) and the local Environment Agency (EA), which stemmed directly from the storm surge. The paper is free to view until 28 March 2015, after which you will require a subscription to Estuarine, Coastal and Shelf Science to view.

Hull's flood defences overtopping on 5 December 2013 (by @tom_coulthard)

Estuaries are very complex environments. There is a lot going on, beginning with the inputs of often several rivers, and the sea in the form of tidal flows. The relative influence of these on when and where the water and sediment moves in the estuary depends on the tidal cycle and the discharge levels of the rivers. It is a to and fro tug of war between these for influence within the estuary.

If that was not complex enough, there are secondary flows within the estuary. River water is fresh and sea water is salty, making the two flows a different density along with water that is mixture of the two in between. The two water types are often different temperatures too, again resulting in different densities and inducing flows from more dense to less dense regions. All of the flows are influenced by Coriolis forces, the deflection of water flow caused by the rotation of the Earth. The shape of an estuary also influences flow, and in combination with the influences above, estuaries like the Humber often show two channels along the bottom - one resulting from tides coming in and one from tides going out. Finally, overlain on these are the winds, waves and pressure influences of the weather.

This makes estuaries very complex and turbulent, and this turbulence can form a layer of thick sediment laden water to form along the bed - this basal mud layer clings to the bottom and effectively lubricates water flows along the estuary and shields the bed from erosion and deposition.  The salinity of the water also causes fine sediment to clump together in a process called flocculation which makes them behave like larger sediment particles.

It is commonly thought that to model the processes in an estuary then you need to account for all of these processes, but doing so is incredibly computationally expensive. It is possible to do, but even on expensive and powerful machines it often takes several days to model a single tidal cycle. Trying to use them to predict the future of an estuary several decades in the future would be almost impossible. Our approach was to use a simpler model, CAESAR-Lisflood, which has been widely used for a similar purpose on rivers for over a decade, to try and model the Humber Estuary successfully without all of this detail.

Animation showing the CAESAR-Lisflood model simulating the 2013 storm surge and associated flooding.

It was during this process when the storm surge struck and the focus of our research switched. We had already tested the model's ability to reproduce tidal flows - rapidly and at small timescales - so we soon tried applying the data recorded by ABP during the surge. This showed that the model could also reproduce the location and extents of the flooding on that night. This was using the latest information on the Humber's flood defences provided by the EA. The quickness of the model to process the data would make it suitable for producing numerous possible scenarios based on live and forecast data, and potentially help predict the extent of future flooding before it occurs.

This work is ongoing. Next week I will highlight how local residents and businesses were affected by the flooding, as discussed at the Humber Conference of December last year. If you wish to view this paper you can do so here.

Skinner, C. J., Coulthard, T. J., Parsons, D. R., Ramirez, J. A., Mullen, L., and Manson, S., 2015. Simulating tidal and storm surge hydraulics with a simple 2D inertia based model, in the Humber Estuary, UK. Estuarine, Coastal and Shelf Science. 155, 126-136 doi:10.1016/j.ecss.2015.01.019

The Surge 2013

by Chris Skinner (@cloudskinner)

This blog post is going to talk about the storm surge that swept along the east coast of the UK on the 5th December 2013, last week. Rather ironically, I was going to post about problems in predicting disasters and how we mitigate against these, but this seems more topical and worthy of a post., and I hope to give you a bit of an insight into how a GEES researcher responds to live events relevant to their field.

The surge seemed to catch everyone by surprise. I checked the forecast on the Monday as my in-laws were travelling to Hull from London to visit us on the 5th, and the Met Office app suggested Thursday was going to be quite nice, but a bit windy in the far North-East. The forecast did evolve over the week, but not so much to suggest the conditions that resulted in them passing at least five overturned lorries on their journey (two and a van on the Ouse bridge alone).

On Thursday afternoon, there were warnings of a storm surge – a temporary increase of sea level caused by low pressure and high winds – that would potentially flood coastal towns on the east coast. Our local news focussed on Grimsby and Cleethorpes as being the most likely to be hard hit. Hull was just at medium risk. Myself and Prof Coulthard (my boss) watched the tide rise on the Immingham tidal gauge and compared it to the data we held from the same site during the 1953 storm surge.

The 1953 storm is THE storm when talking about storm surges in the UK. It was big and it caused extensive damage and over 300 people lost their lives. This storm was being billed as ‘the worst since 1953’, yet to our astonishment we saw the tidal gauge go up and look increasingly like it was going to exceed the level recorded back then.

As we were leaving the office, around five thirty, the first warnings started coming in that Victoria Dock in Hull was at risk. I followed the story unfold on Twitter as photos popped up showing the first signs of water overtopping the defences. The Marina also flooded and water spilled out into the Kingston Retail Park, and the home of the Hull Stingrays and Hull’s most versatile venue, the Ice Arena.

Flooding between the Marine and Kingston Retail Park in Hull Photo by @estuary_ecology

The City of Hull held its breath as high tide approached. Only the tidal barrier stood between the surging sea and thousands of properties in the flood plain of the River Hull behind. The tide crept ever upwards, lapping at the sides of the mighty barrier but could not overcome it. But it was close – only 40cm remained of that barrier, built to defend the city after the1953 surge. It had done its job, just. The tide height of 5.8m is a record high for Hull.

The Saviour of Hull! - The Tidal Barrier holds back the tide. Photo by @Tom_Coulthard (This is just one of many great photos).

The sea water eventually receded at Hull.  High tide was later in the inner estuary and badly flooded South Ferriby and Goole. The flooding continued further south, in Skegness and Boston. Another great tidal barrier, the Thames, was also needed to save large areas of East London.

Now that the waters have passed the data is beginning to be collected and analysed. What seemed to take everyone by surprise was the scale of it. Data from the Immingham gauge stopped when the level reached 8.5m*, but from the curve it looks like it would have continued to around 9.5m – 2m above the predicted astronomical tide (from the pull of the Sun and Moon), and over a metre greater than the highest reading from the 1953 storm surge (at 8.4m).

*I don't know why the gauge stopped, most of them did before high tide that night. My guess is that they either reached the top of their scale, or exceeded a threshold where it is assume too high to be accurate - The Immingham gauge stopped at around the maximum of the 1953 tide level.

This is very significant. I don’t think anyone anticipated it. As I said previously, 1953 was THE storm. For the last few months I have been working on a computer model to simulate the flows in the Humber, with one of the aims to be able to predict the estuary’s response to 1953-like events, especially in the face of rising sea levels. Much of the Humber’s defences were built after the 1953 surge so unsurprisingly the model showed they coped well. Our hypothesis was that the rising sea levels on top of that might cause them some issues, so we wanted to try and model that.

Naturally, first chance on Friday we ran our model with the tidal heights recorded on the evening before. Our model suggests that if we had been able to predict the scale of the surge we could have anticipated the flooding, even just based on this preliminary data (although a large pinch of salt is needed when interpreting the simulation below).

As bad as the flooding was, it has to be said that our infrastructure did a fantastic job. The scale of this surge was unprecedented, quite a bit bigger than 1953, yet there has not been the devastation, and thankfully, the loss of life that followed that storm. If it were not for structures like the Hull Tidal Barrier, it would have been much, much worse.

And that leaves us with a warning. The International Panel for Climate Change (IPCC), uses different models to try and predict future sea level rises for the next 100years, and the 'Best Case Scenario' - where greenhouse gas emmissions are cut immediately - would likely cause a sea level rise of 40cm. This is the capacity left over on the Hull Tidal Barrier. When we consider that an increase of 60-80cm is probably a better estimate, the ability of our infrastructure to manage this size of event in the future needs to be considered. It maybe that this storm surge is an event that won't be repeated in our lifetimes, but it now stands as THE storm we’ll be using the measure future resilience and it pushed us right to the edge.

Getting Animated

Getting Animated by Chris Skinner (@cloudskinner)

The formal presentation of research in academia is pretty traditional. I doubt it has changed much in the last 500 years, if not longer, and for a progressive sector of society it really does not look set to change. Basically, you get your results, write it up as a paper, some experts look it over and request more details or changes, you do them, they pass it, you get published.

The published article then goes into a journal. Most of these are still printed but are available, usually as a PDF file, electronically. This is where the embrace with the modern world ends. I mainly read articles either on my computer or my tablet – most articles are formatted into two columns on a page which makes it very awkward to read off a screen. So optimisation for electronic presentation is not high on publishers’ agendas it would seem.

But are we missing out? A magazine I have been reading since I picked up my first copy in October 1993 has changed many times in the last two decades. It isn’t a science publication but is related to a hobby of mine, and last year they started publishing a version of the magazine optimised for the iPad. They could have just bunged out a PDF of the paper copy, but they knew that the new technology provided them with a platform to support more content. In place of a photo there is an interactive 360º image, instead of a price list for new products there are hotlinks direct to their entry on the online store, plus there’s additional videos, interviews and zoom panels. If the magazine contains typos or erroneous details, it is automatically updated. The company have started rolling out this idea to their other printed materials.

What if these ideas were used in academia? What sort of content could we include? The most immediate thing that springs to my mind is animations. I produce tonnes of them, and conference presentations aside, they rarely get seen outside of my research group. Why do I make them? Because they are useful for very clearly showing how systems work, if your model is operating how it should or demonstrating patterns in data - (*Thanks to @volcanologist for pointing out that animations can sometimes be submitted, and hosted on a publisher's website).

Take for example some work I have been doing on historic bathymetry data from the Humber estuary. Bathymetry data are readings of water depth at the same tide level, and I use the data to create maps that show the shape and elevation (heights) of the bottom of the estuary. To find out more about what estuaries are, take a look at Sally's previous blog.

Provided by ABPMer, the data spans a period between 1851 and 2003 – I processed the data, calculated rates of elevation change between each sampling period, and from this produced yearly elevation maps. By putting these together as an animation I could see the evolution of the data (it is important here to stress the difference between ‘data’ and reality - not all areas of the estuary were sampled by each survey, and the number and locations of reading varied. Much of the change seen in the video is because of this and not because the Humber has actually, physically, changed in that way).

What immediately struck me was the contrast between the middle and the inner estuary. The middle estuary is the part between the Humber Bridge and the sea, where the estuary’s course deviates southwards – it is remarkably stable over the 150 or so years. The inner estuary, from the Bridge towards Goole, sees lots of internal changes – driven by interactions between the river inputs and the tides – but overall very little change. The Mouth of the Humber, the part closest to the sea, looks to see little overall change, but most of the variations seen in the animation are due to differences in sampling point in the data, and not actual changes. Similarly, changes around the banks of the estuary observed in the animation are most likely caused by sampling difference in the surveys, rather than actual elevation changes.

I have recently been continuing work on adapting a landscape evolution model, Caesar-Lisflood, to model the Humber estuary, and a big step towards this is to accurately model the tides as they are observed by tidal stations recording water depths. Numerically we can do this, but it is important to check that the model is representing the tides in a realistic way - this is a very important step in making a model as it has to be able to accurately simulate observed behaviours before you can experiment with them. Again, animations are a really useful tool for doing this.

The video above shows the variations of water depth throughout several tidal cycles, as modelled, with light blues as shallow and dark purple as deep water. The model changes the depth of the water at the right hand edge in line with water depth data recorded from the Spurn Point tidal station near there. The water then 'flows' from there, down the length of the estuary as the depth increases, and vice versa - this simulates the tides going in and out.

From this I can tell that the model is operating well, as the tide is advancing (coming in/going up/getting deeper) and receding (going out/down/shallower) as expected, throughout the whole region and not just at the points where the tidal stations are located. You'll notice that the early part of the animation shows the estuary filling up with water - this is part of something called 'spin-up', where you let the model run for a period of time to get the conditions right before you start the modelling. In this case it is a 'day' as the water levels gradually builds, filling the estuary.

Another check would be the velocity of the flow as the tide floods and ebbs - this is the speed with which the water is moving (both in or out). The velocity should increase as the tide advances or recedes, but slack water (where the water is hardly moving at all) should be observed at high and low waters. If the model is working as expected, the area of slack water should progress from the sea and up the estuary towards Goole. From the video below, this is seen to be the case. Light blue shows low flow speeds, and darker purples higher flow speeds. The video shows the same modelling procedure as the previous video.

This type of content is really useful to me as a modeller. It is also really useful for presentations as I can show a group of people something that takes a few seconds, yet would probably take a lot of slides and quite a bit of explaining. If academic publications were to begin to include enhanced content in peer-reviewed publications, I believe this could advance the communication of research, not only to other researchers but also to the wider public. For now, Blogs, like the GEES-ology one here, are the best outlet. I hope you enjoyed the animations!