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

BETWEEN THE DEVIL AND THE DEEP BLUE SEA: THE DIFFICULTIES OF DEFINING ESTUARIES.

By Sally Little (@estuary_ecology)

I am an estuarine ecologist, which means that I study the relationship between organisms and their environment in areas of the coast where freshwater rivers meet the saltwater flood of the tide. Estuaries are interesting because they are naturally dynamic, high-energy environments, characterised by a specific flora and fauna. Physical processes operating on both short (e.g. tidal cycles) and long (e.g. climate and sea level change) timescales form the driving forces for many of the complex processes that occur in these systems. This means that estuaries are sites of continuous change, experiencing chemical (e.g. salinity, dissolved gases and nutrients), sedimentary (e.g. turbidity maximum), hydrological (e.g. tidal and freshwater flow) and morphological variations over daily tidal cycles.  Few plants and animals can withstand the extremes of these constantly fluctuating regimes, but those that can, commonly achieve high numbers, making estuaries some of the most important and productive ecosystems in the world.

The Humber Estuary, UK from the International Space Station - courtesy of @Cmdr_Hadfield

A miniature estuary at Sanna on the Ardnamurchan peninsula, North-West Scotland.

Estuaries (arguably more than any other aquatic ecosystem) are at the pinnacle of the human-environment interface – providing sheltered locations for habitation with access to inland, coastal and offshore resources and thus acting as focal points of human settlement and development throughout history.  For example, 10% of the global population (640 million people in the year 2000) live in the lower elevation coastal zone (LECZ; land below 10 m), which covers just 2% of the world’s total land area. This area contains two-thirds of the world’s megacities (population in excess of 10 million people) and more than 10% of the world’s wealth. As such, estuaries are subject to dense populations, development pressures and intensely exploited resources; with issues such as pollution, nutrient enrichment, habitat loss and over-exploitation extremely common in these systems today - pressures which are likely to increase with global population growth.

The megacity of Shanghai in China is located on the Yangtze River Estuary - the third largest river in the world

The megacity of New York has grown around the Hudson River Estuary - the Mahican name of the river (muh-he-kun-ne-tuk) represents its partly estuarine nature as "the river that flows both ways"

In addition to human pressures, estuaries and coastal zones are particularly vulnerable to climate change (e.g. including eustatic sea level rise, changes in weather patterns and extreme events). It is therefore important to manage both the impact of human activities and future global climate change upon estuarine ecosystems, though this has raised one of the fundamental issues in estuarine research – what is an estuary?

Traditionally, we, as estuarine scientists, have used the ‘expert-view’ definition that “if it looks like an estuary, smells like an estuary and behaves like an estuary, then there is a good chance that it is an estuary”! However, when we increasingly have to provide information to lawyers, planners and policy makers and are required to rigorously defend our terms in courts of law, the repercussions of poor definitions may be legally and economically costly – therefore everything becomes a little more tricky. For example, a recent court appeal case (12 April 2011) between Western Ferries (Clyde) Limited and The Commissioners for Her Majesty’s Revenue and Customs (HMRC) concerning liability to pay corporation tax, highlighted the problem of a lack of a legal definition of an estuary in the UK. Western Ferries asserted that they operated a crossing outside the Clyde estuary and harbour limits, therefore should be taxed under ‘tonnage’ rather than ‘corporation' tax regulations – at stake was a cool £3 million. The judgement considered a variety of definitions of an estuary, from both scientific literature, management directives and evidence from expert witnesses, one of which included the eminent estuarine scientist Dr Donald McLusky. However, even then the definition of an estuary was not clear-cut and proved problematic to establish on a legal basis.

Gourock and the Firth of Clyde, North-West Scotland

The issue is that estuaries are extremely difficult to define. That is not to say that there aren’t any definitions, in fact there are over forty definitions of an estuary, the majority of which are based upon physical characteristics and all of which differ based upon the research discipline and geographical location of the defining author.  There is however no one definition that provides universal coverage for all the estuaries in the world.

This is the crux of the problem. Whilst estuaries can be characterised in similar ways (e.g. freshwater input, tidally influenced with a salinity gradient), each is physically and biologically different. Often, the further apart estuaries are geographically, the more different they become. The majority of estuaries in the Northern hemisphere, for example, have a tidal range of greater than 4 metres (macrotidal), free connection to the sea, significant freshwater river input and a salinity gradient from fresh to marine waters. In contrast, from a global perspective, very few brackish coastal water bodies match these archetypal classical estuaries of Northern Europe, where the majority of estuarine research has taken place.  In Australia and South Africa for example, a growing number of scientists argue that coastal systems such as intermittently open and closed coastal lagoons and lakes be included in the definition of an estuarine ecosystem.  In these often microtidal (tidal range <2 m), arid systems, tidal and freshwater input can be negligible giving rise to temporarily open/closed systems, where evaporation can lead to hyperhaline areas (salinity greater than 35).  In these systems, the mouth is often marked by physiographic forms (e.g. a sand bar) which serve to close off the estuary from the sea for at least part of the year. However, during these closed phases, these systems have been shown to function normally as estuaries prior to re-opening. Interestingly, even though both South Africa and Australia have legal definitions of an estuary (in contrast to the UK), neither cater for hyperhalinity.

The temporarily open/closed East Kleinemonde Estuary in South Africa (picture courtesy of Michael J Stone)

The temporarily open/closed Brega River Estuary in New South Wales, Australia

To avoid these problems of definition, legislators are increasingly developing new conservation, socioeconomic and legal definitions and classification systems for estuaries – the most widely accepted of which (in Europe at least) is the term ‘transitional waters’ coined by the European Union within the Water Framework Directive (WFD) to define all waters that are neither the open coast or true freshwaters (i.e. fjords, fjards, river mouths, deltas, rias and lagoons as well as the more classical estuaries).  Additional legal definitions have been developed for estuaries worldwide.  The problem with all of these definitions however, is that more often than not, they do not delimit where an estuary starts and where it ends – an issue which will be the subject of my next blog!