Making waves and moving sediment

Dr Hannah Williams has been a Post-Doctoral Researcher in Physical Geography at the University of Hull since April 2017. Hannah is part of the Hydralab+ project, a large European project that brings together researchers to improve experimental hydraulic research to better address climate change adaptation issues. Here she talks about a recent set of experiments carried out at the Total Environment Simulator.

Mixed Sediment Beaches are commonly found at high latitudes around the world, including amongst other locations, along the coastline of the United Kingdom. These types of beaches can consist of a mixture of both sands and gravels, and behave differently under hydrodynamic forcing, such as waves, to those made up of a single sediment size. Although some research, mainly in the 1970s-1980s, has been carried out to gain an understanding of the morphological behaviour of these types of beaches, little is still known about the variations in the morphology of these beaches due to mixed sediment, and how they respond to the hydrodynamic conditions.  The aim of this study was to try and gain some insights into beach response using a physical model.

At the University of Hull, we are lucky that we have a large experimental flume available for research called the Total Environment Simulator (TES). The TES has a working area of 11m by 6m, and is equipped with pumps to allow recirculating flow and sediment, a multi-paddle wave generator for the generation of both regular and irregular waves up to ~0.3m in height (depending on water depth), and finally is equipped with a rainfall generator sprinkler system on the roof. During my time at the University of Hull, I have been involved in experiments using all of these systems, demonstrating just how versatile the flume is. The photo below shows the TES when it first opened in 2000. As a well-used facility, it doesn’t look quite so clean anymore!  

For these particular experiments we were only interested in the beach response under wave loading, so only the wave generator system was required. We constructed a large beach across the opposite end of flume, with a height of 0.8m at the rear, and extending 5m towards the wave paddles. This gave the beach an initial gradient of 1:7.5.To obtain a mixed beach, we chose two different sediment sizes with a large difference in diameter. The fine sediment had a D₅₀=215μm (often known as play sand as it is commonly used in children’s sand pits), whilst the coarser sediment had a D₅₀=1.6mm. To construct this beach, this required over 5 tonnes of each type of sediment (and this including bulking out some of the area deep underneath the beach with breeze blocks), which all had to be lifted into the flume and distributed by hand. The photo below shows the initial smooth beach conditions. 

In terms of measurements, there were two main parameters we were interested in, firstly the incoming wave conditions. To measure these, we had 8 acoustic wave gauges distributed throughout the flume (see below). These recorded information about the wave heights and periods, from which we can gain an understanding of the transformation of the waves as they approach the beach. 

The second parameter we were interested in was the beach morphology. To measure this, we deployed a Terrestrial Laser Scanner. This was mounted from the ceiling above the beach. After each experimental run, the water was drained from the flume, and the scanner carried out a full 360 degree scan of the beach surface.The image below shows an example of a TLS scan, in which you can clearly identify the top of the swash zone, as well as a berm which has formed part way down the beach, and ripples in the lower section. 

For the actual experiments carried out here, we attempted to replicate some of the influence of the tidal cycle on the response of the beach. The experiments were run at three different water depths, namely 0.3m, 0.4m and 0.5m. In three of the experiments, we hit the beach with an initial storm (H=0.18m, T=2.2s, where H is wave height and T is wave period), at different points in the tidal cycle. One at high tide, then one at mid-tide on the flood tide, and one at mid-tide on the ebb tide. The purpose of this was to try and investigate the effect that timing of the storm with relation to the tidal cycle has on the beach response. After each storm a number of recovery events (H=0.10m, T=1.5s) were carried out, at each depth to complete a tidal cycle. The video below shows some of the experiments in action.

Using the laser scans, we can also examine the differences between scans, giving us an idea of the evolution of the beach throughout the experiments. From these we can obtain information about the amount of erosion and accretion at different points of the beach, and examine if this is different depending on when the storm occurred. The image below shows an example of a Digital Elevation Model of Difference, from which a number of interesting observations can be made.  It should be noted that Red shows accretion of sediment, whilst blue shows erosion of sediment. 

The very top of the beach remains white, this shows that the beach level here remains constant throughout the experiments, due to the wave run-up not reaching this point. Just below this section is a large area of erosion, this is the swash zone, where waves are breaking. This is a very energetic area which results in a large amount of sediment transport, mainly transported further down the beach to the zone showing large accretion. This is known as a berm and often forms as the wave deposits sediment. Below this area, it can be seen that ripples form. This is prior to the wave breaking where sediment movement occurs in an elliptical motion, forming small ripples on the surface. These are all features that are not unique to mixed sediment beaches, however, one feature that is, are the beach cusps. These can be identified in the figure by the regular arc shapes present. There is limited information on the origin of beach cusps, but once they have been created they are a self-sustaining formation. This is because as a wave hits the area of the beach with the cusp, it splits at the point and the water is forced either side. As the wave then breaks, the coarser sediment falls out of suspension and is deposited on these points (known as horns), whilst the water flows into the arc (also known as an embayment) where it in turn erodes out the finer sediment.

These experiments have only just finished, so analysis of the results is still on-going, but hopefully we will have gained some useful insights into the behaviour of mixed sediment beaches which can be used to help devise beach management plans in the future.

For more information on the work of the Hydralab+ project, then please visit: https://hydralab.eu/ 

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!