Through The Sandglass

Coastal change and saving sand - the state of the art

 

In my talk earlier this week, Ten Things We Don't Know About Sand, number 6 was "How coasts work." Now of course we know the basics, but coastal systems are so complex and dynamic that the details elude us. I used my postcards of French sandbanks and the fascinating work of Rob Holman at Oregon State University and his Argus program movies to show just how dynamic our coasts are and how difficult it is to describe or model them in a way that not only creates a detailed understanding of their processes, but therefore a means of sensibly managing them and planning for the future. And then I read the announcement of a new USGS publication, Saving Sand: South Carolina Beaches Become a Model for Preservation, in which the introduction includes the following:

Changes in the morphology of coastal landforms and patterns of sediment movement have been stud­ied for over a century, but, in general, scientists have only a rudimentary understanding of the processes that drive coastal change.

Sometimes I think that there must be something to the idea of synchronicity...

The full report, downloadable from the USGS link above, is a case study of the state of the art of our evaluation of how a single stretch of coast works. It's the summary of a six-year multidisciplinary study of the northeastern coast of South Carolina by the USGS in cooperation with the South Carolina Sea Grant Consortium (SCSGC). "The main objective of the study was to determine the geologic and oceanographic processes that control sediment movement along the region’s shoreline and thereby improve projections of coastal change." The stretch of coast studied is the "Grand Strand" segment, three headlands south from Cape Hatteras (where the lighthouse was heroically moved after decades of futile attempts to stop natural processes). It's only a 100 km length of coast, but it's a complex system of beaches, barrier islands, back barrier lagoons and tidal inlets. The variation of shore types is illustrated in the figure below, from the report.

 

And the dramatic rates of shoreline change, both long-term and short-term, are shown in the figure below. Note that the scale is in meters per year, ranging from +10m/yr to -10m/yr, over just this short length of coast.

 

The study is truly interdisciplinary, and employed a wide-ranging set of tools. Sidescan sonar, and the acoustic back scatter that it generates, allowed mapping of sediment types. Ground penetrating radar and different sub bottom profiling techniques revealed the structure and sedimentological/geomorphological history of the sub-seafloor geology. Current meters, sediment traps, wave-height sensors, and other instruments were attached to sturdy metal frames  and anchored to the seafloor to record constantly changing environmental conditions and sediment flux. The operations (below, right) were clearly more successful than those of the traverse and triangulation party of L. P. Raynor in 1924 (below, left, photo from http://www.photolib.noaa.gov/).

 

I won't attempt to describe here the results of this extraordinary programme of research, but they are fascinating. Substantial contrasts in coastal processes arise from sand supply and availability. Sand-starved segments of the coast are characterised by exposure of older sediments and sedimentary rock (as old as Cretaceous in age, from 70 million years ago) and respond to wave and storm action very differently from sand-rich segments. Beach profiles and the dramatic changes to those profiles vary enormously both with sand availability and with exposure to seasonal change and storm events. Sediment budgets and individual littoral cells, along with their specific sediment sources and sinks, change rapidly along the coast and on a scale smaller than we have previously been able to define. Sediment flux is hugely variable with the time of year and wave and weather conditions, displaying complete reversals of transportation direction. The illustration below is just one example of the detailed analysis of shallow offshore processes, sand ridges in this case lying just oceanward of the popular resort of Myrtle Beach.

 

And popular - never mind populous - describes this entire stretch of coast, and the work described in this publication attempts to address the thorny issues of future management of a coast that has seen profound changes as a result of human activity. The entire profile - and the associated sedimentary processes - of much of this shoreline has been fundamentally changed by development and all the sea walls, beach armouring, and attempts at "beach nourishment" that go along with it. But look at the rates of shoreline change illustrated above, and the scale of the challenge becomes clear. Shorelines move - that's what they do. And they do it on our time-scales. And they'll do it regardless of our puny bits and pieces of engineering. They may do it slowly but inexorably, day-in, day-out, or they may do it overnight - for example, as a result of Hurricane Hugo in October 1989 (another anniversary this month, along with the Loma Prieta earthquake), illustrated dramatically in the "before and after" photos of the aptly named Folly Beach, down the coast from Myrtle Beach, shown below (again from the amazing resource that is the photo library at the NOAA).   

 

In a 2002 report titled Coastal Sprawl: The Effects of Urban Design on Aquatic Ecosystems in the United States (written, very appropriately, by Dana Beach of the South Carolina Coastal Conservation League), we read the following:

Coastal counties cover 17 percent of the land area of the United States. Coastal watersheds, as described by the Department of Agriculture, represent just 13 percent of the nation’s acreage. By any measure, the coastal zone is a small part of the country, but it is home to more than half of America’s citizens. Moreover, today’s coastal populations are just the tip of the iceberg. Over the next 15 years, 27 million additional people—more than half of the nation’s population increase—will funnel into this narrow corridor along the edge of the ocean.

And population growth is but a subset of a broader problem that results from rising sea levels, the evolving economic and regulatory challenges, and environmental changes. I'll finish this post with a couple of quotes from the press release for the humbly-titled USGS Circular 1339 - I recommend downloading it - it's a cracking read.

“Effective beach preservation requires knowing the beach’s sand budget and understanding the geology that constrains it,” said U.S. Geological Survey lead scientist Walter Barnhardt. “It takes a systematic approach and strong partnerships at all levels of government with neighborhood associations and universities to keep a beach from simply washing away.”

“The comprehensive nature of this study -- considering the geologic framework, behavior and driving processes regionally -- has resulted in a remarkable baseline for better managing our beach and near- shore resources,” said Paul Gayes, Director of Coastal Carolina University’s Center for Marine and Wetland Studies.

“From inventory of potential future beach nourishment sand resources, to distribution of important hardbottom fish habitat, to models of beach behavior, this study forms the starting point for many present and future efforts. This work is regularly cited as a model approach and result for similar studies and efforts around the country,” said Gayes. 

To "around the country" I might add "and around the world."

The Bayeux Tapestry - the quicksand scene

 

My daughter is a great fan of The Simpsons (recently celebrating their 20th anniversary), and enjoys challenging anyone to name any topic for which she cannot quote an episode in which that topic featured. Sometimes, I feel as if I'm getting that way about sand, given the diversity of contexts in which it shows up, but I'm really not up to her professional level. However, until recently, if someone had challenged me to link the Bayeux Tapestry (which is not, incidentally, a tapestry, but more of an extravagant piece of embroidery) and sand, I would have been stumped: but not now.

I recently had the experience of seeing the Tapestry for the first time. The word "spellbinding" is not one I use often, but it seems the only way of describing it. The Tapestry, all 70 meters (230 feet) of it, is displayed in low light to preserve its extraordinary colours, and this adds to the magic as you walk along the story. It's an ancient graphic novel, an elongate comic book, almost an animation; you half expect to see Spiderman intercepting the Norman invaders, or the Sandman attacking their fleet. Such thoughts did not detract from my sense of wonder, but my reverence was briefly tested when I spotted Harold riding along with a hawk on his arm (as, I suppose, such noble folk generally did in those days); I was suddenly struck by a memory of my father reciting to me the comic monologue from music hall days (does anyone reading this remember Stanley Holloway?) of the Battle of Hastings, in which Harold was routinely depicted "on his 'orse with his 'awk in his 'and." Chuckling would have seemed entirely inappropriate, so I controlled myself.

But, back to the point. The early scenes depict Harold being captured in France by Duke William and, having sworn allegiance to him and giving his guarantee that he would not declare himself king, taking off on a campaign with William against a revolting member of the local aristocracy. In doing so, they had to pass by Mont St. Michel and cross the River Couesnon. Now in those days, the river was still behaving naturally, since it was long before human interference changed the regional sedimentology (see my piece on the massive experiment underway there). And the river contained quicksands. The scene that I've reproduced at the head of this post shows Norman horses struggling in the quicksand and Harold saving a couple of soldiers by dragging them out by hand (he carries one on his back). The Latin description reads "hic Harold Dux trahebat eos de arena" - "here Duke Harold pulled them from the sand."  The round hill with the structure on top of it (upper left of centre) depicts Mont St. Michel.

The story goes on to describe how Harold went back on his word and declared himself King, thereby incurring the wrath of William, who proceeded to invade England and defeat the Saxon army at the Battle of Hastings, in the course of which Harold was shot in the eye by an arrow:

And after the battle were over

They found 'Arold so stately and grand,

Sitting there with an eye-full of arrow

On his 'orse with his 'awk in his 'and.

And thus was our vocabulary doubled - but no need to sweat or perspire over that. 

Manchester Science Festival

Next monday, the 26th, I'll be giving a talk at the Manchester Science Festival (Manchester UK, that is). A year or so ago, on British TV, there was a series of programmes with titles such as "Ten things you didn't know about volcanoes," "Ten things you didn't know about earthquakes," and so on. It seemed to me that not only were these titles rather patronising to the viewer, but that the programmes, although entertaining, failed to convey the excitement of science. After all, what keeps scientists enthusiastic, and what keeps science going, is what WE don't know. And there's an awful lot in that category.

So I decided to talk about how even an apparently humble material has its mysteries, and illustrates a variety of areas of scientific enquiry. The talk is, of course, intended to entertain, and so I'll be stirring the occasional frivolity and magic trick into the mix, but think about it: granular physics, coastal and desert dynamics, meanders, meiofauna, Mars, Titan...... My challenge (as always) is what to leave out. The talk will be at Blackwells bookshop, 6.30 pm. And Jan Zalasiewicz, whose provocative book, The Earth After Us I've written about earlier, will be talking on tuesday evening. 

Liquefaction, boils, and volcanoes - sand's role in the Loma Prieta earthquake.

 

Candelstick Park famously survived the earthquake of October 17, 1989 for one simple reason: its foundations were firmly anchored to solid bedrock. But for large areas of central California, the advice against building a house on sand was dramatically and tragically justified. Areas built on young sands and silts, or on reclaimed land, were fine until the waterlogged sediments were shaken - all cohesion and strength in the sediments was lost and they turned to liquid, in the same way that apparently firm patches of beach suddenly suck at your wiggling feet. Liquefaction, this loss of strength in water-saturated sand, is notorious for being often the major cause of severe damage as a result of an earthquake; the sand not only becomes like a liquid but compacts and expels its water. The foundations of structures sink, rotate, and deform, with catastrophic consequences - in the Loma Prieta earthquake some buildings in San Francisco foundered to the point where their third floors were at ground level. The earthquake that devastated the region around Bhuj in northwestern India in 2001 was one of the most damaging in the country’s history. Twenty thousand people were killed and the havoc caused more than $3 billion of damage, much of it as the result of liquefaction - huge cracks in the earth appeared, and water and sand erupted in volcanic fountains as the ground collapsed back on itself. This fickle behaviour of granular materials is the subject of intensive research by engineers and physicists - and microbiologists. As I described earlier, a humble soil-dwelling bacterium, bacillus pasteurii, can make sandstone out of loose sand, gluing the grains together in a way that offers promise for offsetting the threat of liquefaction.  

 

As in Bhuj, liquefaction during an earthquake also shows up in bizarre ways. Layers of sand below the ground surface will liquefy and, under the pressure of the overlying sediments, will exploit any fissure or other line of weakness to flow upwards and burst out on the ground surface as an eruption of sand and water. Known variously but descriptively as sand volcanoes, boils or blows, these can appear anywhere. The image at the head of this post shows a sand volcano that erupted during the Loma Prieta earthquake in the median of Interstate 80 west of the San Francisco-Oakland Bay Bridge toll plaza. The images below are of sand-inundated fields after the Loma Prieta event (left) and, right and below, the Imperial Valley earthquake of October 15 1979 (this seems to be a good week for California earthquake anniversaries).

 

A principle cause of damage by liquefaction is the phenomenon of lateral spreading as the liquid sand spreads out, clearly demonstrated in the photo below (also after the Loma Prieta earthquake) where the road surface has been torn apart by the movement of underlying sand from right to left.

 

And liquefaction was also amply demonstrated in the most powerful series of earthquakes to strike the lower 48 in recorded history—not in California, but in New Madrid, Missouri, between 1811 and 1812. New Madrid at that time was, fortunately, a small town of four hundred inhabitants, none of whom were aware that they lived above an ancient system of deep fractures in the Earth’s crust beneath the Mississippi River Valley. The fractures do not exercise themselves very often, but when they do it is with extreme violence. In the early hours of December 16, 1811, the terrified residents were awoken by violent shaking, accompanied by an appalling roaring sound. They later reported that the surface of the Earth moved in waves and that cracks opened in the ground from which water and sand erupted. One resident, Eliza Bryan, wrote that “the surface of hundreds of acres was, from time to time, covered over in various depths by the sand which issued from the fissures, which were made in great numbers all over this country, some of which closed up immediately after they had vomited forth their sand and water”

If we excavate a sand volcano and examine its plumbing, the sand-filled fissures through which the eruption took place are revealed. This is again analogous to volcanic eruptions of molten rock, where a volcano is fed via underlying cracks through which the magma forced its way. When the molten rock cools and solidifies, these filled feeder cracks are called "dikes", often harder than the surrounding rock and now forming long walls across the landscape. The examples below, cutting across the Namibian desert, record the igneous activity associated with the opening of the southern Atlantic Ocean (photo by the author).

 

The same process, on a more modest and cooler scale, can be seen below sand volcanoes, where the fissures are plugged with sand, silt, and mud. We see these things too in the geological record - clastic dikes (since they are formed of clastic, fragmentary, sediments0 cutting through the surrounding rock. They record past episodes, sometimes on a large scale, of sediment liquefaction. The examples below come from the Badlands National Park of South Dakota (from a good article by the University of Nebraska at Omaha, Dept. of Geography and Geology). These things, for obvious reasons, are sometimes also referred to as injectites.

 

And then there's Sodom and Gomorrah. We are often tempted, for dramatic purposes, to compare the destructive power of natural events such as earthquakes to the biblical demise of the twin cities. Ironically, work by David Neev of the Geological Survey of Israel and K.O. Emery of Woods Hole Oceanographic Institution, together with research by engineers and geologists in the United Kingdom, has suggested that, assuming the cities existed at all, they may well have been destroyed by earthquakes and liquefaction. The area around the Dead Sea, the likely location for Sodom and Gomorrah, lies across the boundary of two rapidly shifting segments of the Earth’s crust and has experienced significant earthquakes over recorded history. Lying below sea level, it is also the ultimate destination of vast volumes of sand and mud, and in many of these ancient layers are the telltale signs of violent water expulsion from the sediments. The photos below show two examples - a sandstone dike on the left and, on the right, incredible folding of the sand and mud layers as a result of sliding like a crumpling table cloth. Altogether, these forensic clues point to an obvious culprit for the destruction. But what about the fire and brimstone? Bitumen, natural asphalt or tar, was much prized during ancient times for medicine, the caulking of boats, and the preservation of mummies, and sulphurous bitumen, together with lighter oil, has for thousands of years been leaking from fractures around the Dead Sea, seeping into the ground and the water. All that would be required would be gas leaking along with the oil and a spark as the ground catastrophically gave way—fire and brimstone?

So, from the Loma Prieta earthquake to Sodom and Gommorah - wherever will sand take us next?

[There is lots of information on the internet and the geoblogosphere about the Loma Prieta anniversary; Andrew Alden on About Geology has been collecting personal stories of the event. For liquefaction, there is, as always, no better resource than the USGS - go to their website and enter "liquefaction" or sand plus "volcanoes" or "boils" or "blows" and you'll find excellent articles and images; for images of the Loma Prieta earthquake, go to the USGS gallery] 

Earth Science Week - sand and the nine big ideas

 

This is Earth Science Week, in the US at least. It's an idea that deserves, through globalisation or international intellectual contagion, to be celebrated worldwide. Sponsored by the American Geological Institute, and supported by a consortium of earth science organisations, it is also underpinned by the Earth Science Literacy Initiative funded by the National Science Foundation:

The Earth Science Literacy Initiative (ESLI).... has gathered and codified the underlying understandings of Earth sciences into a succinct document that will have broad-reaching applications in both public and private arenas. It establishes the “Big Ideas” and supporting concepts that all Americans should know about Earth sciences. The resulting Earth Science Literacy framework will also become part of the foundation, along with similar documents from the Oceans, Atmospheres and Climate communities, of a larger geoscience Earth Systems Literacy effort.

The primary outcome of the Earth Science Literacy Initiative is a community-based document that clearly and succinctly states the underlying principles and ideas of Earth science across a wide variety of research fields that are funded through the NSF-EAR program, including Geobiology and Low-Temperature Geochemistry, Geomorphology and Land-Use Dynamics, Geophysics, Hydrologic Sciences, Petrology and Geochemistry, Sedimentary Geology and Paleobiology, and Tectonics.

It goes (I hope) without saying that the fundamentals of Earth Science Literacy are principles that all human inhabitants of our planet should be familiar with - a utopian aspiration perhaps, but it would arguably make our planet a better place. The fundamentals have been distilled into nine "Big Ideas," each with its own set of supporting concepts, and the summary and guide can be downloaded from the website as a pdf. And it's a compelling document; an Earth Science literate person is defined as someone who

• understands the fundamental concepts of Earth’s many systems

• knows how to find and assess scientifically credible information about Earth

• communicates about Earth science in a meaningful way

• is able to make informed and responsible decisions regarding Earth and its resources

As I read through the nine Big Ideas (and being the arenophile that I am), I was struck by the way in which the stories that sand has to tell can form a theme, a narrative thread that weaves through each of the ideas, illustrating and developing. So below are the nine Big Ideas, each one illustrated by sand, together with links to posts on Through the Sandglass that are but a minor sampling of those stories. This is, on the one hand, something of an indulgence, a sort of retrospective of this blog so far, but, on the other hand, I would like to think that it might be an inspiration, a different, and entertaining, route to Earth Science Literacy. 

1. Earth scientists use repeatable observations and testable ideas to understand our planet. And a large variety of scientific principles are harnessed to reveal the complexities of the earth system. 

Whether it's in the field or the laboratory, as providing vital testaments to the Earth's history or revelations on surface processes and the bizarre behaviours of granular materials, sand and sandstones provide crucial evidence and insights. Sedimentary structures allow reconstruction of past environments and events, a typical integration of fundamental physics, engineering, mathematics, and fieldwork led to our first understanding of how deserts work, and analysis, through granular physics, of the underlying laws of natural systems sheds light on a wide range of phenomena. 

2. Earth is 4.6 billion years old. And throughout its history enormous changes have occurred through gradual and catastrophic processes.

The age of our planet is determined from meteorites and lunar materials, but the oldest bits and pieces formed on the earth itself are sand grains - zircons from Australia dating back more than 4.2 billion years. And zircon grains have many tales to tell of our planet's tumultuous history of change. And then the entire sedimentary record of the earth's history, incomplete and variable as it is, represents the fundamental ledgers of the saga.  

3. Earth is a complex system of interacting rock, water, air, and life.

Whether inorganic in origin, formed from the disintegration of rocks under the assault of atmospheric weathering, or biogenic, formed from shell and other organic debris, sand tells the stories of chemical and hydrologic cycles, energy and material transfer, and mass balances of natural materials. The size of sand grains influences the appearance of the entire planet.

4. Earth is continuously changing.

Sand, by its very nature, is perhaps the most dynamic of natural materials, and geological change on a human timescale is daily illustrated by shape-shifting bodies of sand on our coasts, in our oceans, and in our rivers and deserts. 

5. Earth is the water planet.

And sand is a ubiquitous participant in the great game that water plays in earth processes. Go to the beach and watch the game in action in miniature, or wonder at the huge scale of marine sand transport, the complexity of coastal processes, or the changing dynamics of rivers (with or without human interference).

6. Life evolves on a dynamic earth and continuously modifies earth.

Evolution is a fact. Sand preserves the evidence and can drive ongoing evolution today. Microorganisms are the most widespread, abundant, and diverse group of organisms on the planet, and many of them spend their lives in the company of sand grains. Our beaches host biodiversity possibly greater than that of the rain forest; bacteria turn sand into sandstone, and countless critters build their homes from sand. And then there are the provocative ideas on the interactions between organic and inorganic evolution.

7. Humans depend on earth for resources.

And sand represents a huge resource in countless ways. Electronics, concrete, and precious minerals are but a few examples of sand as a resource - and think how much of our water and hydrocarbons reside in the pore spaces between the grains of sand and sandstone reservoirs.

8. Natural hazards pose risks to humans.

The residents of New Orleans or Samoa whose houses were filled with sand following hurricane Katrina and the recent tsunamis are only too aware of this. The good folk of the Red River Valley, desperately filling sandbags during the floods of earlier this year know all about natural hazards. Desert towns around the world face the threat of moving sand every day. But careful earth science investigation of past catastrophes - for example, tsunami forensics - can help us understand these risks and prepare for future events.  

9. Humans significantly alter the earth.

Boy, do we ever. Earth Science Week this year is dedicated to climate change, but there are endless other examples of our footprint. We dramatically change sediment budgets, altering our coasts on a daily basis; we completely disrupt the natural dynamics of rivers and their sediment transport role. And we do these things often for the most trivial and short-term reasons.

A different kind of sand - on the road again in the volcanoes of the Auvergne

I've just spent several days rambling around the geology of the Puy de Dome and the surrounding volcanoes in the Auvergne region of France's Massif Central. These are extraordinary and surprising landscapes - volcanicity with a long history extending up to a mere five thousand years ago, leaving one to wonder whether or not it is indeed all over and done with. But this is not the only fascination of this area; it is one of the less-heralded, but nevertheless key, cradles of geology in the early part of the nineteenth century. Much more on this later, but meanwhile an example of volcanic ash - a different kind of sand - and a sample of the volcanic crater landscape. 

Two tourist lessons in sand dynamics

A few days ago, I described the fractal sand landscapes on the beach of Cap Ferret on the Atlantic coast west of Bordeaux. The swirling sand banks that shape-shift around the cape are but part of the great drama of shifting sand around the Bay of Arcachon and southwards past the great dune of Pyla, the largest in Europe. I left Cap Ferret and navigated around the bay to visit the dune where I found a tourist attraction not much less teeming than that of Mont-Saint-Michel (my previous post). Getting to the dune required passing through endless stalls selling the predictable claptrap and tat, but I did find a good selection of postcards that showed aerial views of the dune and the offshore sandbanks. I bought several, but it was only later, when I laid them out, that I realised how vividly they illustrated the real-time dynamics of this great sand system. Each postcard view was taken at a different time (although, irritatingly, not documented) – and every one is visibly and dramatically different. One is shown at the head of this post, the others below, together with the Google Earth view – a lesson in the futility of nautical charts in the arenas of the great games that sand plays.
 

I also found, amongst the claptrap, large numbers of “sand picture” souvenirs, plastic rotatable frames within which lurid varicoloured sand is set in an oil of some kind – a sort of arenaceous lava lamp. Turn the frame and the sand drifts down to create a new “landscape.” I bought one of the least lurid and I have to admit that I have enjoyed experimenting with it. As shown below, I have created some vivid cross-bedding reminiscent of my kitchen physics experiments I described back in April. So, search among the tourist tat and you can find lessons in marine sand dynamics and the behaviour of granular materials!

 

[On the base of my sand landscape generator, I found the website of its maker, Baz-Art]