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."