I’m in the US at the moment (outside Philadelphia to be more precise), visiting family and preparing to give a talk in a couple of weeks time at the aquarium in Long Beach. Readers of this blog can probably guess the general topic, but I wanted to give the talk a Californian flavour (or flavor), as well as putting more of an emphasis than usual on things marine. The California coast is a varied and dramatically dynamic one, and is much-studied. In looking into data on sediment movements for the region, I came across several extraordinary resources on the web. First of all, of course, the USGS: Open-File Report 2006-1219 documents historic shoreline change and sediment movements along the California coast. Then there are a number of reports from the Institute of Marine Sciences at the University of California, Santa Cruz, together with the California Coastal Sediment Workgroup and the state Department of Boating and Waterways, for example Littoral Cells, Sand Budgets, and Beaches: Understanding California’s Shoreline and the California Coastal Sediment Master Plan Status Report. These, and the associated publications, are incredible goldmines of information, massive amounts of data distilled, explained, and discussed that evaluate the ledgers of sediment movement along the Californian Coast. They define an measure sediment budgets, the incomings and outgoings for segments of the coast.
As with any budget, there are inputs and outlays: sand is added and removed. If there is a surplus, sand is deposited; if there is a deficit, erosion and land loss prevail. Understanding the sediment budget for a given location on the coast is key to defining both natural processes and the consequences of human interference, the budget measuring input of sediment versus removal, directions of transport, and volumes and variations with time of year. Here are some headline numbers for the average annual sand contribution to the California Coast (the standard measurement is in cubic yards: 100,000 cubic yards of sand would build a mile of beach 100 feet wide and five feet deep, or fill 5,000 large dump trucks):
- rivers – 5,695,000 cubic yards per year (72%)
- cliff erosion – 335,000 cubic yards per year (4%)
- reworked out of coastal sand dunes – 528,000 cubic yards per year (7%)
But you have noticed that these numbers don’t add up to 100% – where does the rest of the sand, 17%, come from? Well, us – through what is euphemistically called “beach nourishment.” To maintain or enhance our beaches in the way that we like them, or to make up for the removal of sand that our interfering in natural budgets has caused, we find, every year and at some expense, 1,338,000 cubic yards of sand somewhere else and scatter it along the coast. Well, “scatter” is hardly the right term - “industrially pump” might be more appropriate (see the picture at the head of this post). This is not, of course, just a Californian habit, but an important activity on the other side of the country too. Sand in gigantic quantities is dredged or pumped via monster hoses from offshore and sprayed onto the eroded beaches, at a cost of up to $10 million per mile. On the east coast barrier island chain, from the southern end of Long Island to southern Florida, the equivalent of 23 million dump trucks of sand has been spread over 195 beaches on more than seven hundred separate occasions. Virginia Beach has been “re-nourished” more than fifty times. A fundamental problem is not simply that this is treating the symptoms rather than effecting a cure, but that the wrong kind of medicine is often used. Beach sand is, after all, the specific local product of local processes, and replacing it with sediment dredged from offshore is replacing it with something else entirely—finer-grained sand, differently sorted sand, or mud. A typical “nourished” beach erodes much faster than a natural one. There are examples where the best sand has correctly been identified as that in tidal inlets—after all, it’s sand that derives from longshore drift along the beach. But remove that sand, and the only thing that is created is another problem—the next beach downstream from the inlet has a reduced supply: the budget is broken.
Any coast can be divided into natural segments within which the sediment budget is essentially self-contained, sediment transport in (sources) and out (sinks) are roughly in balance on average, and the component movements can be measured. These segments are littoral cells. The fundamental processes and numbers for any cell are naturally highly complex, and we can only begin to scratch the surface, so to speak, of quantifying sediment movements. But the kind of work described in the publications mentioned above is, even with all the associated uncertainty and unknowns, critical for understanding how our coasts work. And how we interfere with how they work. Dams impound sediment, sand and gravel are extracted from the system (some for beach nourishment), rivers are controlled, and urbanization stops sediment movement: we impact the budgets in a major (and rarely helpful) way.
This has been merely a surface scratching of the fascinating subject of sediment budgets and littoral cells, and I will no doubt return to these topics. But for now I’ll conclude with a graphic that I put together for the talk in Long Beach. I used data from the publications I mentioned to look at the San Pedro Cell, within which Long Beach is situated. If that particular stretch of coast were in its natural state, rivers would contribute essentially 100% of the incoming sand. As it is, they contribute just 41% – the rest is beach nourishment.