Beach Nourishment and Sediment Budgets

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 Shorelineand
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. SIGNATURE

Comments

BrianR (2009-05-18):

great post … and nice collection of links

malpais beach costa rica (2009-11-19):

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Miami Hotels (2009-12-13):

I was born in orlando and I always wanted to move to miami. I felt like Miami would be more suitable for me . Now I understand why. Amazing!.

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/05/beach-nourishment-and-sediment-budgets.html